bathroom cross bonding

[theboyracer]

ok.. i admit.. i never get the hang of the why's of cross bonding.. yeah.. i understand its to keep me alive... anyway..

moved into this place and the bathroom isnt crossbonded.

so what i need to do is crossbond all the metalparts incl my bath and connect it to the earth in the nearest socket, right?

and since all the pipes run as copper to the kitchen i wont need to crossbond the kitchen pipes (i think its not a legal requirement anyway?) apart from the gas pipe, right?

oh and please if any of you has the patience to explain why we're cross bonding again... yes i read through all the threads.. but in plain english pls? what i dont understand is where is the current that we're trying to lead away potentially coming from?

will get my sparky to check its all cool when he comes round. but its a simple job i'd expect to be able to do myself...

 

[philrosenberg]

The way I alway imagine cross bonding is like this. Your having a wash in the morning and just as you turn the taps off your washing machine shakes a live wire loose inside and it touches an earthed bit. The current flows to earth by the path of least resistance. Because you are wet and have a low resistance that path could potentially be through your bonding to the cold water pipes, through your hand on the cold tap, through your hand on the hot tap down the hot water pipes through the hot water pipe bonding and finally to your CU. If, however, you attach a nice thick copper cable in your bathroom between the hot and cold pipes that has a lower resistance than you do then the current will flow through there instead.

The current source might more realistically be your shaver socket or light fitting in the bathroom but same ideas apply. You have to connect all metal pipes together and also to any electrical stuff in bathroom eg shaver socket non selv lights, heaters etc. If you can't get access to your bath pipes without a tool these are excluded.

I'm not sure if you actually need to connect it all to a socket in the end (although if you are cross bonding shaver socket/lights this will happen anyway)

 

[JohnD]

You have to bond it to the Earth wire of each electrical circuit that enters you bathroom.

This will usually be a lighting circuit, sometimes a socket circuit (e.g. if you have a shower pump or a shaver socket on this circuit), may also be an electric shower and an immersion heater. If the pump for the boiler is in the bathroom, that too.

Note that you bond to circuits not outlets. So if you have a pull-cord lightswitch and eight lamps on the same circuit, you only need to bond to the lighting circuit once. (all the lamps will already be earthed on the lighting circuit)

You can often conveniently bring the wire from the lighting circuit down inside the airing cupboard. This is often a convenient point to connect the wires from the immersion heater, pump and socket circuit as well, as it is tidier than running several into the bathroom. If the airing cupboard is adjacent to the bathroom, or inside it, you can bond to pipes in there before they pass into the bathroom. You have to bond central heating pipes as well as pipes for the hot and cold.

The objective is to make sure that all the things that might bring a voltage into the bathroom from outside are bonded. So for example a brass towel rail that is not connected to a pipe or wire, and is wholly inside the bathroom, does not need to be bonded.

If you have an old house with iron soilpipes or lead wastepipes, you have to bond them too where they come into the bathroom. You do not have to bond plastic pipes.

 

[theboyracer]

hmm.. getting there!

but if and socket, appliance or lighting circuit is already earthed to the consumer unit why is the cross bonding necessary?

if my wm was to devellop a fault i suppose it would send the loose current down the earthwire to the cu and trip the mcb?

ok i might just be thick but i sense im close to getting it...

 

[JohnD]

Supplementary bonding inside the bathroom removes dependency on anything outside the bathroom.

Even if the fault is (e.g)

burst pipe makes wall or ceiling wet round light switch and makes pipe live

plumber damages cable under floor and conductors touch copper pipe

rat gnaws through immersion heater cable

Stupid DIY person breaks the earthwire at the shower switch

Decorator drives screw through cable, severing earth wire.

Clumsy person trips over under stairs and grabs hold of cables to steady themselves, pulling earth wires out at consumer unit

or combination of all of the above...

Suppplementary bonding in the bathroom means that even if you touch the tap with one wet hand and the radiator with the other, they are all at the same potential so you will not get a shock.

