My Workshop conundrum

[JonL]

So I'm sat here with nothing to do on a wet day. I can't go out to the workshop because when it rains the electric trips the RCD in my house. My house is about 30m from my house, with my neighbours beautifully kept garden between the two. Guess where the armoured cable runs.

So I need to either:

a) run another cable, via another route, and get it signed off by a qualified electrician. Total amount of cable required to go from fusebox to fusebox, at a guess 100m. It's at the front of my house, the workshop is a distance away at the back.

b) Get a small generator for the rainy days, enough to run my lighting (low powered LED) and lathe/mill ideally (Myford ML7, power meter says it draws 300w loaded) and Mill (Seig, quoted power was either 750w or 1kw, I can't remember). Both of these are obviously inductive loads.

c) get a small inverter to run the workshop off during rainy days, I have lots of old batteries that would run it.

The problem I'm having apart from the usual (money) is that I don't think its as straight forward as 1kw of load = 1kw of inverter/generator capacity required.

Lets assume I had the mill or lathe and the lighting running, so probably 1kw of draw worst case. As the machines are an inductive load, does that mean its not as straight forward as requiring a 1kw supply?

If anyone knows enough to steer me in the right direction I would be very grateful, I'm glumly looking out the window at a workshop I can't use with jobs stacking up.

Jon.

[don9f]

Have you tested, or had the cable tested, to prove that it is actually the cause of the “tripping”, ie disconnected at both ends & megger tested etc ? Is it a modern steel wire armoured cable?

It reminds me of something from many years ago, when a farmer friend was using an ancient Black & Decker metal cased grinder in his workshop, that was powered by a cable running through the trees and plugged in, in his kitchen.
It started tripping his earth leakage breaker in the house, so he disconnected the earth wire in the grinder’s 13A plug, only to then receive an electric shock when he switched it on....his solution was to stand on a piece of wood when he wanted to grind something !

Hope you get it sorted,   Don

[timb]

Hi Nobby, an isolation transformer might solve your issue, at least in the short term.

See this for an explanation. www.powerinspired.com/isolation-transformer-need-know/

You are probably losing the insulation resistance due to a split cable jacket or just damp due to the age of the cable and it constantly being in the ground. If you fit an isolation transformer at the house end of the cable then you are isolating the supply to the workshop from the house - the house RCD will not trip. You then fit an RCD to the workshop fusebox to protect you in the workshop and the cabling(fuses/Circuit breakers protect cables not machinery!) will be protectd from overload by the other fuses/MCB's. RCD's limit the amount of current allowed to flow in the event of a fault in the cable. The average human body can survive a current of 40-50mA give or take, most RCD's are limited to 30mA for this reason. The isolation transformer will look after itself, most have a reset button of some kind for overload and over temp.

You need to size it for less than you think, a 1KW motor takes just over 4 Amps at full load - mostly startup and for less than a second. After it is moving it will take probably 0.5 to 1 amp running. If you have an inverter fitted you can display the current on the front of the drive.

You dont say what size MCB is connected to the feed at the workshop. As a rough guide multiply this by 240 to get the Watts of power you have available - eg 10 Amp = 2500W or 2.5KW.

The size of the cable dictates what size fuse/mcb you can fit. There are factors to apply for the length of the cable and how it is run, in your case underground. This affects the way the cable can dissipate heat. Personally I would not run anything less than a 4mm to a shed or outbuilding. This could be raised to 6mm sdepending on the length of run. 4mm gives you a max current of 32A in ideal circumstances according to the regs.

By fitting an isolation transformer, at least if you have any issues you will not trip the house.

This gives you an idea of price but RS are very expensive and you will find one cheaper locally I think.

uk.rs-online.com/web/p/safety-site-transformers/2604238/

Having said all this it is unusual for modern day cables to decay. Your issue is most likely due to some sort of junction or connection that is failing somewhere. Check that the cable that leaves the house looks the same as the one going into the workshop. This might lead to a junction somewhere that needs attention.

Sorry for jumping around but there is a lot that can go wrong. If I lived a little closer I would come round and have a look for you!

Tim

[timb]

Just another thought,

Is your workshop damp when it is raining? If the consumer unit in the workshop is not sealed to air then any dampness can cause spurious RCD trips.

I will leave you alone now!

Tim

[JonL]

All very good tips, thank you. I've found it still trips the house when the workshop MCB is off, so I figure it's under the ground. There are no physical joints that I can see, it leaves my fusebox, goes underground and appears through the floor in the garage. House is around 17 years old. The breaker in the house is 16A.The workshop is completely bone dry as far as I can see.

Thanks for the tips, I need to do a bit more investigation around the connections between the house and workshop I think.

[oldnorton]

Excellent answer from Tim. Just to say Jon that I fully agree with him. It is most unlikely that the cable has failed or is leaking in itself. Isolate it totally and perform a mega test between its three strands. It is most likely to be the termination points at either end, or a piece of connected machinery in the workshop that is sensitive to the damp days. House RCDs are sensitive so Tim's suggestion of an isolating transformer is a good one to stop the house tripping while you track down the fault.

