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I have a fairly elderly MEM 8 way board with a 100A 30mA RCD main switch which takes out the whole board when it trips. About a month ago I put in a new circuit to an upstairs annexe in a garage (16mm SWA, TT rod and RCBO's in a separate CU). About 2 weeks ago the electricity board were doing some work so the power went on and off a couple of times, the last time it went off it tripped the RCD, I'm guessing this might have been coincidental but ever since the RCD will trip randomly, ranging from 3 days to 3 minutes,I have done a ramp test on the main RCD which is fine (trips at 29mA) and tested the circuits individually, again with no apparent problems. If I turn off one circuit (doesn't matter which one) this seems to be enough for the RCD not to trip, so I guess the problem is cumulative earth leakage??

I have disconnected as many external appliances as I can (e.g. Alarm box that was full of spiders outside) but still it randomly trips, I was wondering if the wise Sages of this forum had any experience of this kind of thing and if they cured it, what was the main culprit. I know ultimately I will have to change the board but it would be nice if I could fix this.

For info I am a Electrical Trainee and all this work will be signed off by a qualified spark.

Thanks for any help

D2E
 
Why is the DB completely protected by a 30mA RCD?

Can you not replace the main switch with a 100mA or 300mA RCD and individual 30mA RCBO's for any circuits that require them.

If its an MEM Memshield 2 DB, you can get 30mA RCD pods that you can fit onto the MCB's making them RCBO's and replace the main switch with a standard main switch (if all circuits are RCD protected) or a 100 / 300mA RCD.
 
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  • #3
Why is the DB completely protected by a 30mA RCD?

Can you not replace the main switch with a 100mA or 300mA RCD and individual 30mA RCBO's for any circuits that require them.

If its an MEM Memshield 2 DB, you can get 30mA RCD pods that you can fit onto the MCB's making them RCBO's and replace the main switch with a standard main switch (if all circuits are RCD protected) or a 100 / 300mA RCD.
Replacing the main switch and RCBO'ing the other circuits was another option, cost wise I wondered if I might as well replace the board, however, time is not on my side so I will probably do as you say. Thanks
 
With regards to costs, if you decide to replace the whole DB, what DB would you install?

Dual RCD: May still have nuisance tripping.
Individual RCBO's: DB will be costly anyway.

If it is an MEM 2, how many 30mA RCD's do you need?
I do have some spare MR30's that I would be willing to sell (at a reasonable price, most likely cheaper than any other supplier). They are brand new in boxes, and never used.

I may also have a 100mA Main Switch (Used) and a standard main switch (Used).
 
D2E, do you have or can you lend a clamp meter and test the whole leakage and then each individual circuit for earth leakage whilst all appliances are on? You will narrow down your fault finding this way. You may need to pat test appliances.
 

Jimmy Boy

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Arms
I have disconnected as many external appliances as I can

That wont cut the mustard if you want to find the fault you will need to disconnect everything from line and neutral pulling a fuse on a FCU will still leave the N intact, once ALL vulnerables are removed IR it at 250V first see if it complains if clear up it to 500V if not clear then split the circuits and IR each, for a TT you want to go with a time delayed 100mA up front with 30mA split load or rcbo's

J

 
As Paul says, best to get a clamp meter that will go down to mA. It does indeed sound like cumulative leakage. I bought the dilog unit and it's very useful.
The other reason the RCD trips is that it could be faulty. It wouldn't be the first time an RCD was damaged following leccy board work nearby.
 

Jimmy Boy

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Arms
Whilst I have the utmost respect for guitarist and Pauls advice, where I beg to differ is that you should make sure the fixed installation is fault free before worrying about appliance leakage, they may well be proved right, none the less fault finds should start at the origin of the installation not assuming it may be ancillary equipment, you mention cumulative leakage this could be from an appliance but it could also be leaking from the fixed installation contributing to the overall problem.

J
 
From the op:-

"I have done a ramp test on the main RCD which is fine (trips at 29mA) and tested the circuits individually, again with no apparent problems."


Looks like he has tested the circuits that's why I went down the appliances leaking route. But as Jimmy says IR test each circuit, its all a matter of systematically eliminating it down to the actual fault.

Good advice from everyone :).
 
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D2E, do you have or can you lend a clamp meter and test the whole leakage and then each individual circuit for earth leakage whilst all appliances are on? You will narrow down your fault finding this way. You may need to pat test appliances.
I think I know a chap with one, I will try and borrow it and track it down, trouble is there aren't enough hours in the day at the moment.
 
S

sparky-s-w

could this be an overload problem, not an earth fault. you've go 8 circuits on a 100amp 30ma rcd, + what you installed. what is you max demand. what are your circuits pulling combined.
 

Rockingit

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Mentor
Arms
Esteemed
I've had a few cumulative leakage problems before - a few mA here, a few there, and suddenly you've got 40-50 mA across one side of a split board. It's made by all the little digital things we plug in..routers, flat tv's, laptops etc etc. Only real solution is to split the circuits up more and more and put them on rcbo's.
 
