About circuit breaker derating

Update date:2018-05-23 Source:MAXGE

When the operating environment temperature of the circuit breaker is greater than the specified value, derating is required. The general rule is 40°C (temperature exceeds 65°C for motor protection). Under what circumstances need to use derating?

circuit breaker

1. The power distribution cabinet is installed outdoors, with high temperature in summer; the ambient temperature exceeds 40°C.

2. It is installed indoors, but the indoor ventilation and heat dissipation conditions are poor. Although the indoor temperature does not exceed 40°C, the temperature inside the distribution cabinet has exceeded 40°C.

3. Even if the distribution box is installed in an air-conditioned room, there are many circuit breakers in the distribution cabinet and the heat dissipation is poor. Derating should also be considered.

circuit breaker

4. At high altitudes, because of the thin air in high altitude areas, the decrease in air density will reduce the convection of heat dissipation and increase the temperature rise. Although rising altitudes will also lead to a decrease in temperature, which partially compensates for the effect of rising altitude on the rise in temperature, this effect is insufficient to offset the increase in temperature rise and the product has to be derated.

circuit breaker

How much?

Derating is based on the installation location and full load rate, but the exact number is difficult to give. Under normal circumstances, when it is determined that the ambient temperature is high and the heat dissipation is not good, we will take a derating factor of 0.8, which would be extremely severe with a value of 0.7. If the circuit breaker is too large, the cable specification will increase, which will have a great impact on the cost, and it will affect the sensitivity of the short circuit protection. Therefore, we do not use excessive derating.


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