Lack of supplementary bonding - what’s the danger ?

[equitum]

So I know why it is done, where it is done and how it’s done, and when it isn’t required, 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) and there is no additional protection provided by 30mA RCD .

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 ?

 

[flameport]

Metal waste pipe to the bath, which is at earth potential. Not bonded but the mixer shower above the bath is.

Some fault occurs elsewhere in the installation which causes a voltage of 150 volts to appear on the bonded metalwork of the shower.
The person standing in the bath with their foot on the metal waste gets a shock when they turn on the shower, as there is now 150 volts between their hand and foot.

If supplementary bonding was in place, the shower and waste would be at the same voltage (potential) and therefore no shock would occur.

 

[equitum]

…ok thanks for posting John, but how could a metal waste pipe be an extraneous conductor ? Waste pipes are connected to the drains via plastic / ceramic underground pipework and as such are not going to be earth potential? I don’t think I’ve ever actually seen a metal waste pipe from a bath shower other than maybe a fancy bit of chrome on a roll top bath connected in close proximity to PVC waste pipe . Secondly what kind of fault could raise the potential of the shower metalwork to 150v when it is bonded to the supplier earth via main bonding conductor ? Loss of PEN conductor ?

 

[flameport]

Modern pipes are usually plastic, but plenty of old ones were metal (such as lead or copper) into a cast iron stack.

Any line-earth fault will cause a voltage on the main earth terminal and all conductive parts connected to it.
The voltage only exists until the fault is cleared by a fuse or circuit breaker, but with certain installations that could be up to 5 seconds. Much longer if the fuse or circuit breaker doesn't clear the fault due to some other problem.

Bathroom supplementary bonding is less of an issue with modern installations due to the extensive use of plastic pipes, class II equipment and RCDs for everything. It originated long before all of those things existed.

 

[JohnW2]

Modern pipes are usually plastic, but plenty of old ones were metal (such as lead or copper) into a cast iron stack.

Indeed. It happens that I've been removing some of them recently!

However, unless the trap of the bath (or a basin/shower/bidet) is so ancient as to also be metal, there will be no electrical continuity between the metal pipe and the touchable waste within the bath/whatever. I imagine that metal traps must be pretty rare, since I don't think any will have been installed for well over 50 years (although I confess that I have also recently been removing some of those ).

In the situation you describe, the metal waste pipes would obviously be extraneous-c-ps which would require main bonding - but even if such bonding were in place, that wouldn't prevent dangerous pds arising between the pipes and exposed-c-ps when there were faults affecting the latter

he voltage only exists until the fault is cleared by a fuse or circuit breaker, but with certain installations that could be up to 5 seconds. .... Bathroom supplementary bonding is less of an issue with modern installations due to the extensive use of plastic pipes, class II equipment and RCDs for everything. It originated long before all of those things existed.

As you say, an RCD, if present, will hopefully limit the duration of any electric shock. However, what I've written above reminds me that nothing other than local supplementary bonding can prevent a shock occurring (during the period until a device clears the fault) in the vent of an L-E fault in something with an exposed-c-p if there are simultaneously touchable other things that are (directly or indirectly) bonded to the MET or simultaneously touchable exposed-c-ps of other circuits.

 

[equitum]

I was about to make the point that if the metal waste pipe and trap were indeed continuous into the ground they would be /should be main bonded . So we are down to shock protection during a LE fault duration of typical 0.4s for final circuits. Thinking aloud, is there anything more dangerous during this period in a zone than in say a bedroom because we are talking 240V touch voltage here - maybe more exposed pipe work - but a radiator would have the same effect I’m sure in another room . In any case what is the supp bonding achieving here that isn’t already achieved by all of the CPC’s being combined at the CU and all of the extraneous services being connected be main bonding connections?

 

[equitum]

Bathroom supplementary bonding is less of an issue with modern installations due to the extensive use of plastic pipes, class II equipment and RCDs for everything. It originated long before all of those things existed.

I’d still be interested to understand the object of it and the scenarios they were intending to mitigate because I can’t get my head around the usual “to prevent PD between extraneous conductive parts” whilst the zone occupant is performing some odd pose whilst wet and unrobed. I’ve always wondered how large this PD could possibly be if main bonding in place, and in all likelihood even if it isn’t ?

 

[ebee]

Indeed, I`ve come across quite a few metal wastes on metal baths and favour supp bonding and would still install it even if a RCD is present.

