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Earth Leakage Current – How much is too much?
Graham J
5 Posts
Question
Hi All, I’ve recently started out and have my first customer with an intermittently tripping RCD and relatively high earth leakage current.  I’d appreciate some opinions on my diagnosis and thoughts on a solution that will guarantee to fix the problem and also be cost-effective.   Here are the details…
Installation:
Domestic, overhead PME supply
Consumer unit configuration:
80A Main Switch/30mA RCD combined (MK 7880s) feeding 11 MCBs i.e a single 30mA RCD protects all 11 circuits, date fitted estimate 1990.
Fault Symptoms: 
(1) RCD Main Switch trips intermittently, randomly (i.e not coincident with equipment or appliances switching on/off, often when no-one is home), infrequently but frequently enough to be a real nuisance as it disconnects the whole installation.  On average the trip occurs once every two weeks, but a month could go by with no trips then, for example, it may trip three times in a weekend.  Tripping has occurred in all weather conditions but more often in stormy weather.  This has been happening since the customer moved in six years ago.
(2) RCD trips very occasionally when garage lights (5x fluorescent tubes) or tumble drier are turned on – this is much less frequent than the random tripping described in (1).
Investigation:
Earthing Conductor and Main Equipotential Bonding all present and correct.
All circuits have Zs within spec, measured at far ends of circuits.  Insulation resistance test of the whole installation (L & N to E at 250V) gives 37kohms. 
There are no obviously faulty accessories or equipment and the installation is generally in good condition.
The RCD operation seems to be good; with all circuits disconnected it passes the no-trip test at 0.5I-delta-n and the disconnect times are 15.6ms and 11.1ms at I-delta-n and 5I-delta-n respectively; ramp test shows that the RCD trips at 23mA.
Earth leakage current (measured with an earth leakage clamp meter around the tails) with all MCBs closed and the installation “as found” (i.e nothing explicitly turned on or off) is 17mA.  A repaired ring circuit in the kitchen draws 6mA of the earth leakage, a garage circuit draws 3.3mA and the remainder is divided roughly equally between the other nine circuits (0.5 – 1.5mA each).
My Diagnosis: 
The installation is not unsafe.
The RCD is (quite severely) sensitised by the 17.0mA standing earth leakage current, it will trip when a further 6mA of leakage current is drawn. 
The fluorescent lights and tumble drier are likely to draw a transient earth leakage current >6mA when they start which explains symptom (2).
The random tripping, symptom (1), is being caused by supply side transients (or maybe even an intermittent fault in the overhead cable) and/or a high impedance neutral-to-earth fault somewhere in the installation.
Question 1:
What is the maximum level of earth leakage acceptable in any given circuit for it to be considered safe?    The 30% limit (9mA in this case) of 531.3.2?
Question 2:
How should I proceed to be sure of resolving the problem, and in a cost-effective way?
  1. Change the consumer unit for a split load dual RCD configuration – with careful assignment of the circuits each RCD will carry roughly 8.5mA of standing earth leakage, but will this give immunity to further tripping, maybe a three-way split required?
  2. Change the consumer unit to all-RCBOs – this is expensive for the customer and I can’t give a 100% guarantee that the tripping will cease… how much earth leakage could a mains-borne transient or fault cause?  It has the benefit that, if I have missed a circuit fault, it will be isolated by an RCBO when it next occurs.
  3. Get the DNO to check the overhead cable before changing the consumer unit.  Will they do that?
  4. Chase down and reduce the earth leakage current?  Would you consider this a “faulty” installation? 
  5. Any other ideas?
Any thoughts will be gratefully received, and I’ll be sure to let you know how it goes.
 
33 Replies
Hi Graham
Welcome.
You need to isolate/unplug all loads and try again. You should then find the leakage is substantially zero. Then it is a case of finding the high leakage loads one at a time, and working out why they leak. However, if this is worse in damp weather, I think you may have a damaged cable problem or something similar allowing water to get the the electricity! If all the wiring seems to leak somewhat then insulation testing is the order of the day, you should find the problem fairly quickly.

Regards
David
mapj1
4317 Posts
Your approach in isolating the leakage in each cct is correct, the next step is to divide to the device level. A test extension lead with the 3 cores split out for a clamp meter may be useful.

Be wary that in a PME house the current down the CPC may not be the same as the leakage an RCD sees, try to measure the way it does threading L and N together through the core of an AC clamp meter.

17mA is quite a lot for a house at idle, note that it does not stack with the 37k ohms (230/37 is about 6mA - high but not the full story ). For IR and leakage currents not to be consistent  is common, and is telling you that you have a mix of capacitance between L and E, and resistance.

