RCCD tripping in Domestic TT circuit.

[corvus13]

Sparks and Sparkettes, Last week the 100A 30mA RCCD(Merlin Gerin) tripped for no obvious reason and it reset with no problems. The next evening it tripped again but wouldn't reset. I opened the MCB's feeding the final circuits one at a time until I disconnected the faulty circuit and I could reset the RCCB. Only it woulndn't. All outgoing circuits disconnected and the RCCB would not reset. Clearly a faulty RCCB but a guy at work said that a neutral 'faulting' to earth would stop the RCCB from reseting. As I far as I understand this, even though the neutral(s) are still connected to the common neutral bar, the open MCB stops any volts to cause a differential current in the RCCB. Neutal shorting to earth or not. Am I right or tripe?

As it happened the trip did reset after 20 attempts(mindless desperation) but tripped before I could close a MCB. More mindless desperation and the RCCB eventually reset together with all the MCB's. Another trip after 30mins but solid since then.

So is the RCCD faulty or do I have an earthy neutal despite the MCB's being open?

Also, bearing in mind this is a TT installation, can I replace the 100A RCCB with a 100A trip and used RCBO's on the final circuits? Is this allowed?

Thanks in advance for any help- I'm off into town now to see a Punk tribute band, ACE! Cheers, Tim

 

[topquark]

May I ask what test equipment you have at your disposal?

 

[GLENNSPARK]

so if i turned up for work tomorrow with a pinny on do i qualify as a sparkette?..:tounge_smile:

 

[topquark]

so if i turned up for work tomorrow with a pinny on do i qualify as a sparkette?..:tounge_smile:

Eeeek!

 

[Engineer54]

Also, bearing in mind this is a TT installation, can I replace the 100A RCCB with a 100A trip and used RCBO's on the final circuits? Is this allowed?

This would be a far better set-up for a TT based installation, but the 100mA RCD will need to be an S type unit (time delayed) or the chances are, it will trip with any one of the RCBO's tripping on a leakage fault!!!

As for the other matter, you need the proper test equipment to find the cause of your problem, there are no short cuts there i'm afraid, but yes a neutral earth fault will cause your RCD to trip and/or hold off any reconnection attempts on the RCD!!!

 

[Deleted member 9648]

You need to IR test final circuits...should be the first thing you do.

 

[Specialist]

so if i turned up for work tomorrow with a pinny on do i qualify as a sparkette?..:tounge_smile:

You'll have to post pictures for us to vote on Lol :wink_smile:

 

[clicon357]

Also, bearing in mind this is a TT installation, can I replace the 100A RCCB with a 100A trip and used RCBO's on the final circuits? Is this allowed?

This would be a far better set-up for a TT based installation, but the 100mA RCD will need to be an S type unit (time delayed) or the chances are, it will trip with any one of the RCBO's tripping on a leakage fault!!!

As for the other matter, you need the proper test equipment to find the cause of your problem, there are no short cuts there i'm afraid, but yes a neutral earth fault will cause your RCD to trip and/or hold off any reconnection attempts on the RCD!!!

A few terminology issues in the above! "100A RCCB" - I don't think so" - try 100mA. Then "100A trip" - you mean main switch not "trip". So, the preferred solution here would IMO be to get rid of the 100mA RCD altogether and replace with a 100A main switch THEN have all circuits on RCBOs off of that? No need for a 100mA S type at all IMO.

 

[Engineer54]

A few terminology issues in the above! "100A RCCB" - I don't think so" - try 100mA. Then "100A trip" - you mean main switch not "trip". So, the preferred solution here would IMO be to get rid of the 100mA RCD altogether and replace with a 100A main switch THEN have all circuits on RCBOs off of that? No need for a 100mA S type at all IMO.

Not on a UK based TT system consisting of one 1m 3/8'' rod!! I'll stand by my original reply post, and stay with the 100mA RCD up-front isolator!! lol!!!

 

[MarkieSparkie]

Clearly a faulty RCCB but a guy at work said that a neutral 'faulting' to earth would stop the RCCB from reseting. As I far as I understand this, even though the neutral(s) are still connected to the common neutral bar, the open MCB stops any volts to cause a differential current in the RCCB. Neutal shorting to earth or not. Am I right or tripe?

You could be wrong and your mate right …

A N-E fault on a single pole MCB, isolated final circuit, creates a parallel path for a proportion of any N current flowing in the other un-isolated final circuits. If this fault current is greater than the RCD tripping current (typically 20mA for a 30mA nominal rated RCD) the RCD will trip even though this current bypasses the RCDs toroidal core, because remember, the fault current is a proportion of the un-isolated N load current and there is a resulting, automatic out of balance condition, that therefore trips the RCD.
On a TT system with a low enough earth electrode resistance, this N-E fault current will flow due to the small potential that typically exists on the distribution board N busbar resulting from the voltage drop along the N conductor of the DNO supply network. This is the reason why 537.2.1.1 effectively requires double pole isolation (e.g. Main Switch L&N) on a TT system.

 

[clicon357]

Not on a UK based TT system consisting of one 1m 3/8'' rod!! I'll stand by my original reply post, and stay with the 100mA RCD up-front isolator!! lol!!!

I don't have my OSG to hand but sure that a 100mA S type used as a main switch followed by a line of RCBOs would be well........pointless - unless you can tell me otherwise. Interested to know the thinking behind it. Cheers.

 

[malcolmsanford]

I don't have my OSG to hand but sure that a 100mA S type used as a main switch followed by a line of RCBOs would be well........pointless - unless you can tell me otherwise. Interested to know the thinking behind it. Cheers.

Not entirely pointless as you could use the 100mA S type protection for say a distribution Circuit to a garage or shed. Perhaps for outside lights that don't require additional protection by a 30mA RCD.

Though not in the regs, on a TT system I prefer to use double pole protection devices, if you never had this upfront 100mA S type RCD then I would have to fit double pole RCBO's

Often if the CU is metal clad you would use a 100mA S type RCD to protect the tails coming into it.

The 100mA RCD can give you certain other options on an installation, but often as they are expensive you would not get many customers approving the cost.

 

[Engineer54]

I don't have my OSG to hand but sure that a 100mA S type used as a main switch followed by a line of RCBOs would be well........pointless - unless you can tell me otherwise. Interested to know the thinking behind it. Cheers.

As well as the points brought up By Malcolm, the 100mA RCD would stand as a secondary protection to the downstream RCD/RCBO's. The failure to trip on a leakage fault of any, would be backed up by the up-front unit.

If we were talking anywhere else on a TT system, i'd be inclined to agree with you. But as UK TT systems are generally so badly constructed and left with useless Ra values, that give zero protection in fault conditions, .....I'll remain in standing by my original reply!! lol!!!!

 

[Guest112]

I tripped an rcd main switch to an entire school by simply touching neutral to earth accidentally while the circuit was isolated, whoops

 

[Engineer54]

I tripped an rcd main switch to an entire school by simply touching neutral to earth accidentally while the circuit was isolated, whoops

Surely, that would on a ACB/MCCB that would have adjustable settings for both time delay and mA values. If it hasn't such facilities on it's main or sub-main breaker then it's a P**s poor design that school has!!! lol!!