Think of that bird, high up on the power grid lines, as happy as a sand boy called Larry because everything it can touch is the same potential.
(What is a sand boy and who the heck was Larry?) , anyway if everything is the same or substantially the same voltage we can be happy.

 

[equitum]

Indeed, I`ve come across quite a few metal wastes on metal baths and favour supp bonding and would still install it even if a RCD is present.

Think of that bird, high up on the power grid lines, as happy as a sand boy called Larry because everything it can touch is the same potential.
(What is a sand boy and who the heck was Larry?) , anyway if everything is the same or substantially the same voltage we can be happy.

Well in this case we are deliberately making as large a potential difference as possible between class 2 metalwork and EC parts with respect to line voltage to ensure ADS works, which is providing a nice path through our bodies should we happen to be in contact with both of them during a fault. Can’t see a parallel to Larry. In that case we’d be isolating all exposed metalwork from earth by use of class 2 and plastic service pipes wouldn’t we ?

 

[ebee]

Sorry I think I`ve missed your point here.

what I was describing with the bird on the line was that every piece of conductor that bird could touch was at the same voltage. It is the difference in voltages (Electrical potential) that is the problem.

so bonding makes everything within reach the same (or substantially the same) voltage, hence no danger.
Whether that voltage is the same as the ground outside (Earth potential) or 400,00 volts or any voltage, if it`s all the same then no problem.
That`s the purpose of bonding, to keep all we can touch at the same voltage.

if we decide to insulate all voltage sources so we can not touch them, yes good.
If we decide to earth things to cause a fuse to blow, breaker to trip then yes good. Bonding holds all conductive parts together ubtil disconnection of supply happens.
So, if you grab an appliance and it gets connected to Line via a fault (say around 240V when measured to Earth) then until that fuse/breaker disconnects then it might be at around 120V to earth but if every single conductive thing you can touch during that time is at the same 120v then not really a problem. Anything at a different voltage to the bit you are touching is a problem, this could be your metal bath and metal pipework.

 

[equitum]

Yes that’s the purpose of an equipotential zone - So back to my original point - if main bonding is in place, how will there be any significant differences in potential between extraneous conductive parts with the omission of supplementary bonding ? Here’s another thought - where all or some of the extraneous conductive parts in a bathroom were actually NOT ECP’s because they are isolated from ground via plastic pipes or fittings, by adding or the presence of supp. bonding to CPC’s you are creating hazards that otherwise wouldn’t exist ….

 

[ebee]

If main bonding is in place then circuits in a bathroom during a fault can rise and often do. Seeing it is more critical to equalise them because a bathroom is a place where someone could be hot naked and sweaty then the risks are greater than in normal dry conditions. So supp bonding in bathrooms (and some other special locations) was mandated to further reduce risks.
However, if all circuits are RCDs nowadays then supp bonding is not now considered as important providing certain other conditions are all met.
Personally, I carried on supp bonding even if not mandated.

Yes, if something is not earthed because it does not require it, then you do earth it then you could be introducing another fault path to those already present so you don`t do that , only do it if it requires it.
If I remember correctly, the Mary Wherry case was a metal hook rack in the kitchen that was live (may have been live for quite a time previously) and she touched something in the kitchen that was earthed, example, washer/dryer/fridge and that completed the circuit and resulted in tragedy, if those items had not been earthed then this incident might not have occurred .
So if it needs earthing or bonding then do it but if it does not need it then don`t do it.

I have seen a case where an electric shower in a bathroom was not earthed (Earthing conductor omitted from the one way consumer unit (Switchfuse) supplying it but fortunately was earthed by connection to other supp bonding in the bathroom circuits.
It have even been inspected and tested and given a Zs reading by a firm doing a PIR (EICR as it is nowadays) because nobody noticed the missing earthing conductor.

 

[Harry Bloomfield]

Think of that bird, high up on the power grid lines, as happy as a sand boy called Larry because everything it can touch is the same potential.
(What is a sand boy and who the heck was Larry?) , anyway if everything is the same or substantially the same voltage we can be happy.

My bathroom, and everywhere there are taps etc., are well and truly earth bonded, including every individual radiator, despite the use of copper pipework throughout.

 

[equitum]

Why ? Do you doubt the conductivity of the copper ?

 

[jurassic sparkk]

My bathroom, and everywhere there are taps etc., are well and truly earth bonded, including every individual radiator, despite the use of copper pipework throughout.

Even if you cannot see visible (supplementary bonding) most likely whatever pipes are in the bathroom would be still bonded by the main bonding (if there is no plastic pipes of course)