RCBOs vs RCDs Pt2

[davesparks]

Is disconnecting the L on a N-E fault a problem?

It's completely pointless, you might as well have a device which switches the gas off when the water main bursts!

It could also give a false sense of security as someone might assume that since a protective device has operated whatever fault is present will have been disconnected and so not worry about investigating in a hurry. But a N-E fault will still be connected which could present a massive fire risk through diverted neutral currents

 

[Deleted member 26818]

Wouldn't the N & E already have those diverted neutral currents present if the system is TN-C-S?

 

[davesparks]

Wouldn't the N & E already have those diverted neutral currents present if the system is TN-C-S?

No, the current would only divert through the fault once the fault occurs!

 

[Deleted member 26818]

I think I need you to explain where these diverted neutral currents originate.

 

[davesparks]

I think I need you to explain where these diverted neutral currents originate.

The whole installation!

 

[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.