16th Ed Theory: Supplementary Equipotential Bonding back to MET not allowed?

[eveares]

Putting aside the fact that Supplementary Equipotential Bonding is often not needed these days in accordance with the 17th edition providing that all circuits in the bathroom are protected by a 30ma RCD, all incoming services are mains bonded back to the MET, and that all circuits meet the require disconnection times; I have the following question:

Assuming a bathroom is wired to the 16th edition and that Supplementary Equipotential Bonding is required; from what I can find online, your not supposed/allowed to link the bathrooms Supplementary Equipotential Bonding directly back to the MET.

Now providing you link/bond all extraneous conductive pipe work and parts in the bathroom to the CPC's of all the circuits/applnces in the bathroom using 4mm solid core, thus ensuring they are at the same potential; why is it dangerous and thus presumable not allowed to at the same time take this 4mm bonding conductor back to the MET as well?

I'm struggling to think how a potential difference between two points in the bathroom may occur in this situation as the CPC of the bathroom circuits and the extraneous conductive pipe work would be at the same potential as they would be directly linked??

All I can think of is that you would get a difference in potential between the 4mm bonding conductor going back to the MET and the bathrooms circuits/applainces CPC's, even though they would be directly linked together in the bathroom.

Also this situation would also occur if the mains water stop cock enters the property in the bathroom as well as you would have main and supplementary bonding in the bathroom, what are you supposed do do then!?

Can anyone please clarify my senario?

Reagrds: Elliott

 

[Lectrician]

In the 16th, we had a site with 20 odd identical/mirror image flats. The bathroom backed the airing cupboard. The consumer unit was in the airing cupboard. The Immersion cylinder was in AC. All pipe work copper. Bathrooms had light, heater circuit, most had showers, or were having them fitted.

The regs used to say to bond "in close proximity" I recall.

When adding showers to these flats, we took a 4mm from CU, and just bonding the hot and cold in the airing cupboard. This was after consultation with the NIC.

 

[Adam_151]

As above, There is nothing to say it *can't* go back to the MET, but

a) It doesn't need to, and in most cases this is a waste of copper / labour
b) It does need to pick of the CPCs of the final circuits close to the bathroom. Going back to the MET does not avoid the need for this. In LECs case the CU was in close proximetry to the bathroom, but this is only true of a small number of properties.

 

[EFLImpudence]

Assuming a bathroom is wired to the 16th edition and that Supplementary Equipotential Bonding is required; from what I can find online, your not supposed/allowed to link the bathrooms Supplementary Equipotential Bonding directly back to the MET.

It's not that you are not supposed/allowed to link the bathrooms Supplementary Equipotential Bonding directly back to the MET,
It depends what you mean. Supplementary Bonding is to link parts in the room together. There is no point doing this with many metres of separate cable which would have to be quite thick to achieve a low enough impedance.
They should already be effectively connected to the MET; if not, they there may be something wrong with the Main Bonding.

Now providing you link/bond all extraneous conductive pipe work and parts in the bathroom to the CPC's of all the circuits/applnces in the bathroom using 4mm solid core, thus ensuring they are at the same potential; why is it dangerous and thus presumable not allowed to at the same time take this 4mm bonding conductor back to the MET as well?

It's not dangerous; just not needed.

I'm struggling to think how a potential difference between two points in the bathroom may occur in this situation as the CPC of the bathroom circuits and the extraneous conductive pipe work would be at the same potential as they would be directly linked??

If you mean directly linked by the SB then why would you want to run a conductor back to the MET?

All I can think of is that you would get a difference in potential between the 4mm bonding conductor going back to the MET and the bathrooms circuits/applainces CPC's, even though they would be directly linked together in the bathroom.

You would not get a potential if the parts are connected.

Also this situation would also occur if the mains water stop cock enters the property in the bathroom as well as you would have main and supplementary bonding in the bathroom, what are you supposed do do then!?

Well, the stop cock is irrelevant and nothing to do with the positioning of the MB; the pipe should be bonded where it enters the premises, where practicable.
If this is in the bathroom and SB links other parts then you likely are not supposed to do anything else.

Supplementary Bonding is to reduce potential between parts of the location (bathroom); it doesn't matter what is happening elsewhere.

 

[bernardgreen]

You would not get a potential if the parts are connected.

There could be a potential difference with a very high current through the connection. Probably less than a volt but with a resistance of 0.01 Ω a potential difference of 1 volt will generate a current of 100 amps.

Fires have been started when the bond wires connecting PME METs to metallic water supply pipes have carried enough current to have been glowing red hot. ( Network Neutral open circuit so bond and water pipes provide route for neutral current to sub station )

 

[EFLImpudence]

You would not get a potential if the parts are connected.

There could be a potential difference with a very high current through the connection. Probably less than a volt but with a resistance of 0.01 Ω a potential difference of 1 volt will generate a current of 100 amps.

