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
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