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Old 02-11-2012, 06:59 AM   #1
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open neutral problem, please help


Hi,
I have an open neutral problem within CU from the main-power supply to FCU (3A) to boiler. The voltgages of L(brown) and N(blue) on my boiler both appears as 238 v against earth. I tried to find where the neutral is open. I disconnected the three main-power lines (one red supposed to be live, one black supposed to be neutral and the yellow/green supposed to be earth) on the FCU (the other three lines to boiler also disconnected from FCU). I checked the voltage of these three lines from main power against the other earth which is grounded by connection to the metal panel of the boiler in the end.

Red the live against earth on boiler: 238
Black the neutral against earth on boiler: 147
yellow/green the earth from main-supply against earth on boiler: 178

Black against yellow/green the earth from main-power: 18
Black against red: 51
Red against yellow/green the earth from main-power: 36


I did some more tests. I connected both earths in FCU together and grounded it by connection to the metal panel of boiler.

Red against earth: 238
Red against black: 174
Black against earth: 37

I get confused now. Is the lines from main-power to FCU ok? Does the test suggest a short cut between the live and neutral somewhere from the air-switch box? why the voltage between the two earths isn't 0? Does it mean the earth from main-power isn't grounded?

Please help me. Thank you so much

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Last edited by beresford; 02-11-2012 at 07:42 AM.
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Old 02-11-2012, 07:29 AM   #2
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open neutral problem, please help


What country are you in? What is your nominal supply voltage/frequency?

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Old 02-11-2012, 07:46 AM   #3
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open neutral problem, please help


Hi, thanks
I am in UK. I assume the residential electric supply is 240V/50HZ. I did more test and edited my poster above a bit.

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What country are you in? What is your nominal supply voltage/frequency?
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