SWA cable - best to earth at both ends ?

[wobbin]

Hi all, hope you can help me please ?

I have an outbuilding at the bottom of my garden which has a SWA cable (2 core 6mm I think but possibly 4mm) running to it which was installed when I bought my house about 10 years ago. The SWA outer wires are being used as the earth, and the cable is approximately 25m long.

I had a ground floor extension built for my disabled son around 4 years ago, and as we hadn't enough spare ways on the distribution board I had a new one fitted which is a split rail jobbie with a 30mA RCCB. Previously there was no RCCB.

Lately I have an issue with nuisance tripping of the RCCB which relates to the outbuilding - specifically where I have a particular laser printer plugged in (but not switched on) in the outbuilding. The odd thing is that if I power the printer up on the downstairs ring in the house it doesn't trip.

I disconnected everything at both ends of the SWA cable, and have meggered the cable, which gives a resistance in excess of 25MOhms between either core and the earth cores. With this printer (physically)disconnected I don't get the trips.

I guess that the printer may have a leak between L or N and earth, but why doesn't it cause the tripping when located in the house ?

Is the lack of an earth rod at the outbuilding end likely to cause this problem ? I really hope that I don't ever have to have the SWA cable replaced as it will be the mother of all nightmares routing a new one.

PS I'm an electronics engineer by trade so well aware of the dangers of electricity. Happy sorting out microprocessors etc but I'll leave the high voltage / high current stuff to you guys

Kind Regards,
Robin

 

[baldersj]

Is the downstairs ring definitely protected by the RCCB? It's not unknown for them to end up on the wrong side of a split board. Just a guess...

Also, excessive moisture in the outbuilding could be another possibility?

Happy hunting!

 

[wobbin]

Hi - thanks for your reply.

No, both circuits on on the protected side of the RCCB.

The outbuilding is a proper brick built structure with a flat roof and had all mod cons, central heating, Saniflo loo etc.

I keep it fairly warm as it houses my electronics workshop and I don't want my testgear to get damp, so unlikely to be moisture.

Very odd

 

[Adam_151]

Interesting one

Have you tried switching off the breakers for the other circuits protected by the RCCB and seeing if it still trips? I'm thinking it may be right on the verge of tripping all the time and maybe a long length of cable and a switchmode powersupply that could be putting odd spikes on the waveform might be conspiring to allow capactive coupling between the phase and the cpc just push the rccb over the edge

(well its the only vaguely possible explanation I can think of without resorting to the paranormal )

 

[securespark]

Can you get the printer PAT tested just to eliminate it from your enquiries?

 

[Spark123]

Could be something along the lines of a mains filter on the printer, can be a bit of a nuisance with RCDs.

 

[plugwash]

have you checked for live-neutral reversals?

i could see a filter having different ammounts of leakage depending on if live and neutral are reversed or not.

 

[wobbin]

Thanks all for your thoughts on this.

@Adam_151 - Quite possible, originally I replaced the RCCB with a new one, only to find the problem was worse as we had more nuisance tripping, so I ended up putting the original back in place. I've read that the breaking point can vary more than you'd think and that as long as the RCCB has tripped by the time 30mA leakage is detected then it passes spec. Unfortunately tripping at 20mA is also in spec and as our house has several networked PC's (Kids eh ?) we have more switched mode psu's than might be the norm so the cumulative leakage may be quite high under "normal" circumstances. The odd thing is that this usually happens at night - the first thing we know about it is my 4 yr old coming in to tell me that his night light has gone off, bless him.

I take your point re capacitive coupling - hence my original question, I was wondering if having an additional earthg rod fitted at the far end of the cable might improve the situation, though I think that would normally only be done to improve safety earthing ?

@Spark123, Plugwash & securespark - I guess I could try meggering the printer with the printer's mains in L&N connected together, then megger between the junction and earth. Should be OK to do that I think. It'll be a big paperweight if not

Just had another thought - I have a 1:1 isolation transformer available. I Could try plugging the printer in via that to see if I still get the trips, though I'll need to make sure that it's rated suitably first.

Failing that I'll call in Ghost Busters

Kind Regards,
Robin

 

[Adam_151]

You are right, the testing procedures for RCDs require we test them to make sure they don't trip at 1/2 the nominal current, but trip in 200ms at the nominal current (and for 30ma devices, there is another test at 5xIn with a max allowed result of 40ms)

So that means that it could be anywhere from round about 15.5ma to 29.5ma

If you have lots of electronics like PCs, the filters leak down to earth and add up putting the RCD on edge, I found this when our RCD had to be swapped out for one with 80A contacts for a shower upgrade, the new one was more frisky than the old one, and because we too have a network of computers, and the electronic controls of the kictchen applicances also contained filters that leaked, the RCD just wouldn't stay in for more than half a day or so, you almost just had to breathe on a plug and it'd go... The solution was to have the kitchen ring final circuit moved off the RCD and onto a separate RCBO on the other side of the split board.

I expect the filters are generally much more significant generally IT equipment in causing earth leakage than capacitive coupling in the supply cable, but on the end of a long circuit where you have a higher capacitance value than shorter circuits anyway, and if you plug something in the end of this that tries to take its power in 'spikes' and the because its a longer cable with more resistance its easier for this to drag the voltage down and create an effective frequency higher than the nominal 50hz, etc just pointing out that its generally the filters in these devices which conspire to trip earth leakage devices as as opposed to the switch mode nature of them (the switchmode nature does create many problems of its own though, casuing neutral currents in polyphase systems to add rather than balence, etc)

Separating it off into its own TT system wouldn't help, what would help though, is putting it on the non rcd side of the board and fitting an RCD in the board in the workshop

 

[securespark]

Need to do a ramp test: if it is tripping below 15mA then it's faulty. But I prefer a minimum of 18mA, much lower, and there could be problems.

You could do with a bit of kit I'm trialling at the moment. a mA sensitive clamp meter.

Stick it on the main earthing conductor, and it will give you the current drain in mA. Then you can power down circuits to find the culprit.

If you do a ramp test first, you will find out at what level your RCD trips, then put on the clamp meter to find out how much is leaking to earth.

 

[wobbin]

Thanks guys.

Oh, don't - I'm a sucker for shiny test gear, the Mrs would be on my case

I think what I'm going to do is have the outbuilding moved to the Non RCCB side on the main board and have the dis board in the outbuilding replaced with a new RCCB type. It's a really old metal boxed, plug in fuse type so is probably overdue for the great pylon in the sky anyway.

Cheers all

Kind Regards,
Robin

 

[donglebang]

as its an out building your swa should not be earthed at both ends any way and you should have an earth rod fitted fot the outbuilding ..

 

[securespark]

Not necessarily, DB.

BTW, did you realise this thread is ancient?

 

[ban-all-sheds]

as its an out building your swa should not be earthed at both ends any way and you should have an earth rod fitted fot the outbuilding ..

Very poor advice. I know this is an old topic that you've resurrected for GKW reason, and so the OP won't be doing anything in response to it, but other people might find this topic via searching and to make out that the answer is always the one you've given is just plain wrong.

 

[donglebang]

hmmm i.v just been in touch with the nic and its right out buildings need there own earth ans should not be joined to the house . if the outbuilding is seperate from the house and it has a fuseboard in there