Shouldn't that be the other way round?
If there is only 1V then the reason for supplementary bonding (ensure <50V until disconnection) would seem to have been achieved.

Fires have been started when the bond wires connecting PME METs to metallic water supply pipes have carried enough current to have been glowing red hot. ( Network Neutral open circuit so bond and water pipes provide route for neutral current to sub station )

Is that the reason supplementary bonding is applied?

Are you not doing what you usually do in citing an occurrence where it would be better to have NO bonding or earthing?

 

[bernardgreen]

Are you not doing what you usually do in citing an occurrence where it would be better to have NO bonding or earthing?

What I am doing is considering the problems that the bonding / earthing arrangement has to cope with when there are faults external to the property. In the case of a TT supply with RCD protection on all circuits ( either RCBOs or circuits protected by one or RCDs ) there is no way that there can be high currents in any of the earth or bond cables. Provided the bond / earth conductors can carry the trip current ( 30 mA ) of th RCD they can be small cross section.. In PME systems where the MET is in fact the Neutral the bond cables have to be able to carry very high current that can be caused a fault in the Neutral external to the property. ( metal theft at the sub station for example ) Hence bond cable of 10 mm² cross section area are specified.

It is never better to have no bonding or earthing but is is necessary to design the bonding / earthing arrangment in view of wht can happen inside or external to the property.

 

[EFLImpudence]

Are you not doing what you usually do in citing an occurrence where it would be better to have NO bonding or earthing?

What I am doing is considering the problems that the bonding / earthing arrangement has to cope with when there are faults external to the property.

Yes, but that is not a consideration for supplementary bonding but why the main bonding should be large enough to carry the expected current.

In the case of a TT supply with RCD protection on all circuits ( either RCBOs or circuits protected by one or RCDs ) there is no way that there can be high currents in any of the earth or bond cables. Provided the bond / earth conductors can carry the trip current ( 30 mA ) of th RCD they can be small cross section..

So are you suggesting it would be better to have high impedance earth connections to reduce lost neutral current in the main bond with the consequent loss of adequate current to operate the protective device(s)?

In PME systems where the MET is in fact the Neutral the bond cables have to be able to carry very high current that can be caused a fault in the Neutral external to the property. ( metal theft at the sub station for example ) Hence bond cable of 10 mm² cross section area are specified.

Precisely, but that is the main bonding. What do you think would be the effect on the supplementary bonding?

It is never better to have no bonding or earthing

Well, in general that is obviously true, but in some of your extremely unlikely "what if' scenarios it would be, wouldn't it?
It is all a compromise to mitigate the effects of the most likely faults.

but is is necessary to design the bonding / earthing arrangment in view of wht can happen inside or external to the property.

Is that not done?

 

[eveares]

Now providing you link/bond all extraneous conductive pipe work and parts in the bathroom to the CPC's of all the circuits/applnces in the bathroom using 4mm solid core, thus ensuring they are at the same potential; why is it dangerous and thus presumable not allowed to at the same time take this 4mm bonding conductor back to the MET as well?

It's not dangerous; just not needed.

That's what I suspected, and I can see why it would be dangerous if you took the supplementary bonding back to the MET without also bonding to the CPC's of the circuits/appliances in the bathroom as you could get a significant difference in potential between the CPC's of the bathroom circuits and the bonded extraneous conductive parts.

But by simply taking the 4mm² bonding conductor back to the MET as an added extra in addition to the SEB as Bernard pointed out, the difference in impedance and thus potential between the CPC's/earthed appliances and the bonded extraneous conductive parts is likely to be very minimal as the bonding conductor will likely bring the CPC's/earthed appliances to near the same potential as the bonding conductor it's self that is going back to the MET.

Just a ohms law case really with working out the impedance between two points.

 

[EFLImpudence]

That's what I suspected, and I can see why it would be dangerous if you took the supplementary bonding back to the MET without also bonding to the CPC's of the circuits/appliances in the bathroom

Yes, I suppose so but that wouldn't actually be supplementary bonding which you are taking back to the MET.

as you could get a significant difference in potential between the CPC's of the bathroom circuits and the bonded extraneous conductive parts.

Again, yes but they would not be (supplementary) bonded if they are not connected to the CPCs in the location.

But by simply taking the 4mm² bonding conductor back to the MET as an added extra in addition to the SEB as Bernard pointed out, the difference in impedance and thus potential between the CPC's/earthed appliances and the bonded extraneous conductive parts is likely to be very minimal as the bonding conductor will likely bring the CPC's/earthed appliances to near the same potential as the bonding conductor it's self that is going back to the MET.

If the extraneous and exposed-conductive-parts have negligible (or low enough) impedance between them because of the supplementary bonding then connecting one of them to the MET is not going to alter it.

 

[eveares]

Again, yes but they would not be (supplementary) bonded if they are not connected to the CPCs in the location.