Subdividing over more  RCDs or retrofitting RCBOs in place of the MCBs  if available from that maker,  or at the top of the scale  a whole new CU, are the nuclear option(s). Lets keep them in mind at however many pounds an hour,  but try to avoid it. The halfway is to fit Henlys in the tails and have another CU for some of the stuff.

Things that leak to earth resistively,

1)anything with a metal sheathed heating element, cookers, (spilt liquid getting inside the works ) immersion heaters , kettle, dishwasher etc. (sometimes the jacket of the element develops a pin-hole corrosion and the inner mineral insulation, often magnesium oxide powder tight packed then slowly gets wet.)
2) Any appliance where water could get in, things with outside lights, outside cable joints ... poor sealing where cables go through walls. Almost anything under a leaking radiator or pipe.

Things that leak to earth capacitively by design,
motors with suppression capacitors, IT equipment like laptops and monitors (a few mA each is possible) Some LED drivers - the ones with an 'electronic transformer' - really a switch mode power supply.

How much of this sort of stuff is there ?
Let us know how you get  on.
Mike.

PS EDIT
I'd not be happy  with more than about 10mA total leakage on any one  30mA RCD, ideally less, and I''d like a much higher DC result for the insulation test - ideally a few megohms, or at least being able to point to one appliance. As an aside, some designs of induction hob are quite bad and end up being on their own RCBO.



 
Chris Pearson
3186 Posts
"Insulation resistance test of the whole installation (L & N to E at 250V) gives 37kohms."

Well, that's not right. Assuming that only one side has a low resistance, that is 6 mA. As a first step, find out which circuit is responsible.
ebee
1368 Posts
37K global L+N to E whilst not a trip in itself is not so good. Any " leakage" in  addition to that on any circuit or appliance whilst running current could easily throw any RCD into trip therefore with a few circuits intermittently adding a bit can cause a "random" problem, fridges, freezers and CH to name a few. I usually talk the end user into RCBOs if possible. Having said that I`ve had a front end RCD set up for decades now with very few problems causing RCD trips, usually a kettle or a steam iron but watch ou for toasters full of toast crumbs and even the odd ali milk bottle top 

 
AJJewsbury
3059 Posts
+1 for 37kΩ insulation resistance being a red flag. I'd expect something at least three orders of magnitude higher than that in healthy domestic (at least with appliances disconnected).

That low, I'd expect something semi-conducting - a lot of damp having got into somewhere it shouldn't (outside lights/PIRs?) or rodent damage or a charred cable - just the sort of thing not to be stable but vary with the oddest conitions (e.g. after rain, but only with the wind in a certain direction, or at the whim of a mouse or when an odd corner of the house gets to a certain temperature).

   - Andy.
ebee
1368 Posts
There are another couple of things ref loose connections on supply side adding their tenpennorth too
Graham J
5 Posts
Thanks all for your thoughts and suggestions - really helpful.
I'm going to re-test circuit-by-circuit to find out where the leakage current, both capacitive and resistive, is going - then decide on the correct fix.  It should be an interesting piece of technical detective work!
I'll post the results as soon as I have them.

Graham  
Chris Pearson
3186 Posts
Graham J:
Thanks all for your thoughts and suggestions - really helpful.
I'm going to re-test circuit-by-circuit to find out where the leakage current, both capacitive and resistive, is going - then decide on the correct fix.  It should be an interesting piece of technical detective work!
I'll post the results as soon as I have them.

Wouldn't it be easier to check the IR circuit by circuit?

Graham J
5 Posts
Yes, that's what I meant really. I'm just interested in seeing how the earth leakage current I measure with the clamp meter reduces once all the appliances are disconnected, then comparing that measurement with the IR test results.  I will be IR testing circuit by circuit.

Graham
mapj1
4317 Posts
Also note the make and model of the consumer unit - you may (or not  😓)be able to get an RCBO to retrofit  for it, if say you find that it is just the outbuilding power that is touchy,
Mike.
Dave Thomas
33 Posts
I'd agree with your assessment that the installation is likely safe for use. The problem is likely a combination of all these things mentioned: damp in outside stuff ( do you have DP isolation for all outside circuits, could you just try leaving these all off for a couple of weeks), not the best insulation values overall),  but a single RCD is always going to prove problematic, and although RCBOs seem expensive: how many times can you go back? If you could switch some or all of it over- RCD to control internal fixed appliances: Boiler, shower, RCD; yes these can still cause tripping but can usually be double pole isolated if they fail, and RCBO for all socket and lights circuits; possibly only 5 RCBOs for a modest house. Are your meters calibrated? can you try a ramp test? If you're sure you're right, you need to discuss properly with the customer, or ask them to get a second opinion. Their system is safe, the RCD is doing it's job; Its going to cost them money if they want it to stop it  tripping.
Graham J
5 Posts
Well, I’ve now been back to the intermittently tripping installation and here’s an update for those that offered their advice.

