RCBOs vs RCDs Pt2

[Deleted member 26818]

Here's my understanding of diverted neutral currents:
These are currents caused by unbalanced 3 Phase loads, or when there's a supply neutral fault.
In any event these currents are always present on both N & E, due to the fact that the majority of the UK distribution is TN-C.
Not too sure how the diverted neutral currents can originate from one particular installation?

 

[davesparks]

Neutral currents exist in all single phase installations and any unbalanced three phase installation.

As SP RCBOs are being discussed then we are almost certainly looking at unbalance TP or single phase.

So there is a neutral current which normally returns to the cutout via the neutral conductors.
A fault occurs between N-E on a final circuit.
This presents another route back to the cutout via the E conductors.
Part of the neutral current of the installation will divert through the fault instead of through the normal neutral path.

This is not the same as the neutral currents which become diverted through external faults on a TNCS supply.

The term 'diverted neutral current' does not refer to one specific scenario, any neutral current can be diverted away from its normal path.

 

[Deleted member 26818]

I do understand how part of the current will flow along the E, which will cause the RCBO to operate.
The thing is, that as the RCBO has operated, there should be no more current to be diverted, so how is this a danger or fire risk?

 

[davesparks]

I do understand how part of the current will flow along the E, which will cause the RCBO to operate.
The thing is, that as the RCBO has operated, there should be no more current to be diverted, so how is this a danger or fire risk?

But it has only disconnected the L, the neutral is still connected straight through to the neutral bar so neutral current from every circuit in the installation will partially divert through back through that neutral and to earth via the fault

 

[Deleted member 26818]

That neutral current would be present on the earth conductors anyway, if the system is TN-C-S.
This is why we are required to have 10mm bonding conductors with PME.

 

[davesparks]

That neutral current would be present on the earth conductors anyway, if the system is TN-C-S.
This is why we are required to have 10mm bonding conductors with PME.

No it wouldn't, otherwise we would be installing 10mm cpcs in every circuit!

 

[Deleted member 26818]

I'm afraid it is.
the reason why we don't require 10mm CPC is because whilst there will be diverted current on the CPCs, it does not flow anywhere.
Whereas with bonding conductors there will be current flowing from the extraneous conductive-parts to the installation MET and vice versa.

 

[davesparks]

I'm afraid it is.
the reason why we don't require 10mm CPC is because whilst there will be diverted current on the CPCs, it does not flow anywhere.
Whereas with bonding conductors there will be current flowing from the extraneous conductive-parts to the installation MET and vice versa.

But with a N-E fault it will divert through the fault and down the cpc to earth.

 

[Deleted member 26818]

But with a N-E fault it will divert through the fault and down the cpc to earth.

What current?
The N & E are already connected on the distribution side, so where is this current coming from to flow up the N to the fault then back down the E to meet the N & E link on the distribution side?
Rememeber there will only be current flow if there is a difference in potential.
If both conductors already have the same current on them, it won't flow anywhere.
If there's a supply neutral fault, then yes there could be current flow.
Which is where the 10mm bonding conductors come into play.
They provide a path via whatever services they are connected to for the installation's neutral current

 

[davesparks]

The neutral current will divert through the fault to earth in accordance with the laws of resistances in parallel.

 

[Deleted member 26818]

Take one bit of 2 core flex, connect both cores at one end to a neutral, then connect the other ends of the two cores together.
What current will flow?
Where will it flow to.
That is essentially what you have with the N & E linked on the distribution side, and your N-E fault in the installation.

 

[ImpededLoop]

The neutral current will divert through the fault to earth in accordance with the laws of resistances in parallel.

I would agree with this.

However, how many circuits are there in existence that are not RCD protected but have had neutral to earth faults for years with no apparent effects?

A single pole RCBO would hopefully provide protection to somebody receiving an electric shock, so I think their use is beneficial, but not as good as a device that disconnects all live conductors.

 

[davesparks]

Take one bit of 2 core flex, connect both cores at one end to a neutral, then connect the other ends of the two cores together.
What current will flow?
Where will it flow to.
That is essentially what you have with the N & E linked on the distribution side, and your N-E fault in the installation.

No it isn't, the whole of the installation is connected into it too.

In your example you would have to connect the neutral side of a load to one of the cores halfway along its length to simulate this. The neutral current from the load will flow to neutral directly down the shorter path and through the longer path going to the far end of the flex and back to neutral.

The ratio will be determined by resistance of each path

 

[davesparks]

I would agree with this.

However, how many circuits are there in existence that are not RCD protected but have had neutral to earth faults for years with no apparent effects?

A single pole RCBO would hopefully provide protection to somebody receiving an electric shock, so I think their use is beneficial, but not as good as a device that disconnects all live conductors.

And how many have had disastrous effects? How many times have fires started from N-E faults?

You have only seen the ones which haven't caused a problem because the ones which have caused a problem have been fixed or burned out already.

Obviously you rarely see the ill effects of them in domestics because there is naff all neutral current to start with !

 

[ImpededLoop]

And how many have had disastrous effects? How many times have fires started from N-E faults?

I doubt anybody really knows.

Obviously you rarely see the ill effects of them in domestics because there is naff all neutral current to start with !

I've never seen it in commercial either, but perhaps I've been lucky.

 

[davesparks]

I doubt anybody really knows.



I've never seen it in commercial either, but perhaps I've been lucky.

Nobody will know, but that's no excuse for not understanding the simple theory and choosing the best protection based on that.

 

[Deleted member 26818]

No it isn't, the whole of the installation is connected into it too.

In your example you would have to connect the neutral side of a load to one of the cores halfway along its length to simulate this. The neutral current from the load will flow to neutral directly down the shorter path and through the longer path going to the far end of the flex and back to neutral.

The ratio will be determined by resistance of each path

What does this diverted neutral current do when it meets the diverted neutral current coming from the N & E link up the CPC to the N-E fault?

 

[davesparks]

What does this diverted neutral current do when it meets the diverted neutral current coming from the N & E link up the CPC to the N-E fault?

Wtf?

Current doesn't flow from the cutout up the cpc to the fault!
It flows via the fault then through the cpc back to the cutout

 

[Deleted member 26818]

Why doesn't it?
If there was no fault it would, so why doesn't now that there is a fault?

 

[davesparks]

Why doesn't it?
If there was no fault it would, so why doesn't now that there is a fault?

Current flowing through the cpc from the cutout in normal operation ???
Where did you get that one from?

The only thing flowing in a cpc under normal conditions will be leakage current from smps's and similar.