Dangers from the neutral

[La Poste]

Greetings one and all.

I'm fairly new to this game and was wondering what dangers can be introduced from the neutral cable.
I was at the wholesalers today and the guys were saying to me that there is no need for double pole RCBO's only singles are needed.

I then asked if there was no danger from the neutral cable why do we have double pole RCD's in fuse boards.

I understand that with a TN-S system if we lose the supply neutral the neutral bar in the fuse board will jump up to 230 Volts.

I also understand that with PME the neutral can float above 0 Volts and this is why we have an equipotential zone and that losing a supply neutral will make all the metal work jump up to 230 Volts as the neutral and earth are the same.

But what other dangers can we get from the neutral cable?
When installing a TT system I have been told everything should be double pole switching I guess because the metal work in the house is tied to true earth through the rod and the neutral cable can float above true earth thus creating a potential difference between the neutral and earth.

Can anyone offer any advice on what dangers can be introduced through the neutral cable?
Are double pole RCBO's really necessary when I keep being told that there is no real danger from the neutral cable at all.

Thanks.

 

[ackbarthestar]

Ok,
The first test
1/ turn on circuit 1 - lets say its called 'lighting'
2/ Disconnect lighting neutral
3/ What happens - Ok, the lights go out
4/ Would you now be comfortable touching that neutral conductor?

So
1/ DP RCBOs offer isolation to both Phase and Neutral of that circuit
2/ If the neutral becomes loose or disconnected with a DP RCBO then the circuit will automatically disconnect when any thing on that circuit is switched on.
3/ You cannot borrow neutrals across different circuits when using DP RCBOs

and as far as the guys at the trade counter, I would ask them what would happen if two neutral were crossed on two lighting circuits
firstly, when both lights are covered by 1 RCD and secondly, when the two lighting circuits are placed on different circuits.

If the PME neutral fails then there is a strong possibility that the metalwork will rise to whatever the supply voltage is at the time.

If you're interested there is a very interesting thread IET Forums - Message for Mr Cronshaw if you get through all the banter !

 

[oldtimer]

Listen rule number one never believe a clown who stands at a wholesalers counter and bumps his gums this is where urban legends are born and grow if you are going to learn anything then realise that conversations like this are pure strutting and teresterone on full pelt. Also the guys behind the counter are great when it comes to service but never ask them a technical question as they will feel the need to spout utter dribble because they have an audience.

 

[fazhols]

You can not get a single pole - RCBO's. RCBOs are "Risidual Current Circuit Breakers" - the clue is Residual Current. They work by measuring the difference in current between Line and Neutral. When it measures a difference greater that the designed maximum (e.g. 30mA) it breaks the circuit. Without Neutral it would have nothing to measure so would not switch on. The differnce means that current is escaping from the circuit elsewhere - could be to earth through a person(?) or even through the neutral of another circuit. Either way, not a happy state of affairs.

Other risks from Neutral include:
SURGE: in a TN-C earth arrangement (even from another building on your phase) a lightning strike would find its way from the ground into the Neutral conductor.
A risk to the safety performance of a RCBO could also arrise if, whilst you are touching the live cable, you simultaneously touch the neutral (while standing on an insulated surface): you would be fried without the RCBO noticing a difference so it would not break!!
There are others mainly relating to voltage "potential difference" caused by other circuits- the electrons try find the easiest path to ground and will "try all circuits" (in this case Neutral via busbar) to find an easy way- the greater the resitance in the original circuit the greater the possibility of a higher flow coming through another neutral that is "grounded".

Greetings one and all.

I'm fairly new to this game and was wondering what dangers can be introduced from the neutral cable.
I was at the wholesalers today and the guys were saying to me that there is no need for double pole RCBO's only singles are needed.

I then asked if there was no danger from the neutral cable why do we have double pole RCD's in fuse boards.

I understand that with a TN-S system if we lose the supply neutral the neutral bar in the fuse board will jump up to 230 Volts.

I also understand that with PME the neutral can float above 0 Volts and this is why we have an equipotential zone and that losing a supply neutral will make all the metal work jump up to 230 Volts as the neutral and earth are the same.

But what other dangers can we get from the neutral cable?
When installing a TT system I have been told everything should be double pole switching I guess because the metal work in the house is tied to true earth through the rod and the neutral cable can float above true earth thus creating a potential difference between the neutral and earth.

Can anyone offer any advice on what dangers can be introduced through the neutral cable?
Are double pole RCBO's really necessary when I keep being told that there is no real danger from the neutral cable at all.

