Earthing... yes another thread

[EFLImpudence]

So it would seem that the same piece of wire can be either a bond or an "earth" depending on which fault occurs to affect the bath.

Not really. Perhaps if you were to purposely touch the bath with a live conductor.

If there is a LOW impedance fault connection between a live conductor and the bath then the wire is an EARTH that carries enough current to trip an over current device.

The bath, which is not part of the electrical installation, is unlikely to come into contact with a live conductor.
Should you have a bare live conductor left under the bath which is likely to 'curl up' with the heat or moved by a mouse then you may consider it advisable to 'earth' the bath even if it does not need 'bonding'.

If there is a HIGH impedance fault connection between a live conductor and the bath then the wire is an BOND that carries enough current to keep the bath at the same potential as other metal work in the bathroom. This current may not be enough to trip an RCD

No, if you consider the HIFC directly between the live conductor and the bath likely that would still be 'earthing' - had you considered it necessary even though 'bonding' was not required.

The purpose of 'bonding' is to equalise potential between simultaneously accessible extraneous conductive parts in the bathroom when a fault occurs elsewhere (not at the bath).
The purpose of 'earthing' is to cause the OPD to operate in the event of a fault in an accessory or appliance.

 

[JohnW2]

The purpose of 'bonding' is to equalise potential between simultaneously accessible extraneous conductive parts in the bathroom when a fault occurs elsewhere (not at the bath).

What is perhaps a little more difficult (at least for me) to understand about the 'purpose' of supplementary bonding is the requirement that, if such bonding is undertaken, that those parts must also be connected ('bonded') to the CPCs of the installation, even if there are no exposed-conductive-parts (indeed, maybe nothing electrical at all, apart from lighting far too high to touch) in the bathroom. How does that fit with your statement of the 'purpose'? ...in teh sort of situation I've described, it sounds almost like a requirement to 'earth the bonding' !

Kind Regards, John.

 

[Electrifying]

The purpose of 'bonding' is to equalise potential between simultaneously accessible extraneous conductive parts in the bathroom when a fault occurs elsewhere (not at the bath).

What is perhaps a little more difficult (at least for me) to understand about the 'purpose' of supplementary bonding is the requirement that, if such bonding is undertaken, that those parts must also be connected ('bonded') to the CPCs of the installation, even if there are no exposed-conductive-parts (indeed, maybe nothing electrical at all, apart from lighting far too high to touch) in the bathroom. How does that fit with your statement of the 'purpose'? ...in teh sort of situation I've described, it sounds almost like a requirement to 'earth the bonding' !

Kind Regards, John.

I wonder if that's where the term 'earthed equipotential bonding comes from.

I don't know why you always seem to find a problem understanding earthing/bonding.

In the scenario that you describe with just lighting in the bathroom, the taps, due to main bonding, could rise in potential under fault conditions.

Additionally, a radiator in the bathroom could also rise in potential during the same fault - but there could be a difference in potential between the taps and the radiator.

With RCD protection the fault would be cleared quick enough to prevent danger.

Without RCD protection, we supplementary bond the bathroom metalwork to ensure that there is no difference in potential.
Connecting to the earthing of any bathroom circuits ensures that 'everything' in the bathroom remains at the same potential.

I really don't see the difficulty.

 

[bernardgreen]

With RCD protection the fault would be cleared quick enough to prevent danger.

Not true..... The RCD only operates to disconnect when there is a fault current flowing that creates un-balance in the Live and Neutral currents through the RCD sensor. The danger exists from the moment two things that can be touched at the same time are at different potentials. The RCD MAY operate when that danger becomes an active danger ( current flowing through a person or other route ). While the RCD may react in 0.4 of a second the person taking the current is likely to react quicker and while an electric shock of less than 0.4 seconds duration may not be fatal the muscle spasms may lead to a fall or spillage that causes injury.

The basic is that a functioning RCD will only reduce the the duration of an electric shock to what is considered an "acceptable" duration, A functioning RCD does NOT prevent electric shock to the person. It operates on the un-balance that results from the fault current passing through the person's body.

Without RCD protection, we supplementary bond the bathroom metalwork to ensure that there is no difference in potential.

Working on the known fact that an RCD may fail to operate when needed then isn't it sensible to treat the installation as if there is no RCD and therefore bond in order to prevent prolonged, possibly fatal, shock in the event that the RCD does fail to operate.

 

[JohnW2]

I don't know why you always seem to find a problem understanding earthing/bonding..... Without RCD protection, we supplementary bond the bathroom metalwork to ensure that there is no difference in potential.

