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Topic Title: Rcd issue
Topic Summary: RCD not tripping
Created On: 04 November 2012 09:15 PM
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 04 November 2012 09:15 PM
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pixelmonster

Posts: 7
Joined: 08 July 2012

Sorry let me post that again.
Hi, I went to test circuits at a cafe in a public garden. The system is TT on a 3 phase wylex board.Ze is 0.67. Ra is 1.07. Main earth was disconnected during test.
Circuits tested fine apart from the RCD tests. The 30ma RCBOs on the sockets trip at 19ms at x1 but not at x5 and the 100ma main switch wont trip at all under x1 100ma test.
My meter is telling me that the contact voltage is exceeding the pre set limit of more thanm 100v.I recalled that due to higher Ze readings on tt systems the x5 test doesnt work. Can anyone shed some light on this or am I looking at a fault somewhere.
Many Thanks. Confused in Cornwall!!
 04 November 2012 10:11 PM
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daveparry1

Posts: 6382
Joined: 04 July 2007

Are you sure the Ra is 1.07 ohms and not something much higher? If it really is that low you must have a very efficient earth spike!

Dave.
 04 November 2012 11:42 PM
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sparkingchip

Posts: 6338
Joined: 18 January 2003

I have a Megger RCD tester that locks out if it considerer's the touch voltage will rise above 50 volts, though a loop test prior to RCD testing often leaves the meter's maths looking either dodgy or erring distinctly on the side of caution.

Having considered the loop test result and having given careful consideration to safety issues my normal course of action if the Megger locks out as it considers the touch voltage will be to high is to get my Robin 1620 tester out of its box and use that has it does hot have any qualms about getting on and doing a test in such circumstances.

If your installation is TT then you have some parallel earth paths bringing the Ze down to avery low level and as Dave said your Ra is exceptionally low, so a high touch voltage really should not be an issue.

The 100mA RCD doesn't need a x5 test and it would put too much current into the earthing system at 500mA (0,5 amps) for safety.

I would suggest you spend a little time at home doing a calibration check on your meters at home and at the same time review your testing regime, then you can return to site and test again having checked your meters and your testing procedures which may result in more acceptable results. You could even try knocking a temporary earth rod in at home and do some testing with it along with a extension lead and a plug in RCD just to check you can get your meters to perform in the expected manner.

I hope that doesn't sound patronising, but you need to review three things:

1. Your meters.

2. Your testing regime.

3. The results you are getting on site.

Regards Andy
 04 November 2012 11:46 PM
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sparkingchip

Posts: 6338
Joined: 18 January 2003

Having said all that if the problems persist then maybe check insulation test results for the indication of a neutral/ earth fault
 05 November 2012 08:34 PM
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pixelmonster

Posts: 7
Joined: 08 July 2012

Hi Andy, Thanks for your advice and taking the time to reply. There is some more testing history.
The cafe is on a submain from the farm office. I tested the rcds at 0.5, x1 and x5 at the farm office. Tested fine.

I did these tests with the cafe submain disconnected and connected.I also tested on a socket on another workshop submain. Tested fine.

I then disconnected every circuit at the main DB in the farm office except for the cafe submain and went back to the cafe and re-tested. RCDs dont trip contact voltage exceeding100vs etc.

I will go back and re test the Ra. It does seem very low but I dont think their is an issue with the tester.

Sparkingchip suggested a neutral earth fault. This is what the helpline at Elecsa said. I understand that this can cause damage to equipment if voltages get to high.

I will return to the site and retest Ze/ Ra and the distribution supply for the cafe.
Many thanks
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