RCD Trip on isolated circuit

[Alan Shaw]

Hi,
I am fairly new to this having only just completed and passing my Part P qualification so please bear with me. I decided to tidy up some wiring carried out in my house by a "professional" bathroom installer. The circuit in question is a 16amp radial with only one outlet supplying an immersion. The MCB was turned off but when I disconnected the immersion the RCD tripped. when line and earth wires came into contact with each other. I checked and found 5 volts between the 2 conductors and believing there may be an insulation fault ran a 1000v insulation test and all was in order. I disconnected line feed from consumer unit but still the RCD trips when conductors come in to contact. The reason I mentioned a 5 volt reading is on checking other circuits that have been isolated no voltage is evident.

Ive spent a long time investigating without success so any advice would be appreciated

 

[GLENNSPARK]

you have gone neutral earth....
as the neutral will still be connected....as will the CPC....the RCD has seen it as an imballance.....and thus a fault..


this caused it to let go..

electrical disconnection/seperation means just that:

both/all live conductors....this includes the neutral as its a current carrying conductor...and thus is considered live...

 

[GLENNSPARK]

did you do one of these "part P" courses then...
or college backed up with time on site over 3/4 years?

 

[Alan Shaw]

Reading between the lines, do I take it that there is not fault as such. Thanks for your very prompt reply it is really appreciated.

 

[telectrix]

this might help. probably no time on a 5 week course to learn how they work.

http://www.google.co.uk/url?sa=t&rc...gbgUJrhGfLJEnPIcgpDTV7w&bvm=bv.55980276,d.ZG4

 

[Lucien Nunes]

Do you consider that turning off an MCB constitutes safe isolation for a circuit prior to work? Is it sufficient for conducting a 1000V insulation test? Your post raises many points about the sequence in which one tackles the job. But as Glenn says, there is little question about why the RCD tripped, as the neutral would still have been connected.

 

[Alan Shaw]

Worked some 40+ years ago in industry before changing careers. Decided to freshen up out of interest now I am retired and get up to date with 17th edition really as a hobby.

 

[GLENNSPARK]

Worked some 40+ years ago in industry before changing careers. Decided to freshen up out of interest now I am retired and get up to date with 17th edition really as a hobby.

you cannot really just treat electrical installation/testing as a "hobby"...

you know what they say about a little knowledge....

 

[telectrix]

Reading between the lines, do I take it that there is not fault as such. Thanks for your very prompt reply it is really appreciated.

correct. do a forum search on phantom voltage and use of analogue meters to prove/disprove.

 

[GLENNSPARK]

talking about someone who got started about 40 years ago...or was it 60..

morning Tel...

 

[telectrix]

morning glenn. that comment has gone in the little black book. a bit like : http://www.google.co.uk/url?sa=t&rc...A2TVMpJ7fR46GFWlw_FcV4g&bvm=bv.55980276,d.ZG4

 

[Alan Shaw]

So by your definition a hobbyist by definition less knowledgeable than the professional who carried out the work in my house. Wish I had taken a photograph of his work. By submitting my post I am gaining knowledge. I did at least put my cards on the table when I posted so I am not trying to be clever.

 

[LeeH]

Do you consider that turning off an MCB constitutes safe isolation for a circuit prior to work? Is it sufficient for conducting a 1000V insulation test? Your post raises many points about the sequence in which one tackles the job. But as Glenn says, there is little question about why the RCD tripped, as the neutral would still have been connected.

Why 1000v?

I'm a bit rusty since I did my 2391 but I thought it was 500V test voltage fit anything up to 500 line voltage?

Let me know if I'm not up to date!

 

[GLENNSPARK]

So by your definition a hobbyist by definition less knowledgeable than the professional who carried out the work in my house. Wish I had taken a photograph of his work. By submitting my post I am gaining knowledge. I did at least put my cards on the table when I posted so I am not trying to be clever.

i am not saying that...
theres some rite emptyheads about carrying out installation work...who are neither competant..nor intelligent enough to properly grasp what we are dealing with....

so dangerous...

 

[Alan Shaw]

Thanks Tel.

 

[Alan Shaw]

I share your point of view, thats why I didn't get this clown back to remedy the problem. Thanks to you all for your help.

 

[Alan Shaw]

Did both 500v and 1000v just to be confident.
PS did the insulation test with both conductors disconnected from consumer unit.

 

[Lucien Nunes]

correct. do a forum search on phantom voltage and use of analogue meters to prove/disprove.

You can also get digital meters with a special low input impedance voltage range, to distinguish between energised circuits and ghost voltages. E.g. the Fluke 117. See Dual impedance digital multimeters - What's the point?

This range probably would not have shown up the false 5V reading you saw, which, as typical ghost voltages go, is pretty low. It's not uncommon on long runs of conduit cables, embedded amongst lots of energised lines, to get a reading of 100V or more open-circuit on a Hi-Z DMM.

 

[GLENNSPARK]

You can also get digital meters with a special low input impedance voltage range, to distinguish between energised circuits and ghost voltages. E.g. the Fluke 117. See Dual impedance digital multimeters - What's the point?

This range probably would not have shown up the false 5V reading you saw, which, as typical ghost voltages go, is pretty low. It's not uncommon on long runs of conduit cables, embedded amongst lots of energised lines, to get a reading of 100V or more open-circuit on a Hi-Z DMM.

you can also get it between neutral & earth on a TN-S Lucien....
when the referance (star) point is a long way off...

- - - Updated - - -

a couple of volts that is...

 

[telectrix]

most stars are a long way off. light years away.:banghead: