Need some ideas - RCD tripping but everything tests fine....

[weevilward]

Hi all,
Customer reports sporadic nuisance tripping of RCD. I cannot reproduce it, or fault anything, except SOMETIMES on EFLI test it trips the RCD (megger mft 1553 'no trip' test).
I'm out of ideas here, can anyone help? This is what I've got:
- TT installation (small domestic);
- 30mA RCCB main switch incomer (Crabtree starbreaker);
- all circuits IR test 8MOhm to L&N-earth;
- RCD testing off load and on load all fine, ramp test 23mA off load 19mA with everything on.
- Ze about 160 ohms down the rod, drops to 12 ohms with all bonding connected.

 

[Jacek Buczak]

I think in TT system we need to use RCD just 100mA or 300mA.

 

[westward10]

So does a main switch.

Isolation was probably the wrong wording. A single pole rcbo may disconnect the circuit as required under certain fault conditions but does not disrupt the neutral which is considered a live conductor, a double pole upstream rcd will.

 

[marc8]

Ebay about £40 notes, works well no problems. Brand new

 

[Deleted member 9648]

Isolation was probably the wrong wording. A single pole rcbo may disconnect the circuit as required under certain fault conditions but does not disrupt the neutral which is considered a live conductor, a double pole upstream rcd will.

Tin hat on I've never quite got the significance of disconnecting the neutral on a TT. If an RCBO wont disconnect it on a TT it wont on a TN , so whats the difference?>

 

[westward10]

Neutral is deemed being at earth potential for TN systems.

 

[Midwest]

Coming into the discussion a bit late, but isn't 8 megohm a bit on the low side (I relegalise its within acceptable limits) for a two up two down. Wouldn't it be an idea to get that to a higher figure, then dodgy appliances wouldn't have such an impact?

OP did you say you've measured 19 mA? Think most 30mA RCD start tripping at 21, so you've not much scope.

 

[Deleted member 9648]

Neutral is deemed being at earth potential for TN systems.

Why is there nothing in 7671 about sp RCBO's on TT systems?

 

[Midwest]

Cos I suppose there's a main switch that isolates both Live & Neutral, but I'm no expert here!

 

[Deleted member 9648]

Cos I suppose there's a main switch that isolates both Live & Neutral, but I'm no expert here!

I know I've read somewhere about SP RCBO's not being suitable for TT systems but cant remember where and am still sceptical. In fact on our last NICEIC visit I asked the question and got a blank look.
If anyone can shed any light......

 

[westward10]

I know I've read somewhere about SP RCBO's not being suitable for TT systems but cant remember where and am still sceptical. In fact on our last NICEIC visit I asked the question and got a blank look.
If anyone can shed any light......

It is a much discussed grey area. An rcbo is not there to necessarily provide isolation but disconnection in the event of a fault which a single pole device will do. It confuses me and it is not clear anywhere with mixed opinions given.

 

[weevilward]

Coming into the discussion a bit late, but isn't 8 megohm a bit on the low side (I relegalise its within acceptable limits) for a two up two down. Wouldn't it be an idea to get that to a higher figure, then dodgy appliances wouldn't have such an impact?

OP did you say you've measured 19 mA? Think most 30mA RCD start tripping at 21, so you've not much scope.

No, it's tripping at 19mA on ramp test with everything on. (23mA disconnected). So there is nornally 4mA leakage.
I realise 8M is a bit low....
Think I'm gonna invest in a leakage clamp meter and have a deeper look - if I get called back that is - still no tripping 48 hrs on......

 

[Midwest]

Perhaps an investigation to that 8mO figure might be worth looking at?

 

[static zap]

Any industrial workshops ,garages nearby ?

(or road dug up for new street lights ?)

 

[davesparks]

RCBOs with a solid neutral are far from ideal with any earthing system.
If there is a neutral to earth fault then the rcbo will react by disconnecting the line and not actually disconnect the fault.
Thus it appears to the user that a fault has occurred but it is now disconnected because the protective device has operated, when in actual fact the fault is still there.
Plus of course you now have a portion of the neutral current from the entire installation diverting through the fault.

Additionally manufacturers have their own requirements for maximum Zs for correct operation of their single pole RCBOs, which may not be achieveable on a TT system.

 

[weevilward]

Any industrial workshops ,garages nearby ?

(or road dug up for new street lights ?)

Not that I can see. Why do you ask - can a dirty supply operate RCDs????