Discuss Neutral to earth fault affecting both RCDs in the Electrical Forum area at ElectriciansForums.net

Reaction score
12
Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:  American Electrical Advice Forum

Hello everyone, I'm newly registered having read the boards from the shadows for a while. I have a question about neutral to earth faults and RCDs. I have read around this as much as I can but being relatively newly qualified my experience isn't as much as the collective members here so I thought I'd ask.

I had been asked to install a new extractor fan in a kitchen, I have done some pre work testing on the cabling present and it all looked good so I proceed to fit the extractor. When filling out the minor works cert I have struggled to get the Rcd to trip. I know I should have tested this as well before I started the work but as I said I'm new and clearly a bit naive. Initially the Rcd auto test didn't trip at 1/2 on both sides of the wave, tripped the Rcd both times at 1x then gave me a 50v error at 5x, the test button worked.

I did a bit of reading and found that the fault could be a neutral to earth fault. I pulled out each neutral at the board and IR tested between individual neutrals and the earth bar of all the circuits on that side of the board and found no problems >200M on each circuit. When carrying out a global IR test (neutral bars to earth bar) I see there is a fault but on the other side of the board. I pulled all the neutrals out on the other side and identified a neutral to earth fault on the sockets ring upstairs. I have chased this to a cable between two sockets and will obviously now have to replace this but my question was, can a neutral to earth fault on the other side of the board affect both RCDs? Was the 50v warning on the rcd I was initially testing due to the neutral to earth fault I have identified on the other side of the board?

From my reading I have seen that this type of fault can cause the RCDs to not trip at all and wondered if the fault could be affecting my ability to test this RCD?

Any help would be appreciated and any additional info will be provided swiftly.

If it helps anyone the RCDs are actually Lewden type A RCCBs (63A 30mA)

Thanks
 

timhoward

-
Esteemed
Arms
Reaction score
2,256
Initially the Rcd auto test didn't trip at 1/2 on both sides of the wave, tripped the Rcd both times at 1x then gave me a 50v error at 5x, the test button worked.
Hi and welcome.
Just checking you know it's not supposed to trip at 1/2.
The 50v error at x5 indicates you have not got a supply voltage at the output terminals and the test cannot proceed; this can be that the 'jump' of the trip slightly shook one of the probes off.
It might be worth trying that one again.

It might be worth pointing out that regulation 415.1.1 says that an RCD must operate within 40 ms when tested at a current equal to or higher than five times its rated residual operating current.
So if you get <40ms at X1 (which invariably you do these days) it's already in fact passed.

I'm just reading and digesting the other bit - well done for finding the N-E fault - they can be a nightmare to find.
 

timhoward

-
Esteemed
Arms
Reaction score
2,256
I did a bit of reading and found that the fault could be a neutral to earth fault. I pulled out each neutral at the board and IR tested between individual neutrals and the earth bar of all the circuits on that side of the board and found no problems >200M on each circuit. When carrying out a global IR test (neutral bars to earth bar) I see there is a fault but on the other side of the board. I pulled all the neutrals out on the other side and identified a neutral to earth fault on the sockets ring upstairs. I have chased this to a cable between two sockets and will obviously now have to replace this but my question was, can a neutral to earth fault on the other side of the board affect both RCDs? Was the 50v warning on the rcd I was initially testing due to the neutral to earth fault I have identified on the other side of the board?
Assuming the RCDs are working properly, in normal circumstances a N-E fault on one side of a dual RCD board should only affect that side.
I'm wondering if as you've had most wires out of the board to fault find (sensibly!) then is there a possibility one of the Neutrals is now in the wrong bar, which is very easily done.
I only went in to put a new extractor up 🤦
From time to time this sort of thing happens and its frustrating for you and the customer. I'm not trying to be patronising but you are handling it very well, most newly qualified people don't cope too well when testing goes off-script and the wrong number appears, and you've worked out the fault and found it. Don't beat yourself up!

