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hi why do the volts go through the roof when the neutral is suddenly lost, frying everything? is it because there is no return path?
 
exactly. as there is no current flow, the neutral side of a lamp, say is at the same potential as the live feed. volt drop is 230 x 0 = 0
 
If a supply transformer loses its neutral then the star point is no longer referenced to earth and floats. If the load is balanced then there is no problem but if it is unbalanced then it results in the voltage measured between the floating neutral point and each phase being different and can vary between 0V and 400V
In a domestic situation if the incoming supply neutral is lost the voltage will not rise but will have no return path. This can result in earthed metalwork becoming live up to 230V. If an appliance is switched on and has electronic circuitry especially filter circuits tied to earth then the return voltage will try to find a way to earth and fry these circuits.
 
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think i misread the post, was thinking of lost neutral on a circuit, not the supply, sorry
 
IF you have a TNCS 3 phase installation what happens if you loose your neutral you can get back feed up the phases with loads then recieving 400 volts ,causing them the basically blow up ,can be like bonfire night , if you have a shop office using switch start lighting you may find that all the lights flash and dont start , it also as has been said make all the bonded metal work become live ,round here its quite common as they have used some dodgy cable connectors
 
One of our electricians was running a 120mm SWA in to the back of a switchboard. It was a bit of a tight squeeze so he disconnected 2 95mm earth cables while he wrestled the cable in to place. Cable in he replaced the earths. In the mean time all the office equipment had gone bang. He left the switch room jumped in the van and drove off without anyone seeing him.
I’d installed the 500KVA transformer and board, where the two main earths that came from the earth nest and the 11KV cable to the neutral / earth connection I’d placed an engraved 6” X 4” label “NEUTRAL / EARTH CONNECTION. DO NOT REMOVE”.
“I didn’t see that” says he! “I thought there was a bit of a spark”.
 
OOPS. Should have sent it to you been framed
 

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