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LSI"G" - Electronic Trip Units / Releases and Earth fault protection

Nick Parker

When do I use LSI “G” breakers? I have a TN-S system. The fault current would be high enough that for earth fault, over current protection also operates. Except where the code requires RCD (like sockets & heat tracing), I plan to use just thermal/ magnetic overcurrent protection for all the breakers in the switchboard.

Also would like to know, What's in vogue?

 

  • 0

    Which country are you in ?

    In a UK domestic setting the nearest thing is the RCBO, programmable units are more the domain of specialist kit like UPS installations or industrial units with lots of VSD etc; as they have higher CPC current by design and such things need someone skilled to set them up safely.

    my understanding is that the abbreviations you have used are more american-centric.

    • L = Long  (overload protection, usually an inverse-time like a thermal/magnetic breaker)
    • S = Short Time (short circuit protection of low level faults)
    • I = Instantaneous (short current protection of high level faults, analagous to the instantaneous magnetic trip of a thermal/magnetic breaker)
    • G = Ground Fault (equipment ground fault protection)

     

    Mike.

  • 0

    I'd agree you'd usually just need simple overcurrent protection on conventional TN-S systems.

    You're likely to need additional earth leakage units where the earth fault current might be too small to trip and overcurrent protective device quickly enough - TT systems obviously, but also TN systems where the supply might not be able to deliver sufficient energy (even occasionally) - e.g. parts of a system that might have to run off a backup generator or downstream of a UPS.

       - Andy.

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