A
anglebox
Guest
In a domestic installation where all circuits including lighting are protected by RCD at the main circuit board what purpose does earth bonding water pipes serve? Is it necessary?
It is a requirement of BS7671 that exposed and extraneous conductive parts (ie pipes and the like) are bonded to earth. This is to ensure that there is no potential differnce in the event of a person touching a fault and earth. RCD's rely on the bonding else they wouldnt work. The main earth bond is the 16mm2 main earth from the origin to the fuseboard. Then, depending on the type of supply you need 10mm2 bonds to your gas and incoming water. Then you need supplementary bonding in the bathroom....bonding is VERY IMPORTANT and must not be omitted.
Lectric man
New Member
agree with most of what you say but that bit about rcd needing bonding to work I think you will find that put very simply rcds work on in balance
sparky (Original)
New Member
Whilst the current flowing down the live conductor is balanced by that in the neutral the RCD takes no action but if part of the current flows down to earth through some metalwork or through a person's body an unbalance occurs between live and neutral which trips the RCD
ban-all-sheds
New Member
agree with most of what you say but that bit about
rcd needing bonding to work I think you will find
that put very simply rcds work on in balance
Indeed, and as sparky says:
...if part of the current flows down to earth
through some metalwork or through a person's body an
unbalance occurs between live and neutral which
trips the RCD
I imagine that what unphased was saying was that in the event of a fault it's better that the RCD should trip because of current flowing in an earthing wire than in some poor sod's body
rcd needing bonding to work I think you will find
that put very simply rcds work on in balance
Indeed, and as sparky says:
...if part of the current flows down to earth
through some metalwork or through a person's body an
unbalance occurs between live and neutral which
trips the RCD
I imagine that what unphased was saying was that in the event of a fault it's better that the RCD should trip because of current flowing in an earthing wire than in some poor sod's body
M
MICC
Guest
Agree here, RCD works on balance between conductors, but earthing is still required as per IEE regs. and must not rely on the RCD
C
Caveman
Guest
A questionable practice, this one - obviously exposed metalwork should be earthed mainly to prevent it becoming live due to faults ( a live conductor touching plumbing in a house not bonded makes most exposed metal live!) but at the same time, say you picked up a faulty kettle in a kitchen - do you really want to be surrounded by 'hard' earths? Many moons ago my Father ended up at an inquest because a customer picked up an old brass bedside lamp to inspect some plumbing under a bath - he grabbed the taps to help himself up. He died BECAUSE the plumbing had been bonded. Tricky debate.
ban-all-sheds
New Member
That example show why there are rules about electrical appliances in bathrooms, not why earthing is a bad idea.
It also hows why RCDs are so valuable.
It also hows why RCDs are so valuable.
Brightspark
New Member
RCD's are used for supplementary protection from direct contact. The reason the pipes are earthed is to comply with EEBADS, where the benefits of main bonding is to reduce the prospective touch voltages in the premise.
Basically by having a good earth potential the protective device should disconnect in a given time.
Basically by having a good earth potential the protective device should disconnect in a given time.
ban-all-sheds
New Member
Indeed - the purpose of earth bonding is to prevent all that metalwork in the bathroom, where it can easily be touched by wet hands, from becoming live. A good earth will cause a sufficiently large current to flow to operate the protective device on the circuit (fuse or MCB), thus preventing your bathroom fixtures and fittings from becoming live. Or at least from rising above 50V.
The point I was trying to make was that the death of the man holding a faulty appliance in one hand and a good earth in the other does not mean that earthing is a bad idea, it means that portable appliances in bathrooms are a bad idea. To suggest that pipework etc should not be earthed in case some idiot is holding a faulty appliance is utter madness.
And although RCD protection is "secondary" it is also very important. I realise that the example quoted probably dates back to pre-RCD days, but the same scenario could arise today, and if the (probably) upstairs sockets had been protected (a practice which some people here don't believe is necessary), then the man would probably not have died.
The point I was trying to make was that the death of the man holding a faulty appliance in one hand and a good earth in the other does not mean that earthing is a bad idea, it means that portable appliances in bathrooms are a bad idea. To suggest that pipework etc should not be earthed in case some idiot is holding a faulty appliance is utter madness.
And although RCD protection is "secondary" it is also very important. I realise that the example quoted probably dates back to pre-RCD days, but the same scenario could arise today, and if the (probably) upstairs sockets had been protected (a practice which some people here don't believe is necessary), then the man would probably not have died.
Brightspark
New Member
Good post Ban-all-sheds. There could be many reasons why the poor man past away (heart problems, age etc) the Zs on the circuit the lamp was feeding could have been high or the protective device not selected right.
But without doubt in most domestic installations you must earth the water, gas and any structural steel work.
There are cases when an earth free environment is used but that falls into it's own category and is classed as a special installation.
But without doubt in most domestic installations you must earth the water, gas and any structural steel work.
There are cases when an earth free environment is used but that falls into it's own category and is classed as a special installation.
hmm. the issue still applies if the man was holding his faulty lamp in the bedroom and touched the radiator, or in his lounge and touched the metal caase of the hi-fi.First line of defence is supposed to be that that the metal lamp is earthed. So the lamp really had two faults. one that exposed metal wasnt earthed. two that exposed metal got in contact with live. Might happen if the flex got pulled out, especially if the earth wire was the shortest connection and came off first!
a lot of lamps dont have earths...they are double insulated instead, whichever way, you are reckoning that the lamp should fail and get fixed properly before it becomes live.
electricity isnt dangerous unless you complete the circuit! so if you are working in an environment where theres lots of lives about for some reason (!!!) then it might be a good idea if nothing was earthed. we just generally choose to connect one end of the circuit to earth so that anyone putting their body between earth and live automatically gets a shock. ..except using an isolating transformer. then you have to join both ends of the circuit to get the shock, which is less easy to do. this is fine, but as the installation gets bigger, chance of an accidental earth gets bigger too and we are back where we started with earth-live shocks
a lot of lamps dont have earths...they are double insulated instead, whichever way, you are reckoning that the lamp should fail and get fixed properly before it becomes live.
electricity isnt dangerous unless you complete the circuit! so if you are working in an environment where theres lots of lives about for some reason (!!!) then it might be a good idea if nothing was earthed. we just generally choose to connect one end of the circuit to earth so that anyone putting their body between earth and live automatically gets a shock. ..except using an isolating transformer. then you have to join both ends of the circuit to get the shock, which is less easy to do. this is fine, but as the installation gets bigger, chance of an accidental earth gets bigger too and we are back where we started with earth-live shocks
correct me if i'm wrong here but i see a possible issue
suppose a 2 core flex runs up a metal lamp to a plastic lampholder
as the flex is double insulated the system would count as double insualated afaict
but if the flex is pulled out of the lampholder it can still make the lamp live
suppose a 2 core flex runs up a metal lamp to a plastic lampholder
as the flex is double insulated the system would count as double insualated afaict
but if the flex is pulled out of the lampholder it can still make the lamp live