Selecting a Thermal Magnetic Circuit Breaker

Quote of the Day

Go out and smell like the flock.

— Pope Francis to parish priests, admonishing them to go out and know how people live their lives.

Introduction

Figure 1: Current Rating Multiplication Factor Versus Temperature (Source).

I have an application that requires a thermal magnetic circuit breaker that will provide a given level of protection at an elevated temperature. Most thermal magnetic circuit breakers have a current rating that is specific to a stated temperature, usually room temperature (~23 °C). In my case, I need a circuit breaker with a 15 A break current at 50 °C.

In the case of E-T-A  breaker shown in Figure 1, the labeled current ratings holds for 23 °C. For other temperatures, a "multiplication factor" is given. For my 50 °C application per Figure 1, I need to specify a breaker with a labeled rating 1.16 times my 15 A requirement or 17.4 A.

This actually makes sense because these breakers sense current by measuring the internally temperature change with a bimetallic strip. This strip is similar to the bimetallic strip in a thermostat. In a circuit breaker, the overload current increases internal temperature of the breaker, which causes the strip to bend and eventually trip the breaker.

If the breaker must function at a temperature higher than its rating, the bimetallic strip bends because of the higher than specified temperature. This would cause the breaker to open at 50°C at a lower current than at 23 °C. Hence, we must compensate for the higher temperature by using a circuit breaker that is rated for more current at room temperature than we need at 50 °C. This is the purpose of Figure 1. The opposite effect occurs if the breaker must function below room temperature.

Here is a useful Youtube video that does a nice job of demonstrating the different operating modes of a thermal magnetic circuit breaker.

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