UK Scenario where lack of supplementary bonding is dangerous

[Falkus]

So I know why it is done, where it is done and how it’s done, but I’m yet to see a scenario where there is shock danger in a typical bathroom in a dwelling provided a) the main bonding is in place and b) the CPC’s of all circuits are connected together in the CU (as they will be).

Consensus is on an EICR code 2 where there is no RCD for additional protection and supplementary bonding is not in place. What is the potential danger ? What am I missing ?

 

[Pretty Mouth]

I usually find that 415.2.2 is satisfied without supplementary bonding being in place in/around the bathroom.

 

[Falkus]

I usually find that 415.2.2 is satisfied without supplementary bonding being in place in/around the bathroom.

As I said I know why and how it is done and I should have stated where it is not required- I.e where the circuits in the bathroom have additional protection from a 30mA RCD but my question was, and remains, where supplementary bonding is required, what danger exists where it is omitted, assuming all circuits have CpC and main bonding it in place ?

 

[timhoward]

where supplementary bonding is required, what danger exists where it is omitted, assuming all circuits have CpC and main bonding it in place ?

Well the assumption is really the point....
One example, someone DIY changes a light fitting with one they got from Ikea and doesn't connect the CPC's properly. Every light after it now has no CPC.
It's then possible to have two different versions of "earth" in the bathroom if someone touches a metal light fitting and an electric towel rail at the same time.
Supplementary bonding is belt and braces making sure that whatever else happens, they all have the same zero potential between them.

At least, that's my take on it. It's an interesting question.

 

[Falkus]

Well the assumption is really the point....
One example, someone DIY changes a light fitting with one they got from Ikea and doesn't connect the CPC's properly. Every light after it now has no CPC.
It's then possible to have two different versions of "earth" in the bathroom if someone touches a metal light fitting and an electric towel rail at the same time.
Supplementary bonding is belt and braces making sure that whatever else happens, they all have the same zero potential between them.

At least, that's my take on it. It's an interesting question.

It’s a reasoned response however if the metal light fitting is not earthed then touching it will not be introducing any potential will it ? If it’s faulty with a live/earth fault then yes it (metal parts of the luminaire) will be at 240v buts that’s already covered under the requirement to have a cpc connected to class 1 fittings or the use of class 2 where no cpc ?

 

[timhoward]

It’s a reasoned response however if the metal light fitting is not earthed then touching it will not be introducing any potential will it ? If it’s faulty with a live/earth fault then yes it (metal parts of the luminaire) will be at 240v buts that’s already covered under the requirement to have a cpc connected to class 1 fittings or the use of class 2 where no cpc ?

The thing is, it's 'supplementary'. It's additional protection. It's an extra safety net. We could say there's no need for all sorts of things if everything is installed perfectly and maintained perfectly.

We also need to remember that it pre-dates RCDs and it was the best way at the time to make sure that you can never have two pieces of exposed metalwork in a bathroom at different potentials to each other, when the resistance of the body is at an all time low as it's soaking wet.

It has a further role of adding possible earth paths so a phase to earth fault in any circuit in the room will cause ADS even if the faulting circuit has no upstream CPC continuity.

e.g. In the example I gave above, suppose they then like their Ikea light and install another one beyond the bathroom fitting but their twist and tape joint ends up touching the casing and the casing ends up live.
They'll turn the power back on and be none the wiser, but there's no fault protection and it's sitting there with a live case.

However in this case supplementary bonding saves the day and we'll get ADS. Without it, it will still sit there live.

Don't get me wrong, I prefer making sure every circuit in the room is RCD protected.
But I wouldn't say that supplementary bonding is pointless.

 

[Falkus]

The thing is, it's 'supplementary'. It's additional protection. It's an extra safety net. We could say there's no need for all sorts of things if everything is installed perfectly and maintained perfectly.

We also need to remember that it pre-dates RCDs and it was the best way at the time to make sure that you can never have two pieces of exposed metalwork in a bathroom at different potentials to each other, when the resistance of the body is at an all time low as it's soaking wet.

