Circuit Breakers, what are they and what do they do?

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A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected.

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Why do we care?

Circuit breakers are important because we want to avoid damage to persons and electrical safety is of paramount importance concerning electrical infrastructure. We must abide by all relevant standards, rules and legislation and also the ramifications around potential warranty claims.

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AC, 3 phase v single phase

A basic 3 phase MCB is three single MCB's, formed together  and linked to ensure isolation of all live conductors occurs and the maximum size that is available is 125A but usually 80A is the maximum rating commonly used. 

AC 3 phase breaker: derating

When sizing the circuit breakers you must take into account various factors such as the actual temperature of the installation location, derating due to proximity of other protective devices and derating due to airflow, inside an enclosure. Heat is the number one enemy of most things electrical and circuit breakers are no exception. 

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AC 3 phase breaker: compliance

So how do they do the tests? 

Basically a body called the International Electrotechnical Commission conducts tests, IEC 60947 low voltage switchgear, to determine the correct ratings. These tests are performed under laboratory conditions, at 40°C ambient temperature with a cable length of two metres so the question must be asked, is this a realistic scenario and if not do the results obtained have to be derated?

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Derating tables

Most manufacturers have derating tables, so it's a good idea to check them and, if in doubt, go to the next sized breaker and at the same time look at the CCC (current carrying capacity) of the cable under consideration.

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AC 3 phase breaker: The inverter

So we need to size the circuit breaker for the inverter so will need to determine AC output of the inverter in watts or the current output from the data sheet, factor in the nominal three phase voltage in Australia, 400V and incorporate the square root of three, which is 1.732.

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Now for the maths. . . . . .

The calculation is the output of the inverter in watts, divided by (the square root of three x nominal three phase voltage):

• Ac output of the inverter in watts, for example 27000 
• 27000/(1.732 x 400)
• 27000/(1.732 x 400) = 39A

So we round up to 40A but we can’t select a 40A breaker but can factor in the resistive load factor of 1.25 which results in a breaker selection of 50A, Fronius Eco 27, 40A x 1.25 = 50A.

AC 3 phase breaker : MCB sizes*

Conclusion

Basic function of a circuit breaker is to break the current after a fault is detected and when selecting 3 phase breakers for inverters you will need to take derating into account. Need to know the output of the inverter, then divide by (1.732 x 400V, nominal three phase voltage) and remember, where and how the circuit breaker is installed will determine how much to derate.

It is up to the designer/installer to correctly ascertain all the relevant factors that influence a breaker’s ability to correctly perform its allocated functions.