# Overcurrent Protection

### Lehman Engineering Power Quiz "Overcurrent Protection"

Consider the following distribution line.
A bolted ground fault occurs from CØ to ground at F1:

• G = generator, grounded Y connected (Ignore its impedance in your calculation)
• TX = transformer, 3Ø, Grounded Y- Delta , 13.2 kv / 480 volts,
1000 KVA, 3% impedance (ZTX)
• ZXL = system line reactance, ZXL = 2% on 1000 KVA base
• F1 = bolted ground fault from CØ to ground (Assume no fault impedance; ZF1 = 0)
• M = 10 HP, 3Ø, 480 volt, Power Factor = 0.8, Transient Reactance (x'd) = 5%
• (All impedances are on 1000 KVA base and 480 base volts)

Question #1:
Calculate the short circuit current for the ground fault at F1.

Question #2:
What current will flow in the unfaulted phases (A & B) during the fault?

Hint: Remember that the transformer secondary is Delta connected.

### Solution

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