This is for a UK wiring issue, but I suppose the issues here are universal.

Every since I bought my home, my outdoor garden lighting circuit trips the RCD when it rains. After multiple expensive visits from 2 different electricians, without success and before I spent money on rewiring the whole thing, I decided to look at this myself first. Bear with me, I have a basic understanding of circuits and DIY enthusiast, but not electrician.

Quick overview of the outdoor wiring. Armoured cable goes from the house to summerhouse into breaker box with RCD and circuits for summerhouse interior lighting and sockets , one circuit for outdoor lighting, and one circuit for outdoor pond pump. Its definitely the outdoor lighting circuit tripping the RCD when it rains. The RCD never trips when that circuit is OFF.

Worrying thing #1 I noticed is that the pond pump circuit and outdoor lighting go through the same 4 core flex cable going outside. Black and Blue for the lighting, Brown and Gray for the pump. There is no earth connection between the earth terminals in the breaker box and the outdoor circuits. That didn't seem right or safe to me, how are my metal encased outdoor lights grounded?

OK, so I followed that 4 core flex cable to the first IP67 junction box from which many connection spur off to the pond pump and lighting. I wanted to see how the ground wires where connected.

Worrying thing #2: the ground wires from the lighting and pond pump where all connected to a hole in the back of the junction box into the wooden fencing. My outdoor lights with metal casings and pond pump are grounded by my wooden fence. I'm no expert but wooden fences seem like a poor choice grounding.

See the ground wire going through a drilled hole into the wood fence: asd

It's not even a conductor right? Unless it's wet and soggy by rain, then it becomes slightly more conducting, which brings me to Worrying Thing #3, my fence lights up my volt pen from about 1 foot away from the junction box:

The red light on my volt pent, that's not normal right

OK, so I have an electrified wooden fence, which means that something live in my outdoor connections is touching the ground, which is "fine" in dry conditions, but as the fence gets wetter it conducts more and more until it trips the RCD. Am I correct?

It seems to me that whoever wired this couldn't be bothered to have a dedicated 3 core wire for the lighting circuit and one for the pond circuit, connecting them to the fancy ground terminals in the breaker box, and avoid frying small mammals like my kids. Am I correct?

In terms of fixing this, what should I do? It seems the right fix would be to get rid of the shared 4 core flex wire and get me two dedicated 3 core flex wire coming from the breaker box , one for each of the two outdoor circuits , so I can ground both circuits to the terminals in the breaker box. Having that in place, I will test the ground connections for voltage and if they still light up my volt tester, I will disconnect parts of the outdoor lighting circuit to try and isolate where the issue is. Does that sound right, so far?

Also those metal duct clamps on the pictures, they are not grounded either. You can get special metal grounding plates for these IP67 boxes for these type of metal connectors, to which you can attach a grounding wire using a screw. I quite like the ducts, but does anyone know if these ducting clamps exist in plastic? Sounds better to me if they exist. Should I get a bigger junction box? It seems unsafe to have so many wires in there.

Should I be suing those 2 electricians (I'm not), that supposed to check the whole wiring , but at best came up with saying I have a couple of dodgy light fittings needing replacing , or am I making a fuss about nothing?

Rest assured, I will have work I do certified.

  • I am in the U.S. But have seen close to what you have. I drove a new ground rod at the waterfall pump and lighting box and turned it into a small 30A sub panel with its own GFCI breaker(I think the same as your RCD). At this point I put a standard breaker in the main panel and ran the hot's and a neutral on the original feed(wire size was 10 so that met code here). In the sub panel the ground was isolated from the new ground. It did pass inspection and the owner sent me t thank you card later that winter. Making a sub panel with a new ground rod may solve the issue.
    – Ed Beal
    Commented Feb 20, 2016 at 20:45
  • Thanks Ed. I will look into the option of a ground rod.
    – Maxm007
    Commented Feb 21, 2016 at 2:03
  • How is the earth to the fence terminated? Surely not just a screw clamp? I'm more concerned, however, that your IP67 box is as wet as an otter's pocket - all that mould! It's doesn't look very IP67 to me. No wonder the RCD is tripping, there will be current leaking to earth and neutral all over the place. I'm suprised if the SWA is not terminated properly though. Ignoring the dampness, you may be earth safe as the electrician may have determined that the lights are double insulated and everything else is plastic. Not ideal but might be ok, just fix the water ingress. Commented Oct 12, 2016 at 20:38
  • I recently had a roof leak which leaked inside a wall and got water into an outside receptacle. This tripped the RCD breaker because it enabled leakage to ground. I do not believe that any water ingress is allowable in a RCD protected box with a receptacle. As a temporary solution until I got the roof leak fixed I removed the receptacle and connected the wires with twist on wire connectors and pointed them upward. Commented Jan 7, 2017 at 17:47

1 Answer 1


After a bit more research, I understand that since the supply wire into the junction box is SWA steel armoured cable, I can use the armour as ground for the whole circuit. Just need to check it is properly terminated in the breaker box so its earthed on that side , and also properly terminated in the junction box with an earthing plate I need to buy. I can then connect all the ground wires to the earthing plate and we should be good, no extra cable.

I'm buying a bigger junction box that comes with earthing plates to terminate the SWA cable. I'll come back to this thread when all the components arrive and it's installed with some nice pictures.

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