I did insulation resistance tests on all of the circuits, individually, with loads disconnected – recall that an insulation resistance test of the whole installation (L & N to E at 250V) gave 37kohms.  The tests found four faults (i.e readings of less than full scale Mohms):
Kitchen sockets circuit:  L-E  230k varying;  L-N 237k varying
First floor sockets circuit:  L-E  67k
First floor lights circuit:  L-E  67k

I then spent a few happy hours looking for the faults, ruled lots of things out including severe condensation in a kitchen socket back box.  But access is a serious problem – the consumer unit is mounted one inch off the ground and the cables promptly disappear beneath the laminate-covered floorboards.  A large group of them re-appear in the loft where the wiring is very untidy – cables strung around diagonally on top of the insulation, emerging from oval conduits that rise a foot above the joists, taped to the joists, slung between pipework, groups crammed into tight gaps in the structure etc.  There’s lots of scope for nicked insulation, misplaced nails, water ingress, mouse nibbles etc but in an hour of searching I didn’t find one.    The house is a small Victorian semi with two extensions - the wiring to the most recent extension particularly looks like it was done in a hurry. 

About 6 years ago (according to the customer) the kitchen sockets ring circuit tripped and couldn’t be reset. The electrician suspected an inaccessible fault under the floor and fixed it by disconnecting one of the ring circuit cables and running a new 4mm cable from the consumer unit to the kitchen, on the opposite side of the house.  The new cable travels via a length of flexible conduit pinned to the skirting board, exits the house via a TV aerial box, travels around the corner of the house in more flexible conduit then up the wall in rigid conduit to the loft through a hole in the soffit.  Once in the in the loft the cable is slung up into an open-ended length of conduit that’s clipped to the rafters, bridging the loft hatch and all the wiring carnage, plumbing, insulation and stored items below.  It exits the conduit, swings down freely again, exits the loft and makes its way down the outside wall to the kitchen via another arrangement of rigid and flexi conduit… it’s far from pretty, but I guess the requirement was to get the kitchen working quickly and with minimal cost and disruption.

Given what I’ve learned about the installation and the advice from this forum, I think the correct course of action this time is to change the consumer unit to an all-RCBO board.   This should fix the immediate problem of the whole house tripping out intermittently and will be the first step on the way to sorting out the installation’s faults as they get worse or as the opportunity arises, circuit by circuit.  For example, it’s possible that the faults revealed by the insulation test results have the same root cause as the old kitchen circuit fault under the floorboards under the laminate and could be fixed if one day the laminate comes up - then the over-the-house cable could be removed.   

My conclusion is that the disproportionate cost and disruption of fixing the root cause of faults is going to be a common conundrum in older installations!

Graham J
 
Chris Pearson
3186 Posts
Graham J:
Kitchen sockets circuit:  L-E  230k varying;  L-N 237k varying
First floor sockets circuit:  L-E  67k
First floor lights circuit:  L-E  67k

Graham, thank you for the update.

The problem now, as I see it, is that you cannot connect these circuits to a new CU. When you produce an EIC, you are certifying that the installation complies with BS 7671 when it clearly does not - 643.3 . I do not think that a departure would be appropriate.

ETA: What value do you get with L&N together to E? Are you sure that you have removed every last load including neons in switches, etc.?

mapj1
4317 Posts
Thanks for the update, sounds quite frustrating.
There is something odd about the lights and power both showing 67k ohms-  perhaps the neutrals are conjoined somewhere - are there wall lights ?

Neons draw a current that is very voltage dependant. so if you see the same resistance reading at 500V and at 250, and if you have an ordinary meter at half a dozen volts as well, then it is not a neon indicator . (typically neons are open circuit up to about 70 to 90V, then they strike and start to conduct - the rest of the voltage is then dropped across a series resistor of  100k ohms or so for the small ones, and a bit less for the big ones.) So if you want to know if it is a neon to ground instead of to neutral, vary the test voltage.

That conduit lark sounds like a total hack - the sort of thing that was always supposed to be temporary that becomes permanent.

Personally I prefer to test L+N to E - as even if there is some hidden load wired L-N in a cupboard, it will not be damaged by the test voltages, as both sides of it are at the test voltage at once , and there is no question that nothing much should be wired either L-e or N-E so testing with L and N in parallel together is useful.


