Is Equipotential bond to water Required

[PeatBog]

Scenario:

TT Electrical Installation in 2 rooms above carport which is outbuilding to main house.

Main room and smaller toilet with wash hand basin (may be converted to shower later).

Incomming water supply in plastic changes to copper for short length to pressurised water tank (90L) with Immersion heater. Most onward plumbing in plastic, however, hot water from tank to basin tap is copper. All pipe work in cupboard or boxed in. No pipe work leaves the toilet!

The tap is obviously exposed and connected to copper pipe work.

Copper pipe work has continuity with MET (0.02Ohms), earthed via the electrical connection at Immersion heater.

The cooper is not an extraneous metallic part as it doesn't introduce any external potential into the room.

I bonded the copper to the MET just after the stopcock in 10mm2 (just because I'm naturally cautious.

Question:

The question is was it necessary according to the Regs?

(Actually, I think I have answered my own question - It's not extraneous, so doesn't require a bond!)

 

[Apache]

Is this a current job - wired to the 17th with RCD's etc?

when you say copper to MET - you mean MET of submain? You've seperated earthing from house and outbuilding?

Copper pipe work has continuity with MET (0.02Ohms), earthed via the electrical connection at Immersion heater.

why have you done this?

:|

 

[PeatBog]

Yes sorry - Continuity with Main Earth Marshalling Point (or what ever it is called in this situation).

Yes 17th Ed Twin RCD etc.

Lights in Toilet share RCD with Sockets in main room and Waterheater shared RCD with main room lights.

The Main house is also a TT installation. SWA armour earthed at head but isolated at outbuilding. Decided to isolate the two earths as the main house is being redeveloped with Service head position moved etc. It thought it would give me more flexability?? Perhaps, it would have been better to connect the two earth via the armour - they then would have provided a back up to each other. I did check that the armour was 10mm equivalent, i.e. capable of taking fault currents. I still worry a little about them being at slightly different potential and having a equipotential zone spanning 2 building 20m apart!

What do you think?

 

[green-hornet]

If the main building is TT then I would suggest putting in an earth rod for the building that is connected by the swa.

This would give a greater path to earth, and the multiple rods would bring down the Zs for the whole installation.

As for the bonding, if most of the pipework is in plastic I would not bond, there is a possibility of raising the potential if you did.

 

[steptoe]

Scenario:TT Electrical Installation in 2 rooms above carport which is outbuilding to main house.

Main room and smaller toilet with wash hand basin (may be converted to shower later).

Incomming water supply in plastic changes to copper for short length to pressurised water tank (90L) with Immersion heater. Most onward plumbing in plastic, however, hot water from tank to basin tap is copper. All pipe work in cupboard or boxed in. No pipe work leaves the toilet!

The tap is obviously exposed and connected to copper pipe work.

Copper pipe work has continuity with MET (0.02Ohms), earthed via the electrical connection at Immersion heater.

The cooper is not an extraneous metallic part as it doesn't introduce any external potential into the room.

I bonded the copper to the MET just after the stopcock in 10mm2 (just because I'm naturally cautious.

Question:

The question is was it necessary according to the Regs?

(Actually, I think I have answered my own question - It's not extraneous, so doesn't require a bond!)

I thought the mini would be downstairs in the car port.!

but yes, the copper is extraneous, it will introduce a potential under fault conditions, that is the whole point of bonding, protection under fault conditions. it MUST be bonded.

 

[sparkyork]

its all about the numbers. below 22k ohms and it needs a bond, if its more then it dont (i only base this on plastic incoming water pipes) i personally bond up to 30 k ohms and not if its anymore.

 

[batty]

Is this a current job - wired to the 17th with RCD's etc?when you say copper to MET - you mean MET of submain? You've seperated earthing from house and outbuilding?

why have you done this?

:|

Patch RCD'S are not relevent to bonding you cannot realie on RCD for total protection they are for supplementary protection. All main bonding is still needed.

Batty

 

[steptoe]

if you are not exporting your earth, TT, then the outbuilding is classed as a separate installation, and therefore it requires main bondoing as per 7671,

UNLESS,

that is of course you wish to deviate from the BRC that is.

 

[PeatBog]

Thanks for the replies so far. So I now understand the following, please correct me if I'm wrong:

1) Before I connected the Immersion heater (IH), the copper pipe and tank were NOT extraneous so didn't require bonding (OSG Page 27 bottom of page). If I had tested the resistance to MET/MEMP it would have been high (>22KOhmns)).

2) Once I had wired the IH it became "Earthy" due to the Internal design of the IH connecting the CPC to the tank. The resistance to MET now being much lower than 22KOhms (0.02Ohms!).

It is now extraneous and needs to be bonded (equipotential bonding, that is, not supplementary bonding).

Sparkyork - How is the magic number of 22KOhms derived?

 

[sparkyork]

since 411.3.1.2. does not define a numerical criterion, we apply Ohm's Law: if RCP > ((UO/IB) - ZTL) then the conductive part is not liable to introduce an earth potential and does not constitute an ECP.

where RCP is the resistance between metalwork and MET. Typical values are: UO (phase voltage), 230 volts;

IB (current through the human (or livestock) body which should not be exceeded), 10 mA; and ZTL (human/livestock body resistance), 1000 .

Substituting in the equation, RCP must be > ((230/0.01) - 1000), ie > 22 kil, for ECP bonding not to be required.

If water is involved (eg a bathroom) it may be safer to use IB = 5 mA and ZTL = 200 , which requires bonding below RCP = 45.8 Kil.

copied and pasted from other thread next doorGuiness Drink

 

[green-hornet]

Thats why some schools and hospital rooms are protected by 10mA rcd and not 30mA.

If I am rambling its because its friday and I have had a drink.

 

[PeatBog]

Thanks for the clear explainantion of 22kohms. I would do a proper electronic thankyou if only I knew how!!!

 

[Apache]

Thanks for the clear explainantion of 22kohms. I would do a proper electronic thankyou if only I knew how!!!

at the bottom right hand side of the helpful post there is a thanks button

click it