Strange RCD tripping issue

[Milleniumaire]

I have isolated a RCD tripping issue to the garden lights/water feature, but it seems to be random and I'm unsure how to go about troubleshooting futher.

A double switch allows the garden lights (all LED's) and water feature to be turned on separately. Had no issues since they were installed a couple of years ago.

With the water feature off, I can flip the switch on and off for the garden lights with no issue. With the garden lights off I can also flip the switch on and off for the water feature with no issue.

However, SOMETIMES, when both are on and I switch off the water feature, the RCD trips. I've been unable to determine a pattern and this particular sequence may be a red herring, but it is all I have to go with.

When the RCD first tripped a couple of weeks ago, we were sitting in the garden with the lights and water feature on and the RCD tripped. This was the first time. I was able to replicate the tripping by simply turning on the garden lights (with the water feature off) - the water feature seemed to be okay, so back then I assumed it was an issue with the garden lights only, and I would need to troubleshoot which bulb/junction box might be causing the issue. However, today I'm unable to replicate this behaviour and the only time the RCD has tripped is as a result of turning off the garden feature when the lights were on!

Any idea where I start to isolate the problem?

 

[Harry Bloomfield]

That sounds like a bit of leakage to earth, on both. One alone doesn't have enough leakage to trip the RCD, but the cumulative leakage of both, is enough.

 

[chivers67]

Start from the double isolator, check if there is damp or water ingress somewhere in the first instance.

 

[Milleniumaire]

I know there are a number of underground cable connectors, so I guess it could be one of these.

Had a couple more trips while testing this evening.

The water feature has been on for a few hours without issue, so I turned the garden lights on as well. No problem until my wife turned the garage light on and RCD tripped. Turned the water feature off and the lights back on. After about 40 mins the RCD tripped again. Not sure what caused it, possibly a sensor light coming on or the boiler.

Unable to flip the RCD on without turning the garden lights off.

There's a large wiring box in the garden into which all the armoured cable is connected up so tomorrow I'll disconnect all but one and add them in one at a time every hour in the hope of identifying the cable causing the issue. Then I'll dig up the two junction boxes for that cable. Find it hard to believe water has got into the junction boxes as they are crammed full of that water resistant gunk!

 

[chivers67]

Can you Disconnect the neutrals and earth to the outside stuff and see if anything happens in the next week?

Have you access to an Insulation Resistance Tester of any description?

 

[Taylortwocities]

An RCD looks for an imbalance on both the live and the neutral wires. The switch, and probably your investigations to date, have only disconnected the live connections.
As Chivers has suggested, you’ll need to start by disconnecting both the live and neutral connections of various sections to try and narrow it down.
This is a long and tortuous method. Better to use an Insulation Resistance tester.
PS buried boxes filled with gunk are not necessarily waterproof. It depends on how well they were installed.

 

[ericmark]

Sounds like an earth neutral fault. When there is no load, earth and neutral are the same voltage so a leak between the two will carry no current so nothing will trip, as the load increases the voltage between earth and neutral will increase, so any leak will now trip the RCD.

So if you have some damp toast stuck in the toaster the toasted may not draw enough current for the voltage across that damp toast to trip the RCD however turn on the kettle and it trips, making the user blame the kettle, when it's the toaster at fault.

Switch on or off an appliance will likely never find the fault, there is only one way, use an insulation tester, they use a high voltage normally 250 or 500 volt, so multi meter is no good, however not too expensive start around £35 but clearly needs so skill to use them, so may be prudent to use an electrician.

 

[Harry Bloomfield]

Sounds like an earth neutral fault. When there is no load, earth and neutral are the same voltage so a leak between the two will carry no current so nothing will trip, as the load increases the voltage between earth and neutral will increase, so any leak will now trip the RCD.

Often there will be a small voltage differential between N & E, even when there is no load on that particular circuit. The voltage can arise from other circuits and other circuits in different premises. Even those small voltages and resultant current flow are quite capable of tripping an RCD.

 

[Milleniumaire]

Thanks for the feedback. It does look like I will have trouble isolating the issue without getting some help as I don't fully understand your explanations of what might be causing the issue and I don't have the equipment or the knowledge to test it myself.

Having said that, I've been isolating the cables that run into the garden wiring box in an attempt to pin down where the fault might be, if this is even possible with this type of fault (which I think you guys are saying it probably isn't).

There are two cables providing power, each controlled by a switch. There are 5 cables running to various points in the garden; one for the water feature (small submerged pump in a reservoir). This is powered by one of the cables off the switch. The remaining 4 cables are powered off the other switch and each one powers two LED lights, all of which are connected by underground aluminium connection boxes waterproofed using gel.

So, I started by disconnecting all the lighting cables in the wiring box except one. Turned on the garden water feature and single connected lighting cable. Left it for an hour, but randomly turned on/off lights etc in the house. Nothing happened.

Added a second lighting cable, so now 4 LED lights connected. Same again, no issues. The third lighting cable caused no issue. Added the fourth lighting cable and straight away the RCD tripped. So, I thought the issue must be with the fourth lighting cable!

