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Transformer Fault Current Calculation
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Transformer Impedance
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Transformer Impedance is measured in Percent Impedance, this is the Percentage of Rated Primary
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 Voltage applied to the Transformer's primary winding in order for the rated secondary Full Load Current to
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flow in the secondary winding (this test is preformed with the primary winding connected to a Variac
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or variable supply and the secondary winding shorted-out).
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Transformer Impedance values may vary but typically Blakley Electrics transformers will be between
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4 to 5% of primary Voltage. For further details please contact Blakley Electrics' Technical department.
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Impedance Voltage Vz =Primary Volts V x Percent Impedance Z%
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100
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Click-On Cells with Blue Text to Modify Values, Results will be Displayed in Green
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Any Warnings will be Displayed in Red and Full Results are Displayed Below.
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Primary Volts (V)
10000Vp
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Secondary Volts (V)
400Vs
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Transformer Rating (VA)
1000VA
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Transformer Configuration
Three
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Percent Impedance (Z%)
6Z%
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Impedance Voltage (Vz)600.00
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Transformer Maximum Fault Current
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Transformer Secondary Fault Type
Line to Earth
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Maximum Fault Current A=100x Secondary Full Load Current I
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Impedance Volts Vz%
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Maximum Fault Current (A)
23.3
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Maximum Fault Current (kA)
0.021.44
Secondary Current (I)
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This is the Maximum Current that can be achieved at the Transformer's Secondary Terminals and
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doesn’t allow for any Impedance in the primary or secondary circuitry to and from the transformer.
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Transformer Maximum Fault Current Allowing for Loop Impedance
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To Calculate the Maximum Fault Current that can be achieved in a circuit fed by a transformer
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we refer to the below formula. This formula calculates the loop impedance at the end of a circuit fed
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 from the secondary winding of a transformer.
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When calculating Fault Loop Impedance Select the Correct Transformer Type Above.
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Three Phase Transformers Divide the Values for Vs secondary Voltage by √3 and the VA by 3
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For Centre Tapped to Earth (CTE) transformers halve the values for Vs and VA .
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For RLV / 110V CTE Disconnection Times & Zs figures refer to BS7671:2008, 411.8 Table 41.6
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Zsec = Zp x Vs ² VpZ%Vs²+ (R1 + R2)
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100VA
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0.46Ω
Zp = Loop impedance of primary circuit including Source (Ze + 2R1)
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10000V
Vp = Primary Voltage
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230.9V
Vs = Secondary Voltage
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333VA
VA = Transformer Rating
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6%
Z% = Transformer percentage Impedance
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0.2Ω
(R1 + R2) = Resistance of Secondary circuit Phase and Protective conductor
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Primary Circuit Impedance (Ze + 2R1)
0.46
Zp (Ze + 2R1) Ohms
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Secondary Circuit Impedance (R1 + R2)
0.2
(R1 + R2) Ohms
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Total Impedance (Zsec)
9.8065Ohms
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Transformer Actual Earth Fault Current
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To calculate the Actual Fault Current in a transformer we can refer to the below formula taken from
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BS7671:2008, 411.4.5 and use the above figure for the transformers Total Impedance (Zsec).
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Earth Fault loop Impedance Zs = Voltage to Earth UoFault Current Ia = Voltage to Earth Uo
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Fault Current IaLoop Impedance Zs
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Actual Secondary Fault Current (Ia)
23.5Amps
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Primary Fault Current I = Secondary Fault Current Ia xSecondary Voltage Vs
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Primary Voltage Vp
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Actual Primary Fault Current (I)
0.9Amps
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Blakley calculator (version 2.0) Blakley Electrics Ltd.
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