 Limited Time Offer: Save 15% on Pro Plan with discount code: NTX78. Study specialized LV/MV/HV technical articles and guides.

# Fault Calculations

### Formulas

The different types of short-circuit fault which occur on a power system are:

• single phase to earth,
• double phase,
• double phase to earth,
• three phase,
• three phase to earth.

For each type of short-circuit fault occurring on an unloaded system:

• the first column states the phase voltage and line current conditions at the fault,
• the second column states the phase ‘a’ sequence current and voltage conditions at the fault,
• the third column provides formulae for the phase ‘a’ sequence currents at the fault,
• the fourth column provides formulae for the fault current and the resulting line currents.

By convention, the faulted phases are selected for fault symmetry with respect to reference phase ‘a’.

I f = fault current

Ie = earth fault current

Ea = normal phase voltage at the fault location

Z1 = positive phase sequence network impedance to the fault

Z2 = negative phase sequence network impedance to the fault

Z0 = zero phase sequence network impedance to the fault

Single phase to earth – fault from phase ‘a’ to earth:

 Va = 0Ib = Ic = 0I f = Ia = Ie Ia1 = Ia2 = Ia0 = Ia / 3Va1 + Va2 + Va0 = 0 Ia1 = Ea / (Z1 + Z2 + Z0)Ia2 = Ia1Ia0 = Ia1 I f = 3Ia0 = 3Ea / (Z1 + Z2 + Z0) = IeIa = I f = 3Ea / (Z1 + Z2 + Z0)

Double phase – fault from phase ‘b’ to phase ‘c’:

 Vb = VcIa = 0I f = Ib = – Ic Ia1 + Ia2 = 0Ia0 = 0Va1 = Va2 Ia1 = Ea / (Z1 + Z2)Ia2 = – Ia1Ia0 = 0 I f = – jÖ3Ia1 = – jÖ3Ea / (Z1 + Z2)Ib = I f = – jÖ3Ea / (Z1 + Z2)Ic = – I f = jÖ3Ea / (Z1 + Z2)

Double phase to earth – fault from phase ‘b’ to phase ‘c’ to earth:

 Vb = Vc = 0Ia = 0I f = Ib + Ic = Ie Ia1 + Ia2 + Ia0 = 0Va1 = Va2 = Va0 Ia1 = Ea / ZnetIa2 = – Ia1Z0 / (Z2 + Z0)Ia0 = – Ia1Z2 / (Z2 + Z0) I f = 3Ia0 = – 3EaZ2 / Szz = IeIb = I f / 2 – jÖ3Ea(Z2 / 2 + Z0) / SzzIc = I f / 2 + jÖ3Ea(Z2 / 2 + Z0) / Szz

Znet = Z1 + Z2Z0 / (Z2 + Z0) and Szz = Z1Z2 + Z2Z0 + Z0Z1 = (Z2 + Z0)Znet

Three phase (and three phase to earth) – fault from phase ‘a’ to phase ‘b’ to phase ‘c’ (to earth):

 Va = Vb = Vc (= 0)Ia + Ib + Ic = 0 (= Ie)I f = Ia = hIb = h2Ic Va0 = Va (= 0)Va1 = Va2 = 0 Ia1 = Ea / Z1Ia2 = 0Ia0 = 0 I f = Ia1 = Ea / Z1 = IaIb = Eb / Z1Ic = Ec / Z1

The values of Z1, Z2 and Z0 are each determined from the respective positive, negative and zero sequence impedance networks by network reduction to a single impedance.

Note that the single phase fault current is greater than the three phase fault current if Z0 is less than (2Z1 – Z2).

Note also that if the system is earthed through an impedance Zn (carrying current 3I0) then an impedance 3Zn (carrying current I0) must be included in the zero sequence impedance network.

 NOTATION The symbol font is used for some notation and formulae. If the Greek symbols for alpha beta delta do not appear here [ a b d ] the symbol font needs to be installed for correct display of notation and formulae. B C E f G h I j L P Q susceptance capacitance voltage source frequency conductance h-operator current j-operator inductance active power reactive power [siemens, S] [farads, F] [volts, V] [hertz, Hz] [siemens, S] [1Ð120°] [amps, A] [1Ð90°] [henrys, H] [watts, W] [VAreactive, VArs] Q R S t V W X Y Z f w quality factor resistance apparent power time voltage drop energy reactance admittance impedance phase angle angular frequency [number] [ohms, W] [volt-amps, VA] [seconds, s] [volts, V] [joules, J] [ohms, W] [siemens, S] [ohms, W] [degrees, °] [rad/sec]

1. 