Stupid Question On Bonding?

[DevonSally]

Hi everyone, I am new to this forum, so sorry for asking a daft question. I have checked this site elsewhere (reference and other posts) as well as books, but have some points I hope someone can clarify. In my house I want to check that the bonding is up to scratch. I believe the main bonding is O.k (Do Not Remove labels, correct cable sizes etc, etc.) but appear to have no local bonding in my bathroom. I have not taken up floorboards, or checked in loft yet, so the bonding cables may be hidden in these places (would that be acceptable?) Also, how would I bond my shower to my batroom radiator, about 4 foot away? Surely I dont need to have a 4 foot cable running from my shower pipe, down the wall and joining the radiator pipe as this would look awful? Other people don't seem to have that kind of set-up. Could I bury the cable in the wall? And lastly, does every pipe need to be connected to every electrical item, (shower, Shower pull cord, light, light pull cord) or should each pipe be connected together then just joined to the earth terminal of one electrical item.
I have done small electrical jobs before, and understand the theory of bonding, just not how it would be done practically and neatly/ discreetly.
Sorry for the long question, and thanks in advance.
Sally.

 

[ChrisRogers]

do you have plastic piping? or plastic piping below the floor joining onto copper for exposed runs to basins ect?

 

[Lectrician]

Photos of the layout, scetch of the bathroom/airing cupboard etc would also help if you are able to post them.

 

[DevonSally]

Thanks for the rapid response, guys.
Chris, have copper pipes above floor, but not sure what lies below. (Copper I assume)
Lectrician, sorry but unable to post photos etc, as I am ashamed to say I am very computer illiterate.
All i have in bathroom is basin (metal taps and drainage) bath (metal pipes and drainage,) shower with pull cord, two lights with pull cord and one radiator. Sorry I know this doesn't help much.

 

[kevplumb]

if you read forum info how to post a pic is in there
if its any consolation your not the only one

 

[mapj1]

It is perfectly acceptable to have the bonding in places where it is not on view, such as in a neighbouring airing cupboard, but any joints/clamps etc should be inspectable, although having to remove the side of the bath or something similar to do the inspection is common. Also note that before the most recent editions of the regs, such supplementary bonding was not common practice.
More generally so if the house is before the late 1960s, there will probably be no earth in the lighting circuits, before 1980 ish ether will be no cross bonding to incoming gas and water mains, and before the 1990s no suplementary bonding in bathrooms. Installations made at the height of the 'earth everything' mania of the late 15th early 16th edition may have bonding in kitchens too, though this is no longer considered essential.
Being wired to an older edition does not make the house dangerous, per se, but may mean extra work bringing it up to date with the latest fashion when new circuits are added.
Note that things that are metal but cannot become live by any reasonable fault (shower trays and radiators fed only by plastic pipes for example) need not be bonded, even today.
M.

 

[DevonSally]

Cheers Mapj1, thanks for the response, it was most helpful. It's certainly cleared up a few points.
Regards, Sally.

 

[kendor]

It is perfectly acceptable to have the bonding in places where it is not on view, such as in a neighbouring airing cupboard, but any joints/clamps etc should be inspectable, although having to remove the side of the bath or something similar to do the inspection is common. Also note that before the most recent editions of the regs, such supplementary bonding was not common practice.
More generally so if the house is before the late 1960s, there will probably be no earth in the lighting circuits, before 1980 ish ether will be no cross bonding to incoming gas and water mains, and before the 1990s no suplementary bonding in bathrooms. Installations made at the height of the 'earth everything' mania of the late 15th early 16th edition may have bonding in kitchens too, though this is no longer considered essential.
Being wired to an older edition does not make the house dangerous, per se, but may mean extra work bringing it up to date with the latest fashion when new circuits are added.
Note that things that are metal but cannot become live by any reasonable fault (shower trays and radiators fed only by plastic pipes for example) need not be bonded, even today.
M.

Now this is where i disagree, as far as i'm concerned Bonding was never about stopping live metalwork that was the job of a cpc. Bonding was introduced to make sure extraneous metalwork is at the same potential and so to avoid pd's(potential differences) which i believe can still occur if one lot of extraneous metalwork is at earth potential and other pieces of extraneous metalwork(unbonded) is floating at an unknown potential.
This hazard is more likely these days with the predominant use of plastic piping in water distribution.

