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I had two power supplies fed from the DB protected by an ELCB. Yesterday I had the power supplies burnt at the incoming end and no breaker was tripped. I also had couple of phase dimmers fed from two separate phases protected by a breaker each. They also were damaged at the incoming end but the breakers never tripped. What could be the cause? I checked all the connections , didn't find any short circuit. enter image description here

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Most probably you have had a voltage surge on the incomming power line. There can be many reasons for this.

  1. A bad neutral connection at your place if you have a 3 phase supply.

  2. A bad neutral connection or connection mistake during work outside (third party)

  3. A voltage surge due to lightning. Could be a direct hit but does not need to be.
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  • I agree especially if your power is 3 phase. – Ed Beal Apr 22 '17 at 15:08
  • Or 2 of 3 phases would suffice, because if a neutral floats you get 400V. That's a little high for any of the switching power supplies I know of to ride through; they generally max out at 306 (110% of 277V, a leg of 480 wye). – Harper - Reinstate Monica Apr 22 '17 at 16:32
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It's no surprise for a breaker to not trip. Breakers aren't universal failure detectors.

CB's (breakers) care about overcurrent - either a high overcurrent e.g. 300 amps, or a small overcurrent for a certain length of time (e.g. 40A for 3 minutes). That would be a very high-energy failure, which could start a fire even with a breaker trip.

ELCB's/RCDs (GFCIs) care about leakage to earth or other unintended current path (i.e. a ground fault). Rather than rely on ELCBs, The power supply and the system also have protective design to prevent it faulting to unintended places.

Arc fault detectors have a limited ability to detect certain types of internal failures, but even then I would not be surprised if a power supply burnt up without tripping the arc fault detector.

Product engineers work hard to make devices "fail soft", i.e. to fail safe without causing overcurrent or ground faults, and thus relying on those other devices to save the building and its people. This is "defense in depth" or "belt and suspenders and fashion tape".

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