Whilst I have the utmost respect for guitarist and Pauls advice, where I beg to differ is that you should make sure the fixed installation is fault free before worrying about appliance leakage, they may well be proved right, none the less fault finds should start at the origin of the installation not assuming it may be ancillary equipment, you mention cumulative leakage this could be from an appliance but it could also be leaking from the fixed installation contributing to the overall problem.

J
It's nice to know that you have the utmost respect for us jimmy, and I for one respect your opinions just as much.
From what the op had said, I also assumed that he had carried out all the basic tests you refer to.
 
could this be an overload problem, not an earth fault. you've go 8 circuits on a 100amp 30ma rcd, + what you installed. what is you max demand. what are your circuits pulling combined.
RCD main switches have no overload capability, they are just a main switch, in this case rated at 100A.
 

Jimmy Boy

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Arms
could this be an overload problem, not an earth fault. you've go 8 circuits on a 100amp 30ma rcd, + what you installed. what is you max demand. what are your circuits pulling combined.
Stand alone RCD's don't trip on over load only fault leakage but it could burn out

J
 

Strima

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Arms
Esteemed
I'm with Jimmy on this, disconnect everything and IR test the individual circuits. Could be a cumulative leakage fault or a circuit starting to fail. No point looking for other faults until the whole installation is proven to be good.
 
I'm with Jimmy on this, disconnect everything and IR test the individual circuits. Could be a cumulative leakage fault or a circuit starting to fail. No point looking for other faults until the whole installation is proven to be good.
He says that he has already tested individual circuits mate.
 

Jimmy Boy

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Arms
He says that he has already tested individual circuits mate.
Well as I was hinting at earlier he doesn't say he has IR'd the circuits just tested them for RCD tripping I assume by isolating them individually..

I have done a ramp test on the main RCD which is fine (trips at 29mA) and tested the circuits individually, again with no apparent problems.

J
 

Strima

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Arms
Esteemed
The OP hasn't mentioned the type of tests he carried out, that's the way I read it. If the circuits have been IR'd then I'll shut up and move onto clamping the earth.

If D2E could list the specific tests carried out it would be easier to advise. Also incoming voltage and Ze etc.
 

Jimmy Boy

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Arms
Looking back at the first post it states he has run a second CU to a garage, presumably from the original CU so I hope he either split the tails to serve this or perhaps he managed to get a pair of 16's in the incoming side of the main switch/RCD otherwise there would be no discrimination

J
 

Strima

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Arms
Esteemed
If I turn off one circuit (doesn't matter which one) this seems to be enough for the RCD not to trip, so I guess the problem is cumulative earth leakage??
Just re-read your Op, Think of how each circuits neutral and earth are terminated. This is why I'm wondering if an IR test has been done on each circuit, a global one may be within the required limits however resistance in parallel could give false readings.
 
Guys, you are probably right. I assumed (maybe wrongly) that the op had carried out all the usual tests. I realise that we should never assume anything, but I still stand by the fact that clamping for leakage would probably be the quickest thing to test. As he doesn't have a clamp meter, then it's not gonna be quick :)
 

Strima

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Arms
Esteemed
I'm never right, I just like to add to the confusion... :lol:
 

Jimmy Boy

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Arms
Strima your quite right parallel resistances will lower the figure but you would need a hell of a dis board to have enough parallels to reduce it to <1M

J
 

Strima

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Arms
Esteemed
Edit, as I'm confusing myself now... Not hard before you say it. On a global test the measurement may be >1M however the leakage across one or several circuits may be enough to trip the RCD especially as loads increase such a refrigeration/PC's etc kick in.
 
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Jimmy Boy

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Arms
What do mean by a bad one? What I meant was that if all insulation was in good order ie not breaking down you would need a huge amount of parallel resistances via circuits to lower an otherwise good IR reading, if you had one leaking like a sieve on a bit of old rubber of course it would drag the overall down and you would start splitting them

J
 

Strima

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Arms
Esteemed
As I said, I'm confusing myself. I meant to say accumulative effect over all circuits.

And you may have been posting when I edited my original post.
 
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Looking back at the first post it states he has run a second CU to a garage, presumably from the original CU so I hope he either split the tails to serve this or perhaps he managed to get a pair of 16's in the incoming side of the main switch/RCD otherwise there would be no discrimination

J
The garage CU is fed from a 63A MCB in the main house CU so discrimination would be an issue until I change the main RCD switch to a non RCD one and change the house MCB's to RCBO's. The fact that there is no discrimination at the moment, that presumably wouldn't be the cause of the tripping?
 

Strima

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Arms
Esteemed
The annex that has been wired, although it is on a TT has the earth been exported to the new CU as well? Or is the SWA armouring/earth only terminated at the supply end?
 
  • Thread Starter Thread Starter
  • #33
The annex that has been wired, although it is on a TT has the earth been exported to the new CU as well? Or is the SWA armouring/earth only terminated at the supply end?
The armouring earth has only been terminated at the supply end, I haven't exported the earth.
 

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