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]

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...

 

[Mr Mark Sparks]

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]

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]

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

 

[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.

 

[Mr Mark Sparks]

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

 

[Mr Mark Sparks]

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

 

[Mr Mark Sparks]

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

 

[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 ?

 

[Mr Mark Sparks]

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

contactum taken out and new wylex dual board fitted house built mid 80s and suspect its always been like it

 

[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.

 

[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.....

 

[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.

 

[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...