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Q: How do you disconnect a live or energized power line automatically from the power distribution transformer for preventing electrocution when it gets snapped and falls down or start shooting current to the earth?
Victor Winston Biveira, India

A: Detection and disconnection of downed electric power lines of any voltage is a particular problem, even on multi-grounded wye-connected systems. If the line is a secondary-level line from a distribution transformer the problem is compounded by the relatively low voltages and the high load currents that must be accommodated by any protective equipment (e.g., breakers) during normal service. The contact resistance to earth of downed conductors is commonly exceeds 100 ohms. If the conductors are covered, contact may be intermittent and the resistances much higher.

With the phase-to-neutral voltage of distribution transformers being either 120-volts or 240-volts, the current through the fault is on the order of 2 amperes or less. The load current on a typical 100 kVA transformer at 415/240-volts is 139 amperes and 278 amperes for a 200 kVA unit. Since the unbalanced (neutral) load current on a 100 kVA transformer (@ 240 volts) can approach 100 amperes, it is impractical for even a hypothetical neutral-current detection system to discriminate between unbalanced-load and ground-fault currents.

In the absence of a secondary circuit breaker the only protective device will be the primary fuse. On an 11 kV system, the 100 kVA delta-wye transformer would have a load current of about 5.3 amperes and a primary current for a secondary fault on the order of 0.05 amperes. Therefore, the primary fuse also could not provide detection for this condition.
Ed Thomas
President, Utility Electrical Consultants, P.C.

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