Just read your explanation on bathroom bonding. I totally understand the need but I am a little confused on the tie back to the Main Earth Terminal business.
In my bathroom with an electric shower all metal ,pipes,rads,towel rails, even the bath and sik taps are all bonded in and bonded in to the water supply pipe to the shower and eventualy to an earth block in the loft which in turn is connected to an earth bloch tied to my main TT earth armouring on my main supply cable.
My confusion is this:- My Mira electric shower has a pre-installed earth wire connecting the internal shower water inlet pipe to the earth terminal of the shower and stressed this must be done.
But the shower supply cable's Earth is connected to the CU Earth terminal so as I see it the bonding system has a link to the CU earth.
Am I missing something.
The main problem over the years has been where a fault in a remote item like a boiler or immersion heater has resulted in something in the bathroom becoming live.
The conducting of this power can be through pipes, or even the building fabric so the idea is to connect all metal items together so even if it does become live every item will be live and so like the bird on the wire one will not get a shock.
Years ago this got rather silly with requirements to even earth door handles but now it is restricted to items which may either transmit live or earth into the bathroom.
The word is equipotential bonding and as it suggests idea is to have everything at same voltage even if this is not the voltage of the ground outside so there is no need for any earth wire to go from bathroom to main earthing coming into house unless we have something electric in the bathroom.
Where electric items are used in the bathroom there supply cables will provide them with earths and again no extra cables need go from bathroom to supply earth.
However with showers and other high power items cross bonding could mean under fault conditions the high current could try to return to earth through some wire not designed to take that amount of current so the rules were changed in 2008.
A house built today needs to have all items in a bathroom protected by RCD and as a result the rules on earthing have been further reduced and now one does not have to do all the cross bonding required years ago.
But this only applies to premises where the system complies with latest regulations BS7671:2008 throughout and since pre-2008 lights were not RCD protected in bathrooms many older premises have to still abide by older regulations.
Since there is no requirement to remove earths, where not sure easiest method is to earth.
Note:- All earth wires not contained within cables or conduit need to be at least 4mm CSA. Also the Part P regulations refer to BS7671:2001 and not BS7671:2008 so inspectors from the LABC may still require earth bonding even where the regulations don’t so I would always ask the LABC before you start or of course where submitting plans tell the LABC what you intend to do. Then at a latter date they can’t change their mind. All health and safety has to be in writing don’t tell or ask any health and safety verbally without written conformation.
your correct in as much as the shower circuit earth (CPC) is connected back to main earth bar, however it is only a 2.5/4mm max with relatively high resistance.
So if the shower element burnt out and touched the copper casing of the heater many amps would flow until the protective device operates.
During this period the showers copper pipe work would form part of a series circuit of resistance which by ohms law would raise the touch voltage of it above a safe level ref to true earth.
If other pipe work was bonded back to true earth via a direct low resistance path such as a 10mm conductor there would be possibilty of touching two pipes/taps etc at different potentials.
so by local bonding only this is prevented as even if the afore mentioned fault occurs all metalwork in the room will be at same potential (which may well be many volts above true earth) but you will not be subject to shock risk as you would also be at that same potential.
Imagine this real situation:
Young electrician [me] working in large London hotel, rewiring circuit at a time whilst guests out sightseeing.
Wall switches in bathrooms! yes really in 1960's.
Thick carpets! rubber soled work shoes, 'inconvenient for staff' to have circuits dead but no problems due to carpets/boots!
So when straightening ends of conductors to go into new switch I was 'live' at 240v - no problem - unless someone/some thing earthed touched me.....
back of knees made contact with rim of iron bath.... woke up, face down, in middle of passageway!!
Not sure if if you see the point but basically it's all about Einstein's theory of relativity, every thing depends on your reference point.