rcbo's tripping randomly

[fylthespider]

Hi all, new to the forum and got a cracker for you.
story so far...
100 year old mid terrace house. was asked to inspect/test 30year old wiring. turns out ok so recommend new cu, all rcbo's and DNO to install PME.
shower, kitchen rm, house rm, lights up and down.
all goes well, everybodys happy.

about a month later i get a call.... the dreaded nuisance tripping ARRGH!!!!

so, perform another full test and pat test all appliances and again, all good. insulation test good, no shared/crossed neutrals/earths etc.
so leave with instructions to keep a log of tripping.

after about a month i went back to find completely random tripping, any and all rcbo's in any combination at random times etc.

tested again and still no joy.

tried substituting mcb's in various combinations, isolated each circuit one at a time, all the usual tricks.
even had shower rcbo trip when outgoing cable isolated ?????.

starting to loose the will on this one.

any ideas ?
only thing i can think of is transients on the supply as its the only thing common to all rcbo's, but of coarse the DNO dont want to know.

I rate myself as a good sparky, been part p since it started, 2391 since 2001, Ive never been stumped before but this got me tearing my hair out (and i havnt got that much left)
only good thing is the customer is a friend so she's not dancing on my neck, but i got to figure this out cos its been going on for months.

any thoughts appreciated cos i'm ready to give up and get a job at tesco's.

cheers phil.

 

[jaresquire]

Do you have an earth-leakage clamp meter you can check each circuit with?
Were the RCBOs a known brand?

 

[IQ Electrical]

Hi all, new to the forum and got a cracker for you.
story so far...
100 year old mid terrace house. was asked to inspect/test 30year old wiring. turns out ok so recommend new cu, all rcbo's and DNO to install PME.
shower, kitchen rm, house rm, lights up and down.
all goes well, everybodys happy.

about a month later i get a call.... the dreaded nuisance tripping ARRGH!!!!

so, perform another full test and pat test all appliances and again, all good. insulation test good, no shared/crossed neutrals/earths etc.
so leave with instructions to keep a log of tripping.

after about a month i went back to find completely random tripping, any and all rcbo's in any combination at random times etc.

tested again and still no joy.

tried substituting mcb's in various combinations, isolated each circuit one at a time, all the usual tricks.
even had shower rcbo trip when outgoing cable isolated ?????.

starting to loose the will on this one.

any ideas ?
only thing i can think of is transients on the supply as its the only thing common to all rcbo's, but of coarse the DNO dont want to know.

I rate myself as a good sparky, been part p since it started, 2391 since 2001, Ive never been stumped before but this got me tearing my hair out (and i havnt got that much left)
only good thing is the customer is a friend so she's not dancing on my neck, but i got to figure this out cos its been going on for months.

any thoughts appreciated cos i'm ready to give up and get a job at tesco's.

cheers phil.

We had a callout to a day centre a few weeks back, RCBOs randomly tripping, rather similar to what you describe.

The weird bit was the caretaker saying that the problem always happened when the council were cutting the grass outside!

Anyway, it turns out that the DNO neutral meter connection (external supply) was loose and every time a decent load kicked in or a vibration took place outside, a few RCBOs would trip!

I was quite proud of that one but to be honest, if it wasn't for the caretaker mentioning the grass cutting, it would have taken a hell of a lot longer to find, especially as the neutral loop impedance reading was exactly as I would have expected!

Worth a shot anyway as it probably won't show up in your conventional testing regime!

 

[jaresquire]

Ok, as IQ says, do you have a lawn mower

 

[topquark]

We had a callout to a day centre a few weeks back, RCBOs randomly tripping, rather similar to what you describe.

The weird bit was the caretaker saying that the problem always happened when the council were cutting the grass outside!

Anyway, it turns out that the DNO neutral meter connection (external supply) was loose and every time a decent load kicked in or a vibration took place outside, a few RCBOs would trip!

I was quite proud of that one but to be honest, if it wasn't for the caretaker mentioning the grass cutting, it would have taken a hell of a lot longer to find, especially as the neutral loop impedance reading was exactly as I would have expected!

Worth a shot anyway as it probably won't show up in your conventional testing regime!

Classic, seen it on the main incommer and RCD connections but never on the DNO side, top marks for getting to the bottom of that one IQ!

 

[IQ Electrical]

Classic, seen it on the main incommer and RCD connections but never on the DNO side, top marks for getting to the bottom of that one IQ!

Yes, it was one of my 'lucky' days!

 

[fylthespider]

definitly not the lawn mower, she got concrete back yard.
not got clamp meter, but left multi-meter (with max reading hold) inline with earth for couple hours and nowt. would like to leave it for couple days but wouldnt be very safe.
rcbo's from denmans, control gear i think. used loads of them in similar situations and no probs. all hold fine on 15ma test so i dont think it that.

i'd like the dno to monitor the line for a while but since it took 3 weeks for them to change a metal clad neutral fused head i dont think i'll have much luck there.

I thought about floating neutral but as IQ said, perfect neutral readings.

Any one used transient suppressors on incoming line?

getting tempted to do a rewire then shout at dno about compo, but need to be sure where the fault is first otherwise eggy face.

thanks for the replys tho.

 

[Flying Ninja]

As mentioned above but have you double checked connections in the incoming suppy such as the meter, fuse ,Henley blocks, main switch and that the rcbo's are not not clamping on the bus bar properly. Know they are simple ones but sometimes it's the simple ones we miss.

 

[GLENNSPARK]

Maybe a reversed polarity someware within the install? I know you tested it all but...........

 

[Plonker 3]

Have you carried out a ramp test on each RCBo to ensure they aren't tripping at very low earth leakage?

 

[ezzzekiel]

Minor neutral to earth fault not being detected by ir testing

 

[alarm man]

mice having a chew,

 

[the pict]

I had this once with one rcbo on a board I fitted turned out the N condutor copper bit sheard off and had enough of a contact to test ok, and to make it worse I had cut the pvc to short and the clamp had a hold of it so it felt tight, I only found it as I was replacinging it with a MCB as a temp mesasure,
Pict

 

[Paul.M]

I've had this before:- hallway landing floor joist notched out and packed with cables, floorboard pushing on the cables and pvc being worn away slowly over time. Random tripping? Going to the toilet at 2am stepping on the floorboard giving enough to trip rcd (in my case) but not a short circuit therefore mcb not tripping.

 

[Knobhead]

You have a recording meter set it to record N-E for a while. You say the DNO replaced the service head. There’s good chance it’s the culprit.