LSIG Breakers

[NickParker]

When do I use LSI “G” breakers? I have a solidly grounded system. The fault current would be high enough that for ground fault, that over current protection also operates. Except where the code requires ground fault protection (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?

 

[waross]

Background
LSIG (long time, short time, instantaneous, ground) -equipped circuit breakers are often required by code in the application of larger UPS systems. Specifically, UPS systems fed by 480 volts (up to 1000V) and protected by circuit breakers of 1000 amps or greater, must generally have a means of ground fault protection in the service or feeder for that UPS. Note that certain exceptions in the NEC allow for non-LSIG circuit breakers to be used. An example is shown below:

Check your code.
Generally:
The feeder,
Downstream of an isolation transformer.
Downstream of the UPS.
Note: This may not be specific to UPS systems.
Again, check your local code.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[unclebob]

Be aware that if you only use a ground element on a main breaker, any downstream ground faults will get detected by the main but your thermal/mag breakers may not.
Finding where the GF occured gets tricky.

 

[dpc]

Coordination (and protection) will be greatly improved by using ground fault protection as far downstream as you can. The only real downside is cost. As unclebob says, if you install only the code-required minimum on the main breaker, it creates the potential for serious miscoordination for downstream ground faults.

 

[che12345]

NickParker (Electrical)(OP)27 Oct 21 15:24
" 1) When do I use LSI “G” breakers? I have a solidly grounded system. 2)The fault current would be high enough that for ground fault, that over current protection also operates. 3) Except where the code requires ground fault protection (like sockets & heat tracing), I plan to use just thermal/ magnetic overcurrent protection for all the breakers in the switchboard."
1) In LV installations, you can use LSIG on [ACB/MCCB] or [RCBO or MCB+CRD] in any circuit where you wish to trip out the breaker when the earth-leakage current reach a certain potential damaging level. Take the case of a 2000A incoming with ACB. Earth-fault "G" may set to say 10% of incoming rating but not exceeding say 100A with a short time delay say 0.5s. In the case of MCB, the set up is [MCB+RCD] say 30mA, which offers the earth-fault protection. The requirement/Code, practice varies between IEC and ANSI/NEC world; location/application/installation etc.
2. You must be aware that irrespective of the earthing system i.e. (TT, TN, IN etc), the earth-fault current usually is very much LOWER than the breaker rating. Say an earth resistance of 0.5 Ohm on a 230Vac L-E. The earth-fault would be only 460A. A breaker say 1000A with LSI would NOT operate/trip on If=(460A). A persistent EF current of 460A is certainly undesirable, which is a potential of fire risk.
3. It is certainly desirable to have LSIG even at a higher cost, instead of only LSI; when the conductors leave/go out of the Switchboard.
4. BTW: If the breakers are within the Switchboard and <3m , EF protection may be exempted. Refer to local Code/Regulation.
Che Kuan Yau (Singapore)
,