Shower Supplementary Bonding

[baggy99]

Hopefully a simple question......
New electric shower fitted in bathroom. The work is notifiable as there has been other rewiring done too. All accessible metalwork will be eq bonded, running back to the shower mains supply (ceiling mounted dp switch) BUT.....
do I need to bond the water inlet to the shower INDEPENDENTLY from the shower itself? The short piece of visible pipework where it enters the bottom of the shower IS accessible, but the shower itself IS obviously earthed via the cpc. (However, the feed is 6mm T&E which has a 2.5mm cpc doesn't it? Given that the recommendation for eq bonding is 4mm csa, does that mean the cpc in the 6mm T&E isn't up to the task, even though the main local earth is only 3' away??)
Any help appreciated! Thanks.
PS I know an electric shower is supposed to go an an RCD, but the new CU has a split rail and the shower obviously runs off the RCCB side. Am I right in thinking this is sufficient?

 

[sm1thson]

^bonding and earthing are different, they both use G/Y wires and generally end up at the same place, but earthing is about a path to earth to cause current flow and trip the MCB, bonding is so anything metal which may introduce a voltage from else ware will be the same voltage as other like items so there is no voltage between them to get a shock from.

Weather or not you can put the shower on the RCD side of the splitload depends on the current capacity of the present RCD, if this is two low for all the circuits on that side then you will either need to replace it or put the shower on the none RCD side but on its own RCBO (better IMO).

If all bathroom circuits are RCD protected you can do away with bonding in the bathroom. Putting the lights on the RCD side of your splitload wont do this as then that breaks the regulation (ok its debatable, but I doubt Mr BCO will accept it, even if he does its crap so you don’t want it) about minimizing disruption (shower trip=no light =not good). Again an RCBO (for the lights) on the none RCD side of the split load will sort this out.

 

[baggy99]

V interesting - thanks.
I am intrigued by the possibility to do away with the bonding altogether (would save me a pile of work.) You appear to be saying that if I put the shower & the lights off the non-RCD side of the rail (both on separate RCBOs,) I don't need ANY supplementary bonding?? Literally none - radiator, sink/bath taps & shower?
I think I read this elsewhere but I assumed it must be wrong or I had misunderstood! Isn't it still possible, in an extreme case, for say the taps & the rad to be at different potentials if they are not earthed locally? I thought that was the whole point of using a "local" earth and not trailing the g/y cable back to the CU? Similarly, if the cold water feed to the building and the CH & DHW pipes are bonded at the boiler, why (under the old scheme) would you ever have needed to bond locally?
I'm not doubting what you're saying - I'm just interested in the theory. Is this a 17th ed change?
Thanks

 

[Spark123]

You say the shower if fed from the RCCB side of the board? An RCCB is an RCD (RCD is just a generic term which covers a range of devices including RCCBs and RCBOs.)
Supplementary bonding can be ommited from bathrooms under the 17th edition regulations as long as certain conditions are met. ONE of these conditions is that all circuits within the location are all RCD protected (30mA max).
Another condition is that all circuits must meet disconnection times (not much of an issue if they are RCD protected).
Another condition is that the effectiveness of the bonding of any extraneous pipework within the location is confirmed.
//www.diynot.com/wiki/electrics:supbond17th

 

[ricicle]

Why does the wiki say that an RCBO cannot be used for protection in a bathroom?

 

[Spark123]

Where?

 

[ricicle]

//www.diynot.com/wiki/electrics:earthing_supply_types_and_bonding:supplementary_bonding

 

[RF Lighting]

How odd.

Maybe something to do with SP RCBOs not being allowed in TT installations when backed by a 100mA type S, and someone getting all muddled up? (Eh Paul?)

 

[Spark123]

This is for the other type of supplementary bonding which limits touch voltages within the installation to 50v to the MET in the event of a fault when ADS cannot be met, not bathroom supplementary bonding.
iirc RCBOs are normally electronic hance need about 50v for the RCD aspect to remain operational which is why they get a mention in there.

 

[ricicle]

May be I should have read a bit more carefully

 

[baggy99]

Oh dear - and I thought I'd asked a simple question !!

I am now confused by the following:

1) Are we now saying I can't run the shower & the bathroom lights of RCBOs? If so, what are the options? Separate RCCDs with separate MCBs? (In which case I may not have room in the existing 10 way 16th ed CU......)

2) How am I supposed to measure the resistance between the extraneous/exposed metalwork and the MET? I can cope with measuring between the metalwork and the local earth, but surely the very act of adding a 50' extension off the meter probe (ie back from the bathroom to the MET) will introduce it's own resistance. Do I simply sum this out (ie deduct the resistance of the probe extension from the readings?)

3) Presumably this info will need to be recorded somewhere to satisfy the BCO that local bonding is not necessary? Where? No obvious place for it on the Domestic Electrical Installation Certificate?

Thanks

 

[Spark123]

1) No, they are fine for use with what you want. There are two main applications for supplementary bonding, the RCBO issue is to do with the other type - not the bathroom type. (Ignore the last 6 posts before your last one!)

2) Yes, that is called the wander lead method. Subtract the lead length resistance from the result.

3) You can make a comment on the EIC if you wish.

 

[baggy99]

That's great - thanks all. So, to clarify, I'm OK with the following course of action:

- Gas & water bonded to the MET where they enter the building. (10mm csa back to a labelled earthing terminal, then 16mm csa back to the supplier's MET.)
- Bathroom light & shower (the only electrics in the bathroom) run from RCBOs off the non-RCCB side of the CU.
- Resistance readings taken from all exposed metalwork in the bathroom back to the MET and readings noted on the EIC. Provided all are <1666 Ohms there is no need for supplementary bonding. (Absolutely NONE - no clamps on hot/cold pipes, radiator, shower feed, etc.)

Is that all as per 17th ed - not likely to raise any issues with the BCO?

Thanks again.

 

[Spark123]

Think you have summed it up OK, if the resistance is between 1666ohms and 22k ohms then it needs to be supplementary bonded.

 

[baggy99]

Brilliant. Thanks again. The cost of a couple of RCBOs is peanuts compared with the hassle involved in fitting earth bonding now the floors are down & the walls plastered!

Just one final question - how should the RCCB be labelled? Obviously every MCB & RCBO will be labelled with the name of the circuit it supplies. Is it OK just to label "RCCB" under the device or should there be a fuller description? (I am aware of the need to place an RCD test label in the vicinity, but I am unsure about how the device itself should be labelled.)

Cheers again.