RCD Keeps Tripping

[ColJack]

switching off a circuit doesn't stop N-E faults, only disconnecting the relavent neutrals and earths from the bars and switching off the breker does that.

 

[JohnD]

also look for mouse damage or damp

has it rained recently?

have you got any outdoor circuits?

 

[RF Lighting]

I'm going to replace the 30mA RCD, with a 60 or 80mA RCD, and see if that works.

You shouldn't do that. Any RCD over 30mA will not provide adequate protection against electric shock.

 

[sparkwright]

Have you tried unplugging (not just switching off at the wall) all appliances?

Any damaged cables anywhere? Cables drilled through. Cables crushed in loft space. Neutral touching earthed box lug.

 

[LoveRocket]

N-E fault.

You need someone with an insulation resistance tester.

 

[sparkwright]

Any obvious diy electrical work been undertaken do you think, like putting up a light fitting or an extra socket? It's sometimes this kind of 'improvement' where careless diy work can cause such problem.

 

[firebladerrw]

Thanks for all the great replies and advice.

Clearly this isn't simple, and I think I've exhausted the simple things to try:

Isolating each circuit at the breaker (noting comments about neutral and earth disconnections)
Inspecting wiring & outside light installations, dont see anything wrong, but some wiring is hidden.
It hasn't been raining, nor is anywhere damp.

I think (using my mech. eng. logic !), that its either:

N-E fault - likely
Faulty RCD - although I dont know if this is common

So I think before I get an IR check done, I'm going to change the RCD ?

Question, I assume that if there is a build up of earth leakage, that occasionally pushes the RCD over 30mA, wouldn't it be possible to fit a slightly higher rated one? I note that some CU's have 60, 80 & 100mA RCD's fitted?

 

[JohnD]

I'm going to replace the 30mA RCD, with a 60 or 80mA RCD, and see if that works.

I think you have misread the labels. 30mA and 30mS gives good personal protection against fatal shock, which is why it is the most common one domestically.

RCDs have two ratings:


The sensitivity, most often domestically 30mA (which protects you from a life-threatening shock) and 100mA (which will probably protect your life, but might be unpleasant, but will cut other faults even if you happen not to be touching them,and also gives protection from some faults that may cause fires)

The power-handling capacity (in domestic installations, this is commonly 63A, 80A or 100A). It is unusual for a domestic installation to have loads greater than these. The variation is also by brand. The higher ratings ought to have bigger contacts inside that can withstand a higher current without arcing. They also tend to be slightly more expensive as the rating goes up.


(RCDs are also available in 10mA (used in hospitals) and lower sensitivities; and in 300mA, 500mA etc, used industrially, to protect against faults and fires, but these do not give personal protection. They are also available with different tripping speeds - domestic ones usually trip within a nominal 30milliseconds, but this depends on the current flowing through, but are also available with a preset delay, made to not trip for 100 milliseconds, to avoid nuisance tripping under some circumstances. This means that you might get a nasty shock, but it will quite likely not kill you.

For your old MEM enclosure, if you can find a replacement RCD to fit (probably second-hand) you will probably only be able to get 30mA or 100mA. I think it will also be money down the drain.

OOI, if you get a new MEM CU, as I suggested, it will also give you access to a very wide range of RCDs, because the MEM industrial RCBOs will also fit. From memory, if you happen to want it, you could get anything from 5mA (useful if you have a Dialysis machine) to 500mA (useful if you have an induction furnace).

 

[1john]

If you get someone to do an insulation resistance test, they will be able to check the RCD for you too, they will check it trips out at 30mA within the specified time, at 150mA within the specified time and that it doesnt trip out at 15mA.

 

[firebladerrw]

I'm going to replace the 30mA RCD, with a 60 or 80mA RCD, and see if that works.

I think you have misread the labels. 30mA and 30mS gives good personal protection against fatal shock, which is why it is the most common one domestically.

RCDs have two ratings:


The sensitivity, most often domestically 30mA (which protects you from a life-threatening shock) and 100mA (which will probably protect your life, but might be unpleasant, but will cut other faults even if you happen not to be touching them,and also gives protection from some faults that may cause fires)

The power-handling capacity (in domestic installations, this is commonly 63A, 80A or 100A). It is unusual for a domestic installation to have loads greater than these. The variation is also by brand. The higher ratings ought to have bigger contacts inside that can withstand a higher current without arcing. They also tend to be slightly more expensive as the rating goes up.


(RCDs are also available in 10mA (used in hospitals) and lower sensitivities; and in 300mA, 500mA etc, used industrially, to protect against faults and fires, but these do not give personal protection. They are also available with different tripping speeds - domestic ones usually trip within a nominal 30milliseconds, but this depends on the current flowing through, but are also available with a preset delay, made to not trip for 100 milliseconds, to avoid nuisance tripping under some circumstances. This means that you might get a nasty shock, but it will quite likely not kill you.

