Supplementary bonding in a bathroom

[waterworks]

Hi everyone, please could someone please explain to me whether an earth cable is required from the consumer unit to the bathroom or can you just bond all metal parts in a bathroom together?.

Under the 17th edition regulations (BS7671:2008) supplementary bonding in a bathroom may be omitted where certain criteria are met. These criteria are as follows:

All circuits comply with the requirements for disconnection times
All circuits are RCD protected by a 30mA device maximum
All extraneous conductive parts in the location are effectively connected to the protective equipotential bonding.

The disconnection times for a TN or a TT system should not be difficult to meet, especially as all circuits need to be RCD protected. The effectiveness of the bonding can be determined by measuring the resistance between the extraneous conductive part and the MET. The product of the resistance and the current required to disconnect any circuit within the location should not exceed 50v. Where an RCD is employed, the current required to automatically disconnect the circuit can be taken as the operating current of the RCD, i.e. 30mA. This means if the resistance between the MET and the extraneous conductive part does not exceed 1666Ω then it can be considered to be effectively bonded.
If the resistance exceeds 22KΩ then the part can be taken as not an extraneous conductive part therefore will not need bonding.
If the resistance is between 1666Ω and 22KΩ then supplementary bonding will need to be employed as it is an extraneous conductive part not effectively connected to the protective bonding.
If there are circuits within the location which are not RCD protected i.e. installed under the 16th edition then supplementary bonding is still required.

 

[Taylortwocities]

You do not run a cable from the consumer unit. This could introduce an unwanted voltage into the room.

The purpose of equipotential bonding is to connect together extraneous conductive parts so that there can be no voltage difference between them should a fault occur.


BS 7671 defines an Extraneous Conductive Part as "a conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation".


So that is radiators, water pipes, metal waste pipes etc and also the cpc of ligthing, shaver points etec.

 

[ericmark]

The supplementary protective bonding conductors as shown fig 2.1 will connect to the circuit protective conductors and so back to the main earth and also through the pipes main protective bonding conductors so again will connect back to main earth.

But as the name suggests equipotential bonding gets all exposed parts to the same voltage and does not need to return to the main earth. So with plastic pipes and SELV lights it could be the bathroom does not connect to the main earth although this would be unusual.

Water is contra to popular belief is not a good conductor and 1 foot of plastic pipe will result in near electrical isolation. The problem arrives where a plastic connector is used on metal pipe where the distance is very small then the two sides need bonding together.

I seem to remember there was something on IET web site giving details on earthing of plastic pipes. Thickness also makes a difference and also type of water. I seem to remember reading if there is an earth fault on a boat in fresh water it can kill a swimmer in the water near the boat but not with salt water because it conducts that much better and therefore the gradient is higher. And in salt water you need to touch the boat to get a shock. I know SLD pumps had a problem somewhere in Scotland using a pump with two sensors on the top which should start the pump and a current transformer arrangement which would stop it again when load reduced. It seems the water was too pure and it would not start the pump.

So although 1 foot may isolate in one area may be different in another or when the local water board use different dosing i.e. add fluoride.

Seems odd that a bath room is a special location under Part P in a domestic building but a narrow boat completely surrounded in water is not covered by Part P even in the bathroom!

 

[Taylortwocities]

Here's that IET article - interesting stuff http://www2.theiet.org/Publish/WireRegs/EarthingPlasticPipes.pdf
Edit: Note this was written in 16th edition times. Zones and RCD requirements have changed in the 17th.

PS, ERIC - how did you get a narrow boat into your bathroom?

 

[ban-all-sheds]