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I am trying to install a UPS in my home .. diagram of the circuit attached. UPS is in first floor. Both floors have separate main circuit breakers .. Ground floor has Earth leakage breaker while First has MCB 63Amp. UPS is powering ground floor and taking input line from first floor with common ground in first floor. When I turn the switch ON (any light, button in the ground floor (with UPS installed), earth leakage breaker trips immediately. What is the problem here ?

Earth leakage breaker trips

O Ishtiaq
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  • I'm not sure what your "T"-shaped symbol for the neutral connections is supposed to represent. Any current supplied to the ground floor lights from the 1st floor UPS needs to be returned to the first floor neutral bar, not the ground floor neutral bar. If that's what you're doing, that explains why the RCD/GCFI is tripping -- it's seeing an imbalance between line current and neutral current. – Dave Tweed Jul 06 '23 at 10:01
  • Ts are nothing but actually neutral connection of electrical items likes Lights, Fans, AC etc. What is the solution here ? I want to run UPS on both floors.. Do I need to replace Earth leakage breaker to ordinary MCB so that both Neutrals join. – O Ishtiaq Jul 06 '23 at 10:49
  • I can't give you that kind of advice, because I don't know anything about the regulations that apply where you are. But I would be surprised if you were allowed to remove an earth leakage breaker. Better to keep the neutrals for the UPS-powered lights separate and return them to the first floor. – Dave Tweed Jul 06 '23 at 11:00

2 Answers2

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Common causes of earth leakage breaker trips are:

  • Neutral-ground (or neutral-earth) faults. The neutral and ground should only be connected at the origin of the supply, and nowhere else.
  • Borrowed neutrals. Your diagram suggests that the lights are powered by the first floor supply, but return through the ground floor neutral. That's a problem.

Don't assume that all the wiring is good. Test it to make sure.

Simon B
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So you appear to be powering some loads on the ground floor, using power from the UPS on the first floor. Am I correct?

So those loads that are taking "live"/"hot" from the UPS. Where are they getting neutral???

The first rule of AC mains power is that it's wired like an "isolated system" (except for safety earthing). Live and neutral must always run together. Anything that taps a "Live" wire must tap its associated "neutral" wire and nothing else. Neutral is not "common" or GND or backplane as in electronics. It's not chassis as in cars.

If you put a clamp meter on any cable (or conduit). the net current (sum of phase and neutral wires) must always "sum to zero" i.e. be equal and opposite. If live current comes down one wire of a cable, neutral current must return on another wire of that same cable. If this is not proper, then the cable will throw a very strong magnetic field (which a clamp ammeter will detect as current - and so will an RCD, which will trip it).

Why must neutrals be monogamous to their partner hot? Several reasons, but one is Neutrals do not have circuit breaker protection. If properly wired, neutral current cannot be higher than live wire current, so the live breaker protects the neutral also. This still works in split-phase and 3-phase wiring: the neutral current will not exceed the current of any live wire because of physics.

So I hope you see the problem here. The UPS is delivering its "live" to the 1st floor but delivering its output neutral to the 1st floor neutral bar, a different place. The circuits on the ground floor on UPS are taking their "Live" from the UPS but stealing "neutral" from another place. Aside from risking overload of that neutral, it also imbalances the current in the RCD, correctly causing a trip.

Re-arrange your wiring, and use legal in-wall cabling methods, to bring UPS live and neutral together from the UPS to the circuit breakers illustrated. Keep their neutrals separate from any others. It is alright to tie earths together, that will not affect any of the above.

Harper - Reinstate Monica
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  • Thank you for your valuable advice. After reading above comments / suggestions, I have decided to power First floor from Ground floor breaker instead of direct line from electricity meter ... this would keep Neurtral same on both floors and would definitely prevent breaker to trip. – O Ishtiaq Jul 08 '23 at 05:37