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!!

 

[clicon357]

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

Out of interest is the CU metal clad or insulated?

 

[corvus13]

First off Thanks for all the replies. I've just spent an age typing a responce then I noticed there is a second page.



Clicon357: The RCCB I have is a 0.030A and 100A Merlin Gerin 'Multi 9'. This means, as I understand it, the device will trip on seeing a difference in the Live-Neutral currents of 30mA, and is only designed to carry 100A load current before overheating. Is this right? When I was thinking of replacing this with a trip I meant an overload that would trip at a load of 100A but had no earth leakage trip facility. The CU is metal Clad and the RCCB is double pole.

Topquark-Wirepuller, I can get hold of a 500v Insulation Resistance Tester. As for fractional ohm meters, would have to see what turns up at work. In this situation I would be looking for low(ish) resistance between earth and the neutrals as they return to the common bar in the CU. All switches, MCB's etc off ofcourse. If these circuits were ok then I'm looking at fridges, errr and things plugged in, no electric shower/immersion heater....

MarkieSparkie, I was thinking of the neutral voltage difference to earth on the way to the pub last night but dismissed this. Wrong again.

So to confirm, is it ok or better to replace the 30mA 100A RCCB with:
a 100A Main Switch or
a 100A Overload Trip or
a 100mA S-type RCD rated at 100A- which then feeds the final circuits via 30mA RCBO's?
Sorry if all this is plainly stated but I'm unfamiliar with a lot of these terms. Cheers Tim

 

[malcolmsanford]

First off Thanks for all the replies. I've just spent an age typing a responce then I noticed there is a second page.



Clicon357: The RCCB I have is a 0.030A and 100A Merlin Gerin 'Multi 9'. This means, as I understand it, the device will trip on seeing a difference in the Live-Neutral currents of 30mA, and is only designed to carry 100A load current before overheating. Is this right? When I was thinking of replacing this with a trip I meant an overload that would trip at a load of 100A but had no earth leakage trip facility. The CU is metal Clad and the RCCB is double pole.

Topquark-Wirepuller, I can get hold of a 500v Insulation Resistance Tester. As for fractional ohm meters, would have to see what turns up at work. In this situation I would be looking for low(ish) resistance between earth and the neutrals as they return to the common bar in the CU. All switches, MCB's etc off ofcourse. If these circuits were ok then I'm looking at fridges, errr and things plugged in, no electric shower/immersion heater....

MarkieSparkie, I was thinking of the neutral voltage difference to earth on the way to the pub last night but dismissed this. Wrong again.

So to confirm, is it ok or better to replace the 30mA 100A RCCB with:
a 100A Main Switch or
a 100A Overload Trip or
a 100mA S-type RCD rated at 100A- which then feeds the final circuits via 30mA RCBO's?
Sorry if all this is plainly stated but I'm unfamiliar with a lot of these terms. Cheers Tim

Because your CU is metal clad you will have to protect the tails and have an upfront RCD 100mA S type would be the preferred weapon of choice.

So you will need to fit the 100mA S type RCD in the tails before they enter into that metal clad CU. You can then fit a 100amp double pole main switch inside the CU and then your RCBO's

This will then give compliance to Regulation 531.4.1 for the metal clad CU regulation 531.2.9 for compliance of RCDs in series and 537.1.4 for isolation

 

[corvus13]

Thanks for that but, and I know this sounds like I'm ungreatfull but it's not the answer I was hoping for. TT and the 17th Edition have done for me. Bah! Expect further irritations from me when I get myself together for an adventure into testing earth spikes on TT systems. I'll blast my final circuits this weekend and see what turns up.
By the way, with regards to the feed into the metal clad CU that is not protected by a S-type (only the 100A fuse), is this illegal concidering this installation was done around 20 years ago. Cheers to every one Tim

 

[corvus13]

Right then Lads, to cut a long story short I blasted the Neutrals to the Earth Terminal in the CU @ 500v and got the following results:
Downstairs sockets(ring main) - 2.3MOhms.
The Neutral Bar with all other Neutral connected- 20Mohms.
I then plugged in some appliances(suspect devices) eg an electric radiator, table lamps and a 'Stella' bottle cooler and still got 2.0Megs.
At least if I become plagued by further trips I know where to look but still disapointed by a dicky 20 year old installation. Cheers and Thanks