Bonding bath

[bureaucrazy]

I need to replace a wooden shower cubicle with a bath. From what I understand, If I get a steel (not plastic) bath, I need to get the bath equipotentially bonded with other metal in the bathroom. The only metal existing is the copper pipes that supply the basin and toilet fill. These have safety earth clamps on them in the cellar and are earthed back to the consumer unit.

I have read the posts already on this forum about this topic and there seems to be far too many different opinions for me to be sure about what work is specified so I hope you can staightforwardly clear this up for me.

It would help if someone could explain why this bonding is necessary although several ideas spring to mind, please correct the following if my understandig is wrong:

1. Metals must be earthed in case they somehow contact a live wire and would electrocute someone who touched a live metal bath. e.g. someone uses an extension lead to bring a hairdryer into the bathroom and drops it into a full bath and then grabs hold of the bath. The earth connection causes a current in excess of the rated fuse of the circuit with the hairdryer on to will trip the switch on/blow fuse on the consumer unit switching off the live voltage and saving the person from death.

2. Static charges can build up on insulated metalic objects causing differences in potential so if someone touches 2 objects with diferent potential they get a shock (but not usually life threatening?)

I read that all circuits in the bathroom must be bonded together e.g. the light, the shower plus the metal objects. Is this done by running a wire from the earth on the light fitting to the earth on the shower pull cord and then to the copper pipe and then the bath? Or do I just need to put an earth clamp on the copper pipe and attach an earth wire from that to the bath? If the circuits for the shower and light and the copper pipe are earthed onto the consumer unit then they must be at the same 0v earth potential right? Therefore there will be no potential difference between those earths and also the any live voltage coming into contact with an earthed part will flow to earth and blow a fuse. If this is so why is it necessary to run extra bonding wires between earth terminals of the circuits and the pipe and the bath aswell? Could bonding the bath to the wrong thing be dangerous? e.g. workmen dig up road and live wire contacts with water main, copper pipe is then live and so is bath? Surely the earth link back to the consumer unit will be the shortest path?

Also, I noted that the shower circuit and the lighting cicuit (which is for the whole house) only have mcbs but no rcd. Is it simply a matter of replacing each mcb with an rcbo of the correct value and I can forget about the bonding?

Which parts of this job need part p? I am hoping the answers will be very straight forward at least so that I am confident to ask an electrician for exactly what needs doing as I will just be using the yellow pages and won't know them from adam.

thanks

 

[philip962]

The D/B does need an rcd not sure if replacing with rcbo will do basiclly you have got it right about the bonding but even if you do upgrade to rcbo you still need to have the bonding done any work in the bathroom needs labc sign off via napit elecsa niceic as it is a special location so get a spark that is registered and check with the scheme what they are able to do

 

[JUD]

If all the circuits in the bathroom are RCD protected and all disconnection times are met then supplementary bonding is not usually required.

 

[sparkylad]

1st Step : Confirm if the copper pipes are what are termed extraneous conductive parts...this means that they are actually introducing a potential which will allow a harmful current to flow in the event of fault. To do this you will need to measure the resistance from the pipes in the bathroom back to the Main Earth terminal at your fusebox position using a test meter and long lead. If your reading is less than 22000 ohms then supplementary bonding will be required between the pipes and to the metal bath and to the Shower cpc and the light cpc.......if the reading is more than 22000 ohms then you will require supplementary bonding only between the Shower and light Cpc s.
In your case you have no RCD protection to your bathroom circuits ,if you did provide them with rcd protection, and the disconnection times were met and the Main Equipotential Bonding is installed then supplementary bonding can be omitted.

 

[dichroic]

Supplementary equipotential bonding (‘supplementary bonding’ for short) is considered as additional protection against electric shock under fault conditions, which is required by BS 7671 in certain situations. Supplementary bonding involves connecting together the conductive parts of electrical items and non-electrical items, to prevent the occurrence of a dangerous voltage between them under earth fault conditions.

There are two situations in which supplementary bonding may be required by BS 7671. The first is in locations where there is considered to be an increased risk of electric shock . The second is where the conditions for automatic disconnection of Regulation Group 411.3.2 cannot be achieved by a protective device.

Regulation 701.411.3.3 requires additional protection against electric shock to be provided for all circuits of the location, by the use of one or more RCDs having the characteristics specified in Regulation 415.1.1 (that is, RCDs having a rated residual operating current (

) not exceeding 30 mA and an operating time not exceeding 40 ms at a residual current of 5

).

Supplementary bonding is permitted to be omitted by Regulation 701.415.2, where the location containing a bath or shower is in a building with a protective equipotential bonding system installed in accordance with Regulation 411.3.1.2 and all of the following conditions are met:
•all final circuits of the location comply with the requirements for automatic disconnection according to Regulation 411.3.2•all final circuits of the location have additional protection by means of an RCD in accordance with Regulation 701.411.3.3•all extraneous-conductive-parts of the location are effectively connected to the protective equipotential bonding according to Regulation 411.3.1.2.


Regards

Dichroic