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Thread: Circuit breakers

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    Diamond Member adrianh's Avatar
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    Circuit breakers

    Lets say you have a number of breakers in series / parallel:

    60A
    | | |
    20A 20A 20A
    | | ...etc
    10A 10A

    What would be a rule of thumb to determine the downstream max breaker current / breaker to ensure that the upstream breakers do not also trip on a dead short. These are standard B-curve breakers.
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    Diamond Member AndyD's Avatar
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    I'm not sure I'm understanding your question Adrian. Are you trying to find a breaker arrangement whereby if there's a short circuit fault on say one of the 10Amp circuits it won't trip the upstream 20Amp and 60Amp breakers which are supplying the faulty 10Amp circuit?

    No such arrangement exists, in the case of a direct short circuit the only factor limiting the current is the impedance of the infrastructure supplying the fault ie the resistance of the cable and terminations etc. The current that flows on a short circuit fault even as far downstream as one of the 10Amp circuits is massive (several hundred or thousand amps depending on cable lengths) momentarily. Although the fault current is only for a short period it will cause several upstream breakers to trip no matter what curve they are rated at.

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    Diamond Member adrianh's Avatar
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    Series circuit Breakers

    Ok, but then how come if I short an electrical appliance in the house the 16A breaker goes but the main 60A breaker stays on. (Ok, it might be that it is not a complete dead short - maybe that's why)

    Here is the problem:

    A device is connected to a DB board. The DB board has a breaker specifically for the device. The device has a breaker and a PLC and the PLC switches 2 other devices via relays. The one output gets a dead short and trips out the entire panel plus the breaker in the DB.

    Is there not a way to localize the trip on the business end of the relay so that the entire panel does not trip?

    Could this be why fuses are used?
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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by adrianh View Post
    Ok, but then how come if I short an electrical appliance in the house the 16A breaker goes but the main 60A breaker stays on. (Ok, it might be that it is not a complete dead short - maybe that's why)
    Yep, short circuit is a very loosely used term. A 'short circuit' in an appliance is often just a poor contact to earth that will cause enough current to flow for a flash to occur and the breaker to trip. If the fault occurs on the load side of a thermostat or relay for example then the contacts of the thermostat or relay can present sufficient resistance as the fault current increases to limit current flowing and prevent the larger breakers in the circuit tripping.

    Quote Originally Posted by adrianh View Post
    Here is the problem:

    A device is connected to a DB board. The DB board has a breaker specifically for the device. The device has a breaker and a PLC and the PLC switches 2 other devices via relays. The one output gets a dead short and trips out the entire panel plus the breaker in the DB.

    Is there not a way to localize the trip on the business end of the relay so that the entire panel does not trip?

    Could this be why fuses are used?
    Fast blow fuses could work to reduce the cascading trip. Depending on the equipment your supplying a transformer or some other supply arrangement may be used to limit the fault current on the secondary circuit.

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