RCBOs vs RCDs Pt2

[rodgearing]

OK we are told RCBOs are the way to go.
Agreed.
Because this will do away with using a RCD but, and I guess this has been questioned and possibly answered before, why buy an RCBO that only protects phase side (single pole RCBO) of the cct?
Have I missed something?
Do we put in one RCD!?
Fred

 

[Octopus]

Why would you put in 1 RCD only??

 

[Engineer54]

If you're that worried, use SP+N RCBO's that are becoming increasingly available!!

SP RCBO's are not as bad as some make them out to be, we have been using them for a good number of years now, (without any major safety concerns being reported) and SP MCB's and fuses for a damn sight longer!!

 

[telectrix]

double pole RCBOs or single pole with switched N are the best way, but cost is a factor.

 

[rodgearing]

Why would you put in 1 RCD only??

It was just a statement
Does one put any RCDs in a unit that has all SP RCBOs?

 

[Deleted member 9648]

OK we are told RCBOs are the way to go.
Agreed.
Because this will do away with using a RCD but, and I guess this has been questioned and possibly answered before, why buy an RCBO that only protects phase side (single pole RCBO) of the cct?
Have I missed something?
Do we put in one RCD!?
Fred

It's the phase side that bites,thats why.

 

[telectrix]

It was just a statement
Does one put any RCDs in a unit that has all SP RCBOs?

only if you want a 100mA type S upfront.

 

[richy3333]

Why would you put in 1 RCD only??

Agreed and isn't Amend 3 stopping this?

 

[ipf]

Agreed and isn't Amend 3 stopping this?

Tell you when it arrives on my desk..

 

[Knobhead]

If you're that worried, use SP+N RCBO's that are becoming increasingly available!!

SP RCBO's are not as bad as some make them out to be, we have been using them for a good number of years now, (without any major safety concerns being reported) and SP MCB's and fuses for a damn sight longer!!

You’ve changed your tune.

The trouble is the SP RCBO just don’t match what it says on the tin. For this reason I think many have accepted them as an instant cure all in every situation. This can only be attributed to lack of knowledge on the part of the electrician. (The ones that bite my head off now will be the ones in the know.)

Some manufacturers do point out the short comings of the SP RCBO in handling a N→E fault. To my mind it should be made clear to everyone. As we’ve often discussed it seems to be the doctrine of colleges to preach fitting RCD’s to everything.

 

[Deleted member 9648]

You’ve changed your tune.

The trouble is the SP RCBO just don’t match what it says on the tin. For this reason I think many have accepted them as an instant cure all in every situation. This can only be attributed to lack of knowledge on the part of the electrician. (The ones that bite my head off now will be the ones in the know.)

Some manufacturers do point out the short comings of the SP RCBO in handling a N→E fault. To my mind it should be made clear to everyone. As we’ve often discussed it seems to be the doctrine of colleges to preach fitting RCD’s to everything.

A single pole MCB wont react to a N-E fault. To my mind the RCD element of an RCBO on a TN system is likely to be additional protection only,not fault protection.

 

[davesparks]

A single pole MCB wont react to a N-E fault. To my mind the RCD element of an RCBO on a TN system is likely to be additional protection only,not fault protection.

An mcb is not designed to react to N-E faults so there is no issue there.
It reacts to L-L, L-N or L-E faults by disconnecting the L or Ls

A SP RCBO does react to N-E faults, but the way it reacts is to disconnect the L which has no fault in it.
This is the problem.

 

[Deleted member 9648]

An mcb is not designed to react to N-E faults so there is no issue there.
It reacts to L-L, L-N or L-E faults by disconnecting the L or Ls

A SP RCBO does react to N-E faults, but the way it reacts is to disconnect the L which has no fault in it.
This is the problem.

None of which really matters if the RCBO is simply used as an MCB with additional protection.

 

[davesparks]

None of which really matters if the RCBO is simply used as an MCB with additional protection.

What on earth are you talking about?

If an mcb is needed you use it as an mcb.

If RCD protection is needed in addition you either install an RCCB before or after the mcb or an RCBO to provide both functions in one.

Whatever reason you install an RCBO for then if it is SP it will detect N-E faults and not disconnect them, it will disconnect the L.

 

[Deleted member 26818]

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

 

[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.