Difference between Fault Current and Short Circuit Current

Introduction:

  • There is a difference between “Fault Current” and “Short Circuit Current” in electrical system. Both parameters are important while selecting an Equipment or designing a Network, however both terms are misled in Electrical engineering.
  • In very simple language “Short” means less (shortest distance, time or circuit), Short circuit Fault means least resistance or no resistance in circuit and Current is high due to less resistance. This high current convert into heat energy. The opposite of a short circuit is an “open circuit”, which is an infinite resistance between two nodes.
  • While Fault means wrong. Fault Current means Current pass in to wrong path.

What is Fault Current

  • A fault current is a current which takes the wrong path instead of using the normal conducting path during Fault condition.
  • Under normal condition, the electric equipment operate at normal voltage and current ratings. Once the fault occurs in a circuit or device, voltage and current value deviates from their nominal Value. This may be high or Low Values.
  • The fault may be occurred due to insulation failures, Wrong Connection or conducting path failures, which further convert in Open Circuit, Short Circuit and Ground Fault.
  • A fault current can either current being more or less than the normal rated current.
  • In Three phase power system, there are basically three types of Fault Current.
  • Open Circuit Faults
  • Short Circuit Faults (L-L / L-L-L)
  • Ground Circuit Faults (L-G / L-L-L-G)

What is Short Circuit Current:

  • When a two or more conductors of differential potential comes to contact with each other (one phase comes in contact with other Phase, Neutral or Earth) gives the electricity to a path of less resistance hence a large current flow in the un-faulted phases, such current is called the short circuit current.
  • When Short circuit occurs, current returns to its source without passing to the load. It caused zero or very little resistance and No Voltage drop in that circuit.
  • This Current will be the maximum that the source can deliver for a very small time before the protection device operates. The current is limited only by the resistance of the rest of the circuit.
  • We know that V (Voltage) =I (current) x R (resistance of Circuit).
  • When short circuit occur, resistance is very small and can be considered as negligible. We can consider R=0. This means I = V/0, which means infinite current will Flow so the conductor must have the capacity to allow this huge current to flow. In most of the cases breakdown happens.
  • The resistance when short circuit occur is very small and can be considered as negligible. We can consider R=0.
  • This means V=Ix0, which means Voltage at Short circuit is very Less.
  • V(drop)=0 and current(I)=infinite
  • Short circuit gives thousands time larger Current than the normal current and Zero Voltage at Fault Point. This will produce more heat and result in burns and fires.
  • Short circuit faults are also called as Shunt faults.
  • Causes:
  • Over Loading of Equipment: Overloading of equipment and insulation failure due to lighting surges and mechanical damage.
  • Loose Connections:Due to Loose Connections, Sometimes Neutral and Phase wires to touch.
  • Faulty or Wrong Connections: Wrong Connections make Short circuit in Circuit.
  • Failure / Ageing of Insulation:Old or damaged insulation makes neutral and Phase wires to touch, which can cause a short circuit. Punctures in Insulation can damage insulation and makes short circuit.
  • Harmful Effects:
  • The short-circuit produces the arc that causes the major damage of equipment such as transformers and circuit breakers.
  • The short circuit causes a heavy current in the power system which produces excessive heat and hence results in fire or explosion.
  • The short circuit affects the stability of the network which disturbs the continuity of the supply.
  • The operating voltages of the system can go below or above their acceptance values that creates harmful effect to the service rendered by the power system.

Open Circuit Faults:

  • Open Circuit Faults occur due to the Failure / Open of one or more Phase Conductors in Circuit.
  • In Open Circuit Fault, Current cannot flow hence Current is Zero and Voltage become Infinite.
  • V(drop)=infinite and current(I)=0
  • Open circuit faults are also called as series faults. These are unsymmetrical or unbalanced type of faults except three phase open fault.
  • Causes:
  • Broken Conductor, Failure of Conductor Joints and malfunctioning of circuit breaker in one or more phases.
  • Harmful Effects:
  • Abnormal operation of the system.
  • Danger to the Human and Animals.
  • Exceeding the voltages beyond normal values in certain parts of the network, which leads to insulation failures and developing of short circuit faults.

Difference between Fault Current and Short Circuit Current:

Circuit Resistance:

  • A short circuit has zero resistance between two Wires / Circuits / Systems, on the other hand a Fault current has a resistance that draws current. The amount of resistance decides how much current is drawn and is usually caused by a breakdown in the insulation of a system.

Amount of Current:

  • Fault Current: it is the current exceeding the equipment current rating e.g. motor rated 25A, then more than this will be the fault current.
  • Short Circuit current: it is the maximum current which can flow when the equipment is short circuited & it can withstand. above this the current will damage the equipment.
  • Fault current is the current that flows during an Open Circuit or Short Circuit Fault condition so each time it is not necessary that Fault Current is a Short Circuit Current (It may be Open Circuit Fault).
  • A short-circuit current will flow when there is short-circuit in the system, and it will represent the highest possible fault current that a system can experience.
  • Therefore, a fault current can be less that the short-circuit current, and a short-circuit current will represent the highest fault current in the system.
  • A fault current can either current being more or less than the normal current while Short Circuit Current is higher than Normal Current.

  • A Fault Current is not necessary a short circuit Current but Short Circuit Current is always a Fault Current.

Comparison of Fault Current -Short Circuit Current

Basis For Comparison Fault Current (Open Circuit Fault) Short Circuit Fault Overload
Meaning In the Open circuit the voltage at the fault point is high up to infinite and current is zero through the faulty point of the network. In the short circuit the voltage at the fault point decreases to zero and current of irregular high value flow through the faulty point of the network. The overload means that load greater than the desired value have been imposed on the system.
Resistance High Zero
Current Zero High Low as compared to short circuit.
Voltage High Zero The voltage becomes low, but cannot be zero.
Occur It occurs when the neutral and live wire Break or Open. It occurs when the neutral and live wire touch each other. It occurs when a large number of devices are joint in a single socket.

Importance of Fault Current and Short circuit Current for designing of System or Panel.

  • The safety of the system is decided by short-circuit current rating (SCCR) of the Equipment with the reference of the available fault current where the Equipment is installed. 
  • The short circuit current rating gives a baseline for the fault current that an equipment can withstand for a specific amount of time, or until it clears the circuit with opening of a circuit breaker.
  • The short circuit current rating of a panel is the amount of energy, usually expressed as a value in kilo-Amperes (kA), that the panel can handle without causing fire, a shock hazard, or explosive danger.
  • In equipment with higher short-circuit current ratings compared to Fault Current is not an issue.
  • The available fault current of panel can be decided by the size of the upstream transformer, size of the electrical conductors / Cables up to the Equipment.
  • If the System Fault Current at the Location is 20KA to 50KA and if we use Equipment having short circuit current of 5KA to 10KA may cause damages of equipment or network in fault condition.
  • If the System Fault Current at the Location is 5KA to 10KA and if we use Equipment having short circuit current of 65KA to 100KA will not create any issue but it will unnecessarily increase the price of equipment hence short circuit level of the equipment is not too much high with respect of fault current.
  • We have to ensure that the Short Circuit Current is equal or more than Fault Current available at the point of Equipment.

About Jignesh.Parmar (B.E,Mtech,MIE,FIE,CEng)
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

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