help with rcd problem

[Mr Mark Sparks]

evening all,
went to look at a mates cu the other day tt system 30ma rcd main switch tripping out randomly
earth loop to stake 150 ohms.
earth loop with everything connected 1.30 ohms.
before i did any insulation tests i did a full set of rcd tests and it wouldnt trip via my tester (mft1552) however test button did work,

so said i would change cu as replacement main switch for old cu not available,
replaced cu today with wylex dual rcd cu tested circuits as i put them back.

now the odd thing there is one lighting circuit that tests >299 l-n >299 l-e >100 n-e when i connect this it stops the rcd from tripping on test.
swapped rcds round and tried circuit on both side of cu but with this circuit connected rcd will not trip, with it removed it will.

what sort of issue can stop the rcd from tripping

for those who dont know me i am 22yrs time served with 2391



at the mo the circuit is just via mcb not on either rcd
and i am waiting to here if random tripping has stopped

thanks mark

 

[sparks1973]

i`m wondering if its a problem being caused by a faulty Dimmer switch....or if i`m just being a dimmer?

 

[Plonker 3]

Most gadgets and appliances create "electronic noise" on a circuit which can affect rcd testing - which is why mcbs should be switched off at the time of testing.

My ELECSA assessor, told me to test at a socket or light even on a DB change where you might not be 100% certain of having everyting unplugged or disconnected.

 

[sparks1973]

it has to be something that is common to BOTH RCDs from this one lighting circuit for them both not to want to trip....that means the earth bar........

 

[Guest55]

.....i mean this is standard fault finding practice surely guys...

Nothing is straightforward or standard in trying to diagnose a fault over the internet.

 

[sparks1973]

My ELECSA assessor, told me to test at a socket or light even on a DB change where you might not be 100% certain of having everyting unplugged or disconnected.

yes...its better to be downstream of the device to test...rather than right on it...

 

[Guest55]

My ELECSA assessor, told me to test at a socket or light even on a DB change where you might not be 100% certain of having everyting unplugged or disconnected.

I couldnt give a rats --- what your assessor says lol.
And if all the mcbs are turned off it doesnt matter whats plugged in or not ???

:41:

 

[sparks1973]

Nothing is straightforward or standard in trying to diagnose a fault over the internet.

oh come on Biff....you should know me better than that by now...lol...

 

[Guest55]

yes...its better to be downstream of the device to test...rather than right on it...

Why is that ??

 

[Plonker 3]

yes...its better to be downstream of the device to test...rather than right on it...

But what about unknown loads that may contain a capacitor which would give false readings?

- - - Updated - - -

Why is that ??

On a new build then yes I would say so, but a refurb not possible really which was what I was referring too.

 

[Des 56]

I made the suggestion about equipment perhaps being the problem because,I trust Mark has carried out all the usual tests we would normally do,he has happened upon an unusual problem that requires an unusual cause

I made the reference to the no trip test of a loop tester to give comparison to what I was dreaming about

Dreaming,because I have no technical know how whatsoever, regards the intricacies or the workings of the rcd tester

I did however find this bit of information from a study into the effects

DC in Residual Current Devices
The widespread use of semi-conductor
devices in domestic, commercial and industrial equipment may result in the
flow of non-sinusoidal earth fault currents in the event of equipment insulation
failure. This can occur in certain circumstances where the waveform may be
rectified or chopped and these waveforms are said to contain a pulsating d.c.
component which may desensitise a standard Type AC RCD.

The above is somewhere along the lines of what I was thinking,it may be utter bullcarp and the problem will likely be standard, but its certainly an unusual cause


Some of the equipoment listed includes Phone chargers invertors,transformers etc

 

[sparks1973]

Why is that ??

because some devices dont test properly up close....

 

[sparks1973]

I made the suggestion about equipment perhaps being the problem because,I trust Mark has carried out all the usual tests we would normally do,he has happened upon an unusual problem that requires an unusual cause

I made the reference to the no trip test of a loop tester to give comparison to what I was dreaming about

Dreaming,because I have no technical know how whatsoever, regards the intricacies or the workings of the rcd tester

I did however find this bit of information from a study into the effects

DC in Residual Current Devices
The widespread use of semi-conductor
devices in domestic, commercial and industrial equipment may result in the
flow of non-sinusoidal earth fault currents in the event of equipment insulation
failure. This can occur in certain circumstances where the waveform may be
rectified or chopped and these waveforms are said to contain a pulsating d.c.
component which may desensitise a standard Type AC RCD.

The above is somewhere along the lines of what I was thinking,it may be utter bullcarp and the problem will likely be standard, but its certainly an unusual cause


Some of the equipoment listed includes Phone chargers invertors,transformers etc

hey no probs Des...i wasn`t having a go mate....it was just a bit of info i picked up some time ago about the old robin D-loc...thats all mate...

 

[Guest55]

because some devices dont test properly up close....

Only mem rcbo's have that problem , most other manufacterers advise the testing to be done at the output terminals.

 

[Jimmy Boy]

The OSG does advise testing as near as practicable to it's point of installation, and load supplied should be disconnected, but struggling to find that in the BGB
J

 

[Mr Mark Sparks]

just to clarify as of now the board has 4 circuits on one rcd and these all test and behave normaly

3 circuits on the other rcd and they also test and behave normally

the problem circuit is just on mcb at mo but would effect either rcd if connected

have just had text to say nothing has tripped since i left at 1pm and on the old board it was going upto 10 times a day

so i am wondering if i have a transformer or something that tested ok cold after cu change but is stopping rcd tripping and also when it gets hot causes insulation fault remember l-e 299 n-e 100