I wonder if, with an isolation transformer installed, an RCD in the workshop serves any purpose?

[timb]

The RCD is more important with an isolation transformer in the line. If you think about it, it is a complete power source which itself is isolated from anything else. The earth is the only common thing, the live and neutral effectively floating until the neutral is grounded to the earth. Any earth faults therefore would go undetected unless an RCD is fitted. OK the probability is that one of the MCB's will go before any harm is done BUT any two wire devices that are connected COULD be dangerous, see the note about DonF's farmer friend above.

Tim

[oldnorton]

If the workshop neutral is grounded to a new earth then I agree. Perhaps needs a new ground anyway to ensure noise suppression capacitors work. Then an RCD will detect any leakage and be an important safety addition.

But if it is floating? Its just like isolation transformers for outdoor power tools.

Perhaps Jon needs some tips on whether or not to use his cable earth back into the house if he instals an isolation transformer. I have no idea what the professional advice would be.

[timb]

In my experience the earths are common in most isolation transformers. RCD's monitor for an imbalance in current flowing through the live and neutral wires.

Tim

[JonL]

I agree on the whole with what is being said, but it is still tripping with the workshop isolated at the internal switch, so I suspect it is nothing actually inside the workshop.

[John Baguley]

If the cable runs under your neighbours garden then there is always the possibility that it has been damaged by someone's enthusiastic use of a spade, especially if it is not very deep. SWA is pretty tough but it's not indestructable.

It may not be all in one piece if whoever ran it was using what cable they had and there may be an underground joint in it which is asking for trouble (don't ask me how I know!).

Is it a three core cable or only a two core with the armour sheath used as the earth? I've seen that done before although there is nothing really wrong with that approach provided the sheathing is terminated properly.

Has the armoured been terminated with the proper gland fittings so that the sheath is properly earthed?

If everything checks out ok then might be worth swapping the RCD in the consumer unit (get someone to do it for you if you're not competent to do stuff like that). It could be over sensitive and giving false tripping.

John

[keith1500]

I am with John on this one. Suspect damage to cable or joint. Damage most likely to be due to something added in the garden like a shed base, fence post or pergola perhaps? Odd that your cable goes via a neighbours garden?


Anyway, a few more questions if that’s ok,

are you able to carry out a mega test on the cable?

What’s the set up in the house? Does the workshop have its own RCD and MCB or is the RCD part of the house distribution?
What’s the set up in the workshop? Small fuse board with its own double pole isolator?

Keith

[andyhigham]

The cost of the cable is relatively low digging a 100M long trench to lay it in is a major hurdle

[JonL]

I live on a housing estate built between 15-20 years ago; all the houses were built at the same time. The cable was laid then, its been in place from new. The I have two circuits in the house, mcb controlled and not. The garage has its own smaller fuse board with two circuit breakers, one for lights and one for the ring main. I believe the cable to be armoured but that is speculation as its entirely hidden from the box in the house until it sprouts from the workshop floor. I can borrow a megger from work and check it out, that shouldn't be too difficult.

All of these hurdles have made me think picking up a second hand generator and just running my lights/lathe from that when the rain is heavy would be my best bet... it only happens when the ground is saturated. I can go months with not an issue, then have a week where its completely unusuable.

[keith1500]

It does sound like a ground fault.

Does it take long following rainfall before you can switch on again?

I would suggest isolate the incoming supply to the workshop using the isolator in the workshop dist board. Then check if the supply still trips the RCD.

This should definitely eliminate any live to earth, or neutral to earth faults in the workshop beyond the dist board.

[keith1500]

If it still trips

Is the cable a two or three core?

[ettingtonliam]

I once had a problem in a house I lived in, in which the RCD would trip for no apparent reason. Tried all the usual things, such as switch everything off, then switch back on one at a time. Eventually I was talking to an old electrician, who told me that the RCD itself might be at fault. He came round and replaced it, didn't cost very much, and that cured the problem.

[JonL]

I believe it to be three core but I'll have to have a look. It's normally relatively quick to get working again once things have dried out.

[gwr7800]

I remember it was common practice some years ago to use the armour outer cable as a means of a earth connection then just live and neutral on the inner core, not now!

For what it’s worth I would get a qualified electrician just to check it over and test all of the instillation it’s about an hours work.
Which area do you live Nobby?

My brothers nephew lives in Fareham he’s qualified,may be able to help if needed, he signed my work off just recently! My daughter wanted some work done! ( I am now retired! )
Regards Chris

[keith1500]

If relatively quick to get working once things have dried out it’s unlikely to be In the ground?

One of the great nuisances of RCD protection is where one RCD is used to protect several circuits. A circuit that is switched off at the fuse or MCB can still trip the RCD by shorting the neutral to earth. This because it unbalances the current flow through the RCD which is coming from the other circuit that are on.

With this in mind you need to really home in on whether it’s the workshop or the cable.

I would isolate the workshop dist board thus disconnecting its live and neutral conductors ( by using the dist board main switch should do) in at the board and then power up the cable and see if it trips the RCD. That should identify whether it’s the cable or the workshop.