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]

So leaving the circuit unsafe is a better option is it? At least the RCD is present and can be tested and certified as such.

1) the RCDs dont work with the circuit connected
2) whoos to say that just putting the offending circuit on an RCBO will give THAT circuit additional protection?....the RCBO may not work due to THAT circuit being fed from it....so how does that comply?!!

 

[Jimmy Boy]

1) the RCDs dont work with the circuit connected
2) whoos to say that just putting the offending circuit on an RCBO will give THAT circuit additional protection?....the RCBO may not work due to THAT circuit being fed from it....so how does that comply?!!

It doesn't the point was it would need to be tested, if it failed back to square one, if it passed yippee lol.
J

 

[Guest55]

Well I was wondering what the next 18th might ask for.............. !
J

it will be asking for another £80 lol.

 

[Des 56]

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

I have had a think about Marks problem and have now come to the conclusion he doesn't have a problem :smile5:


With this subject of test procedure in mind
I am aware the test procedure doesn't recommend for circuit loads to be catered for in the test

I have just read the contents of this link,which was found on the IET forum, it seems to give the reasons for no load testing of an Rcd, for anyone interested the conclusion at the end sums it up
http://www.esc.org.uk/fileadmin/user_upload/documents/industry/rcd_research/TaskAEffect of Load.pdf

 

[topquark]

Does the "errant" RCD fail to trip at BOTH 0deg and 180deg with the suspect lighting circuit connected?

 

[Mr Mark Sparks]

Does the "errant" RCD fail to trip at BOTH 0deg and 180deg with the suspect lighting circuit connected?

ah interesting
have to say ran auto test so fail at 0deg didnt try 180
however did try x5 manual 0deg

why do you ask

 

[topquark]

ah interesting
have to say ran auto test so fail at 0deg didnt try 180
however did try x5 manual 0deg

why do you ask

I suspect it's what Des was alluding to earlier with DC saturation. That would only affect one side of the waveform. If it works on one half but not the other then I'd suspect a combination of what Des and spark1973 (possible faulty dimmer/wiring issue (to explain the poor IR)) said earlier.

Edit: In case that's not clear it would need to be a faulty dimmer/switch and the wiring fault (to allow DC from a transformer into the circuit).

 

[Mr Mark Sparks]

I suspect it's what Des was alluding to earlier with DC saturation. That would only affect one side of the waveform. If it works on one half but not the other then I'd suspect a combination of what Des and spark1973 (possible faulty dimmer/wiring issue (to explain the poor IR)) said earlier.

Edit: In case that's not clear it would need to be a faulty dimmer/switch and the wiring fault (to allow DC from a transformer into the circuit).

ir fine at time of test readings at start of thread

 

[Mr Mark Sparks]

ok so problem solved but not sure how

went back and fitted rcbo to problem circuit and it popped out instantly
when it stayed on long enough it wouldnt trip under test
re tested insulation and there was a n-e dead short 0.0 (this wasnt there before but had suspected intermitant fault)

removed lights and switches and found trapped wire behind light (this light was fitted a long time ago by others)
cleared fault and re tested all ok and rcbo now tripping under test.

can only assume fault was coming and going with expansion and movement in the house in the warmer weather (cant think of any other reason)

so one question remains with no lights turned on and testing at the rcbo why wasnt it tripping under test before the fault was cleared.