Selection of Surge Protective Device (SPD)- (Part 4)

Typical System Voltage & MCOV rating (As per IEEE):


Typical IEEE System Voltages
Normal (Line to Line) Voltage (KV rms) Maximum ( Line to Line ) Voltage (KV rms) Maximum (Line to Ground) Voltage (KV rms) Min MCOV
kV rms kV rms kV rms kV rms
2.40 2.52 1.46 1.46
4.16 4.37 2.52 2.52
4.80 5.04 2.91 2.91
6.90 7.25 4.19 4.19
8.32 8.74 5.05 5.05
12.0 12.6 7.28 7.28
12.5 13.1 7.57 7.57
13.2 13.9 8.01 8.01
13.8 14.5 8.38 8.38
20.8 21.8 12.6 12.6
22.9 24.0 13.9 13.9
23.0 24.2 14.0 14.0
24.9 26.2 15.1 15.1
27.6 29.0 16.8 16.8
34.5 36.2 20.9 20.9
46.0 48.3 27.9 27.9
69.0 72.5 41.9 41.9
115.0 121 69.8 69.8
138.0 145 83.8 83.8
161.0 169 98 97.7
230.0 242 140 140
345.0 362 209 209
500.0 525 303 303
765.0 800 462 462


Typical System Voltage & MCOV rating (As per IEC):


Typical IEC System Voltages
Normal (Line to Line) Voltage (KV rms) Maximum ( Line to Line ) Voltage (KV rms) Maximum (Line to Ground) Voltage (KV rms) Minimum Uc
kV rms kV rms kV rms kV rms
3.3 3.7 2.1 2.1
6.6 7.3 4.2 4.2
10.0 11.5 6.6 6.6
11.0 12.0 6.9 6.9
16.4 18.0 10.4 10.4
22.0 24.0 13.9 13.9
33.0 36.3 21.0 21.0
47.0 52 30.1 30.1
66.0 72 41.6 41.6
91.0 100 57.8 57.8
110.0 123 71.1 71.1
132.0 145 83.8 83.8
155.0 170 98.3 98.3
220.0 245 142 142
275.0 300 173 173
330.0 362 209 209
400.0 420 243 243


SPD and Fuse / CB co-ordination chart:

Fuse/CB co-ordination chart
Incoming feeder fuse rating (A) Incoming feeder CB Rating (A) SPD fuse rating (A) SPD CB rating (A)
16 6 10 4
25 10 16 6
32 16 20 10
40 20 25 16
63 32 40 20
80 40 50 25
125 63 80 40
160 80 100 50
250 125 160 80
500 250 320 160


Sample Specifications of SPD for 277/480V Supply System

  • Voltage :277/480V 3Ø WYE, 480V 3Ø Delta
  • Frequency: 50/60Hz
  • Surge Technology: 40mm MOV
  • Nominal Discharge Rating (IN):20kA
  • Maximum Continuous Operating Voltage (MCOV): 320V
  • L-L=640V
  • L-N=320A
  • L-G=320A
  • G-N=320A
  • Maximum Surge Current, Per Mode (Per Phase) : 200kA (400kA
  • Voltage Protection Rating (VPR) (Clamping) : 800V(L-N)/700V(L-L)
  • Short Circuit Current Rating (SCCR): 10kA
  • Connection Type: Parallel Connection

  Reason for Failure of SPD :

Most SPDs will last for many years. The things that cause sudden failure are

  • External supply faults such as overvoltage -faulty transformer, MV lines
  • Local supply faults -broken or ungrounded neutral.
  • Wrongly-selected SPD voltage.
  • A surge in excess of the SPD’s rating.


Classes’ of surge arrestors according to impulse current:


 1 = Test impulse current for lightning current arresters

2 = Test impulse current for surge arresters

  •  There are 3 x main categories of lightning surge arresters.
  • Class 1/A – (10/350) lightning current arresters, which can withstand direct lightning
  • Class 2/B – (8/20) surge arresters, to protect against induced surge currents
  • Class 3/C – (8/20) surge arresters, to protect against induced surge currents

 Meaning of 20kA (8/20μS) impulse current.

  • In 8/20μ The first value (8) is the rise time (from 10% to 90% of peak). The second value (20) is the duration for the test transient to decrease to half its peak value.

Standard for SPD:

  • Underwriter laboratories—UL 1449 (3rd Edition 2009)
  • IEEE C62.45 (2002)
  • NECT National Electrical Code Articles 245, 680 and 800.
  • NFPAT 780 Lightning protection code recommendations for the use of surge protection devices at a facility service entrance.

Selection of Surge Protective Device (SPD)- (Part 3)

Type of SPD:

Type 1 SPD:

  • Protection for : Transient Over voltages due to Direct Lightning Strokes
  • Location : It is installed at any location between the secondary of the utility service transformer and the service entrance primary disconnection
  • It is installed in the main electrical switchboard when the building is equipped with a lightning protection system.
  • It protects against external surges caused by lightning or utility capacitor bank switching.
  • These devices to discharging a very high lightning current from earth to the power distribution system.
  • Current ratings: 10Ka to 35Ka – 10/350µs wave form.
  • Required Dedicated Fuse / Circuit Breaker for SPD : No
  • Risk Factor : Very strong risk Area

Type 2 SPD:

  • Protection for : Transient Over voltages due to Switching and Indirect Lightning Stroke.
  • Location: It is installed in the main distribution switchboard.
  • It is designed to discharge the currents generated by indirect lightning strokes and causing induced or conducted overvoltage on the power distribution network.
  • It protects against residual lightning energy, motor driven surges and other internally generated surges.
  • Current ratings: 5Ka to 200 Ka – 8/20µs wave form.
  • Required Dedicated Fuse / Circuit Breaker for SPD : May or May Not
  • Risk Factor : Common risk Area

Type 3 SPD:

  • Protection for: Sensitive Loads.
  • It is installed as a supplement to Type 2 devices and to reduce the overvoltage at the terminals of sensitive equipment.
  • Their current discharge capacity is very limited. As a consequence they cannot be used alone.
  • Installed at minimum conductor length of 10 meters (30 feet) from the electrical service panel to the point of utilization
  • Provides point-of-use protection, easily replaceable and it provides the last line of defense against a lightning strike.
  • Risk       Factor : Very strong & common risk Area

Connection of SPD in Distribution Box.