 

[theboyracer]

do you know what???

i've finally got it!!

thanks john and phil!!

have a great w/end!!

 

[Spark123]

Can you follow this or did I make it too technical?
//wiki.diynot.com/electrics:sp...l_bonding:supplementary_equipotential_bonding

 

[theboyracer]

yep.. think that helps too spark123... although if i had been reading this bit in a couple of hours i'd think we were speaking about sth else...

The theory behind this type of bonding is if a person who has lower body resistance due to wet skin and lack of clothing who simultaneously comes into contact with two extraneous/exposed parts can’t form a circuit between the two with enough current flowing to cause harm.

sorry.. maybe if i started concentrating it wouldnt have been so hard to understand in the first place.

thanks again!!!!

 

[Spark123]

Yeah, bit of a mouthfull, maybe I should change it

 

[mikely]

but if and socket, appliance or lighting circuit is already earthed to the consumer unit why is the cross bonding necessary?

To understand the requirement for supplementary bonding you need to consider what happens during an earth fault. During the fault, before the protective device operates, the voltage developed at the fault rises to approximately half the supply voltage because the resistance of the phase conductors and earth conductors in the fault loop circuit is roughly equal and they act as a voltage divider. The main bonding will cause bonded metalwork to rise to the fault potential developed at the MET, however, a significant voltage can still develop across the CPC resistance between the fault and the MET. This will appear between any Class 1 appliance connected to the CPC and any earthed metalwork (e.g. a radiator, or a Class 1 appliance on another circuit). This is the dangerous voltage that the supplementary bonding is designed to eliminate.

This is why the CPCs of all circuits supplying equipment in a bathroom are connected to the supplementary bonding, because they are the source of the hazard. They are not simply providing a path to the MET.

 

[philrosenberg]

if my wm was to devellop a fault i suppose it would send the loose current down the earthwire to the cu and trip the mcb?

Ideally yes, but the current follows the path of least resistance. It could be that because your copper water pipes have a huge cross section compared to your ring earth conductor the current may pass through the water pipe insead. As John D says, there are loads of things that can cause faults where stuff earth's through possibly exposed bathrom parts. However provided all these things in the bathroom are connected together with something of lower resistance than a wet person (ie suplimentary bonding) then the current will pass through the supplimentary bonding insead of the person.

 

[twocom]

ok.. i admit.. i never get the hang of the why's of cross bonding.. yeah.. i understand its to keep me alive... anyway..

moved into this place and the bathroom isnt crossbonded.

so what i need to do is crossbond all the metalparts incl my bath and connect it to the earth in the nearest socket, right?

and since all the pipes run as copper to the kitchen i wont need to crossbond the kitchen pipes (i think its not a legal requirement anyway?) apart from the gas pipe, right?

oh and please if any of you has the patience to explain why we're cross bonding again... yes i read through all the threads.. but in plain english pls? what i dont understand is where is the current that we're trying to lead away potentially coming from?

will get my sparky to check its all cool when he comes round. but its a simple job i'd expect to be able to do myself...

Jumping on this thread, I've got a probably stupid question. Our bathroom doesn't have any bonding on any pipes. My question is, as the boiler is in the bathroom and therefore all water, gas, CH pipes are in 1 place, can I simply bond all those together onto the lighting circuit and be done with it? Or do I have to connect them all seperately (sink taps, rad, bath taps etc)?

Paul.

 

[Spark123]

Providing the pipework is electrically continuous you can bond at the boiler, you also need to supplementary bond the pipework to the CPCs (earths) of any circuits supplying class 1 or 2 equipment in the zones (such as lighting ckt, shower, heater etc.)

 

[twocom]

Providing the pipework is electrically continuous you can bond at the boiler, you also need to supplementary bond the pipework to the CPCs (earths) of any circuits supplying class 1 or 2 equipment in the zones (such as lighting ckt, shower, heater etc.)

There is no class 1 or 2 equipment in the bathroom.

I'm sure this is a stupid question but how can I tell if the pipework is "electrically continuous"?

 

[JohnD]

soldered copper pipe.