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Thread: ground fault setting on circuit breakers

  1. #1

    ground fault setting on circuit breakers

    Is there a rule of thumb in the setting of the ground fault protection for circuit breakers? Any percentage in relation with the symetrical fault calculation or it should always be computed in the short circuit calculations :confused:

  2. #2
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    Re: ground fault setting on circuit breakers

    Depending on the application, it is sometimes set @ 10% of the "handle value" of the breaker/fuse overcurrent setting.
    Ron

  3. #3

    Re: ground fault setting on circuit breakers

    Thanks Ron,

    Would you recommend a reference material on this? For now I will follow your advice.

    Thanks again Ron

  4. #4

    Re: ground fault setting on circuit breakers

    Hello Ron,

    Just to clarify "handle value" is the Ampere rating or the interrupting rating of the circuit breaker?

    Thanks in advance Ron

  5. #5
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    Re: ground fault setting on circuit breakers

    For example if it is a 1200A service main breaker, then you might choose 120A as the GFI pickup. The delay is also another setting on many GFI's and it is set to your taste.
    The setting is definitely based in science, but as many coordination studies, it is somewhat of an art too!
    230.95(A) gives some limitations to the setting.
    Ron

  6. #6
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    Re: ground fault setting on circuit breakers

    If you are trying to coordinate the upstream breaker with GFI, with a downstream breaker, you will have trouble if the GFI is set lower than the downstream breaker setting. If the downstream breaker is a 200 amp device, and you have your GFI set at 120 amps, you won't have selective coordination. The GFI will beat the downstream device. This is often an undesirable situation.

    The best way is time consuming, but it will give you good answers. You need to obtain the TCC, time current characteristic, curves of all of the downstream breakers and plot them against the curve for the upstream breaker. By choosing the adjustable settings carefully you can see thow the series device srelate to each other, and you can avoid nuisance trips.

    No rule of thumb will give you proper settings.

    Jim

  7. #7
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    Re: ground fault setting on circuit breakers

    Jim is absolutely correct if there is more than one GFI activated breaker/switch.
    I made a poor assumption that there was only one device.
    Ron

  8. #8
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    Re: ground fault setting on circuit breakers

    I would like to add to Ron's note about there would be trouble with any GFI activated downstream device. Actually, any device, breaker, fuse, etc., that should operate on a ground fault needs to be considered, not just devices with GFI settings.

    I have been called to more than a few installations where the ground fault, is set too low, and a simple 20 amp single pole 277 volt lighting breaker is slower than the settings on the main GFI. Even with a main setting of 100 amps or so, you often won't let the 20 amp downstream breaker operate.

    Another common problem I encounter is an HVAC compressor motor going bad, and the GFI on the service main will beat the downstream HVAC breaker. I finally convince the Contractor to raise the GFI setting, and all of a sudden the HVAC breaker trips.

  9. #9

    Re: ground fault setting on circuit breakers

    Thanks Ron and jtester for the information.

    Well appreciated. Would you recommend reading material about ground fault/

    Thanks again

  10. #10
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    Re: ground fault setting on circuit breakers

    With regard to a simple down stream breaker remember that Breaker respond to either I2t or an instantaneous current. Industrial breakers have an instantaneous trip setting quite commonly of 10x the breaker rating, i.e. a 20at breaker would be 200a. So a main with GF set at 100a will trip first if it has no TD. Even with TD if the magnetude of the GF does not rise to 200a before the GF delay time out the main will trip.
    Residential breakers however, often have instantaneous trip values that are less, in the 6-7x range, which are more apt to pick up a GF sooner.
    In large industrial installation breakers or ground fault relays may even have the ability to communicate with one another such that the breaker closes to the GF is given the opportunity to trip first often referred to as zone interlocking.

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