  • In common mode: Phase to earth or neutral to earth
  • In differential mode: Phase to phase or phase to neutral


  Factors effect on SPD Performance:

(1) Location of Surge Protection Device:

  • Lightning protection should be installed on a overall viewpoint of Protection.
  • For large industrial plants, data centers, hospitals, a risk assessment method must be used to guide in choosing optimal distance.
  • In other cases like housing, offices, buildings Where there is not or less sensitive industrial risks, we may adopt following principle to select SPD.Type 2 surge protective device should be installed in the electrical installation’s incoming Main switchboard.
  • If the distance between that surge protective device and the equipment to be protected is more than 30 meters, than additional surge protective device (Type 2 or Type 3) should be installed near the equipment.


  • When the building is equipped with a lightning protection system, a Type 1 surge protective device must be installed at the incoming Main Switch Board. There exist surge protective devices combining Type 1 and Type 2 in the same enclosure. 2
  • The Lightning rods have to be located on the highest points of the structure, taking into account the location of the grounding, and that the path of the down conductors are as short and straight as possible .

(2) Size of Down Conductor:

  • Lightning is a phenomenon that generates a high frequency voltage. The length of the cables must be taken into account in cases of high frequency.
  • The down conductors may be tapes, stranded wire or solid round.
  • The minimum cross section must be
  • 1 meter of cable crossed by a lightning current generates an overvoltage of 1,000V.
  • Mandatory in Standard IEC 60364-5-534:
  • L (length of cables) < 50cm,
  • Cable cross-section of Cable (S) < 16mm² (Type 1).
  • Cable cross-section of Cable (S) < 4mm²(Type 2).

(3) Placement of Down Conductor

  • Down conductor will be placed on the outside of the structure.
  • When it is impossible to make a down conductor on the outside, conductors can be introduced in a non-flammable insulating pipe, with a minimum section of 2000 mm2, for this purpose.
  • The down conductors on the inside decrease the effectiveness of lightning protection, increase the risk of over voltages penetration of and difficult the verification and maintenance of installation.

(4) Number of Down Conductor

  • At least one down conductor for every lightning rod. A minimum of two down conductor when,
  • (1) The horizontal Projection length of the conductor exceeds its vertical projection length.
  • (2) The height of the structure is greater than 28 meter.
  • Equi potential bonding will be made between the conductors at ground level and every 20 meters.
  • According to UNE 21186:
  • (1) Each lightning rod shall be grounded by two down conductor.
  • (2) It will be necessary 4 down conductors on buildings higher than 60 meters.
  • (3) It should be placed whenever possible in the 4 corners of the building.

(5) Path of Down Conductor

  • The down conductor routes will the shortest path, straight and direct to grounding.
  • We should avoiding elevations above 40 cm with slope equal to or greater than 45°.
  • The radii of curves shall not be less than 20 cm and direction changes less than 90°.
  • The route will be chosen so as to avoid proximity to electrical conduits, telephone, data and its crossing with them.
  • In any case, when we can not avoid an intersection conduits must be placed inside a metallic shield that extended 1 m to each side of the crossing, and the shield should bind to the down conductor.

(6) Safety Distance:

  • According CTE SU8:
  • Safety distance (m) = 0.1 x L
  • L = vertical distance from the point where it is considered the proximity to the grounding of the metal mass
  • Safety distance to outdoor gas pipelines ≥ 5 m.
  • According to UNE 21186:
  • Safety distance for 1 no of down conductor (m) = 0.16 x L
  • Safety distance for 2 No of down conductors (m) = 0.08 x L
  • Safety distance for 4 No of down conductors (m) = 0.04 x L
  • L = length of the down conductor from the point where it is considered the separation distance to the point where is located the nearest equipotential point.

(7) Earthing:

  • There will be 1 grounding system for each down conductor.

(8) Lighting Counter:

  • The lightning counter must be installed over the more direct down conductor, above the joint control, and in all cases, about 2 meters above the ground.

(9) Size of Surge Protection Device:

  • A type 2 surge protective device depends mainly on the exposure zone (moderate, medium, high).
  • Type-2 SPD has discharge capacity (Imax) of 20 kA, 40 kA, 65 kA (8/20 µs).
  • Type 1 SPD has minimum discharge capacity (Imax) of 12.5 kA (10/350).
  • Higher values may be required by the risk assessment when it’s required.
  • For residential or light commercial locations: a surge current rating of 20 kA to 70 kA (8/20 µs) per phase should be sufficient. Installations in
  • For high-lightning areas: SPDs with higher surge current ratings of 40 kA to 120 kA, to provide a longer service life and higher reliability.

