mcb keeps blowing on lighting

[Wayne Hudson]

Hello sparks
I was at a Job earlier today,the customers upstairs lights would not turn on due to the 6A mcb operating so it's a short circuit so I disconnected the supply to the first light the supply disconnected from consumer unit ,then did a insulation resistance which came out clear tried the mcb without any connection it stayed on.so connected the supply at consumer unit and left the other end not connected to anything it stayed on so connected the loop out and switch live it all come back on for 2 poxy minutes then it would come back on .I had to leave disconnected and will be returning back tommorow . Any suggestions would be extremely grateful
Thanks

 

[Engineer54]

I ave just ordered some dprcbos expensive lesson this is but needs to be done . Thanks for your help mostly appreciated

You'd of been better off purchasing SP+N RCBO's which come as single modules approximately the same size as MCB's, and are cheaper than the DP units.

SP RCBO's are not all doom and gloom, they are not recommended for TT/IT systems for obvious reasons. On TN systems although not ideal i would still happily install them. Anyone know of any fires caused by SP RCBO's, because i certainly haven't heard of any?? No different to having a standard SP MCB and solid neutral at the end of the day!! ...lol!!

 

[Knobhead]

You'd of been better off purchasing SP+N RCBO's which come as single modules approximately the same size as MCB's, and are cheaper than the DP units.

SP RCBO's are not all doom and gloom, they are not recommended for TT/IT systems for obvious reasons. On TN systems although not ideal i would still happily install them. Anyone know of any fires caused by SP RCBO's, because i certainly haven't heard of any?? No different to having a standard SP MCB and solid neutral at the end of the day!! ...lol!!

They’re not ideal at all.

As we both know, neutral is capable of carrying considerable current under earth fault conditions and therefore should be disconnected when an earth fault is detected.

A SP MCB with a solid neutral in a domestic environment will nowadays be backed up by a RCD. The neutral will be disconnected, inconvenient it may be to loose half the board but the fault will be cleared safely.

Would you accept a 3 pole MCCB that only disconnected two of the lines? It will disconnect the load, but not in a safe manner.

I will maintain my views. SP RCBO’s are not fit for purpose and should never have been put on the market.

 

[Engineer54]

A SP MCB with a solid neutral in a domestic environment will nowadays be backed up by a RCD.

As i said maybe not ideal, but certainly no different to what has been the norm since we moved away from fusing the neutral conductor of circuits in the year dot... And there are still hundred's of thousands, if not millions of Domestic and small commercial installations in the UK without any form of RCD protection. So really the percieved risks of fires etc, are leaning towards the zone of scaremongering, i'd happily use SP RCBO's on a TN system domestic installation, i'd prefer to use SP+N RCBO's, but i'd certainly not lose any sleep over it!!

 

[HandySparks]

TT system? You are aware that the standard single pole RCBOs are not allowed on a TT system aren't you?

Is there actually a reg for that as such?

They are a problem because they will detect a neutral to earth fault but not disconnect it.

Think of it this way:

A fault occurs between N and E
A portion of the installation neutral current flows through the fault to earth thanks to the effect of parallel resistances.
The SP RCBO detects this fault.
The SP RCBO then disconnects the live of that circuit.
The neutral remains unbroken and so the diverted neutral current continues to flow through it.
The point of the fault and the cable leading up to it becomes very very warm as the diverted current is greater than its CCC.
FIRE!

OK, but put some realistic numbers in for supply impedance, Ra and load current for a domestic premises on a TT supply. I don't think that there's an issue here.

 

[Knobhead]

Is there actually a reg for that as such?.

Manufacturer’s recommendations as RoB2 pointed out.

OK, but put some realistic numbers in for supply impedance, Ra and load current for a domestic premises on a TT supply. I don't think that there's an issue here.

No figures, but I know when I’m looking at a clamp meter reading 35A to earth on a TT system. I need no more proof than that.

 

[Badged01]

Is there actually a reg for that as such?

Depending on how you want to interpret it, you could read 537.2.1.1 as needing DP isolation on all circuits other than on TN ----- the counter argument being that with a double pole main switch you do have that in the domestic environment!!.

Pays your money, reads the regs and makes your choice ......

 

[Engineer54]

Don't get me wrong, i don't disagree with either Tony or Dave, they have made valid points. It has always been better to isolate both the phase and neutral of a circuit for many reasons. Switching multi pole breakers/switches with neutral conductors presents it's own problems, namely the first to make and last to break scenario.

I just think you can go too far with condemning SP RCBO's out of hand, nothing more.....

 

[davesparks]

Is there actually a reg for that as such?



OK, but put some realistic numbers in for supply impedance, Ra and load current for a domestic premises on a TT supply. I don't think that there's an issue here.

Its not so much the TT systems as the TN systems where the big problems can occur. But that's because so many people work to the ridiculous 200ohm Ra.

The manufacturers state they must not be used on a TT system, that's a pretty damned good reason not to!

 

[Wayne Hudson]

Its not so much the TT systems as the TN systems where the big problems can occur. But that's because so many people work to the ridiculous 200ohm Ra.