I then removed all but the fourth lighting cable and ran it with the garden feature on. I confess, I expected this to trip as soon as the garden light switch was turned on, but that didn't happen. I actually forgot about it then my wife informed me the washing machine had stopped and sure enough the RCD had tripped.

I'm currently running ONLY the garden lights on the fourth cable, as I've turned off the water feature.

So, if this trips, does that mean the fault is with the fourth lighting cable, or is it just coincidence that this "caused" the trip or is there some other reason related to the combined load/leakage from all 4 lighting cables and the water feature?

Assuming it does trip during the current test, I assume the issue will be with one of the junction boxes or one of the light fittings or possibly the bulbs connected to the fourth lighting cable?

I was considering changing the junction boxes to use a teetube IP68 3-pole T-splitter from screwfix.

I'm assuming these "may" be more water resistant than the aluminium junction boxes filled with gel that the electrician installed?

 

[Harry Bloomfield]

Thanks for the feedback. It does look like I will have trouble isolating the issue without getting some help as I don't fully understand your explanations of what might be causing the issue and I don't have the equipment or the knowledge to test it myself.

Having said that, I've been isolating the cables that run into the garden wiring box in an attempt to pin down where the fault might be, if this is even possible with this type of fault (which I think you guys are saying it probably isn't).

What we are trying to point out to you, is that either the live or the neutral with leakage to earth, can cause an RCD to trip. So any process of testing will involve disconnecting sections of both L & N. Turning an MCB off, or a switch will not usually isolate both poles.

An RCD trips on a difference in current flowing in L when compared to N. If either L or N has some leakage to earth, that will present enough imbalance (more than 30mA of difference) to cause the trip.

 

[Milleniumaire]

What we are trying to point out to you, is that either the live or the neutral with leakage to earth, can cause an RCD to trip. So any process of testing will involve disconnecting sections of both L & N. Turning an MCB off, or a switch will not usually isolate both poles.

An RCD trips on a difference in current flowing in L when compared to N. If either L or N has some leakage to earth, that will present enough imbalance (more than 30mA of difference) to cause the trip.

Okay, so what I have been doing is simply disconnecting the live wire for each of the 4 cables that feed the garden lights.

Stop press! As I was typing this response, the RCD tripped.

The situation is that only one of the 4 lighting cables has the live connected in the wiring box. All the neutrals and the earths are connected. The garden feature was connected, but wasn't switched on. So, only one of the lighting cables was receiving power and it had been on for around 1.5 hours before the RCD tripped. The TV, washing machine and more recently, the iron were on. Not sure if this is relevant.

To me, this appears to prove that this particular cable is the one causing the fault. Is that correct or because all the neutrals and earths of the other 3 lighting cables were connected, these could still cause an issue even though those cables weren't live?

 

[Harry Bloomfield]

To me, this appears to prove that this particular cable is the one causing the fault. Is that correct or because all the neutrals and earths of the other 3 lighting cables were connected, these could still cause an issue even though those cables weren't live?

Causes of RCD trips can be very difficult to trace, with the proper test equipment, especially when they trip intermitantly. Without, your only way to prove which circuit, is by disconnecting circuits completely, waiting and gradually adding them one at a time. You really do need to disconnect those neutrals too. Don't whatever you do, disconnect the earths.

Even with the earth disconnected, leakage to the actual ground can trip an RCD - that is the whole point of an RCD.

 

[DetlefSchmitz]

To put it in a way that I don't think anyone else has explicitly stated: If you turn off the MCB on any circuit , and then join the neutral to the earth somewhere in the circuit protected by that MCB, in all likelihood the RCD will trip immediately, even though one might instinctively think the two wires were at the same potential. So, as said before, both live and neutral must be disconnected when trying to eliminate a circuit.

 

[Milleniumaire]

.... I forgot to mention, that after it tripped, I was unable to reset the RCD without first switching off the garden light circuit. Doesn't this suggest the same fault was instantly occurring and re-tripping the RCD? Another pointer to the currently only live garden cable?

I understand they are difficult to trace. What I know is it is the garden lights OR the garden water feature pump that are causing the issue. When these are switched off there are no RCD trips.

I'm going to take a punt and replace the underground junction boxes at a cost of around £20 and some of my time. If this works, great, if not I'll probably get an electrician in to find the fault.

I'll try the screwfix connectors, although I've read they are quite tricky to use for thicker wires and one of the boxes has 2 x armoured cable and the light fitting cable.

 

[Milleniumaire]

You really do need to disconnect those neutrals too. Don't whatever you do, disconnect the earths.

So, as said before, both live and neutral must be disconnected when trying to eliminate a circuit.

Okay, thanks guys. Sorry, you must think I'm ignoring your advice.

I will have another test tomorrow and will disconnect all neutrals except the one associated with lighting cable 4, which is the only one currently connected. If the RCD trips again, I will definitely swap out the junction boxes on this cable.