earth bonding

[hoteldave]

I am fitting a wash hand basin in one of my bedrooms.
It has been suggested to me that I need to earth the pipes at the wash hand basin. The copper pipes are already earthed further down the line as I am tapping into these pipes is there a necessity to earth again. If so do I take the earth wire back to the fuse box or can I take it to the nearest power socket.

 

[mapj1]

In a bedroom, unless there is also a shower or bath, bonding not compulsory. Note you only get the full benefit of cross bond elsewhere if the joints are metallic (and no PTFE tape in compression fittings!).

 

[felix]

This whole bonding business seems to be a source of much confusion. After reading many posts on the subject and applying some elementary circuit theory I've come to the following conclusion. The purpose of bonding is to minimize voltage drops between earthed electrical appliances and any other earthed metal within reach.

Consider this scenario. You have a faulty appliance, eg a kettle. It develops a live to earth short and you are holding it at the time. You also have your other hand on a tap. "No problem", I hear you say. "The kettle's earthed isn't it?" Correct, but at that precise moment a very large fault current is running away down the earth wire and dropping a lot of volts; at leat 120 volts. You are holding 120 volts in one hand and an earthed tap in the other. This is not nice.

What you need here is a good solid connection between the earth pin in the socket and the tap, even though both are already earthed somewhere else. Now look what happens:

1) If the tap has a first rate earth through copper pipe all the way back to the electric meter (which it probably does) that fault current can take the easy route along the bonding wire to the tap and down the pipe. The voltage between your hands is now a much smaller voltage dropped in that short, thick bonding wire. Most of the volts are dropped in the longer, thinner live wire(s) supplying the socket.

2) If the tap's earth connection is dubious (PTFE tape in compression joints) there will still be 120 volts on the faulty kettle but, thanks to that short, thick bonding wire, there are also 120 volts on the tap! Unless you have your foot on an unbonded radiator pipe you are floating safely at 120 volts.

Did that make sense?

 

[Big_Spark]

Realistically you have two choices, rely on the bonding already in place and leave well alone, or bond across these pipe, but run a leg of Earth wire for one earth Clamp to the Earth Terminal of the nearest socket to ensure that the pipes are not forming an Earth Free Equipotential zone, something that should be avoided as it increases shock risk considerably.

 

[hoteldave]

Thanks to everyone for there assistance.

 

[Damocles]

Well you could run an extra earth from every single socket in the house to every pipe. This is daft. The earth conductor in the cable has been chosen so that under normal conditions you will come to no harm should you be holding your metal hi-fi while leaning on the radiator. Abnormal conditions means doing all this while water is spraying everywhere from a leaky pipe.

Supplementary bonding of washbasins is not required. I presume because experience shows that people just do not come to harm from incidents like this. It is only considered useful when there is a possibility of people being soaking wet at the time, like in a bathroom.