Supplementary earth Bonding query

[winmill]

my query is about: all circuits entering the bathroom need to be bonded, and all exposed metalwork entering the room, including the radiator.

if this is the case the earth conductor in the electrical circut returns to the earth terminal in the consumer unit, so why is it that the 4mm y/g bonding conductor must not be run back to the earth terminal in the consumer unit?

 

[RF Lighting]

They don't.

In most cases supplementary bonding is not required if certain other regulations are adhered to such as the use of RCDs and the installation of main protective bonding to all extraneous conductiveparts of the installation.

If supplementary bnding is required, it would not be run to the MET as that is not an ECP in the bathroom.

You are BONDING the pipes, not EARTHING them. There is a LOT of dfference.

 

[RMS]

The correct term is 'supplementary equipotential bonding'.

The regulations are clear on the omission of this providing the following conditions are met,

-All final circuits of the location meet the requirements for Automatic disconnection of the supply. (ADS). (A measurement of Zs will confirm this)

-All final circuits have additional protection by mean of an RCD (30mA trip rating).

-All extraneous parts within the location are connected (effectively) to the protective equipotential bonding of the installation.

The first two conditions are clear but the last condition may need you to carry out checks with use of test equipment to ensure this condition is met.

 

[Spark123]

If supplementary equipotential bonding is required, the heating pipes (if metallic) need to be bonded, not the radiator.
You need to be careful with the term "exposed metalwork" as in electrical terms, an exposed conductive part is a metallic object which forms part of the electrical system. Extraneous conductive parts are metal objects which can introduce a potential but are not part of the electrical system such as metal water pipes and metal building structures. You would need to bond the pipework feeding the radiator as these are what can introduce the potential. If the pipework is plastic such that is provides enough insulation from a metal system (and it isn't in contact with a metallic building structure) the metal radiator cannot introduce a potential and doesn't require bonding.
If the criteria which others have said is met then supplementary bonding isn't required anyway.

 

[EFLImpudence]

all circuits entering the bathroom need to be bonded,

You cannot bond a circuit.

and all exposed metalwork entering the room, including the radiator.

'Exposed metal part' is the term given to metal parts of fittings - lights, switches etc. which will already be earthed.
You are asking about 'extraneous metal parts' those which are liable to introduce a potential - pipes and other non-electrical metal parts, but not necessarily just because they are metal.

if this is the case the earth conductor in the electrical circut returns to the earth terminal in the consumer unit, so why is it that the 4mm y/g bonding conductor must not be run back to the earth terminal in the consumer unit?

If you are asking why the supplementary bonding is NOT run back to the earth terminal in the CU (MET), it is because the bonding is to equalise the potential between parts (in a location) which are already connected to the MET.

 

[RF Lighting]

all circuits entering the bathroom need to be bonded,

You cannot bond a circuit.

Yes you can.

 

[bernardgreen]

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.

 

[EFLImpudence]

Well, only the cpc. which isn't really 'the circuit'

 

[EFLImpudence]

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.

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

 

[RMS]

Well, only the cpc. which isn't really 'the circuit'

cpc = circuit protective conductor

 

[EFLImpudence]

Well, only the cpc. which isn't really 'the circuit'

cpc = circuit protective conductor

Semantically - If it protects 'the circuit', presumably it is not 'the circuit'.

 

[Starspark1]

But surely under fault conditions the cpc becomes part of a circuit. Something which is designed for therefore the cpc can be expected to be part of the circuit.

 

[EFLImpudence]

But surely under fault conditions the cpc becomes part of a circuit. Something which is designed for therefore the cpc can be expected to be part of the circuit.

Yes, you're obviously correct.

I just don't think that it is 'the circuit' which is bonded - it is the exposed conductive parts.

My reply "You cannot bond a circuit" related to this query -

my query is about: all circuits entering the bathroom need to be bonded, and all exposed metalwork entering the room, including the radiator.

 

[xr4x4]

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.

 

[RF Lighting]

But surely under fault conditions the cpc becomes part of a circuit. Something which is designed for therefore the cpc can be expected to be part of the circuit.

Yes, you're obviously correct.

I just don't think that it is 'the circuit' which is bonded - it is the exposed conductive parts.

My reply "You cannot bond a circuit" related to this query -

my query is about: all circuits entering the bathroom need to be bonded, and all exposed metalwork entering the room, including the radiator.

Yes, and under the 16th all circuits entering the bathroom did need to be bonded.