Selection of Surge Protective Device (SPD)- (Part 2)

Size of Surge Protection Device (SPD) depends upon Location of Panel:

  • Panel location within the electrical system is more important than the panel’s size.
  • The location of the panel within the facility is much more important. IEEE C62.41.2 defines the types of expected surges within a facility as:
  • Category C: Service Entrance, more severe environment: 10kV, 10kA surge
  • Category B: Downstream more than 30feet from category C, less severe environment: 6kV, 3kA surge
  • Category A: Further downstream, more than 60 feet from category C, least severe environment: 6kV, 0.5kA surge
  • When selecting the appropriate kA rating for an SPD.
  • Category C: 100kA to 200kA per phase
  • Category B: 50kA to 100kA per phase
  • Category A: 50kA to 100kA per phase

Large Size of Surge Protection Device (SPD) does not give better Protection:

  • Most SPDs use a metal oxide varistor (MOV) as the main limiting device. If an MOV is rated for 10kA and having a 10kA surge, it would use 100% of its capacity. The surge will degrade the MOV a little bit.
  • Now if we use 20KA SPD so this SPD has two 10kA MOVs in parallel. The MOVs will equally split the 10kA surge, so each would take 5kA. In this case, each MOV have only used 50% of their capacity which degrades the MOV much less than 10KA SPD
  • Again It is totally misleading that two parallel path (in 20KA SPD) absorb surge faster or better than single path SPD (like 10KA SPD) of same rating.
  • The main purpose of having MOVs in parallel is to increase the longevity or Life of the SPD.
  • Again, It is need to clear that it is subjective and at some point we are only adding cost by incorporating more MOV’s and receiving little benefit.
  • Larger kA ratings are for redundancy & longer life only.

SPD can not give 100% Protection against All Types of electrical disturbance

  •  There is a misconception about SPDs is that they are designed to protect against all Electrical problems.
  • SPD is not designed to protect against excessive voltage at the fundamental power frequency. It is design to give protection against surges (by direct lighting or voltage surges in line at remote location).
  • SPD can not give Protection against Poor Power Quality (Harmonics)
  • Some SPDs contain filtering to remove high frequency noise (50 kHz to 250 kHz), But SPD cannot filter harmonic loads (3rd through 50th harmonic equals180 to 3000 Hz).
  • SPD can not give Protection against Under Voltage.
  • SPD can not give protection against under voltage problems.
  • SPD can not give Protection against direct lighting Strikes.
  • An SPD can not prevent damage caused by a direct lightning strike. A direct lightning strike causes induced surges on the power line that are reduced by the SPD But SPD can not Protect against Lighting Strikes near SPD Location.
  • SPD can not give protection against temporary overvoltage.
  • Temporary overvoltage is caused by a severe fault in the utility power or due to problems with the ground (poor or nonexistent N-G bond).
  • Temporary overvoltage occurs when the Voltage exceeds the nominal voltage for a short duration (millisecond to a few minutes).
  • If the voltage exceeds 25% of the nominal system voltage, the SPD and other loads may become damaged.

Selection of Surge Protection device (SPD):

  • The Size, performance and specification of SPD depend on following characteristics

Current characteristic of SPD

  • I:Surge Current Rating (KA),
  • In: Nominal Discharge Current (In),
  • Imax: Maximum discharge Current (Imax)
  • Short Circuit Current Rating (SCCR).

Voltage characteristic of SPD

  • Uc: Maximum Continuous Operating Voltage (MCOV),
  • Up: Voltage Protection Rating (VPR) or surge voltage rating (SVR) or Clamping Voltage.
  • TOV: Temporary Over Voltage.


(1) Surge Current Ratings (I):

  • The peak surge current ratings of SPD are generally based on the sum of Line-neutral and Line-ground current.
  • A peak ampere rating per phase. (I.e. L-N 100 kA, L-G 100 kA provides 200 kA/phase).
  • Other Specification like MCOV, VPR, In and SCCR that have clearly defined test criteria, but for Surge Current there is no specified Test Criteria or industry-standard hence different SPD manufacturers to create their own definitions of peak ampere surge current ratings.
  • Please note that selection of Higher Surge Current Ratings don’t always gives Better Protection but it is provide loner life.
  • IEEE Clearly states that “The selection of a surge current rating for an SPD should be matched to the expected surge environment and the expected or desired useful life of the device.”
  • Selection of Surge Rating for an SPD depends on The location of the SPD within the electrical distribution & environmental surroundings condition of Site.
  • Following surge current ratings based on SPD location within the electrical distribution.


Surge current ratings based on SPD location
Location Surge Current
Service Entrance Locations 240 kA
Distribution Locations 120KA to 160 kA
Branch Locations 50KA to 120 kA


(2) Nominal discharge current rating (In):

  • The Nominal Discharge Current is the peak value of surge current conducted through the SPD. It has 8/20μs Impulse current Waveform .The SPD must function after 15 applied surges.
  • Nominal discharge Current shows durability of SPD. The highest nominal discharge current rating is 20kA.
  • Example : calculate In for Maximum peak current(Surge Current): I=200 kA (the maximum level of natural lightning where 5% of strikes are bigger than 100 kA)
  • Assume that for perfect current sharing 50 % to ground and 50 % to the electrical network
  • Network configuration is 3 Phases + Neutral (n=4)
    In = Surge Current X Current path to Ground (%) / No of Path =200 x 0.5 / 4 = 25 kA
  • The Nominal discharge current values, with a 8/20μs wave shape as per UL 1449 are
  • Type 1 SPD (In)= 10KA or 20 kA
  • Type 2 SPD (In)= 3KA ,5KA,10KA or 20 kA
  • The Nominal discharge current value as per IEEE C62.41 is 200A to 10KA.
  • The Nominal discharge current value as per NFPA is 20KA

 (3) Maximum discharge current (lmax):

  • The maximum surge current between any one phase and neutral that the SPD can withstand for a single strike of 8/20µs or 10/350μs current is called Maximum discharge current of SPD.
  • This is the maximum value of a surge current that can be diverted by the surge protective device.
  • current surges have two different wave shapes
  • Lightning currents is a long wave shape (10/350μs) which represents direct lightning strike.
  • Short wave shape (8/20 μs) which represents a indirect strike;
  • lmax is the maximum value of a short wave shape current and limp is the value of a long wave shape current; the value lmax or limp has to be adapted to the expected value of the possible lightning currents.
  • Imax > In

 (4) Short circuit current rating (SCCR):

