IEE 18th, 701.415.2 p241 - sup bonding bathroom

[John-]

Hi

Old time 15th Ed. working way through 18th in prep for 18th test, also been a while since i wired in anger so to speak .

701.415.2 states that i can omit supplementary bonding in my bathroom so long as i have a protective equipotential bond compliant to 411.3.1.2 and finals in the location and running through are protected by RCD and all finals in location are compliant with 411.3.2 and that all extraneous conductive parts in the area are 'effectively' connected to protective equipotential bonding according to reg 411.3.1.2 and that effectiveness is assessed based on the 415.2.2 calc.

It is a TN-C-S supply, I have RCBOs throughout, and old bond on my cold water coming in on my side of the water, even though it is plastic pipe into the building. There are older cross bonds in the airing cupboard, main bond on my gas (again plastic coming in) and even another bond at my boiler.

I know i have plastic pipes feeding my large centrally heated towel rail in the bathroom as they were recently changed. When i measured the resistance between the rad and that of a local CPC, it was about 140KOhm, well above 1,666 Ohms as per the calc.

Questions:

Is my non-electrically connected (it has no separate 230Vac heating element) towel rail radiator classed as an extraneous conductive part?

If it is, then does this mean that technically i should provide a bond between it and the CPC of the local LV circuits?

If answers are Yes and Yes, why please?

Thanks

John

 

[Deleted member 26818]

It would be extraneous, but whether it could introduce a potential is another matter.
From what you say, the resistance is such that it would not be likely to introduce a potential, so does not require bonding.

 

[Wilko]

Hi - if it's connected via plastic pipes I don't see how it could be extraneous, maybe I've missed something?

 

[Wilko]

And cheekily, I wonder if the 140k Ohms you've measured is your skin resistance .

 

[John-]

And cheekily, I wonder if the 140k Ohms you've measured is your skin resistance .

Cheekily back, no. Deff just water in the pipes

 

[Deleted member 26818]

Hi - if it's connected via plastic pipes I don't see how it could be extraneous, maybe I've missed something?

It’s extraneous, because it’s not part of the electrical system.

 

[Marcus Vaughan]

It’s extraneous, because it’s not part of the electrical system.

I thought extraneous meant it was capable of introducing an earth potential - if it can't - it's just a piece of metal?

 

[John-]

It’s extraneous, because it’s not part of the electrical system.

Is it though? It is not connected to the building, nor any metal pipe work. It is a lump of metal in a bathroom. It could well be a metal window frame. Whats is it about the radiator that makes it an extraneous conductive part?

 

[Midwest]

If you bond that, you'll be the first person in the country to do so since the infamous 16th was resigned to ignominy of poor taste.

 

[Deleted member 26818]

Extraneous-conductive-part. A conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation.

Definition of Extraneous:
1) irrelevant or unrelated to the subject being dealt with.
2) of external origin.
3) not directly connected with or related to something

 

[Marcus Vaughan]

Where doubt exists about an extraneous conductive part

There may be doubt if an item of metalwork within an electrical installation is an extraneous-conductive-part. For example, it may be suspected that plastic pipework has been installed somewhere along the run of metal pipework. Where there is such doubt, a test instrument can be used in conjunction with the following formula:

Rx ≥ U0
Ilimit – Rb
Where,
Rx = the resistance between the metallic part in question and the MET;
U0 = the nominal voltage line to Earth;
Rb = the resistance of the human body;
Ilimit = the value of current through the
body which must not be exceeded.

The following values are given in British Standard Published Document PD 6519, IEC 60479, Guide to effects of current on human beings and livestock:

U0 = 230V;
Rb = 1,000 ohms (for hand-to-hand
contact in dry conditions);
Ilimit = 0.01A (the let-go threshold).

These values give the following maximum
value for Rx:

Rx ≥ 230
0.01 – 1,000
≥ 23,000 – 1,000 ohms
≥ 22,000 ohms

A reading below 22,000 ohms will mean the metallic item is an extraneous-conductive-part which does require bonding.


Full article can be read in Professional Electrician and Installer. Search for “Effectively Connected Extraneous Metalwork”.

More than 22000 ohms = not extraneous.

 

[Midwest]

I'd just say, if someone installed a cast iron roll top bath, fed by plastic pipes in the middle of a plastic laminated floor bathroom, would you feel inclined to test to see if it was extraneous? Not withstanding its feet might need 'podged'.

 

[Deleted member 26818]

That takes account of the resistance of the body of an average, healthy, adult male (1kohm).
The NICEIC use a formula that brings the figure down even more, to 7.67 kohms if all circuits are RCD protected and using 30mA.
When you also account for the body’s resistance, you get 6.66 kohms.

 

[Marcus Vaughan]

That’s interesting Spin. Thankyou.

 

[Midwest]

There are also those videos by John Ward & Christopher Kitcher, and in Guidance Notes 8 earthing & bonding (I think), which gives guidance on this subject (minimum resistance for electric shock etc). Can't say the page number, haven't my copy to hand.

 

[John-]

Thanks for all the responses. Glad to see some science behind it too.

So. According to one document by calculation when using RCDs if greater than 6.66Kohms (Damien's telephone number) it is not classed as an extraneous part. If not using RCDs if greater than 22KOhms not classed as an extraneous part.

So for my scenario (using RCBOs) if the value was between 1,660Ohms and 6,666Ohms i would have to supply a bond to bring it back down to an acceptable resistance (below 1,660Ohms).

@spinlondon Do you have any reference for the NIC doc, i would like to make some notes in my regs book.

Thank you.

John

 

[Deleted member 26818]

Here’s the thread that mentioned it:
Extraneous....Yes or No.... - https://www.electriciansforums.co.uk/threads/extraneous-yes-or-no.16558/