My bad, I should and meant to say if you only mains bonded the extraneous parts in the bathroom. I know that it is not SEB without connecting to the CPC's/appliances of the bathroom circuits.

If the extraneous and exposed-conductive-parts have negligible (or low enough) impedance between them because of the supplementary bonding then connecting one of them to the MET is not going to alter it.

That may be, but taking the supplementary bonding back to MET as an addition is unlikely to increase the impedance between the extraneous parts and the CPC's/appliances of the bathrooms circuits; that is unless there is a significant difference in the impedance between the item/point that is going back to the MET and the CPC's/Appliances what are furthest away from the item/point that is going back to the MET??

Come to think of it, is it still safer not taking the SEB back to the MET in case of the theoretical situation I mention above where one could become a bridge/shunt for the current under a fault condition to flow from the exposed-conductive-parts furthest away from the extraneous part connected directly to the MET and the extraneous part linked to the MET it's self.

Now where the water service enters the property through the bathroom and thus needing to be be mains bonded back to the MET, I guess you end up in a situation where the mains bonded incoming water service is also electricity connected to the extraneous parts that are supplementary bonded via the pipe work, assuming it's metallic pipework; is essence the same as what I say above.

Plus anyway, I think I have just confused myself even more with trying to Imagen the theoretical fault paths under a variety of situations and theatrical models.

 

[EFLImpudence]

If the extraneous and exposed-conductive-parts have negligible (or low enough) impedance between them because of the supplementary bonding then connecting one of them to the MET is not going to alter it.

That may be, but taking the supplementary bonding back to MET as an addition is unlikely to increase the impedance between the extraneous parts and the CPC's/appliances of the bathrooms circuits; that is unless there is a significant difference in the impedance between the item/point that is going back to the MET and the CPC's/Appliances what are furthest away from the item/point that is going back to the MET??

Mmmm. Not really.
If you have connected an exposed and an extraneous-c-p with a supplementary bond which has a resistance of 0.01Ω (or whatever) then nothing else you connect can increase or decrease that.
As far as the supplementary bonding is concerned it doesn't matter what any other connections to the MET (CPC) are.

Come to think of it, is it still safer not taking the SEB back to the MET

It doesn't matter.

in case of the theoretical situation I mention above where one could become a bridge/shunt for the current under a fault condition to flow from the exposed-conductive-parts furthest away from the extraneous part connected directly to the MET and the extraneous part linked to the MET it's self.

It is only the resistance between the simultaneously accessible (touchable) parts in the location (bathroom) which are of concern.
Using the figures already mentioned - a 100A fault current (B20A MCB instantaneous disconnection) then a resistance of 0.01Ω between two touched parts will result in (100 x 0.01) 1 volt being felt so - alright.
A resistance of < (50 / 100) 0.5Ω would result in a voltage of less than (100 x 0.5) 50V so would be acceptable for the regulations

Now where the water service enters the property through the bathroom and thus needing to be be mains bonded back to the MET, I guess you end up in a situation where the mains bonded incoming water service is also electricity connected to the extraneous parts that are supplementary bonded via the pipe work, assuming it's metallic pipework; is essence the same as what I say above.

Yes, they always are.
Don't forget that supplementary (bonding) is not a magic term; it just means additional where the main bonding and CPCs do not connect together the e-c-ps effectively enough to ensure less than 50V in the event of a fault - during the time it takes to disconnect.

Plus anyway, I think I have just confused myself even more with trying to Imagen the theoretical fault paths under a variety of situations and theatrical models.

I think you have - and also forgetting the simultaneously accessible situation.
It is only the path through the body which we are considering.

 

[eveares]

Thanks for clarifying and clearing things up for me EFLImpudence; I guess I was over thinking it and becoming confused.

Where I said:

one could become a bridge/shunt for the current under a fault condition to flow from the exposed-conductive-parts furthest away from the extraneous part connected directly to the MET and the extraneous part linked to the MET it's self.

I seemed to stupidly overlook and forget that a fault will take the path of least resistance and that as you said the resistance between the simultaneously accessible parts in the bathroom will be extremely low (Fraction of a ohm or less) and thus a person bridging between two simultaneously accessible parts is not going to make a difference.

After all we humans have a resistance a lot higher than 0.01Ω, and thus one will not be the path of least resistance. Now sure in my theoretical situation, some current will go through the supplementary bonding conductor going back to the MET under a fault condition due to it being a parallel path in addition to the joined up CPC's of the bathrooms circuits, but that is beside the point and all accessible parts in the bathroom will be near or at the same potential.

Sometimes it pays to take a break and get a fresh mind.

 

[bernardgreen]

So are you suggesting it would be better to have high impedance earth connections to reduce lost neutral current in the main bond with the consequent loss of adequate current to operate the protective device(s)?

I did say the 30 mA required to trip the RCD, no need to trip an MCB on over current. It would be better if the RCD disconnected Live and Neutral

 

[EFLImpudence]

TT or 7kΩ resistor it is, then.