  • Maximum symmetrical fault current, at rated voltage, that the SPD can withstand without sustaining damage is called SCCR of SPD.
  • Every electrical system has an available short circuit current. This is the amount of current that can be delivered by the system at a particular point in a short circuit situation.
  • SCCR shoes that Measure of how much current the electrical utility can supply during a fault condition.
  • SCCR is not a surge rating but it is the maximum allowable current a SPD can interrupt in the event of a failure.
  • NEC Article 285.6 says that the SPD to be installed where the available fault current is less than the SCCR rating of the SPD unit.
Typical available short circuit currents
Load short circuit currents of SPD
Residential 5KA to 10kA
Small commercial 14KA to 42kA
Large commercial/industrial 42kA to 65kA
Large industrial/utility/downtown in large cities 100kA to 200kA
At a sub panel 120kA to 160kA provides good protection and life
Point of use SPDs 80kA to 100kA perform well


  (5) Calculating Maximum Continuous Operating Voltage (MCOV or Uc):

  • When Surge Protector are installed to protect systems from lightning or switching surges, it should be installed between the phase and earth. Hence MCOV of the installed arrester must be equal or higher to the continuous voltage between the phase and earth.
  • On three phase systems, the line to ground voltage is equal to the phase to phase voltage divided by 1.73
  • For example: on a 440kV transmission system, the nominal system phase to phase voltage is 440kV therefore the line to earth voltage would be 440/1.73=254kV. Since all systems have some regulation error. If the regulation is 10%, then the line to ground voltage could be 254x 1.10 = 280kV. The MCOV or Uc or an arrester for this system at a minimum should be 280kV.
Typical MCOVs
System MCOV
120V system 150V MCOV
240V system 320V MCOV
480V system 550V MCOV


  • Selecting SPD with too low of a voltage rating will result in SPD failure
  • Selecting SPD with too high of a voltage rating will result in reduced protection

 (6) Calculating Line to Ground Voltage:

  • The maximum rms voltage that can be applied to each mode of the SPD is called MCOV
  • When a three phase power system have a fault between one of the Phase to earth, the Voltage of two healthy phases to ground increase. Since Arrestor is mostly connect between Phase and Earth hence Voltage across LA terminals also Increase.
  • This increase in voltage will remain across the arrester until a system breaker operates and breaks or interrupts the fault. This is a very significant event in the life of an arrester and must be accounted for during the voltage rating selection of an arrester.
  • There are some rules of thumb and graphs that can be used, but these are quit crude and difficult at best to use. Annex C of IEEE standard C62.22 and Annex A of IEC 60099-5 cover this subject.
  • For distribution systems where the system and transformer impedances are relatively unknown, a worst case scenario is used for each type of system. The voltage rise during a fault in these cases is determined by multiplying the line to ground voltage by
Type of System Ground Fault Factor
Solidly Grounded 4 wire systems 1.25
Uni-grounded 3 wire systems 1.4
Impedance grounded systems 1.73
Isolated Ground Systems and Delta Systems 1.73
  • For example: In a 440kV multi-grounded system, the maximum continuous line to ground voltage = Phase to Phase Voltage /1.73 =440/1.73=254kV. The voltage during a ground fault on the un faulted phases can reach 254 x 1.25 or = 318kV rms. This is the voltage an arrester will see across its terminals for as long as the fault exists.

 (7) Voltage protection level ( UP at In):

  • This is the maximum voltage across the terminals of the SPD when it is active. This voltage is reached when the current flowing in the SPD is equal to Nominal discharge current (In).
  • The voltage protection level must be below the overvoltage withstand capability of the loads.
  • In the event of lightning strokes, the voltage across the terminals of the SPD generally remains less than Up.
  • While diverting the surge current to the ground Voltage Protection Level (Up) must not exceed the voltage withstand value of the equipment connected downstream.
  • Suppressed Voltage Rating (SVR) was part of an earlier version of UL 1449 Edition and is no longer used in the UL 1449 standard. The SVR was replaced by VPR.

 (8) Temporary Over Voltage (TOV):

  • It is used to describe temporary Surge which can arise as a fault of faults within medium & Low voltage.
  • UTov=1.45X Uo, where Uo= Nominal Line to earth Voltage.
  • For 230/440V System UTov=1.45X230 = 333.33Volt

Selection of Surge Protective Device (SPD)- (Part 1)


  • A device which diverts or limits surge current is called Surge protective devices (SPD).
  • SPD protect electrical equipment against over voltages caused by lightning or Switching. It is wired in parallel to the equipment which is needed to be protected.
  • Once the surge voltage exceeds SPD’s rating it starts to conduct energy directly to the electrical grounding system. An SPD has a very low resistance during this time and give low resistance path the energy to ground. Once the surge is over it gives high resistance path to current.
  • SPD is previously known as Transient Voltage Surge Suppressors (TVS) or Secondary Surge Arresters.
  • Underwriter laboratories ,UL 1449 Listed SPDs are now designated as either Type 1, Type 2 or Type 3 and intended for use on AC power systems rated Less than 1000vrms


  • SPD is used to limit transient over voltages of atmospheric or Switching Surge and gives path to the excessive current to earth hence limit the overvoltage to a value that is not hazardous for the electrical installation.

Causes of Surges:

  • (1) External Surge:
  • lightning strikes :Direct Stroke , Indirect Stroke
  • (2) Internal Surge:
  • Switching Surge:
  • Switching on/off of inductive loads.
  • Tripped circuit breakers and fuses.
  • Short circuits.
  • Malfunctions caused by the power company.
  • Insulation Failures:
  • Arcing Ground:
  • Ignition and interruption to electric arc.

Difference between Surge arrestor (Lighting Arrestor) and Surge Suppressor:

  • Surge arresters and Surge Suppressor both are used to protect equipment from surges. But, there is confusion between the application of surge arrestors / Lighting arrestor and surge suppressors.
  • The main differences between a lightning arrester and a surge arrester are its fault clearing time and it’s position
  • Both are doing the same job, but still both are not same.

