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UPEEC

Instrument Transformers

Current transformers (CT) and voltage transformers (PT/VT) scale high currents and voltages down for meters and relays.

Intermediate
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  • CT: e.g. 400/5 A or 400/1 A; specified by accuracy class (metering 0.2, 0.5; protection 5P10, 5P20 — 5 % error at 10 or 20× rated current) and burden (VA).
  • PT / VT: e.g. 11 kV/110 V.

Never open-circuit the secondary of a live CT. With no secondary current to oppose it, the full primary current magnetises the core, producing a dangerously high voltage and overheating. Short the secondary before disconnecting a meter.

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