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Earth Leakage Current – How much is too much?

Former Community Member
Former Community Member
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?

  • 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.

  • Get the DNO to check the overhead cable before changing the consumer unit.  Will they do that?

  • Chase down and reduce the earth leakage current?  Would you consider this a “faulty” installation? 

  • Any other ideas?

Any thoughts will be gratefully received, and I’ll be sure to let you know how it goes.

Parents
  • Former Community Member
    0 Former Community Member
    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

Reply
  • Former Community Member
    0 Former Community Member
    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

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