 

[kevplumb]

water stopped being a conductor or is it just the regs ?


trays and radiators fed only by plastic pipes for example) need not be bonded, even today.
M

just a question

not a poke

 

[plugwash]

you have to remember that the real danger is current flowing through the body.

To determine the current flow through a load (in this case the load being the victim) we need to know three factors.

1: the open cuircuit votlage of the supply
2: the complex impedence of the supply (which can usually be approximated as a resistance in most practical cases)
3: the complex impedence of the load (Which can again usually be approximated as a resistance in most practical cases)

the impedence of the body isn't usually something we have much control over so that leaves us two variables we can have an effect on.



stick a neon screwdriver in a live terminal.

the metal contact at the back of that screwdriver is now also at mains potential

now touch the metal contact on the back. Although that contact was at mains voltage under open cuircuit conditions it is simply of too high an impedence to give you a shock.

eventually the impedence of a point gets so high that you cannot meaningfully assign an earth relative voltage at all because any attempt to measure it would result in it falling immidiately to zero.


also of importance is that you cannot get a shock from a single point alone you need two points with a potential difference and its the SUM of theier impedences that matters.

if you touch live mains and something that is well isolated at the same time you will not get a shock.

if you touch live mains and something that is poorly earthed at the same time you will probablly get a shock but survive.

if you touch live mains and something that is well earthed at the same time then you will get a much worse shock.

35ma is considered fairly safe by the IEE (look at our rcd specs)

35ma at 240V gives 6.86K

The point of eqipotential bonding is to deal with the situation where you have a number of items which have a low impedence to earth and may have different earth relative voltages.

Since the impedence is already so low decreasing it further will have negligable effect on shock current. furthermore we wan't to prevent sockets from between those items so we bond them together.

Objects that are largely isolated cannot carry enough current to give a dangerous shock so bonding them is pointless.

Furthermore bonding an otherwide largely isolated object INCREASES the danger in the event of direct contact between the human body and a live wire because it allows more current to flow through the body.

so it boils down to

EARTH things that may become live (ie meal surrounding electrical items)
BOND anything that is earthed either intentionally as part of the electrical installation or unintentially through metal pipes etc .

 

[kendor]

The point of eqipotential bonding is you have a number of items which have a low impedence to earth and may have different earth relative voltages.

I felt i had to pick up on what you said here, the term "equipotential" means that all are at same potential equi= equal, so if bonded properly together there should be NO difference in earth relative voltages or "potentials". The Equipotential Zone which i was taught about many years ago when an apprentice was to cover ANY metalwork within Touch of each other so earthed appliances and extraneous metalwork eventually ended up at or near the same potential.
This is why i disagree with what seems to be new directives regarding metalwork "isolated" by plastic piping and not bonded as a risk because of difference in potential.

You mention the risk of earthed metalwork causing greater risk of shock, this was used as a argument whilst at college that if we had no earthing system then no-one would be at risk of shock apart from live - neutral.
The lecturer then went on to say originally that was the case, until 3- phase was introduced and then there was a need for earthing quoting the example:
For distribution purposes Street A is fed from RED phase, Street B from Yellow phase.
Over the years a house in Street A has an electric cooker that develops a live to casing fault the housewife carries on cooking oblivious to the fact her saucepan is live AND herself as there is no return earth path( no Earthing system exists) and no fuse blows either for same reason so for the next few years this cooker stays live no problem!
In street B a cooker all of a sudden develops a live fault to the casing and this is live for months again no fuse blows.
Then on one unfortunate Sunday both housewives decide to cook the Sunday roast at the same time and touch the saucepans at the same time and a 415volt potential then exists from the saucepan through the housewife down to the general mass of earth across the street to the other housewife who obligingly completes the circuit.
This risk of a far higher Voltage potential was one of the reasons in introducing the Earthing system into this country.

