Transformer Overcurrent Settings & OHE Protection Requirements — Traction Distribution Notes

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Earth Leakage Protection — Real Settings

Back-up protection for internal earth faults uses sensitive, high-speed earth leakage relays, provided SEPARATELY for primary and secondary sides, each connected to a separately-mounted CT. These relays are adjustable between 20% and 40% of 5A in equal 5% steps, and must be T-10 class with operating time NOT exceeding 10 ms.

Over-Current Protection — Real Settings Table

Relay Position	Current Setting	Time Multiplier Setting
IDMT overcurrent — primary side	80% to 320%	0 to 3 secs
Instantaneous overcurrent — primary side	400% to 1600%	—
IDMT overcurrent — secondary side	80% to 320%	0 to 3 secs

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Exam Focus — Instantaneous Setting Range Is Exactly 5x the IDMT Range

A genuinely tidy real relationship: IDMT overcurrent is set at 80-320% while instantaneous overcurrent is set at 400-1600% — both the lower bound (80→400) and upper bound (320→1600) scale by EXACTLY 5x. Instantaneous relays are of T-10 class with operating time not exceeding 10ms AT 5 TIMES the current setting — connecting the 5x relationship directly to how the relay's speed is actually specified and tested.

Inter-Tripping — The Real Lockout Mechanism

Differential relays and earth leakage relays (both sides) PLUS over-current relays (primary side) all cause inter-tripping of BOTH the 132/110/66kV and 25kV circuit breakers associated with the transformer. This inter-tripping also fires on Buchholz trip. The secondary-side IDMT overcurrent relay, however, trips ONLY the 25kV-side breaker — a genuinely narrower scope than the other relays.

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Exam Focus — The Real Multi-Mode Lockout

Inter-tripping is achieved through a high-speed tripping relay with a HAND-RESET contact. Once tripped, this relay LOCKS OUT closing of the circuit breaker from ALL modes of closing commands — remote control, local at the panel, AND at the circuit breaker mechanism itself — until the inter-trip relay (or a separate lock-out relay, if provided) is reset MANUALLY. This is a genuinely comprehensive lockout, closing every possible re-energisation path simultaneously, not just the one that caused the trip.

Other Protective Devices — Wiring Detail

The transformer, as supplied by the manufacturer, comes fitted with 4 real warning/protective devices: low oil level alarm, Buchholz relay with alarm/trip contact, oil temperature indicator with alarm/trip contact, and winding temperature indicator with alarm/trip contact. These contacts wire up to a WEATHER-PROOF terminal box mounted on the transformer BY THE MANUFACTURER — the connection is then extended to the alarm/trip circuit and the annunciator on the control board.

25kV OHE Protection — The Real 8-Function Requirement List

The protection scheme for 25kV overhead equipment must genuinely fulfil ALL of these 8 real functions:

Detect ALL short circuits over the zone of overhead equipment fed by the feeder circuit breaker.
Operate with MINIMUM delay in opening the circuit breakers.
Refrain from operating at maximum working current — DISCRIMINATE between max load current and short circuit current, even though load current can sometimes exceed short-circuit current in magnitude, especially when the fault is remote from the substation.
Detect and isolate faults on OHE caused by accidental coupling of two different phases from an adjacent substation.
Provide single-shot auto-reclosing of the feeder circuit breaker after a PRESET TIME INTERVAL adjustable between 0.5 SECONDS and 30 SECONDS — a genuinely wide, specific real range not stated this precisely in earlier chapters.
Reclose the 25kV feeder circuit breaker via single-shot automatic reclosing when it trips on fault through ANY of the three relays: Mho (wrong phase coupling) relay, and instantaneous over-current relay.
Detect and isolate flashover caused by a pantograph on an overlap in front of the traction substation when the panto moves from live to dead section over the insulated overlap — provided by the Panto Flashover Relay.
Detect and isolate a HIGH-RESISTANCE OHE ground fault — a fault type explicitly noted as one that CANNOT be [caught by the standard relays alone, requiring the Delta-I relay covered in the earlier chapter].