It has a further role of adding possible earth paths so a phase to earth fault in any circuit in the room will cause ADS even if the faulting circuit has no upstream CPC continuity.

e.g. In the example I gave above, suppose they then like their Ikea light and install another one beyond the bathroom fitting but their twist and tape joint ends up touching the casing and the casing ends up live.
They'll turn the power back on and be none the wiser, but there's no fault protection and it's sitting there with a live case.

However in this case supplementary bonding saves the day and we'll get ADS. Without it, it will still sit there live.

Don't get me wrong, I prefer making sure every circuit in the room is RCD protected.
But I wouldn't say that supplementary bonding is pointless.

Unless the metalwork of the light fitting in your scenario above is actually earthed (to a cpc) then your supplementary bonding is going to make no difference whatsoever. And if it is earthed it will be at the same potential as all the other circuit Cpcs because they are all connected at the CU - which will be in close proximity to the bathroom , even in a mansion house the potential difference due to circuit length would be in the ELV class

 

[Simon47]

You've missed the key point - CPCs not connected to the MET due to a failure (e.g. the dodgy diy).
In an ideal world we'd not have O/C CPCs and all manner of other faults. But in the real world I would suggest that the JPEL64 committee decided there was evidence of sufficient faults to justify having additional protection.

 

[timhoward]

Going right back to the beginning:

I’m yet to see a scenario where there is shock danger in a typical bathroom in a dwelling provided a) the main bonding is in place and b) the CPC’s of all circuits are connected together in the CU

I agree with you that if everything is perfect, it is indeed supplementary, the clue is somewhat in the name!

Unless the metalwork of the light fitting in your scenario above is actually earthed (to a cpc) then your supplementary bonding is going to make no difference whatsoever

We seem to agree that everything isn't always perfectly installed.

And if it is earthed it will be at the same potential as all the other circuit Cpcs because they are all connected at the CU

In my scenario this isn't in fact the case as the CPC is broken by the first Ikea light upstream.

CU - Lighting circuit - break in cpc - bathroom light - cpc intact - (multiple lights) - live - earth fault
With SB from bathroom light you get ADS as there is a path from the fault to the CU
Without SB you don't and you have an especially dangerous situation in bathroom and a dangerous situation at final light.

If it isn't earthed, as you suggest:
CU - Lighting circuit - break in cpc - bathroom light - cpc broken - live - earth fault
Yes, SB is irrelevant in this case, but you also don't have a dangerous situation in the bathroom which is what the SB is trying to achieve.

Let's not argue too much - I was born in Coventry too.... ;-)

 

[Falkus]

Going right back to the beginning:

I agree with you that if everything is perfect, it is indeed supplementary, the clue is somewhat in the name!

We seem to agree that everything isn't always perfectly installed.

In my scenario this isn't in fact the case as the CPC is broken by the first Ikea light upstream.

CU - Lighting circuit - break in cpc - bathroom light - cpc intact - (multiple lights) - live - earth fault
With SB from bathroom light you get ADS as there is a path from the fault to the CU
Without SB you don't and you have an especially dangerous situation in bathroom and a dangerous situation at final light.

If it isn't earthed, as you suggest:
CU - Lighting circuit - break in cpc - bathroom light - cpc broken - live - earth fault
Yes, SB is irrelevant in this case, but you also don't have a dangerous situation in the bathroom which is what the SB is trying to achieve.

Let's not argue too much - I was born in Coventry too.... ;-)

Hey I’m not arguing and thanks you’ve clarified your scenario which I can see is a valid case and one I hadn’t thought of - in effect loss of a cpc to a class 1 accessory. If then a second fault occurs (making the metalwork live) then ADS will disconnect the circuit removing shock hazard. This is a new one to me thanks because it’s normally stated to do with voltages between extraneous conductive parts. I suspect this scenario is welcome byproduct but not the intended aim ? Because this risk is not made any greater by being in a zone, it could apply to any room couldn’t it and we’d end up supplementary bonding every room in the house ?