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IDG input speed is obtained from the magnetic pickup (MPU) located on the input shaft of the IDG. The Underspeed (US) condition exists when an IDG speed falls to and remains below 4,525 |125 rpm. The US time delay is 150 |50 milliseconds and locks out UF and UV protection functions and trips the GCR and GCB. The MPU is also used for airmode speed which sends a signal to the BCU to inhibit system power transfers from IDG power to external or auxiliary power when IDG input speed exceeds 6,000 |50 rpm.The GCU difference current protection circuitry trips the BTB if the generator output differs from the average of the individual generators operating in parallel by more than 37.5 |2.5 amps. As the difference in the amount of current at the generator output increases, the time between the fault occurrence and the protective trip decreases.
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MAINTENANCE MANUAL

The differential (current) protection (DP) circuitry in the GCUs monitors associated channels to protect against generator, generator feeder, GCB, BTB, and main generator bus faults. Differential protection circuitry compares generator and line currents at the generator current transformer and line DPCT. On any given phase, both generator and line currents should be identical. A low impedance to ground on one or more feeders between the generator and line CT indicates a differential current fault (the generator current is greater than the line current). There are two types of DP faults: differential protection 1 (DP1) and differential protection 2 (DP2). 1) A DP1 trip occurs if the maximum differential fault current
 is greater than 20 |5 amps. If such a condition persists
 for more than 40 milliseconds, DP1 protective circuitry
 trips the GCR and GCB. 2) A DP2 trip occurs if, after a DP1 trip, a differential
 current fault still exists. If such a condition persists
 for more than 52.5 |22.5 milliseconds after the GCR and GCB
 have tripped, DP2 protective circuitry trips the BTB.The generator diode fault protection circuitry uses phase A current of the generator current transformer and exciter field voltage to detect a failed generator diode condition. The protective circuitry compares the calculated exciter field voltage (using generator current as a component) with actual voltage monitored across the exciter field. If actual voltage across the exciter field exceeds the calculated generator voltage, it indicates a generator diode fault, and the protective circuitry trips the GCR in 6.26 |0.75 seconds.Overexcitation (OE) protection is provided to protect against an overexcited generator during parallel operation. The GCU operates together with the OE and UE current transformer and POR to measure the differential reactive current and calculates an effective voltage. At an effective voltage set point level of 130|3 volts ac, the OE protection circuitry will trip the BTB. As the amount of OE increases, the time between the fault occurrence and the protective trip of the GCB and GCR decreases.Underexcitation (UE) protection is provided to protect against an underexcited generator during parallel operation. The UE condition exists if the GCB is closed and the effective voltage is less than 104.5 |1.5 volts ac. The UE protection circuitry trips the BTB within 3.00 |0.25 seconds after a fault is detected.Shorted Permanent Magnet Generator (SPMG) protection uses a ripple detector to determine if the SPMG condition exists. If a SPMG condition exists, the protective circuitry trips the GCR and GCB in 0.2 to 1.2 seconds.
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747-400 MAINTENANCE MANUAL
Open Phase (OP) protection circuitry monitors the lowest phase current and two highest phase currents of a generator. OP protection operates only if the OP protective circuitry measures a current lower than 6 |5 amps on one phase and at least 55 |5 amps on the next higher phase. If these conditions exist, the OP fault isolation circuitry will trip the BTB in 375 |40 milliseconds. If the OP condition persists for a total of 9 |1 seconds after the fault first occurred, the OP protective circuitry trips the GCR and GCB.Reverse power protection is provided to prevent motoring of an IDG. If the current transformer shows 8 |1 kw, reverse power protection will trip the BTB of the motored generator provided an underspeed condition exists. As the amount of current at the current transformer increases, the time between the fault occurrence and the protective trip decreases.The GCU contains circuitry for Central Processing Unit (CPU) fault protection. If the CPU fails, the following occurs: 1) The GCR and GCB trip. 2) The BTB remains closed, or closes if it was not closed when
　
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