Rockwell Automation 1492 APPL.NOTE/PROTECT TRANS.CRCTS. - Manual

ALLEN-BRADLEY
A–B
Using Circuit Breakers
to Protect Transformer Circuits
Application Note
This document describes how transformer inrush current can cause nuisance
tripping of standard trip circuit breakers during start up or application of
power. Recommendations for selecting a high inrush compensated circuit
breaker are provided.
A transformer can generate an almost infinite current spike on start up trying
to overcome the saturation of its core. The large current draw is limited only
by the resistance of the windings and the inductance of the circuit.
Current
I
t Time !
The inrush spikes are of short duration (1/8 to 1/4 of a cycle). The standard
trip circuit breaker, having a quicker reaction time, sees the spike as a short
circuit and trips the circuit.
Intermittent nuisance trips may occur depending upon where the transformer
core inductance resides at the time power is applied. The amount of core
saturation and its location on the hysteresis loop influences the amount of
current drawn at startup. This is why a circuit breaker may only trip
periodically on start up.
Transformer Inrush Current
Nuisance Tripping