Tricky electrickery

[John Rumm]

Why is it the jobs you think are going to be quick and easy, never are?

What follows is a rambling tale of woe of a tedious job that kept on giving!

So the other day, I had someone round to service the aircon on the car. He needed to plug his van in, so I said "no problem, there is a socket just there".

However it turned out that the garden socket near the car was not working, so I had to run an extension lead...

I finally got round to looking at what was going on. Turns out the circuit was tripping. Oddly however it was a MCB trip and not a RCD trip. Resetting the MCB, resulted in it tripping again - but not quite instantly. It took a second or so. This struck me as odd... dampness in the system somewhere could cause a RCD trip. Damage could cause a short circuit and a MCB trip - but that would be an "instant" trip. This had the hallmarks of an overload trip with the MCB tripping on its thermal mechanism. So that would suggest it was sinking 80A or more, on a circuit with one socket, a PIR, and two lamps.

The circuit was originally a 6A circuit for a couple of outside lights controlled from a discrete PIR. Later it gained a double socket, and was uprated to a 16A MCB. A T&E leaves the CU and goes through the wall of the house into a waterproof JB. SWA runs from there to the back of a wall in the garden. A JB there has a T&E that goes through the wall to a socket on the other side, a length of Hi-tuff that goes to a PIR, and a second SWA that runs to the first light. There is another JB there, SWA out to next, and Hi-Tuff to lamp. Then one last JB there, with transition to Hi-tuff for the lamp (with hindsight, running Hi-tuff from the previous JB would have been far less hassle!).

So I took apart the first "waterproof" JB and found it rather full of water, soil, and slugs! It was so wet in fact it had corroded the armour on the SWA, and the gland nuts were well and truly rusted in place. Disassembly was proving to be a non starter, so in the end I attacked the box with side cutters, liberated the cables and replace the whole JB, this time taking the T&E for the socket out via a gland on the side just in case that back connection through the wall was where the water was getting in. I had to cut back the SWA a little bit to remake those connections as well with new glands - there was just enough slack in the cables.

Then I realised that the Wiska box I had used had threaded 20mm knockouts, so you can screw the glands straight in. Handy you might think, but the design is such that there is insufficient space to stick any kind of gland nut inside on the back of the SWA gland or even the banjo tag. So no easy way to make an earth connection to the armour. So in the end I had to take it apart again, put a pair of Banjo terminals on the the outside of the box, overlap the tags, then drill through the box and the tags so that I could put a bolt through both, and give me somewhere to attach an earth wire inside with a eye crimped on the end. I connected it up with wago connectors rather than the supplied and mounted screw terminal block. This also allowed the wires to be dressed so that all the wires lead up to the terminals - hopefully making sure water can't run into them if more gets in.

Tada, job done.

Nope, still trips! So a bit of testing showed it was the link out to the lamps that was the problem. Move along to the next JB. To my surprise, that one was reasonably clean and dry - so no smoking gun yet, but I was getting a low impedance reading between L & N - but insulation resistance to earth was ok.

I had had problems in the past with the JB in the lamps themselves being not really up to the job and allowing insects to migrate stuff into them. So some time back I had taken those out, remade the joints with wagos and cocooned them in self amalgamating tape, before tucking them back into the lamp base. I got the ladder out, and dismounted both lamps, and checked the joints there - still looking good.

So I disconnected the feed to the second lamp, and the problem went away. That had me wondering for a bit, until I remembered there must be another JB before the final connection the the lamp. Next problems was getting at it - a bit of tree had grown past it making it impossible to get to. So I had to get the chainsaw out, and hack away some tree!

Last JB, Aha, stuffed full of damp soil. With one very sorry looking and corroded terminal block embedded in the middle of it. Also the earth connection

Looking at it, this one had a small drain hole in the bottom - obviously large enough to let stuff wander or slither in. So I cleaned that JB out, siliconed up the hole, cut back the corroded wires, and remade all the connections. Normality restored.

So notes to self: Don't install more JBs than you need to, and stick them high enough up the wall to make them easy to work on rather than make it look pretty and have to grovel near the floor when working on them!

[alan_m]

Usually preceded by someone else saying "can you take a quick look at" or "it will only take you 10 minutes" or can you spare a few minutes to...."

On some waterproof fitting there is provision for drilling a drain hole and a hole is recommended. On one fitting I had there were precise instructions for the size of the hole, too small and a drop of water wouldn't drain and too large would allow insect entry. The hole also had to be de-burred. Water in the fitting was more likely to be condensation rather than rain ingress.

[Cursitor Doom]

Your house sounds like junction box central. The entire JB manufacturing industry owes you an enormous debt of gratitude.

[John Rumm]

Ironically the one that was most badly affected was the one with the drain hole - although I think it was too big - prolly about 4 - 5mm

[John Rumm]

There are very few in the house - most of em are in the garden!

(one of the things about SWA (or any other cable that needs a gland at each end), is you need something solid to terminate it on - that that usually means an enclosure of some sort).

[Brian Gaff]

The other issue of course with drain holes is that if the box is near the ground, water bouncing off the concrete can just get in on the bounce. What about putting a bag of silica Gell in the box? Brian PS Military radar components had a scew in De humidifier so one could change them periodically to mop up condensation. Brian

[Tricky Dicky]

Outside connection boxes can be filled with IP68 grade gel especially at ground level to prevent ingress of water.

Richard

[Robin]

£24.75 for 280ml!

I may have to review my opposition to trigger warnings

[John Rumm]

Sealed LRUs are normally purged and possibly refilled with inert gas before final sealing with the desiccant plug - so there is far less available water vapour to condense in the first place. (Conformal coating of all the electronics is also common).

The sealed nature can however have some negative consequences. I recall one unit that had a tactile numeric keypad on the side, which was protected by a hermetically sealed translucent elastomer cover. On warm days this would tend to inflate like a rubber glove someone had blown into!

In the end they had to fit a key pad grid over the top of it to hold it mostly in place (although each "key" could then still bulge a bit)

[John Rumm]

Yup I have looked at those kinds of fillers, but normally I don't have that much problem with keeping the content of outdoor enclosures dry enough. They also add considerable expense to an installation. (although for underground and submerged connections - then fillers are definitely the way to go)

I think in this case, with one box it was the presence of an unglanded connection through the rear of the box that left a path of ingress, and the other had an oversized drain hole.

I did order some ventilation glands. I will give those a try:

[alan_m]

+1

Possibly if you want a completely waterproof domestic type connection use one of the gel filled boxes.

[Fredxx]

CEF are notoriously expensive for non-account holders.

Here is cheaper:

Other places might be cheaper still.

[Robin]

Still £18.60 for enough to fill perhaps 2 Wiska boxes.

[Animal]

Neutral cure silicone & grease are far cheaper.

[John Rumm]

Silicone is not "re-enterable" though (assuming it actually properly cures in that volume), so might not be ideal in cases where you might want to be able to make future changes.

I quite often use silicone grease on sealing surfaces and rubber seals etc.

[wrights...]

On the video bare wires stick out of the connectors. Bill

[Rod Speed]

Brian Gaff [e-mail] wrote

Not going to deal with that amount of water.

Lot of farting around for a domestic situation.

[Fredxx]

Lots of equipment had/has a breathing desiccator. It would normally have an indicator to indicate a change if it went pink.