 

[mapj1]

Indeed true, if a little oversimplified as an educational example in an IT system, two faults to ground are needed to pose a hazard - I believe London Undergroun rails are or were, fed with DC this way earth being connected to neither side, but floated to the middle by high value resistors- the first fault can be detected, as the system pulls from earth centre to earth on one side only, but the trains can still be moved to safety before switching off. Some mission critical medical stuff is also done like this.
I digress.
But in the example, the said cooker can only become dangerous because it can become well connected to live - the live wire goes into the metal box, and may one day the insuulation wears through and touches it. The same is not true of a metal shower door frame, or a radiator fed by plastic pipes (long enough that the column of water within is long and thin enough not to conduct well enough to carry a dangerous current -typically a few feet for 15mm diameter, depends on the state of the water.)
Of course if your metal shower tray or frame did carry a live wire inside for some reason then I agree wholeheartadly, double insulate it or earth it,
but if it requires some really wierd fault involving bringing another defective applience near to it before it even becomes live, then grounding it just makes it more dangerous for someone already live due to this other, (more likely), fault .
(I have seen a leaky shower widdling live water down the wall -yes it can happen,- and the report was 'it tingles when I touch the bath taps'. Luckily as the bath taps were very badly earthed no one was more than very shaken up, that time, but it could have been much worse, had they been. Nowadays a dedicated RCD in the shower feed is something I consider a very good idea).

M.

 

[plugwash]

The point of eqipotential bonding is you have a number of items which have a low impedence to earth and may have different earth relative voltages.

I felt i had to pick up on what you said here, the term "equipotential" means that all are at same potential equi= equal, so if bonded properly together there should be NO difference in earth relative voltages or "potentials".

saorry i misworded a bit

i should have said the point of equipotential bonding is to deal with the situation where you have a number of items which have a low impedence to earth and may have different earth relative voltages.

you can never reduce the potential between two points to zero because you c an never makew a cable of zero resistance you can just reduce it to the point where it no longer poses a danger.

the main point still holds though an object with a sufficiantly high impedence to mains doesn't pose a danger even if it is at full mains voltage and an obect with a low impedence to earth in contact with the body increases the danger from direct contact with an object that has a low impedence connection to mains.

 

[ban-all-sheds]

The Equipotential Zone which i was taught about many years ago when an apprentice was to cover ANY metalwork within Touch of each other so earthed appliances and extraneous metalwork eventually ended up at or near the same potential.
This is why i disagree with what seems to be new directives regarding metalwork "isolated" by plastic piping and not bonded as a risk because of difference in potential

ANY metalwork? Which, of the following, would you bond in a bathroom:

Radiator supplied by plastic pipes
Taps supplied by plastic pipes
Door handle
Stainless steel tumbler
Bathrobe hook
Toothbrush rack
Brass handled razor
Metal comb

???

 

[kendor]

The Equipotential Zone which i was taught about many years ago when an apprentice was to cover ANY metalwork within Touch of each other so earthed appliances and extraneous metalwork eventually ended up at or near the same potential.
This is why i disagree with what seems to be new directives regarding metalwork "isolated" by plastic piping and not bonded as a risk because of difference in potential

ANY metalwork? Which, of the following, would you bond in a bathroom:

Radiator supplied by plastic pipes
Taps supplied by plastic pipes
Door handle
Stainless steel tumbler
Bathrobe hook
Toothbrush rack
Brass handled razor
Metal comb

???

Certainly the Rad would have been an issue and bonding would have been attached to the Rad same with the taps, what about the metal fillings in your teeth Ban?
While i agree that a higher impedance path would reduce the shock, it all seems a bit iffy that you "hope" that the impedance of an unbonded say Rad for instance is high enough not to give a fatal shock, leaving unbonded and unequipotential(if there is such a word) metalwork in a zone makes for a very uncertain installation.
At least with the "old" way you were certain of equal earth potential and the rest of the regs covered fault conditions such as adequate cpc and fusing to try to prevent a shock hazard.
Just for a moment think of ways that metalwork in a zone may become live and stay live if all the other precautions had been adhered to.
To say that an unbonded piece of metalwork is high impedance without having tested it to see if that is the case leaves a lot of uncertainty who knows if the plastic pipe fed rad isn't already at or near earth potential anyway? Maybe a fixing screw has touched earthed metalwork inside the wall say for instance the metal mesh plasterers use on corners this may be in contact with a switch box? Can you see the point i'm making? better certainty than uncertainty.