Supplementary earth Bonding query

[Spark123]

Was there, or is there any reason why you would need to bond hot and cold pipes together and then connect to the MET?

Because Im sure I've seen this on older houses, say 1980s houses.

The cold supply where it enters the premises, if metallic requires connecting to the MET (main protective bonding), hot water is usually generated within the premises so no need to main bond.
Gas usually needs main bonding too.
I have seen some true classic ideas of bonding, from the pipework on a urinal to the uprights holding up a porcelain sink. In the 15th edn regs I think they got a bit silly, or they were not interpreted correctly - these were before my time so not 100% sure on what the 15th actually said.

 

[ricicle]

it is because the bonding is to equalise the potential between parts (in a location) which are already connected to the MET.

But if they are already connected to the MET then how can they be at different potentials......? Only if there are significant currents flowing in the CPC ( "earth" wire ) can there be a voltage drop along the CPC which would be the only way those parts connected to MET could be at different potentials.

The potentials exist due to the resistances between the various exposed and/or extraneous parts (whether or not they are connected by some means or other to the MET). We are providing additional protection by reducing those resistances by connecting them all together locally.
It's all about touch voltages - the greater the resistance between two points, the greater the voltage (created by fault currents)

 

[xr4x4]

In this house it was under the bath, joining the hot cold together, then the earth wire going through the floor to the cu.

 

[RF Lighting]

Sounds like someone has mis understood what bonding is.

 

[xr4x4]

Probaly, I think it was the original suite.

 

[Spark123]

Supplementary bonding in a bathroom does not connect directly to the MET, it's intention is to keep everything (which can introduce a voltage into the room) at a very close voltage so if you touched two parts simultaneously (and a fault occurred on the electrical system at the same time) you would not receive a fatal electric shock owing to your lowered body resistance through being wet.

Supplementary bonding will connect to the MET via circuit protective conductors and pipework/main protective conductors however this connection is not it's primary purpose.

 

[RMS]

Sounds like someone has mis understood what bonding is.

Sounds like it.

The company in which I served my apprenticeship regarded bonding (supplementary and main at the time) as running two 6mm ² earth wires from the consumer unit. One to the bathroom and the other to the kitchen sink.

 

[RF Lighting]

The company I worked for in about 1998 were still cross bonding kitchen sinks and wash basins in school toilets etc. I've seen all the pipes on the bottom of a boiler main bonded too.

 

[bernardgreen]

That's the purpose - to equalise potential under fault conditions.

What about to equalise potential in the case of an accident ?

A metal sink with the taps fitted into the work top and a plastic drain pipe. As I read it there is no need for this to be bonded ( or "earthed" ) as it cannot introduce any potential into the equipptential zone.

The accident is a double insulted hand held food mixer is dropped into the sink when the sink has water in it deep enough to cover the food mixer and the mixer is plugged in and the socket switch is on.

No safety device will operate. There is no path from the sink to the CPC so there is no "earth" leakage so the RCD has no reason to operate. Unless the water is extremely conductive it is unlikely even a 3 amp fuse will blow.

We have a sink with a potential somewhere between neutral and live. The actual voltage depends on the ratio of areas of live parts and areas of neutral parts in contact with the water.

Touching the sink and at the same time any "earthed" item ( direct via CPC or indirect via bond to an "earthed" item will result in a serious electric shock. Bare feet on a damp floor could also provide a second point of contact sufficient for a severe shock. With luck the current through the person may be enough to trip an RCD if fitted.

If the sink was bonded ( or "earthed" ) then the RCD would (should) have operated within seconds of the mixer entering the water. And even in the fault ( accident created ) earth leakage current was not enough to trip the RCD there would be no risk of serious electrical shock as the sink would be prevented from having a potential signicantly different from any other "earthed" or bonded items in the vicinity.

Doesn't common sense apply here ? Or does blind observance of "the regulations" which no longer require sinks to be bonded become interpreted to mean kitchen sinks "must not be bonded".

And consider when the fault is a fault that has driven the CPC above ground potential. If the sink is not bonded then any electrical leakage through damp work surface and damp walls to real ground could mean the sink is grounded with an impedance low enough that someone touching the sink and an "earthed" or bonded item ( no longer at or near ground potential ) will receive a mild to medium shock. In this case because the potential difference is between true ground and the CPC and does not involve live or neutral the RCD will not operate no matter how high the current through the person's body.

 

[ricicle]

The purpose of bonding is not to assist in the operation of protective devices, that is the purpose of earthing. Bonding is there to limit the size of touch voltages. We need to differentiate between the two because those who do not understand it properly are going to become even more confused !

So, Bernard, do you want to earth the sink or bond it ?

 

[bernardgreen]

Ricicle.

Using a length of wire I would bond the sink to adjacent items such as the water pipes which are, via the bond between incoming water supply pipe and the MET connected to the CPC. Whether this is earthing the sink depends on whether the MET is connected to a ground rod ( in which case it is earthed ) or whether the MET is connected to the incoming supply neutral ( in which case it is bonded and may or may not be "earthed" via the ground rod at the sub station. )

 

[ban-all-sheds]

As I read it there is no need for this to be bonded ( or "earthed" )

Bonding and earthing are not the same.

as it cannot introduce any potential into the equipptential zone.

It will do if you earth it.

 

[bernardgreen]

As I read it there is no need for this to be bonded ( or "earthed" )

Bonding and earthing are not the same.

as it cannot introduce any potential into the equipptential zone.

It will do if you earth it.

If you ground it it might but if you "earth" it by connecting it to something already connected to the main "earth" terminal ( MET ) then it is forced to be at the same potential as all other items bonded to the MET.

I said I would bond it to pipework that is already bonded to the MET ( Main "EARTH" Teminal. Since all the pipe work is at the same potential as the main "earth" teminal then how can bonding the sink to pipework that is bonded to the "earth" terminal introduce any potential that is different from the main "earth" terminal into the equipotential area that is at the potential of the main "earth" terminal due to bonding of the water system to the main "earth" terminal.

 

[RF Lighting]

It won't introduce a potential, but it will now be at a different potential to what it would have been. What if not enough current flows to operate the RCD, or there isn't one or it's not working. Now you've livened up all the pipework, and probably the metalwork too which someone else could be touching instead of just the sink.

What if the flex to your double insulted hand held food mixer has become damaged. You accidentally touch it whilst your other hand is touching the sink. You now receive a much worse shock.

If you're stupid enough to try and fish a live appliance out of a sink full of water without unplugging it first, well it's Dawinism at work.

 

[bernardgreen]

What if the flex to your double insulted hand held food mixer has become damaged. You accidentally touch it whilst your other hand is touching the sink. You now receive a much worse shock.

but no worse than if you touch the tap, toaster or any other bonded metal or any appliance with an earthed metal case.

If you're stupid enough to try and fish a live appliance out of a sink full of water without unplugging it first, well it's Dawinism at work.

True, but the chance of touching the sink and something that is at the equipotential zone potential is also significant. A metal fronted socket for example when turning the mixer off and resting a hand on the sink. In fact if standing on a dry floor there would probably be no sensation when touching the sink or water thrown out onto the work surface but still in contact with the sink. That could result in resting one hand on the half live ( 120 volt ) sink while pulling the plug.