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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
 
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S

sparks1973

at what point on the install did you test the RCDs...at the RCD itself...or further downstream...at a socket perhaps....its just i1`v heard of RCBos that wont test upclose to em...but a bit further downstream...
 
  • Thread Starter Thread Starter
  • #3
i tested at various points throughout the house and the result was always the same with the lighting circuit connected they wont trip.
as you say i know that memshield 2 rcbos wont trip properly up close
 

Des 56

-
Arms
Esteemed
I wonder if there are lots of transformers in the circuit,I'm thinking that somehow it is replicating no trip test meter operations, by somehow saturating the circuit with Dc
 

Jimmy Boy

-
Arms
With the L/E/N on the light circuit removed from board did you do a continuity and ohms measurement test from the disconnected ends of the circuit to L/E/N on the board just to double check there isn't a interconnection ?
J
 
O

oldtimer

Why dont you connect the light circuit switch all the lights off at the switch then test the rcd if this works then move on to switching each individual light on until you get the same problem.
 
  • Thread Starter Thread Starter
  • #7
I wonder if there are lots of transformers in the circuit,I'm thinking that somehow it is replicating no trip test meter operations, by somehow saturating the circuit with Dc
yes there is some but not a huge number i did try to trip the rcd both loaded and unloaded

could you explain your comment as i dont understand
 
  • Thread Starter Thread Starter
  • #8
With the L/E/N on the light circuit removed from board did you do a continuity and ohms measurement test from the disconnected ends of the circuit to L/E/N on the board just to double check there isn't a interconnection ?
J

in a word no i did try n bar to n bar to check for borrowed neutrals
 
  • Thread Starter Thread Starter
  • #9
Why dont you connect the light circuit switch all the lights off at the switch then test the rcd if this works then move on to switching each individual light on until you get the same problem.

i did try with all lights on and all lights off still the same but will try again one by one as head was spinning by this point
 
O

oldtimer

i did try with all lights on and all lights off still the same but will try again one by one as head was
spinning by this point
Me thinks that if you switch each light off on the suspect circuit and the rcd should work then switch one on at a time and if you do track it down to a section that has transformers on it then this needs to be highlighted to the manufacturer and even safety standards as there could be a lot of rcds up the country that will not trip and that aint good.

Plus have you thought about writing to the rcd manufacturer I take it , it was the same make and model you replaced ?
 
G

Guest55

Lets deal with this in a logical manner.
So , did you carry out the rcd tests at the CU with all mcbs switched off ?
Did the both rcds test ok with regards to trip times ?
If they did then we know the problem is not the rcds.
 
S

sparks1973

I wonder if there are lots of transformers in the circuit,I'm thinking that somehow it is replicating no trip test meter operations, by somehow saturating the circuit with Dc
But that was D-loc....the Robin testers used it...no good on modern RCDs Des..as they will detect D-loc.....
 
G

Guest55

yes there is some but not a huge number i did try to trip the rcd both loaded and unloaded

could you explain your comment as i dont understand
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.
 
S

sparks1973

look....i think its now clear that something on that lighting circuit is causing the RCDs not to let go....so now its been narrowed down to a final circuit...lets start to look at that.....i mean this is standard fault finding practice surely guys...
 
S

sparks1973

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

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.
 
S

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........
 
S

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...
 
G

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 ass what your assessor says lol.
And if all the mcbs are turned off it doesnt matter whats plugged in or not ???

:41:
 
P

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

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Arms
Esteemed
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
 
S

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...
 

Jimmy Boy

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Arms
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
 
  • Thread Starter Thread Starter
  • #30
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
 
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