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.

 

[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.

 

[Engineer54]

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.

How have i changed my tune??

 

[Deleted member 26818]

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.

I give up.

 

[Geoffsd]

It's similar to this-

 

[davesparks]

I give up.

Have a look at the picture Geoff posted, it's not that clear but does show the diverted current

 

[Deleted member 26818]

Have a look at the picture Geoff posted, it's not that clear but does show the diverted current

I know how it works, just having difficulty making you understand.

 

[davesparks]

I understand perfectly well thankyou, I'm not sure you do.

Neutral current from the whole installation diverts through an N-E fault on a circuit.

Current does not normally flow through a cpc

 

[Deleted member 26818]

I understand perfectly well thankyou, I'm not sure you do.

Neutral current from the whole installation diverts through an N-E fault on a circuit.

Current does not normally flow through a cpc

Like I said, I give up.

 

[Engineer54]

Makes you wonder how we ever managed to get by before the introduction of RCD's, or even before MCB's/MCCB's when fuses were basically the sole means of protection from Main switchboards all the way down to final circuits....

OK, maybe on main switchboards, they could well of had early earth fault relay protection, but then we wouldn't be talking in terms of 30mA or anything close to it!!

Perhaps N-E faults didn't exist in those days!!...lol!!

 

[davesparks]

We got by fine without them, just as we can get by fine without them today if we insisted on better installation standards and stopped treating RCDs as a one size fits all soloution to poor installation work.

Of course neutral faults have always existed, and they will always occur.
But was TNCS, and particularly the PME subdivision of it, so common in those days?

My issue is with the concept of a device which detects a fault on the neutral then reacts by disconnecting the live.
Yes it satisfies bs7671 in terms of protection, but isn't bs7671 supposed to be the minimum standard of acceptability rather than the be all and end all.

 

[Engineer54]

We got by fine without them, just as we can get by fine without them today if we insisted on better installation standards and stopped treating RCDs as a one size fits all soloution to poor installation work.

Of course neutral faults have always existed, and they will always occur.
But was TNCS, and particularly the PME subdivision of it, so common in those days?

My issue is with the concept of a device which detects a fault on the neutral then reacts by disconnecting the live.
Yes it satisfies bs7671 in terms of protection, but isn't bs7671 supposed to be the minimum standard of acceptability rather than the be all and end all.

I'm all for SP+N RCBO's, but as i've stated the present standard SP RCBO's aren't all doom and gloom, they are providing a perfectly acceptable means of additional protection to SP circuits.

Public distribution PME supplies have been around for a lot longer than most imagine, certainly from the middle 1940's with a big expansion in the 50's, 60's, 70's especially in the semi rural areas on the outskirts of towns etc...