Lighting Arrestor / Surge Arrestor:

  •  Surge Arresters are widely also known Lightning arresters.
  • Surge arresters are devices installed on Over head lines, substations etc to avoid a Lighting surge and other Surges of an additional current/ voltage/charge due to various faults occurring.
  • In the past year when nonlinear / solid-state devices (computers, PLC and drives) were not used. The Electrical Load is mostly Linear Load. Utility companies and end users were concerned with how to protect electrical distribution systems from lightning surges to ensure that voltage surges did not exceed the basic insulation level (BIL) of the conductor wires, transformers and other equipment.
  • Hence Surge arrestors / Lighting arrestors were developed for use in low, medium and high voltage applications at various points in the transmission and distribution system.
  • Surge Arrestor provide low resistance path between the phase conductor and ground. LA did not concern with the loads if it cleared within a few cycles.
  • Arrestors are still used in the electrical industry primarily along the transmission lines and upstream of a facility’s service entrance.
  • Arrestors are available in various classes depending upon their withstand capability (e.g., station vs. distribution class). At the service entrance location on low voltage systems (600V and below), Lightning arrestors were designed to protect the electrical distribution system and not the sensitive solid-state equipment.
  • Economically, surge arresters are better than surge Different surge arresters are available based on their withstanding capability. The main problem with them is that they are designed for protecting large electrical distribution systems from lightning surges, and not for sensitive solid state equipment.
  • Applications: The surge arrester is best to protect insulation of transformers, panel boards, and wirings. However, it doesn’t work well for solid state components.

 Surge Suppressor / Surge Protector (called TVSS):

  •  In today’s we mostly use solid-state (nonlinear) loads like electronic equipment, drives, PLCs, computers, electronic ballasts, telecommunication equipment. Non Linear is about 70% of utility loads. The solid-state components will be damaged by the surges.
  • Using Surge suppressors at the service entrance and key branch panels, the surge will be effectively reduced to under 100V.
  • If a TVSS and lightning arrestor are both used at a service entrance switchboard, the TVSS will “turn on” earlier and shunt most of the surge current. Many water-treatment plants, telecommunication facilities, hospitals, schools and heavy industrial plants utilize TVSSs instead of surge arrestors to provide protection against the effects of lightning, utility switching, switching electric motors.
  • Applications: They are used in water treatment plants, hospitals, schools, and telecommunication facilities.

 Size of Surge Protection Device (SPD) does not depend on Panel Size:

  • The kA rating of an SPD (surge rating) is one of the most misleading terms. We normally use 50KA SPD to protect 50KA panel.
  • The kA rating of the surge arresters has nothing to do with the fault current rating of electrical distribution board. We can fit a 40kA surge arrester in a domestic board with a fault current rating of less than 5kA
  • When a surge enters a panel, it does not know the size of the panel. So It is totally miscalculation for use 50KA SPD for 50KA Panel
  • There is a normal Practice that larger panels need larger SPD, but surges are indifferent to panel size.
  • The largest surge that can enter a building’s wiring is 10kA, as explained in the IEEE C62.41 standard. So why would we need a SPD rated for 100KA or 200kA.

Various Routine Test of Power Transformer-(Part-4)

(9) Magnetic Balance Test

 Test Purpose:

  • Magnetic balance test of transformer is conducted only on three phase transformers to check the imbalance in the magnetic circuit.

 Test Instrument:

  • Multi meter.
  • Mill Ammeter

 Test Circuit Diagram:


Test Procedure:

  • First keep the tap changer of transformer in normal position.
  • Now disconnect the transformer neutral from ground.
  • Then apply single phase 230V AC supply across one of the HV winding terminals and neutral terminal.
  • Measure the voltage in two other HV terminals in respect of neutral terminal.
  • Repeat the test for each of the three phases.
  • In case of auto transformer, magnetic balance test of transformer should be repeated for IV winding also.
  • There are three limbs side by side in a core of transformer. One phase winding is wound in one limb. The voltage induced in different phases depends upon the respective position of the limb in the core.
  • The voltage induced in different phases of transformer in respect to neutral terminals given in the table below.
  • 415V, Two phase supply is to be applied to any two phases terminals on HV side of Power transformer and voltages in other two phase combination are to be measured with LT open.
  • Sum of the Resultant two values shall be equal to the voltage applied.


Applied Voltage (415V) Measured Voltage(V1) Measured Voltage(V2) Result


 (10) High Voltage tests on HV & LV Winding:

Test Purpose:

  • To checks the insulation property between Primary to earth, Secondary to earth and between Primary & Secondary.

 Test Instrument:

  • High Voltage tester ( 100KV & 3KV)

 Test Circuit Diagram:

UntitledTest Procedure:

  • HV high voltage test: LV winding connected together and earthed. HV winding connected together and given Following HV Supply for 1 minute.
  • LV high Voltage test: HV winding connected together and earthed. LV winding connected together and given Following HV Supply for 1 minute.
  • 433V Winding =3KV High Voltage
  • 11KV Winding =28KV High Voltage
  • 22KV Winding =50KV High Voltage
  • 33KV Winding =70KV High Voltage.


(11) Di electrical Test:

Test Purpose:

  • To check the ability of main insulation to earth and between winding
  • To checks the insulation property between Primary to earth, Secondary to earth and between Primary & Secondary.

 Test Instruments:

  • 3 Phase Variable Voltage & Frequency Source.
  • Auto Transformer.

 Test Procedure:

  •  The following Dielectric tests are performed in order to meet the transformer insulation strength expectations.
  • Switching impulse test: to confirm the insulation of the transformer terminals and windings to the earthed parts and other windings, and to confirm the insulation strength in the windings and through the windings.
  • Lightning impulse test : to confirm the transformer insulation strength in case of a lightning hitting the connection terminals
  • Separate source AC withstand voltage test: to confirm the insulation strength of the transformer line and neutral connection terminals and the connected windings to the earthed parts and other windings.
  • Induced AC voltage test (short duration ACSD and long duration ACLD ) : to confirm the insulation strength of the transformer connection terminals and the connected windings to the earthed parts and other windings, both between the phases and through the winding.
  • Partial discharge measurement: to confirm the “partial discharge below a determined level” property of the transformer insulation structure under operating conditions.