For your old MEM enclosure, if you can find a replacement RCD to fit (probably second-hand) you will probably only be able to get 30mA or 100mA. I think it will also be money down the drain.

OOI, if you get a new MEM CU, as I suggested, it will also give you access to a very wide range of RCDs, because the MEM industrial RCBOs will also fit. From memory, if you happen to want it, you could get anything from 5mA (useful if you have a Dialysis machine) to 500mA (useful if you have an induction furnace).

Many thanks - I'm on the case to get a new MEM CU as I type this. Any advice on any particular MEM type?

 

[ban-all-sheds]

• For any given circuit how would you go about deciding what cable and protective device to use? You can't just assume that what you already have is correct.
  
  
• How do you calculate maximum demand and how can diversity be used? You'll need to put the maximum demand figure on the EIC.
  
  
• What are the 3 different types of domestic single-phase supplies provided in this country, how would you recognise them, and what differences do each make to the requirements for the rest of the installation, particularly any outdoor supplies?
  
  
• Can you correctly identify all components and connections of a circuit by method of testing or otherwise? In doing so can you identify or recognise anything wrong or dangerous with the circuit?
  
  
• Do you understand how the way in which cables are installed affects how much current they can carry?
  
  
• Where cables need to be joined, how should this be done / not be done and in what circumstances are different methods acceptable?
  
  
• Can you identify extraneous conductive parts, and do you know the requirements for main and supplementary bonding of them?
  
  
• Which circuits should be RCD protected?
  
  
• How do you propose to isolate your supply so that you can connect up your new CU?
  
  
• Do you know what tests you would carry out on the installation - what sequence you'd do them in and at what point you would energise the installation, and for each test do you know what is being measured, why it is important, how you would carry out the test, and with what equipment, and what sort of results you would expect to get if everything was OK?

 

[firebladerrw]

• For any given circuit how would you go about deciding what cable and protective device to use? You can't just assume that what you already have is correct.
  
  
• How do you calculate maximum demand and how can diversity be used? You'll need to put the maximum demand figure on the EIC.
  
  
• What are the 3 different types of domestic single-phase supplies provided in this country, how would you recognise them, and what differences do each make to the requirements for the rest of the installation, particularly any outdoor supplies?
  
  
• Can you correctly identify all components and connections of a circuit by method of testing or otherwise? In doing so can you identify or recognise anything wrong or dangerous with the circuit?
  
  
• Do you understand how the way in which cables are installed affects how much current they can carry?
  
  
• Where cables need to be joined, how should this be done / not be done and in what circumstances are different methods acceptable?
  
  
• Can you identify extraneous conductive parts, and do you know the requirements for main and supplementary bonding of them?
  
  
• Which circuits should be RCD protected?
  
  
• How do you propose to isolate your supply so that you can connect up your new CU?
  
  
• Do you know what tests you would carry out on the installation - what sequence you'd do them in and at what point you would energise the installation, and for each test do you know what is being measured, why it is important, how you would carry out the test, and with what equipment, and what sort of results you would expect to get if everything was OK?

Well I suppose I should thank you for all that...

I could answer a few of your questions, but would a spark realy do all that before fitting a new CU?

I intend getting my mate to fit a new CU for me, but I rather doubt all you have listed would be checked.

Oh and in case you think I'm a luddite, I'm C Eng. Mech.....

 

[ban-all-sheds]

I could answer a few of your questions, but would a spark realy do all that before fitting a new CU?

One who knows what he's doing, and does his job properly, yes.

I intend getting my mate to fit a new CU for me, but I rather doubt all you have listed would be checked.

Then use someone who knows what he's doing, and does his job properly, not your mate.

 

[timtheenchanter]

I would agree with londonboy on this, at least there needs to be some earth leakage checks done, simply upping the RCD in my mind is akin to changing a 6A MCB to a 16 just because it keeps tripping.

 

[bongos]

I could answer a few of your questions, but would a spark realy do all that before fitting a new CU?

They would.

I intend getting my mate to fit a new CU for me, but I rather doubt all you have listed would be checked.

Then get someone who will check it to do it.

Oh and in case you think I'm a luddite, I'm C Eng. Mech.....

I don't think anyone thinks this. But just because you are C Eng. Mech you aren't automatically qualified and competent to complete domestic electrical work!

Ultimately, whether you fit a new CU or stick with the old one, you probably have some sort of N>E fault that will require diagnosing with a insulation resistance tester. If this fault is sorted then there is no reason for you to have to replace your CU at all, although as you have discovered, a fault on your current setup takes out all your circuits which certainly isn't ideal!

It's unlikely (although not impossible by any means) that the RCD is at fault here yet anyone who has these sort of faults always seems to suggest replacing the RCD should be step 1. Whatever you do, you should not and must not replace it with a RCD of a higher rating.