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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.
Transformer Inrush Current
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. I
Current
Time !
Nuisance Tripping
t
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.
Application Note Using Circuit Breakers To Protect Transformer Circuits
Selecting a Circuit Breaker For Transformer Protection
Circuit breaker protection for the primary side of the transformer can be sized per NEC (National Electrical Code) Article 450-3(b)(2) which states: ”All transformers of 600V, nominal, or less, shall be protected by an individual overcurrent device on the primary side . . .” Article 450-3(b)(2) defines guidelines for sizing an overcurrent protection device. The correct sizing of a transformer primary winding depends upon whether or not the secondary windings have an overcurrent device. Overcurrent Device on Primary Winding Only
Source
Primary Overcurrent Device Up to 125% of Load
Load
A primary overcurrent device can be sized at, but not more than 125% of the rated transformer load if the secondary side is not protected. In this application, the primary overcurrent device protects the load as well as the transformer primary windings. Overcurrent Devices on Both Primary and Secondary Windings
Source
Primary Overcurrent Device Up to 250% of Load
Secondary Overcurrent Device Up to 125% of Load
Load
An overcurrent device on the primary side can be rated at, but not more than 250% of the rated transformer load, if the secondary side of the transformer has an overcurrent device rated at but not more than 125% of the rated transformer load. In this application, the primary overcurrent device is protecting the transformer primary windings only.
