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Topic Title: RCD Tripping under Load on one circuit only
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Created On: 02 September 2013 07:05 PM
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 02 September 2013 07:05 PM
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iie628633

Posts: 9
Joined: 11 February 2009

Hi,

I'm having some trouble getting my head round a problem with my wiring at Home.

The wiring is fairly basic, it was in when I moved in and has been done about 20 years ago, with a new RCD CU fitted about 3 years ago.

the mains comes into a 6 way 30mA RCD protected CU. From there it feeds my Garage via a 6mm 2 core SWA cable - about 25mts long with a separate earth, in a duct. the garage has another CU - this time an old Wylex metal clad 6 way with 60898 breakers and no further RCD.

I have in my garage a compressor with a Single Phase 2kW motor on it. when ever I try to use the compressor it trips out the RCD in the house. I've done an IR test on the motor windings and that came out as >200M on each winding to earth. the cabling from the CU to the compressor is also good with no faults. checked all that and its still tripping. its only when I put the compressor on that it trips, the lights and sockets in there are fine.

I decided to test the RCD, but that tests out good, no trip at 1/2 rated, and trips within the specified time at 1x rated. i tried a ramp test without the compressor connected and it tripped out at 22mA, so it doesn't look like there is much additional leakage.

I tried another new motor on the compressor but this trips the RCD too.

I tried putting a 13A plug on the compressor and feeding it via an extension cable from the house - it starts up and runs for about 2 seconds then - predictably blows the 13A fuse - but there unlike from the garage there is no tripped RCD.

I'm finding this hard to understand, the same RCD protects the garage and house, but it trips out when connected to the garage CU and not the house.

the compressor has been working on the above setup without issue for 18 months until now.

Anyone offer any advice as to why this is happening? it's almost like the armoured cable from the house to the garage develops an earth fault when under load.

Andy
 02 September 2013 08:20 PM
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alanblaby

Posts: 344
Joined: 09 March 2012

The obvious thing to do is to disconnect evrything else on the installation, then try the compressor. If it stays on, then it is probably the cumulative earth leakage of all your appliances.
If you can get it running, an earth leakage test of the compressor would give an idea of how much that is leaking.
Testing the motor statically will not give a true reading of its insulation properties - maybe only one of the windings has an earth leakage problem, and you would'nt be able to find that, except by pure chance, if you did an insulation test.

Overall, it all points to a compressor that is leaking a little too much current to earth, possibly carbon brush powder causing a path to earth?
 02 September 2013 08:41 PM
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iie628633

Posts: 9
Joined: 11 February 2009

thanks for the reply...

I thought it all pointed to the compressor, but when I tried a different (new in box) motor on it and it still did it, I drew a blank.

I can't come up with an explanation for it running ok on the house ring main, but not on the garage feeder when they're both on the same RCD

I might have a go at running it up direct on the house consumer unit, and then getting the RCD tester on the board to see what current it leaks.

Andy
 02 September 2013 08:51 PM
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davidwalker2

Posts: 189
Joined: 29 April 2009

Am I missing something here? You say the compressor has a 2kW motor, so it should take 8 amps when running, and should not blow a 13A fuse.

David
 02 September 2013 09:01 PM
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daveparry1

Posts: 6105
Joined: 04 July 2007

It's an inductive load David, the start winding will pull somewhat more than 8 amps I suspect!

Dave.
 02 September 2013 09:05 PM
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daveparry1

Posts: 6105
Joined: 04 July 2007

Andy, I know you said it's been running ok for quite a while but has any work been done on the circuit recently? could it be something really silly like a N-CPC reversal somewhere?

Dave.
 02 September 2013 09:23 PM
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davidwalker2

Posts: 189
Joined: 29 April 2009

Dave,

Accepted! My thinking was that 25 metres of extension lead will introduce a moderate resistive component which would limit the current.

David
 02 September 2013 09:27 PM
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colinhaggett

Posts: 358
Joined: 08 July 2004

Check for a loose connection in the garage circuit.
 02 September 2013 09:36 PM
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iie628633

Posts: 9
Joined: 11 February 2009

Hi Dave,

I've not done a thing to the wiring, much to my missus' annoyance, I try to keep DIY to a minimum!!!

My garage PIR light seems to have packed up recently, I can't imagine it's got anything to do with that though....!

again, thanks for the help, all suggestions very welcome.

I'll give all the connections a good look over from source to final circuit, see what I find.
 02 September 2013 10:25 PM
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antric2

Posts: 1049
Joined: 20 October 2006

Hiya Dave,
It could be accumalative earth leakage and not a single appliance.
I presume that you have had the compressor a while and it has worked fine until recent.

What brand of RCD is it.

Have you a freezer in the garage.This can be a common source of RCD tripping combined with something else on the circuit.
It may be that it has an earth leakage problem that is not sufficient in itself to trip RCD but if something else is added to the circuit then it may cause the trip.
But you mention the PIR as faulty.If its old and the front membrane is damaged or the cable inlet is not sealed properly then it could have let moisture into the sensor.This is very common.
Sorry its not much.
Regards
Antric
 02 September 2013 10:36 PM
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Thripster

Posts: 636
Joined: 22 August 2006

Have you ever noticed that a short between neutral and cpc on an otherwise isolated circuit trips the supplying RCD? I would have a look for chafed cables where vibration is present - at the motor termination box perhaps.

Regards
 02 September 2013 10:36 PM
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statter

Posts: 124
Joined: 06 February 2013

It's not a capacitor motor is it?
.....or a fault on the pressure switch?
 03 September 2013 11:15 AM
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Zs

Posts: 2787
Joined: 20 July 2006

iie,

If I were dealing with this and my client had gone as far as you, I'd be inviting him to help me and refusing to go home until it was sorted.

Without an earth leakage clamp meter, the next thing I would do is put a double socket onto the end of the compressor circuit and put it under load. Maybe a full kettle from cold and a fan heater or a hair dryer ( I'm looking for the moving parts) perhaps the washing machine? But from exactly the same position in the circuit.


My inner detective now has the garage as prime suspect. Further, I'm thinking about he fact that we have had some good rain recently and the spiders have all moved indoors. Any chance you have a cocktail of tired connection exacerbated by some muck in the junction anywhere? What about inside the DB? maybe even in the compressor pipes themselves?

Because you've tried a new motor, my sleuth tells me to look for something in the circuit which I reckon will be really visible and obvious, or something in the electronic switching of the compressor.

Let us know,

Zs

Big Edit; chunk removed re 2 minute fuse blow, I now see it is 2 seconds....My bad. I'll not start looking at the curves.

Edited: 03 September 2013 at 12:40 PM by Zs
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