Agreed - and my understanding is that such is the whole (and only) point of the bonding. Indeed, as one would expect, the regs (e.g. 415.2) call it "Supplementary Equipotentional Bonding", reflecting that (only) purpose.

Connecting to the earthing of any bathroom circuits ensures that 'everything' in the bathroom remains at the same potential. .... I really don't see the difficulty.

You don't? Taking my example (only untouchable ceiling lighting), if all the relevant exposed metalwork in the bathroom is bonded together, then the purpose of bonding is seemingly satisfied, in that one has a local equipotential zone. In what sense does also earthing that bonded metalwork improve that situation and increase safety?

Kind Regards, John.

 

[Electrifying]

With RCD protection the fault would be cleared quick enough to prevent danger.

Not true..... The RCD only operates to disconnect when there is a fault current flowing that creates un-balance in the Live and Neutral currents through the RCD sensor. The danger exists from the moment two things that can be touched at the same time are at different potentials. The RCD MAY operate when that danger becomes an active danger ( current flowing through a person or other route ). While the RCD may react in 0.4 of a second the person taking the current is likely to react quicker and while an electric shock of less than 0.4 seconds duration may not be fatal the muscle spasms may lead to a fall or spillage that causes injury.

The basic is that a functioning RCD will only reduce the the duration of an electric shock to what is considered an "acceptable" duration, A functioning RCD does NOT prevent electric shock to the person. It operates on the un-balance that results from the fault current passing through the person's body.

Without RCD protection, we supplementary bond the bathroom metalwork to ensure that there is no difference in potential.

Working on the known fact that an RCD may fail to operate when needed then isn't it sensible to treat the installation as if there is no RCD and therefore bond in order to prevent prolonged, possibly fatal, shock in the event that the RCD does fail to operate.

Firstly, a 30mA RCD will operate in under 0.04 Seconds, not 0.4 Seconds.

Secondly, if there were a potential on bathroom taps / radiator, this will have arisen from an earth fault within the installation, which would cause an imbalance between line and neutral and trip the rellevant RCD - it doesn't have to flow through a person.

Obviously, the powers that be deem it safe enough to omit 'supplemetary bonding' if main bonding is in place and the circuits are protected by a 30 mA RCD.

 

[JohnW2]

Working on the known fact that an RCD may fail to operate when needed then isn't it sensible to treat the installation as if there is no RCD and therefore bond in order to prevent prolonged, possibly fatal, shock in the event that the RCD does fail to operate.

I have to say that it's very easy to sympathise with that argument. Given that RCDs do sometimes fail to operate (some say quite commonly), it could be said that it is a little odd that the regs now allow one to omit supplementary bonding in the presence of an RCD but still (fairly reasonably) require such bonding in the absence of an RCD. The regs are putting an awful lot of faith in a bit of technology which some believe is not all that reliable.

Also echoing Bernard's comment, whilst supplementary bonding should prevent any electric shock occurring, an RCD merely limits the duration of the shock (which could involve a high current) to a level which will not be directly lethal in most people. However, even if not directly lethal, it would certainly be unpleasant, and could result in serious or fatal secondary accidents (e.g. from falls) - not to mention that, even with a functioning RCD, the shock itself could fatal in a susceptible individual; under some circumstances, a few milliseconds is enough. I've been brought up with the idea that prevention is always preferable to allowing something to happen and then trying to deal with it (particularly if the means of 'trying to deal with it' is not 100% reliable).

Kind Regards, John.

 

[bernardgreen]

Firstly, a 30mA RCD will operate in under 0.04 Seconds, not 0.4 Seconds.

That is only true if there is a 30 mA unbalance for the entire first half cycle of the mains waveform. This is likely if the fault is a low impedance short circuit Live to Earth.

In the case of electric shock to a person the person's impedance may be high for first few half cycles and thus not trip the RCD until electro chemical changes in the body reduce the impedance.

Secondly, if there were a potential on bathroom taps / radiator, this will have arisen from an earth fault within the installation, which would cause an imbalance between line and neutral and trip the rellevant RCD - it doesn't have to flow through a person.

NO The fault may be present but lacking the connection between a falsely live item and earth. Without the connection there is no current. Only when that connection is put in place to allow fault current to flow can the RCD operate.

Obviously, the powers that be deem it safe enough to omit 'supplemetary bonding' if main bonding is in place and the circuits are protected by a 30 mA RCD.

Maybe they do, maybe there are other "considerations" that they have considered as more important than safety. To me common sense is that if there is a risk of two items being at different potentials which then creates a hazard that risk must be remove by bonding to ensure there is a fault current that the RCD can see and then operate.

