MK RCD Ramp test random results

IMAC1959 · Jan 11, 2017

Hi guys I am rather perplexed as I have recently fitted a dual RCD Sentry (App 3 compliant) consumer unit. I am suffering a random tripping pattern with one of the RCD's (63A, 30mA) which sometimes pops the other (80A 30mA) at the same time. I have stripped everything down and insulation resistance tested everything with the lowest reading being 80Mohm L-E and N-E. I also tested with the IR test between neutral bars to ensure I do not have crossed neutrals or some random connection between them. I decided to do a ramp test on both RCD's just to see what they are actually tripping at and was surprised to get random readings on a completely disconnected RCD of 6mA (multiple times) with an occasional 21mA. Both RCDs were similar so I changed both (Screwfix supplied) and they are the same. My tester is new and is a Kewtech KT63. As a matter of curiosity I ramp tested a BG RCD with the same tester in place of the MK ones and the ramp test was consistently 21mA.
Any ideas guys?
My next move is to fit the BG as a temporary test and then call MK Tech Support.
Thanks!

hotfingers · Jan 11, 2017

IMAC1959 said:
Hi guys I am rather perplexed as I have recently fitted a dual RCD Sentry (App 3 compliant) consumer unit. I am suffering a random tripping pattern with one of the RCD's (63A, 30mA) which sometimes pops the other (80A 30mA) at the same time. I have stripped everything down and insulation resistance tested everything with the lowest reading being 80Mohm L-E and N-E. I also tested with the IR test between neutral bars to ensure I do not have crossed neutrals or some random connection between them. I decided to do a ramp test on both RCD's just to see what they are actually tripping at and was surprised to get random readings on a completely disconnected RCD of 6mA (multiple times) with an occasional 21mA. Both RCDs were similar so I changed both (Screwfix supplied) and they are the same. My tester is new and is a Kewtech KT63. As a matter of curiosity I ramp tested a BG RCD with the same tester in place of the MK ones and the ramp test was consistently 21mA.
Any ideas guys?
My next move is to fit the BG as a temporary test and then call MK Tech Support.
Thanks!

hotfingers · Jan 11, 2017

Is there any more rcd,s in the property wired in series? Some times this can trip both

IMAC1959 · Jan 11, 2017

Edit
Forgot to mention our supply is TN-C-S in a rural setting, bulk of installation dates from 1988 when house constructed. Have noticed the lights flicker at times but so does the neighbours. A day or so ago there were two large fluctuations in our supply as confirmed by neighbour. Our RCD's both trippedf on largest dip and "tripiest" tripped on smaller dip. Tonight tripped randomly for no apparent reason, unplugged everything reset RCD and then started putting everything back on. Tripped again with PC plugged in so left unplugged then RCD tripped again. We are now all plugged in and RCD is happily carrying load.

IMAC1959 · Jan 11, 2017

There was a garage RCD unit with a 40A 30mA fitted but I replaced that with a switch and the was also an RCD prot cited socket which has been replaced with a normal twin socket. As the whole installation is covered by the two main RCD's I thought they were superfluous.

Deleted member 9648 · Jan 12, 2017

Have you disconnected/unplugged equipment prior to your IR tests? If so test with everything in place, (not L-N !),you might find you get a different reading. test at 250v initially and see what shows. Also surge protector leads can cause tripping problems, if there are any of these in use try taking them out. Failing that you may be best advised to carry out earth leakage tests,although I do not possess an earth leakage clamp and have never found the need for one.

Octopus · Jan 12, 2017

Best get your earth clamp meter out and see what that says ..... I have one and in cases like this (and before CU changes) its worth its weight in gold!

FYIW I would say you probably have leakage accumulated across a number of circuits and devices PLUS a fault to track down.

DPG · Jan 12, 2017

Dodgy supply neutral?

IMAC1959 · Jan 25, 2017

Well since I posted my original post I have fitted the BG RCD and sent the "duff" one to MK who claim that it is ok. Interestingly the system has been stable since fitting the BG RCD of the same rating as the MK one.
I have an Amecal ST-9810 Leakage Current Tester and have a total leakage of 13-15mA on the RCD circuits in question. This is 2 Ring Final's, 2 Cookers (3Kw each), Garage, Immersion & Kitchen Lights. If I turn off each mcb in turn I end up with approximately 8mA on one of the Rings. This ring has 2 Daikin Air Conditioners on it wired to FCU's on the ring.
I am sure I could track down the leakage and get it closer to zero but clearly the leakage is nowhere near 30mA. I still suspect the low ramp tested RCD as 6mA is very low in my opinion.
RCD testing is always done with all circuits off and neutral disconnected at respective neutral bar.
When I get the tested RCD back from MK I will return it to my wholesaler and try a new one and ramp test that. Hopefully I will find one that is set higher than 6mA!