Thanks.

 

[La Poste]

Never ask them a technical question as they will feel the need to spout utter dribble because they have an audience.

Excellent. :lol:

You can not get a single pole - RCBO's. RCBOs are "Risidual Current Circuit Breakers" -

What I mean is these single pole RCBO's only switch off the line, they leave the neutral still connected.
Even though the circuit may appear dead because the line is broken there could still be danger introduced through the still connected neutral circuit or am I barking up the wrong tree?

 

[fazhols]

I think you are talking about MCB's (Mini Circuit Breakers) - phase only. You are correct a neutral that is not isolated can still be dangerous.

 

[La Poste]

So a single pole RCBO switches both line and neutral?
Is that correct?

 

[oldtimer]

As he said you cannot get a single line RCBO for it to work it needs a balanced load accross both LN

 

[La Poste]

I understand that but once the single pole RCBO has detected an imbalance it only switches off the line and leaves the neutral still connected?

Both line and neutral are connected to the RCBO and it monitors the balance between the two but once it detects an imbalance it only switches the line off right?

 

[danzor]

I always thought RCBO's were single pole isolators

 

[netblindpaul]

LaPoste is correct, "almost" all RCBO's for sale in the UK at the moment are single pole solid neutral, the N conductor is NOT switched.
Look at the little picture on the side and it will explain all.
True 2 pole switching RCBO's are available, but they are unusual.

Trust me on this as I have been through this with most of the manufacturers, NOT the wholesalers.
As I DO have an application for these and I was researching the availability.

 

[WDMDL]

Yes. The Neutral only connects to the RCBO for the residual current sensing - THE NEUTRAL IS NOT DISCONNECTED WHEN THE BREAKER IS TRIPPED.

Think of an MCB, it has thermal overload (low% overload, long term) and electromagnetic (high% overload, instantaneous) links to the tripping mechanism.
An RCBO simply has an additional third link to the tripping mechanism - this time from the residual current trip solenoid.

It's basically an MCB at the bottom, with the residual current toroid and solenoid in the extra tall part at the top.

Simon.

 

[oldtimer]

Yep guys I hear what you say but th RCBO still works the same way as a RCD ie a balanced load via the LN

 

[netblindpaul]

Yes oldtimer it does,
However, it does not disconnect the neutral in the event of a fault.

 

[La Poste]

Thanks for the replies.

Would I be right in saying that these SP RCBO's are potentially dangerous especially when used to supply a garden circuit where outdoor electrical equipment is going to be used, damp garden with bare feet?

 

[oldtimer]

Yes oldtimer it does,
However, it does not disconnect the neutral in the event of a fault.

I cannot understand the preoccupation with breaking the neutral remember a RCBO is a circuit breaker and RCD combined just accept the fact that when you switch a RCBO off it does the same as any circuit breaker ie it breaks the live but the neutral is intact dont get bogged down with this as it will only distract you

 

[netblindpaul]

I believe it comes from the requirement to isolate all live conductors on a TT install.
However, this is not explicit wrt disconnection in the event of a fault, only for isolation in the regs.
It is open to interpretation.
This is due to the fact that the N on a TT may not be at earth potential.
There are times when the N of a TN especially a TN-C-S may not be at earth potential, but in the regs this is considered less of a chance.

 

[telectrix]

so, paul, if you were installing a RCBO board on a TT system, would you then specify 2 pole RCBos, or rely on the main switch for 2 pole isolation?

the reason i ask is: if working on a circuit, you may want to isolate that one circuit while working rather than having to isolate the whole board.

 

[netblindpaul]

Given the choice I would always use 2 pole protective devices on 1ph & 3 or 4 pole on 3ph final circuits however...

Now if we are talking domestic, unless you have someone at home on life support there is no reason to work on a domestic install with the power on to the board, you should be able to isolate the board.

BTW if you have someone at home on life support then there will be other things to consider!!!

Come commercial/industrial, then other issues come into play.

However, it is never this cut & dried.

I have trawled through the regs and I believe that only 1 pole protection is required.
However 2 pole isolation IS required in the regs.

In a domestic situation, what is wrong with killing the whole board & disconnecting the L&N from the board internally, making safe and re-energising. Work on the circuit and then undertake the converse?

 

[ackbarthestar]

Many modern RCBOs have dedicated outgoing neutrals, try and cross them and see what happens to the RCBOs of the two crossed circuits.
The only common reference is the earthing bar.

The double pole isolation factor would be applicable to certain types of installation. However, as Telectrix states they are useful for independent final circuit isolation.