What I'd do next:
I'd be inclined to ramp test both RCDs ideally with all breakers off and the outgoing N disconnected to check they are both tripping at 20-30ma with no loads.
I'd probably remove the N's for faulty socket circuit at the CU, leave that breaker off, and test the RCDs normally again and see what happens.
You could also simulate a N-E fault between each N bar and CPC bar at the CU and see if both RCDs still trip.
 
Last edited:
OP
R
Reaction score
12
Thanks for the advice, I really appreciate it. All neutrals are in the right places as I removed them one at a time to find the circuit. With regards the ramp testing and testing in general across the Rcd without any loads sounds like a good idea. I have bought a new RCD as both are over 11 years old so I can't rule out a failure of both RCDs due to age but I will return that if the testing without loads shows them to be operating correctly. My instinct is that the Rcd on the side I was working on has failed and the Rcd on the neutral earth fault side is not operating due to a fault, the fact they have occured together is hopefully coincidence but I won't be taking anything for granted with it. I can't get back there till Sunday so there's plenty more time for me to consider other possibilities.
 

telectrix

-
Mentor
Esteemed
Arms
Reaction score
65,531
it has been known for a N-E fault on RCD1 circuits to trip RCD1 when a load is switched on on RCD2. this is due to some witchcraft of current flowing down the wrong path. even had 2 RCDs once would not trip either by tests or with the button. was a hard N-E on a redundant but still connected immersion heater circuit.
 

timhoward

-
Esteemed
Arms
Reaction score
2,256
I find it curious that the OP states that the RCD showed 50V error warning on the 5x test. This indicates that the tester is not seeing the required voltage to perform the test.
Agreed. If it's an RCD with deep slots on the terminal screws it can be the GS38 tips prevent proper contact especially if you aim in the middle. I suspect a probe slipped slightly.
Obviously checking for loose connections is wise too!
it has been known for a N-E fault on RCD1 circuits to trip RCD1 when a load is switched on on RCD2. this is due to some witchcraft of current flowing down the wrong path. even had 2 RCDs once would not trip either by tests or with the button. was a hard N-E on a redundant but still connected immersion heater circuit.
Interesting. I wonder if it could be explained by a load with a reasonable inrush current on RCD2 causing a momentary volt drop, in turn causing a momentary current draw rise on RCD1 pushing the leakage current over the threshold...
I almost went for a dual rcd board for an install next week where I'm travelling a fair distance and can't get good information about current circuits other than there are lots of them. My thinking had been I can take lots of MCB's with less outlay than lots of RCBOs. In the end I gave myself a good talking to and went the RCBO route, and this kind of thing reassures me it was the right call!
 
OP
R
Reaction score
12
I ran the test using the plug top on two different socket outlets on the ring main on the same side as the Rcd I was testing as well as from the extractor. Same 50v error 😔

I wondered if it may be some capacitance from a knackered alarm battery which is a fused spur off the ring?
 
Reaction score
58
I ran the test using the plug top on two different socket outlets on the ring main on the same side as the Rcd I was testing as well as from the extractor. Same 50v error 😔
Test instruments meeting the requirements of BS EN 61557 must perform a pre-check when testing to ensure that the earth potential does not rise to a dangerous level during the test. This will apply to earth loop impedance testing and RCD testing since these tests involve producing a current on the earth. If the test current is expected to produce a dangerous voltage, the tester should inhibit the test and show a warning, such as >50 V.
 

timhoward

-
Esteemed
Arms
Reaction score
2,256
It's a Dilog 9110 (can't afford anything premium right now lol) it's ok but prone to being temperamental.
Nowt wrong with that.
This is where the devil is in the detail:
You don't have permission to view attachments. Attachments are hidden.


(On some other makes that message can mean something else.)
For the little time it takes, I'd see if it's now fine with the dodgy socket circuit's neutrals removed.
If still a problem it's IR testing each circuit again I'm afraid, start with L+N together to CPC.
 

