Operation of TLF
 WARNING |
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HAZARD OF INAPPROPRIATE OPERATION
Prior to re-energizing the system, ensure that the fuses
are correctly installed. The circuit breaker may open unexpectedly
under load if the fuses are not replaced before closing the circuit
breaker.
Failure to follow these instructions can result in death, serious injury, or equipment
damage.
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The circuit consists of three 200/100/50/5 A ratio Current Transformers (CTs), Printed Circuit Board Assembly (PCBA) and Mitop coil for tripping on overcurrent and instantaneous earth fault.
Under normal load conditions, a 1000 kVA transformer operating at 11000 V will have approximately 52 A flowing through the CT primaries. If the CT ratio is set on 50/5 A ratio, then approximately 5 A will flow in the CT secondaries. As the trip circuits are shorted out by the fuses, no tripping signal will be sent to Mitop coil. Assuming that there was no earth leakage, the L1, L2, and L3 phases will balance, and no current will flow through the earth fault protection circuit.
In the event of a phase-to-phase fault of 500 A, 50 A would flow through the CT secondaries. This would blow the 15 A fuse links shunting all of the current through the tripping circuit in PCBA, which would operate the Mitop coil and trip the 3-phase circuit breaker. After ensuring the fault is cleared on the network, fuses need to be replaced before closing the breaker.
Two over-current fuses are required to cover all phase fault combinations, that is for a L1-L2 phase fault, the circuit breaker will trip on the L1 phase protection circuit. Under earth fault conditions, there will be an imbalance between the L1, L2 and L3 phases, and current will flow down the residual path of the CT and through the earth fault protection circuit. The unit will trip instantaneously.
Fuse Replacement and CT Ratio Selection
 DANGER |
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HAZARD OF ELECTRIC SHOCK, EXPLOSION, OR ARC FLASH
Failure to follow these instructions will result in death or serious injury.
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The CTs 200/100/50/5 A are fitted as standard, to the ring main units along with a 200 A circuit breaker with TLF protection. Selection of the appropriate CT ratio can be easily achieved by moving the 12 pin connector on the front face of the PCBA. Refer to the Figure – CT Ratio Selection.
Earth fault sensitivity can be selected using the 2-pin connector on the front face of the PCBA. Refer to the Figure – Earth Fault Sensitivity.