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Old 07-31-2008, 08:40 AM   #1
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240v to earth problem


3 bed semi with detached garage wired by underground 4 core armoured cable (only 3 cores used as fitted by previous owner), garage flourescent light failed and was just very dim flicker so i disconnected armoured cable at house consumer unit as i planned to fit new consumer unit next day. Have fitted new house split load consumer unit excluding garage cable and all seemed well. I then re connected garage cable, from RCD side of new house consumer unit. It connects to a small garage consumer unit with its own rcd. However the garage didnt work and ive discovered i only have about 60v between the live and nuetral in the garage. On further investigation i have disconnected garage cable completely at both ends with the following results
house consumer unit working OK
house consumer unit has 240v between live and EARTH
house consumer unit has 240v between live and 'isolated' garage cable ends
house consumer unit has 0v between nuetral and earth

Please help me understand whats happening here as i assumed the garage cable was faulty but its now disconnected both ends.

for information after the new consumer unit was fitted the elec board visited and replaced meter and resealed main fuse

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Old 07-31-2008, 09:12 AM   #2
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240v to earth problem


You're in the UK?

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Old 07-31-2008, 11:17 AM   #3
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240v to earth problem


Uk Yes
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Old 07-31-2008, 11:38 AM   #4
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240v to earth problem


If your testing to the wires of the disconnected armoured cable and are getting 240 volts then you have a fault to the metal of the armoured cable and the metal of the cable is your return path to the house consumer unit. If the metal jacket of the cable is still clamped to the consumer unit and is bonded to the utility neutral there then this would explain getting 240 volts to each wire. Do a continuity test between the disconnected cables wires and the outer metal jacket should be no continuity. Same with between all wires in the garage cable.

What is even weirder is that your rcd isnt tripping?

Last edited by Stubbie; 07-31-2008 at 11:43 AM.
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Old 07-31-2008, 11:46 AM   #5
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240v to earth problem


thanks for helping
Ive just checked and i have no continuity between the armoured outer and any of the four inner cables.
rcd not tripping is bizzare i think
stupid Q but should i have zero volys between live going into main cu switch and earth? cos as posted i have 240v !!!!
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Old 07-31-2008, 12:26 PM   #6
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240v to earth problem


You should have 240 volts between your live incoming and to earth. At the main MCB to your dwelling the earth (Green/yellow) is bonded to the utilities neutral for return to the transformer. Any hot wire to earth should test at line voltage.

Same result that you get when testing neutral to live
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Old 07-31-2008, 12:34 PM   #7
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240v to earth problem


My guess is you have a open or deteriorated neutral in the cable to the garage. If your using a digital voltmeter this would explain the 60 volts. With the cable to the garage disconnected at both ends twist the hot and neutral together at one end and go to the other end and make a continuity check between them. You should have continuity if not then one or the other is opened.

A voltage test at the garage end live to earth with the cable connected to the cu will tell you if you have 240 on the hot out at the garage. You may have already done this test. If so your neutral wire is opened/broken in the garage cable.

This being the case and the neutral not contacting any other metal or wire the RCD will remain closed.

Last edited by Stubbie; 07-31-2008 at 12:40 PM.
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Old 07-31-2008, 12:53 PM   #8
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240v to earth problem


red & black twisted at house - no continuity in garage
red and blue twisted at garage - continuity

luckily i have 4 core laid in by previouse owner so will use blue for N and abandon broken black tomorrow when it daylight

many thanks

was just the 240v live to earth i didnt understand - silly diy ers !
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Old 07-31-2008, 01:02 PM   #9
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240v to earth problem


Hey glad we found the broken neutral (black). As you know there was a color code change in 2004... black is no longer used for neutral. blue is the new color for neutral. So if you have the old colors at the cu at the house take care when connecting the garage cable. If black is being used as neutral at the house cu you will be connecting blue to black. It is wise to color a portion of the black, with blue to show that this is the change over for the neutral.


Have a good day mate.....


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