Attachments

    You don't have permission to view attachments. Attachments are hidden.
Reaction score
31
Are you testing the RCD at the board or on a circuit? No experience with meggers (and we don't install RCD's often for utilities) but I would have assumed that the test had failed because the voltage on the earth conductor was at 50V. I assume the cable resistance was too high to achieve 250mA at <50V. Was there a '>' or a '<' in front of the warning?

I don't understand how a neutral earth fault would cause an RCD not to trip. The only reason I can think of for the fault on the socket circuit not to trip would be if it was sticky from lack of use (or a DC leakage issue).

If the cable isn't accessible and you cant install a new one, could you disconnect the faulty cable and derate it to 20A?
 
OP
R
Reaction score
12
I was reading someones explanation and what I took from it was that the amount of current lost from the neutral was less than 30mA and therefore not triggering the RCD... Didn't fully understand how that happens though.

Yeah I may well have split the ring into two radials, there is a spare way in the board as well.

I have been testing on the circuit and not at the Rcd itself with no load so will try this when I'm next there.

If the Rcd responds at the device itself I will re add the loads but remove the neutral earth fault and see if they continue to operate.

I really appreciate this, being new my confidence when it comes to this sort of fault finding isn't great but I'm keen to make sure I get to the bottom of it, for myself as much as the client.
 

pc1966

Esteemed
Arms
Reaction score
6,232
What was the Zs at the test points used where the RCD failed? (Using the no-trip option, obviously!)

Seeing >50V warning to me sounds like the earth/CPC is exceptionally poor the test current is pushing it up above 50V.
 

pc1966

Esteemed
Arms
Reaction score
6,232
It's a Dilog 9110 (can't afford anything premium right now lol) it's ok but prone to being temperamental.
Have you tried a new set of probes?

I have the 9118 and it is fine (though I question the no-trip accuracy in some low Zs cases) and so far not seen the 50V warning. Probes are "consumables" as eventually the flexing breaks the conductors or the plugs get weak and don't establish as good connection.
 
OP
R
Reaction score
12
Zs was 0.4, on the circuit I was installing the extractor on.

With regards to leads, no I haven't replaced them yet, I will look at picking up a set if I can't seem to find a solution on the system itself, I haven't had them that long but it's a good suggestion for me to look into, thank you
 
Reaction score
31
I was reading someones explanation and what I took from it was that the amount of current lost from the neutral was less than 30mA and therefore not triggering the RCD... Didn't fully understand how that happens though.

Ah that makes sense for why that fault didn't cause the RCD to trip, I was thinking more of a direct short from a screw or the like but you're right it could be a lot lower than 1M and not trip I suppose. Assuming in most cases a 30mA RCD's only operate above 20mA..... 230V/0.02A=11.5kohm. Any chance something could be wet?

Hopefully checking your leads and using a long lead to check the earths should identify the problem with the RCD test.
 
OP
R
Reaction score
12
Ah that makes sense for why that fault didn't cause the RCD to trip, I was thinking more of a direct short from a screw or the like but you're right it could be a lot lower than 1M and not trip I suppose. Assuming in most cases a 30mA RCD's only operate above 20mA..... 230V/0.02A=11.5kohm. Any chance something could be wet?

Hopefully checking your leads and using a long lead to check the earths should identify the problem with the RCD test.
Yes possibly, there is an external wall that the faulty cable is buried on if there's water from the outside I suppose it could be wet but I would probably expect to see that on the plaster on the I side. Will get the damp meter on it next time, thanks
 

Lucien Nunes

-
Mentor
Esteemed
Arms
Reaction score
9,960
To clarify with certainty:

An N-E fault downstream of an RCD can cause it to fail a test. because some of the leakage current generated by the tester can 'get back into' the neutral via the fault and go back through the RCD, hence reducing the imbalance. Equally, the N-E fault can defeat the RCD's own test button, because that might simply send some current from the L-in terminal to the N-out terminal, only for it then to disappear down into the CPC via the fault without returning through the RCD to create the imbalance.

