RCD Tripping

[keithbr13]

Hi Every one, I have been asked to look at a problem in a toilet when the light is turned off the RCD trips intermitenetly but not when it is turned on. There is a fan and small flourescent shaver light in the room . the instalation was rewired by me about 10 months ago all the test results were as you would expect.

 

[Plonker 3]

Have you ramp tested the RCD?

 

[malcolmsanford]

Well have to admit that is a first for me ......turning something off and it trips.

There as to be some sort of in balance here or a dodgy RCD. Have you ramped tested the RCD? What is the protective conductor currents on the RCD circuit(s)?

Just a start

 

[BlueToBits]

A stab in the dark...
Could the act of switching off the shaver flu be causing a high voltage back emf in the choke/transformer which is breaking down the insulation in it's coil winding, or possibly some other point of weakness in the insulation of the circuit?

 

[tony mc]

Seems odd .

Have they left the light switched on for long periods to come to that conclusion.

If not the RCD at fault it may be your looking in the wrong place.


Try clamping the tails with the light on then switch it off to see what the leakage is !

 

[keithbr13]

Thanks for the answers, If it is a dodgy RCD why does nt it trip on any of the other circuits ?

 

[i=p/u]

Maybe bad contacts on switch

 

[davelerave]

A stab in the dark...
Could the act of switching off the shaver flu be causing a high voltage back emf in the choke/transformer which is breaking down the insulation in it's coil winding, or possibly some other point of weakness in the insulation of the circuit?

they mention that specifically in the 'rcd handbook december 2010'
as a source of these transient overvoltages
-switching off fluorescents with no pfc cap in them

dont know if that's the issue here

another poster suggested a snubber across swich contacts for the arc
also ABB make an anti-nuisance trip rcd if no faults show up in regular testing

 

[Rkm45]

I have come across this a few times a snubber will solve it

 

[MarkieSparkie]

Be careful here! Although a snubber might quell the switching transients and prevent the RCD from tripping in the short term, it's only the 'tip of iceberg'.
The under lying problem here is one of high standing leakage on one or more of the final circuits attached to that RCD, the switching transients in the toilet just push the RCD over the tripping threshold, but can't possibly be large enough in this case to cause the tripping alone, as a large reactive load or resonance would be required. Focus your attention on finding and reducing the high standing leakage current. Then the need for a snubber will become unnecessary.

 

[davelerave]

Be careful here! Although a snubber might quell the switching transients and prevent the RCD from tripping in the short term, it's only the 'tip of iceberg'.
The under lying problem here is one of high standing leakage on one or more of the final circuits attached to that RCD, the switching transients in the toilet just push the RCD over the tripping threshold, but can't possibly be large enoughag in this case to cause the tripping alone, as a large reactive load or resonance would be required. Focus your attention on finding and reducing the high standing leakage current. Then the need for a snubber will
become unnecessary.

agree he should test first
i can see how standing leakage might be the underlying issue
in the rcd handbook they say the issue is overvoltage transient+capacitance to earth from wiring and equipment

 

[keithbr13]

Hi All thanks for the answers, can I just ask what a snubber is ? which book are you refering to ? how would you go about testing the other circuits attched to the RCD
Thanks again

 

[davelerave]

Rcd Handbook Dec 2010 49836
9.1 mains borne transients and surges

 

[tony mc]

Hi All thanks for the answers, can I just ask what a snubber is ? which book are you refering to ? how would you go about testing the other circuits attched to the RCD
Thanks again

Do you have a earth leakage clamp meter ?

Disconnect all loads from the RCD and ramp it to find out when it is tripping. (mA)

With your clamp meter on the tails switch on the loads one at a time to find out which if any is excessive.

I take it that all your IR tests for the ccts are good ?

Its very easy to look in the wrong place with RCD faults when they are protecting multiple mcb's!

Have a look at Markies post on this thread re a snubber!
Flashing Light bulb when switchs are off...