 Method No 1 (separate source voltage withstand test) Untitled

  • All the terminals of the winding under test should be connected together and the voltage should be applied.
  • The secondary windings of bushing type current transformers should be connected together and earthed. The current should be stable during test and no surges should occur.
  • A single phase power frequency voltage of shape approximately sinusoidal is applied for 60 seconds to the terminals of the winding under test.
  • The test shall be performed on all the windings one by one.
  • The test is successful if no breakdown in the dielectric of the insulation occurs during test.
  • During the Separate source AC withstand voltage test, the frequency of the test voltage should be equal to the transformer’s rated frequency or should be not less than 80% of this frequency. In this way, 60 Hz transformers can also be tested at 50 Hz. The shape of the voltage should be single phase and sinusoidal as far as possible.
  • This test is applied to the star point (neutral point) of uniform insulated windings and gradual (non-uniform) insulation windings. Every point of the winding which test voltage has been applied is accepted to be tested with this voltage.
  • The test voltage is measured with the help of a voltage divider. The test voltage should be read from voltmeter as peak value divided by2. Test period is 1 minute.

 Method No 2 (Induced source voltage withstand test)


  • The aim of this test is to check the insulation both between phases and between turns of the windings and also the insulation between the input terminals of the graded insulation windings and earth
  • During test, normally the test voltage is applied to the low voltage winding. Meanwhile HV windings should be keeping open and earthed from a common point.
  • Since the test voltage will be much higher than the transformer’s rated voltage, the test frequency should not be less than twice the rated frequency value, in order to avoid oversaturation of the transformer core.
  • The test shall start with a voltage lower than 1/3 the full test voltage and it shall be quickly increased up to desired value.
  • The test voltage can either be measured on a voltage divider connected to the HV terminal or on a voltage transformer and voltmeter which have been set together with this voltage divider at the LV side. Another method is to measure the test voltage with a peak-value measuring instrument at the measuring-tap end of the capacitor type bushing (if any).
  • Test period which should not be less than 15 seconds.
  • It is calculated according ,Test period=120 seconds x ( Rated frequency / Test frequency )
  • The duration of the test shall be 60 second.
  • The test is accepted to be successful if no surges, voltage collapses or extreme increases in the current have occurred.

 Acceptance Criteria:

  • The test is successful if no break down occurs at full test voltage during test.

Method No 3 Lighting Impulse Test:

  • All the dielectric tests check the insulation level of the Transformer.
  • Impulse generator is used to produce the specified voltage impulse wave of 1.2/50 micro seconds wave
  • One impulse of a reduced voltage between 50 to 75% of the full test voltage and subsequent three impulses at full voltage.
  • For a three phase transformer, impulse is carried out on all three phases in succession.
  • The voltage is applied on each of the line terminal in succession, keeping the other terminals earthed.
  • The current and voltage wave shapes are recorded on the oscilloscope and any distortion in the wave shape is the criteria for failure.

Various Routine Test of Power Transformer-(Part-3)

(5) Short Circuit Test

 Test Purpose:

  • The value of the short circuit impedance Z% and the load (copper) losses (I2R) are obtained.
  • This test should be performed before the impulse test-if the later will be performed as a routine test- in order to avoid readings errors

 Test Instrument:

  • Megger or
  • Multi meter.
  • CT ,PT

 Test Procedure:

  • Suitable Low Voltage (3-phase 415V, 50Hz )will be applied to the terminals of one winding (usually the H.V.) with the other winding short circuited with 50 sq. mm. Copper cable. (Usually the L.V.)
  • The applied voltage is adjusted to pass the needed current in the primary/secondary. In order to simulate conditions nearest to full load, it is customary to pass 100%, 50% or at least 25% of full load current.
  • Voltage to be increased gradually till the current in the energized winding reaches the required value (50% to 100% rated current).
  • Measure the 3 Phase line currents at all tap position. If the tap-switch is an Off-Circuit tap-switch, the supply has to be disconnected before changing the tap. A consistent trend in the increase or decrease of current, as the case may be, confirms the healthiness of the transformer.
  • If transformer is equipped with a tap changer, tapping regulations are applied.
  • (1) If tapping range within±5% and rated power less than 2500kAV, load loss guarantee refer to the principal tap only.
  • (2) If tapping range exceeds±5% or rated power above 2500kAV, it shall be stated for which tapping beside the principal tap the load losses will be guaranteed by the manufacturer.
  • Three phase LT supply is applied on HV side of power transformer at normal tap with rated current on HV side and currents measured in all the phases on HV side and phases & neutral on LV side values noted.
  • Readings to be taken as quickly as possible as the windings warm up and the winding resistance increases. Hence, the losses value will increase accordingly.
  • Using appropriate instruments (conventional three watt meter method or digital watt meter with ammeters & voltmeters) measurements of voltage, currents and power can be recorded.


  • Short Circuit Test (Without using CT,PT)
  • To avoid CT’s and PT’s, this method can be used at current levels of 2 to 5 A and measurement of load losses is done at this condition. This measured load loss is then extrapolated to actual load currents to obtain load losses at the operating current.
  • Example: – 11 kV/433 V, 1000 kVA transformer with 5% impedance, the voltage to be applied on H.V. side during load test is estimated below.
  • V. side full load current (I1) = (KVAx1000/1.732xLine Voltage)
  • V. side full load current (I1) =(1000×1000/1.732×11000)=52.5 Amp
  • Line to line voltage to be applied on H.V side for getting 5 A on H.V. side,
  • Line to line voltage to be applied on H.V side Visc= (Line Voltagex1000xZx5/0.866xI1x100)
  • Line to line voltage to be applied on H.V side Visc=(11x1000x5xx/x0.866×52.5×100)=60.5 volts.
  • Since the current drawn on H.V. side is only about 5A in this test, CT’s can be avoided and hence phase angle error is not applicable.


