Instrument Transformers & Protection

It is a protection class. The first number, 5, is the composite error limit in percent at the accuracy limit current, P stands for protection, and 10 is the accuracy limit factor, so the CT stays within 5 percent error up to 10 times its rated current. A higher factor, like 5P20, keeps accuracy at higher fault currents.

With the primary carrying current and the secondary open, no opposing flux exists, so the core saturates and a very high voltage appears across the secondary terminals. This can destroy insulation and is a shock hazard. Always short the secondary terminals before removing a meter or relay from a live CT circuit.

Instantaneous protection trips without intentional delay when current exceeds a set value. IDMT, inverse definite minimum time, trips faster as current rises, giving time grading with other devices. Most overcurrent relays offer both: IDMT for coordination and an instantaneous element for severe faults close to the relay.

Add the burden of every connected relay and meter, plus the lead resistance, which depends on cable length and cross-section. The CT's rated burden should be equal to or higher than this total. A burden that is too high saturates the CT early and a very low burden wastes cost. Using a 1 A secondary reduces lead loss on long runs.

Provide the equipment protected (feeder, motor, transformer), the protection functions required, CT and VT ratios, auxiliary supply voltage, number of contact outputs, and communication protocol. A single line diagram helps. Relay settings should be derived separately from a coordination study by a qualified protection engineer.