But this only happens on circuits protected by the RCD in question. An N-E fault on a circuit protected by a different RCD won't affect the first one. That is logical: N & E are always connected together upstream of the RCD anyhow, whether at the origin (TN-C-S) or the substation (TN-S) so it doesn't matter if they are also connected at an upstream fault.

What can happen that 'crosses over' from circuits on one RCD to another, is the effect of a heavy load current showing up an N-E fault and causing a trip. In that case, it doesn't matter where the load is, whether on the RCD that trips or another. Anything that creates voltage drop in the supply neutral can raise the N-E voltage and drive current through the fault. The fault will however always be on the one that trips.
 
Last edited:

Reply to Neutral to earth fault affecting both RCDs in the Electrical Forum area at ElectriciansForums.net

Similar Threads

S
I have a problem with two separate rcds tripping, both are from the meter, one consumer unit in the house and one in the workshop 30m from the house. If the workshop rcd trips it also trips the house rcd. both consumer units are fed direct from the meter, the workshop having a 80a isolator...
Replies
13
Views
619
arashabb
A
F
My main incoming rcd is tripping out as soon as any load on any circuit. Have pulled each circuit out of the board live and neutral in turn but failed to isolate it to any specific circuit, is this likely to be the rcd at fault?
Replies
12
Views
393
brianmoooore
B
dokkan1080
Is a board layed out like this correct for an eco 7 off peak supply and a peak supply? Two isolators that feed seperate rcds. They have a shared neutral which is split at a Hanley block. The peak live, neutral and earth enter the cu via one gland, And off peak live and neutral enter via...
Replies
10
Views
297
pc1966
P
I
Is a shower rcd, arranged ro the side of the ccu. What the correct procedure. I'm earth on earth bar, neutral on bottom left on rcd, and live on top mcb. The rcd is just tripping once, says trip on the megger. Should I do it inside the shower?
Replies
11
Views
323
Iona222
I
W
Hi, I have an old style fusebox and planning to replace (via electrician) with a modern one (wiring was uprgaded by previous owner) and I noticed when removing a spur line to the outside that was not going to pass muster that my multimeter was showing that earth and neutral were connected. I...
Replies
18
Views
1K
johntheo
J

Electrical News from Electricians Forums and Friends

E
  • Article
Want to become a qualified electrician? Look carefully at the training courses on offer to avoid wasting time and money, is the advice in a new campaign from electrical industry body The Electrotechnical Skills Partnership (TESP). With thousands of searches each year on terms such as...
Replies
49
Views
6K
Marty92
Dan
  • Article
You can now link your account with social media by going to the following page. https://www.electriciansforums.net/account/connected-accounts/ We had this feature some time ago but I removed it when Facebook was found to be selling the data they collected without informing the users of what...
Replies
0
Views
2K
Dan
Dan
  • Article
There was a dodgy advert, typical clickbait, showing "how to save 90% on your electricity bill" which was not the case and it was actually showing some dangerous wiring tactics. Read the actual threads here:- What should be the code for this - C2 or C3 -...
Replies
37
Views
8K
Jurassic Sparks
Jurassic Sparks
James
  • Article
The Go Plus hot tub is being recalled because you could get electrocuted taking a dip, according to press releases from the Chartered Trading Standards Institute (CTSI). An Amazon spokesperson said: "Safety is a top priority at Amazon. "We require all products offered in our store to comply...
Replies
6
Views
3K
Dave Appleby
D
Dan
  • Article
Posted on behalf of @CKI:C.K-Tools Carl Kammerling International Ltd, also known to as ‘CKI’, are the company behind the C.K Tools brand. Founded in Germany in 1904 and with roots stretching back to 1790, we provide a wide range of hand tools, tool storage, power tool accessories and...
Replies
2
Views
4K
telectrix
telectrix

New Posts (Please Reply)

Search Forum

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
Top