  • Short Circuit Test (With using CT,PT)

 UntitledAcceptance Criteria:

  • Measured impedance to be within guaranteed value and nameplate value.
  • Load losses to be within guaranteed values.

 Test can detect:

  • Winding deformation.
  • Deviation in name plate value.

 (6) Open Circuit / No Load Test

 Test Purpose:

  • In this test, the value of No-Load power (Po) & the No-Load current (Io) are measured at rated voltage & frequency.

 Test Instruments:

  • Watt meters.
  • Ammeter , Voltmeter or
  • Power analyses

 Test Procedure:

  • Test is performed at rated frequency.
  • Three phase LT Voltage of 415 V applied on HV side of Power transformer keeping LT open
  • Two voltmeters are connected to the energized winding, one is measuring the voltage mean value and the other is for the Voltage R.M.S value.
  • Voltage applied to winding (usually to H.V. windings).It will be in a range from 90% of winding rated voltage to 110% of the same in steps, each of 5% (i.e. for a 33/11kV transformer, applied voltage values will be 29.7kV, 31.35kV,36.3kV)
  • Readings of watt meters, Voltmeters & Ammeters are recorded to obtain the values of V (r.m.s), Vmean, Po and Io at each voltage step.
  • Test results are considered satisfactory if the readings of the two are equal within 3%. If it’s more than 3%, the validity of the test is subjected to agreement.
  • Measured value of power loss is corrected according to the following formula:
  • Pc=Pm (1+d)
  • D= (Vmean – Vr.m.s) / Vmean
  • Measure the loss in all the three phases with the help of 3 watt meter method. Total no load loss or iron loss of the trf = W1 + W2 +W3

 Test Caution:

  • This test should be performed before the impulse test-if the later will be performed as a routine test- in order to avoid readings errors

 Acceptance Criteria:

  • No Load losses to be within guaranteed values.

 (7) Continuity test:

 Purpose of Test:

  • To know the continuity of windings of the transformer.

 Test Instruments:

  • Megger or
  • Multi meter.

 Test Procedure:

  • Check Continuity of Transformer by using multi meter or by Megger between following Terminals
Transformer P-P P-P P-P Result
HV Side R-Y Y-B B-R Zero Mega ohm or continuity
LV Side r-y y-b b-r Zero Mega ohm or continuity

Test can detect:

  • Open circuit / loose connection of winding

(8) Magnetic Current Test

 Test Purpose:

  • Magnetizing current test of transformer locates the defects in the magnetic core structure, shifting of windings, failure in turn to turn insulation or problem in tap changers.
  • These conditions change the effective reluctance of the magnetic circuit, thus affecting the electric current required to establish flux in the core.

 Test Instrument:

  • Multi meter.
  • Mill Ammeter

 Test Circuit Diagram:


  • Three phases LT Voltage of 415 V applied on HV side of Power transformer and currents are to be measured with mill ammeter.
  • The value shall be = (1 to 2 percent of rated full load current of TC / HT KV ) X Voltage Applied

 Test Procedure:

  • First of all keep the tap changer in the lowest position and open all IV & LV terminals.
  • Then apply three phase 415V supply on the line terminals for three phase transformers and single phase 230V supply on single phase transformers.
  • Measure the supply voltage and electric current in each phase.
  • Now repeat the magnetizing current test of transformer test with keeping tap changer in normal position.
  • And repeat the test with keeping the tap at highest position.
  • Generally there are two similar higher readings on two outer limb phases on transformer core and one lower reading on the center limb phase, in case of three phase transformers.
  • An agreement to within 30 % of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50 times higher than the value measured during factory test, there is likelihood of a fault in the winding which needs further analysis.

 Test Caution:

  • This magnetizing current test of transformer is to be carried out before DC resistance measurement.

Various Routine Test of Power Transformer-(Part-2)

(3) Turns Ratio / Voltage Ratio Test:

 Test Purpose:

  • Turns Ratio Test / Voltage Ratio Test are done in Transformer to find out Open Circuited turns, Short Circuited turns in Transformer winding.
  • The voltage ratio is equal to the turn’s ratio in a transformer (V1/V2=N1/N2). Using this principle, the turn’s ratio is measured with the help of a turn’s ratio meter. If it is correct , then the voltage ratio is assumed to be correct
  • This test should be made for any new high-voltage power transformer at the time it is being installed.
  • With use of Turns Ratio meter (TTR), turns Ratio between HV & LV windings at various taps to be measured & recorded.
  • The turn’s ratio is measure of the RMS voltage applied to the primary terminals to the RMS Voltage measured at the secondary terminals.
  • R= Np / Ns
  • Where,
  • R=Voltage ratio
  • Np=Number of turns at primary winding.
  • Ns= Number of turns at secondary Winding.
  • The voltage ratio shall be measured on each tapping in the no-load condition.