 

[EFLImpudence]

This is all bypassing the main point of the thread (never done that before).

Should anyone wish to apply supplementary bonding because RCDs are not trusted then there is nothing, nor any regulation, to say you may not, but only where required.

But it is not earthing.

It will be there to equalise potential between simultaneously accessible exposed and extraneous conductive parts in one location - if necessary - not to operate the cpd.

Should you wish to 'earth' a metal bath (which does not need 'bonding') because there is an exposed live conductor flailing about underneath then, surely, the cpc associated with that conductor should be used. This may indicate the futility of the exercise - connecting the bath to the electrical installation unnecessarily.

Re above - If the light fitting is the only electrical part in the room someone could stand on the bath to reach it.
If it is 'bonded' by the pipework then we have no choice.
However, if it is unnecessarily 'earthed' it will be more dangerous.

 

[JohnW2]

Should anyone wish to apply supplementary bonding because RCDs are not trusted then there is nothing, nor any regulation, to say you may not, but only where required.

Agreed. I was merely saying that it's fairly easy to sympathise with the point Bernard was making about the surprising lack of caution in the regs. Those regs say that if there is no RCD protection, then the bathroom is not safe without supplementary bonding, yet seem to feel that it is safe with an RCD, even though many people feel that RCDs are significantly unreliable.

But it is not earthing.

Suplementary bonding of extraneous-conductive parts to one another is certainly not earthing - it is simply bonding to equalise the potential between those parts. However, as far as I can make out, 701.451.2 requires that such bonding also be connected to the installation's CPCs even if there are no exposed-conductive parts in the room. I find it hard to regard that connection as 'bonding' in any sense I understand - and the only way I really can describe it is as 'earthing the supplementary equipotential bonding'.

Re above - If the light fitting is the only electrical part in the room someone could stand on the bath to reach it. If it is 'bonded' by the pipework then we have no choice. However, if it is unnecessarily 'earthed' it will be more dangerous.

I'm not sure which 'it' you are talking about. If the light fitting has no exposed-conductive parts (e.g. if it's Class II), I can't see what you would be concerned about touching, even if you were standing on the bath.

To be clear - I am not personally advocating unnecessary bonding, but I do understand why some people consider it as surprising that the regs (a) consider it safe to omit such bonding when there is RCD protection, but not if there's no RCD and (b) appear to require equipotential bonding to be connected to the electrical installations's CPCs even if there are no exposed-conductive-parts in the room.

Kind Regards, John.

 

[Electrifying]

Bernard, if you are going to make these statements as if they were fact (which I dispute), then please try to get the actual figures right.

Firstly, a 30mA RCD will operate in under 0.04 Seconds, not 0.4 Seconds.

That is only true if there is a 30 mA unbalance for the entire first half cycle of the mains waveform.

The RCD will trip in less than 0.04 sec at 5I (150mA) - not 30mA as you are implying.

This is likely if the fault is a low impedance short circuit Live to Earth.

So, you class 1533 ohms as 'low impedance'?

In the case of electric shock to a person the person's impedance may be high for first few half cycles and thus not trip the RCD until electro chemical changes in the body reduce the impedance.

This is only applicable if the person comes into contact with a live part.
If they come into contact with earthed/bonded metalwork under fault conditions, the RCD will clear the fault before the damage is done - that's why it's called 'additional protection'.

Secondly, if there were a potential on bathroom taps / radiator, this will have arisen from an earth fault within the installation, which would cause an imbalance between line and neutral and trip the rellevant RCD - it doesn't have to flow through a person.

NO The fault may be present but lacking the connection between a falsely live item and earth. Without the connection there is no current. Only when that connection is put in place to allow fault current to flow can the RCD operate.

How is it possible for this to happen.....if all metalwork is 'exposed' (earthed) or 'extraneous' (bonded) - What on earth is a 'falsely live' item??

You do realise that beginners and DIYers read these posts.......what are you on??

Obviously, the powers that be deem it safe enough to omit 'supplemetary bonding' if main bonding is in place and the circuits are protected by a 30 mA RCD.

Maybe they do, maybe there are other "considerations" that they have considered as more important than safety. To me common sense is that if there is a risk of two items being at different potentials which then creates a hazard that risk must be remove by bonding to ensure there is a fault current that the RCD can see and then operate.

EFLI has pointed out to you a couple of times that 'bonding' has nothing to do with 'clearing the fault' - yet you still insist on stating things like in your last paragraph above.

Can we at least try and stick to the facts.