Certainly before changing the CU, you will need to make all reasonable efforts to find the problem, you should not in good conscience leave it with known faults, unless there are exceptional circumstances that justify a departure.
Under floor faults  can be very variable - puddles when it rains, mice if the airbricks are damaged,  all out of sight and hard to inspect - and maybe the odd suspended joint box or chock-block flapping in the breeze. And if it is  Victorian there really should be a noticeable breeze or it will have damp and  rot sooner or later. 
It may be possible to gain at least an inspection access under a kitchen  cupboard or in the under the stairs even with a small hole a mirror and a light can sometimes reveal a great deal.


Also if there are conjoined neutrals between circuits then while the affected circuits are on the same RCD you will not realise, but if any of the current that leaves via the live terminal on one RCD or RCBO comes back via the neutral on another, then both will see an imbalance and  pop off.- better check circuit to circuit isolation  for that before fitting that all RCBO board...
For breaking and rejoining circuits quickly for divide and conquer testing, where there is a mass of looses ends, and for things where the probes do not quite fit,  I find these
3 pole lever connectors,  very useful.

M.



 
Zoomup
4089 Posts
There will always be nuisance R.C.D. tripping causing much inconvenience and disruption with consumer units with just one R.C.D. as the main switch, and many final circuits. A change to an all R.C.B.O. board will be much more reliable and cause less trouble. If you tinker about and future nuisance tripping occurs you will most definitely fall out with your customer. Do a good reliable  job and charge the going rate and they will be happy.

Z.

 
Sparkingchip
4710 Posts
Installing a replacement consumer unit with a RCBO for each circuit could conceal over 150 mA of leakage, which is not always desirable.
Chris Pearson
3186 Posts
Sparkingchip:
Installing a replacement consumer unit with a RCBO for each circuit could conceal over 150 mA of leakage, which is not always desirable.

Agreed. And if the electricity is just leaking all the time and doing nothing useful, it will cost about £50/year. Every year!

Zoomup
4089 Posts
Sparkingchip:
Installing a replacement consumer unit with a RCBO for each circuit could conceal over 150 mA of leakage, which is not always desirable.

Agreed, but the leakage can be found and rectified and the nuisance factor of a single R.C.D. board eliminated.  The current system is not fit for reliable service.

Z.

ebee
1368 Posts
Sparkingchip:
Installing a replacement consumer unit with a RCBO for each circuit could conceal over 150 mA of leakage, which is not always desirable.

Correct.
BUT - did we ever bother about this in the Pre RCD era? Consumer units with rewireable fuses and/or MCBs ?

Zoomup
4089 Posts
Zoomup:
Sparkingchip:
Installing a replacement consumer unit with a RCBO for each circuit could conceal over 150 mA of leakage, which is not always desirable.

Agreed, but the leakage can be found and rectified and the nuisance factor of a single R.C.D. board eliminated.  The current system is not fit for reliable service.



Z.

The O.P reports only a 17mA standing leakage.

Z.

Chris Pearson
3186 Posts
ebee:
Sparkingchip:
Installing a replacement consumer unit with a RCBO for each circuit could conceal over 150 mA of leakage, which is not always desirable.

Correct.
BUT - did we ever bother about this in the Pre RCD era? Consumer units with rewireable fuses and/or MCBs ?

I think that in this instance it is wrong to refer to "earth leakage". That occurs in appliances when not all of the electrons taken there by L return by N. I suspect that there were not many leaky appliances 30 years ago. Valved wirelesses and TV sets?

"Earth leakage" should not occur in the fixed wiring. What is described is failing (or even failed) insulation in the wiring or accessories.

I think that much FI is required to find out where the cables are sound and where they are not.

The position of the CU doesn't sound very accessible unless you are a motor mechanic. If it isn't in a cupboard, what protects it from getting kicked and knocked about?

Sparkingchip
4710 Posts
I seem to remember the original acceptable insulation test results were based on the acceptable leakage current as a fraction of the supply amperage.
AJJewsbury
3059 Posts
"Earth leakage" should not occur in the fixed wiring.
I guess we've got used to modern plastics-insulated wiring systems having a near "infinity" insulation resistance, but not all are like that. Mineral insulated (pyro) for instance can have a measurably lower insulation resistance, even when clean/dry/new, and so can have a measurable amount of "leakage" to earth.
   - Andy.
Chris Pearson
3186 Posts
AJJewsbury:
"Earth leakage" should not occur in the fixed wiring.
I guess we've got used to modern plastics-insulated wiring systems having a near "infinity" insulation resistance, but not all are like that. Mineral insulated (pyro) for instance can have a measurably lower insulation resistance, even when clean/dry/new, and so can have a measurable amount of "leakage" to earth.

Probably not pyro in this case, but might there be some tired old VIR in those circuits?

Jaymack
238 Posts
Also do a PAT on 3 core portable ...... and fixed appliances such as a gasboiler, this may help.
Jaymack  

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