 Test Instruments:

  • Turns Ratio meter (TTR) to energies the transformer from a low-voltage supply and measure the HV and LV voltages.
  • Wheatstone Bridge Circuit

 Method No1 Turns Ratio Testing:

 Test Procedure:

  • Transformer Turns Ratio Meter (TTR):
  • Transformer ratio test can be done by Transformer Turns Ratio (TTR) Meter. It has in built power supply, with the voltages commonly used being very low, such as 8, 10 V and 50 Hz.
  • The HV and LV windings of one phase of a transformer (i.e. R-Y & r-n) are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector.
  • Values are recorded at each tap in case of tapped windings and then compared to calculated ratio at the same tap.
  • The ratio meter gives accuracy of 0.1 per cent over a ratio range up to 1110:1. The ratio meter is used in a ‘bridge’ circuit where the voltages of the windings of the transformer under test are balanced against the voltages developed across the fixed and variable resistors of the ratio meter.
  • Adjustment of the calibrated variable resistor until zero deflection is obtained on the galvanometer then gives the ratio to unity of the transformer windings from the ratio of the resistors.
  • Bridge Circuit:


  • A phase voltage is applied to the one of the windings by means of a bridge circuit and the ratio of induced voltage is measured at the bridge. The accuracy of the measuring instrument is < 0.1 %
  • This theoretical turn ratio is adjusted on the transformer turn ratio tested or TTR by the adjustable
    transformer as shown in the figure above and it should be changed until a balance occurs in the percentage error indicator. The reading on this indicator implies the deviation of measured turn ratio from expected turn ratio in percentage.
  • Theoretical Turns Ratio = HV winding Voltage / LV Winding Voltage
  • % Deviation = (Measured Turn Ratio – Expected Turns Ration) / Expected Turns Ration
  • Out-of-tolerance, ratio test of transformer can be due to shorted turns, especially if there is an associated high excitation current.
  • Open turns in HV winding will indicate very low exciting current and no output voltage since open turns in HV winding causes no excitation current in the winding means no flux hence no induced voltage.
  • But open turn in LV winding causes, low fluctuating LV voltage but normal excitation current in HV winding. Hence open turns in LV winding will be indicated by normal levels of exciting current, but very low levels of unstable output voltage.
  • The turn ratio test of transformer also detects high resistance connections in the lead circuitry or high contact resistance in tap changers by higher excitation current and a difficulty in balancing the bridge.

 Test Caution:

  • Disconnect all transformer terminals from line or load.
  • Neutrals directly grounded to the grid can remain connected

 Method No 2 Voltage Ratio Testing:

  •  This test is done to check both the transformer voltage ratio and tap changer.
  • When “Turns Ratio meter” is not available, Voltage Ratio Test is done at various tap position by applying 3 phases LT (415V) supply on HT side of Power transformer. In order to obtain the required accuracy it is usual to use a ratio meter rather than to energies the transformer from a low-voltage supply and measure the HV and LV voltages.
  • At Various taps applied voltage and Resultant voltages LV side between various Phases and phases& neutral measured with precision voltmeter & noted.

 Test Procedure:

  • With 415 V applied on high voltage side, measure the voltage between all phases on the low voltage side for every tap position.
  • First, the tap changer of transformer is kept in the lowest position and LV terminals are kept open.
  • Then apply 3-phase 415 V supply on HV terminals. Measure the voltages applied on each phase (Phase-Phase) on HV and induced voltages at LV terminals simultaneously.
  • After measuring the voltages at HV and LV terminals, the tap changer of transformer should be raised by one position and repeat test.
  • Repeat the same for each of the tap position separately.
  • At other taps values will be as per the percentage raise or lower at the respective tap positions.
  • In case of Delta/Star transformers the ratio measure between RY-rn, YB-yn and BR-bn.
  • Being Delta/Star transformers the voltage ratio between HV winding and LV winding in each phase limb at normal tap is 33 KV OR 33x√3 = 5.196 ,11 KV / √3 11
  • At higher taps (i-e high voltage steps) less number of turns is in circuit than normal. Hence ratio values increase by a value equal to.5.196 + {5.196 x (no. of steps above normal) x (% rise per each tap)} 100
  • Similarly for lower taps than normal the ratio is equal to 5.196 – {5.196 x (no. of steps above normal) x (% rise per each tap)}100

 Test Acceptance Criteria:

  • Range of measured ratio shall be equal to the calculated ratio ±0.5%.
  • Phase displacement is identical to approved arrangement and transformer’s nameplate.
  • The IEEE standard (IEEE Standard 62) states that when rated voltage is applied to one winding of the transformer, all other rated voltages at no load shall be correct within one half of one percent of the nameplate readings. It also states that all tap voltages shall be correct to the nearest turn if the volts per turn exceed one half of one percent desired voltage .The ratio test verifies that these conditions are met.
  • The IEC60076-1 standard defines the permissible deviation of the actual to declared ratio
  • Principal tapping for a specified first winding pair: the lesser ±0.5% of the declared voltage ratio
  • or 0.1 times the actual short circuit impedance. Other taps on the first winding pair and other winding pair must be agreed upon, and must be lower than the smaller of the two values stated above.
  • Measurements are typically made by applying a known low voltage across the high voltage winding so that the induced voltage on the secondary is lower, thereby reducing hazards while performing the test .For three phase delta/wye or wye/delta transformer, a three phase equivalency test is performed, i.e. the test is performed across corresponding single winding.

 Test can detect:

  • Shorted turns or open circuits in the windings.
  • Incorrect winding connections ,and other internal faults or defects in tap changer


(4) Polarity / Vector group Test

 Purpose of Test:

  • The vector group of transformer is an essential property for successful parallel operation of transformers. Hence every electrical power transformer must undergo through vector group test of transformer at factory site for ensuring the customer specified vector group of transformer.

 Test Instruments:

  • Ratio meter.
  • Volt Meter. A Ratio meter may not always be available and this is usually the case on site so that the polarity may be checked by voltmeter.

 Test Circuit Diagram:


 Test Procedure:

  • The primary and secondary windings are connected together at one point.
  • Connect neutral point of star connected winding with earth.
  • Low-voltage three-phase supply (415 V) is then applied to the HV terminals.
  • Voltage measurements are then taken between various pairs of terminals as indicated in the diagram and the readings obtained should be the phasor sum of the separate voltages of each winding under consideration.

 Condition:(HV side R-Y-B-N and LV Side r-y-b-n)

  • R and r should be shorted.
  • Apply 415 Volt to R-Y-B
  • Measure Voltage between Following Phase and Satisfy Following Condition
Vector Group Satisfied Following Condition









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