 

[EFLImpudence]

Should anyone wish to apply supplementary bonding because RCDs are not trusted then there is nothing, nor any regulation, to say you may not, but only where required.

Agreed. I was merely saying that it's fairly easy to sympathise with the point Bernard was making about the surprising lack of caution in the regs. Those regs say that if there is no RCD protection, then the bathroom is not safe without supplementary bonding, yet seem to feel that it is safe with an RCD, even though many people feel that RCDs are significantly unreliable.

The powers that be deem an installatiion 'safe' if the disconnection times can be met.
That is if the Zs is below the maximum value to achieve this.
Effectively the Zs for an RCD is 1666 Ohms - unlikely not to be achieved.

However, this is still an earthing issue (not bonding), i.e. disconnecting supply by opd, albeit being done by an RCD.
Not equalising the potential in the event of a fault Edit - to earth.

I think you are forgetting that even without an RCD supplementary bonding may not be necessary if certain conditions are met: i.e. R<50V/Ia so with an RCD they (conditions) are all but bound to be met.

But it is not earthing.

Suplementary bonding of extraneous-conductive parts to one another is certainly not earthing - it is simply bonding to equalise the potential between those parts. However, as far as I can make out, 701.451.2 requires that such bonding also be connected to the installation's CPCs even if there are no exposed-conductive parts in the room. I find it hard to regard that connection as 'bonding' in any sense I understand - and the only way I really can describe it is as 'earthing the supplementary equipotential bonding'.

Other than noting that the regulations state the bonding must be connected to the 'circuits of the location' I agree it is odd.

Re above - If the light fitting is the only electrical part in the room someone could stand on the bath to reach it(light). If it(bath) is 'bonded' by the pipework then we have no choice. However, if it(bath) is unnecessarily 'earthed' it(bath or general situation) will be more dangerous.

I'm not sure which 'it' you are talking about. If the light fitting has no exposed-conductive parts (e.g. if it's Class II), I can't see what you would be concerned about touching, even if you were standing on the bath.

Sorry, poorly written - see brackets.

To be clear - I am not personally advocating unnecessary bonding, but I do understand why some people consider it as surprising that the regs (a) consider it safe to omit such bonding when there is RCD protection

as above.

but not if there's no RCD and (b) appear to require equipotential bonding to be connected to the electrical installations's CPCs even if there are no exposed-conductive-parts in the room.

as above. I don't know.

 

[JohnW2]

The powers that be deem an installatiion 'safe' if the disconnection times can be met. That is if the Zs is below the maximum value to achieve this. Effectively the Zs for an RCD is 1666 Ohms - unlikely not to be achieved.

No argument with any of that - but that assumes that the RCD works as intended. Given the fact that the regs often seem to consider pretty improbable risk situations, it seems a little surprising that they are prepared to rely on the function of a device which many regard as fairly unreliable in order to remove the need for safety measures which would (probably) be required in the absence of the device.

However, this is still an earthing issue (not bonding), i.e. disconnecting supply by opd, albeit being done by an RCD. Not equalising the potential in the event of a fault.

You are talking to the wrong person here - I have never suggested otherwise.

Other than noting that the regulations state the bonding must be connected to the 'circuits of the location' I agree it is odd.

Exactly my point. If there are no exposed-conductive-parts in the location, it makes no sense to regard that connection as 'bonding'. Like it or not, I think there's little escaping the fact that, in such a situation, the regs are requiring one to earth the supplementary bonding - and for no useful reason that I can think of.

Kind Regards, John.

 

[RF Lighting]

Exactly my point. If there are no exposed-conductive-parts in the location, it makes no sense to regard that connection as 'bonding'. Like it or not, I think there's little escaping the fact that, in such a situation, the regs are requiring one to earth the supplementary bonding - and for no useful reason that I can think of.

Kind Regards, John.

No they are not.

They are requireing you to bond the CPCs as they may be at a different potential to other metalwork in the bathroom.

Wether they are earthed or happen to earth the bonding is purely coincidental.

 

[JohnW2]

Like it or not, I think there's little escaping the fact that, in such a situation, the regs are requiring one to earth the supplementary bonding - and for no useful reason that I can think of.

No they are not. They are requireing you to bond the CPCs as they may be at a different potential to other metalwork in the bathroom.

Hmmmm. As you wrote yourself earlier in the thread, the purpose of bonding is:

It will be there to equalise potential between simultaneously accessible exposed and extraneous conductive parts in one location

If there are no exposed-conductive-parts (i.e. if there is no exposed metal which is connected to the CPCs) in the location, what on earth (sorry would be be the point in bonding the CPCs?

Kind Regards, John.