The manufacturers state they must not be used on a TT system, that's a pretty damned good reason not to!

I looked at the schneider rcbo earlier and manufacturers guide in a bold statement says not to be used on tt or tns ,I looked at wylex proteus lewdon and m2 and none of them state they can't used on a tt wholesellers I go in did not have 1 sp+n rcbos

 

[Tuttle]

They are a problem because they will detect a neutral to earth fault but not disconnect it.

Think of it this way:

A fault occurs between N and E
A portion of the installation neutral current flows through the fault to earth thanks to the effect of parallel resistances.
The SP RCBO detects this fault.
The SP RCBO then disconnects the live of that circuit.
The neutral remains unbroken and so the diverted neutral current continues to flow through it.
The point of the fault and the cable leading up to it becomes very very warm as the diverted current is greater than its CCC.
FIRE!

So there you have it, the device designed to protect against faults to earth does not disconnect them, instead it gives a false sense of safety as to the untrained person it appears that the faulty circuit is 'off' and therefore safe.

How about if every circuit in the installation is protected by a SP RCBO (assuming a TN system)?

The diverted neutral current will be fed from another circuit so the RCBO for that circuit will trip as well.

Not ideal as a single N-E fault potentially trips multiple or all circuits, but if all circuits are RCD protected is this then safe?

I'm only asking because I fit boards with SP RCBOs for every circuit and assume that any/all circuits supplying a N-E fault will trip - please tell me if I'm missing something

 

[davesparks]

How about if every circuit in the installation is protected by a SP RCBO (assuming a TN system)?

The diverted neutral current will be fed from another circuit so the RCBO for that circuit will trip as well.

Not ideal as a single N-E fault potentially trips multiple or all circuits, but if all circuits are RCD protected is this then safe?

I'm only asking because I fit boards with SP RCBOs for every circuit and assume that any/all circuits supplying a N-E fault will trip - please tell me if I'm missing something

No other circuit RCBOs will trip as they will not see an imbalance as all of the neutral current will flow through their neutral.
The paralell path is created after the other circuits RCBO.

Neutral current from the good circuit returns through its RCBO to the neutral bar.
Part of it flows to neutral through the neutral bar link to the main switch.
The diverted part of it flows from the neutral bar back down the neutral of the faulty circuit (and through the SP RCBO) to the point of the fault and then down the CPC and back to earth.

 

[Tuttle]

No other circuit RCBOs will trip as they will not see an imbalance as all of the neutral current will flow through their neutral.
The paralell path is created after the other circuits RCBO.

Neutral current from the good circuit returns through its RCBO to the neutral bar.
Part of it flows to neutral through the neutral bar link to the main switch.
The diverted part of it flows from the neutral bar back down the neutral of the faulty circuit (and through the SP RCBO) to the point of the fault and then down the CPC and back to earth.

Of course, I should have drawn a diagram before I asked a silly question, thanks ds

 

[happysteve]

Its not so much the TT systems as the TN systems where the big problems can occur. But that's because so many people work to the ridiculous 200ohm Ra.

The manufacturers state they must not be used on a TT system, that's a pretty damned good reason not to!

This has been an interesting and educational read. Thanks to all who have contributed, particularly the gentleman I have quoted. Detecting - but not clearing - a N-E fault is a sound reason why SP RCBOs aren't all that great. I understand why this would potentially be worse in a system with lower earth impedance (PME) so as per the quote above you'd actually be "better off" on a TT system.

And I also accept that if the manufacturer says "Don't do it," then that's a compelling argument.*

But (unless I've missed summat), is the general reluctance to use SP RCBOs on TT systems down to the argument of isolation (537.2.1.1)?

"Every circuit shall be capable of being isolated from each of the live conductors. In a TN-S or TN-C-S system, it is not necessary to isolate the neutral conductor where it is regarded as being reliably connected to Earth by a suitably low impedance."

So the fact that it's good practice to be able to isolate individual circuits - rather than the whole lot using the main incoming DP RCD - and on a TT system you need DP isolators - is that the reason?

Question is: is that the same reason why some manufacturers state not to use SP RCBOs on TT systems, or is there some other reason for it?

Just interested. Thanks again

*Mind you, the manufacturer of the fancy light fitting I bought from Homebase said, in the "tools required" bit of the instructions, that I would need insulation tape, which I should "wrap around the terminal block at least twice before placing in the ceiling void."

 

[davesparks]

And if you want another good reason then there is the price.

SPSN RCBO for £16
SP RCBO for >£16

 

[Tuttle]

I fitted a Hager CU with SP RCBOs for every circuit in a house last week. There was a N-E fault on one of the ring final circuit before I changed the board. It was caused by dodgy wiring in a socket outlet causing one of the neutral cores to short back to the back box. I guess there could be other dodgy wiring in that house that I didn't see waiting to cause a similar problem.

It seems that most RCBOs are single pole though, I tend to fit Hager and they're single pole. Is anyone here fitting RCBOs with switched neutrals, if so what's good?