Anybody disagree with any of this?

[ebee]

Supplementary Bonding Bathroom Radiators?​

7 February 2017 xxxxxxx
This may not be an issue in newer bathrooms that comply with current editions of BS7671 as supplementary bonding in bathrooms may be omitted when regulations 701.415.4.5 (iv), (v) and (vi) are met.
However, supplementary bonding the radiator has always been a grey area especially in older bathrooms because the radiator may or may not be an extraneous-conductive part. This is because in certain conditions it may not introduce an earth potential into the environment (for example with plastic heating pipes).
How can we check if a radiator (or anything) is an extraneous-conductive part?
A radiator may be insulated from earth due to the building/construction materials used. To check the effectiveness of this insulation the following formula can be used…
Rcp > (Uo/Ib) – Zt
Where…
Rcp = Resistance Ω of the radiator to the Main Earth Terminal (MET)
Uo = Nominal Voltage to Earth (230V)
Ib = Max value of current through human body, 30mA (lower values of 0.5mA and 10mA can be used)
Zt = Impedance of human body usually 1000Ω
So the minimum resistance between the radiator and the MET of the building is
(230/0.03) – 1000 = 6666Ω or 6.67kΩ
Anything less than this and the radiator can be considered to be an extraneous-conductive part and as such if it also doesn’t comply with 701.415.4.5 (iv), (v) and (vi) then supplementary bonding will be required.
How to check if existing supplementary bonding is effective
To check the effectiveness of the connection of extraneous-conductive parts to earth in a bathroom have a look in Part 7 at the NOTE at the bottom of regulation 701.415.2
This sends you to regulation 415.2.2
R ≤ 50/Ia
If there are RCDs protecting the bathroom then Ia would be 0.03 (30mA) so R ≤ 50/0.03 = 1667 ohms
If it’s an older bathroom and there are no RCDs then R ≤ 50 divided by the current needed to disconnect the protective device (fuse or circuit breaker) within 5 seconds. See the current / time disconnection graphs in Appendix 3.

 

[EFLImpudence]

JohnW2 has (to my satisfaction) previously proved that R<50/Ia is a fallacy and does not work.

That no one else ever seems to have realised this is a mystery.

 

[ebee]

Thanks for the reply ELFI , interesting . I wasn`t particularly thinking about quite that exact part of it but very close though.
I will not say more until I get a few more answers.

 

[EFLImpudence]

How can we check if a radiator (or anything) is an extraneous-conductive part?

Zt = Impedance of human body usually 1000Ω
So the minimum resistance between the radiator and the MET of the building is
(230/0.03) – 1000 = 6666Ω or 6.67kΩ

You do not divide by 0.03 but rather 0.005 or 0.01 which ever you think is the desired value.

 

[ebee]

Yes, I knew you would see it.
I`ve never seen it before that somebody has suggested we actually work on the 30mA figure as being permitted indefinitely although the inference is that it is actually allowed if we consider a 30mA RCD gives protection. In practice 30mA RCDs do tend to trip on less than that figure and as far as I can figure it is somewhere between 21mA and 27mA in most instances (some of you may have more accurate results because my ramp test on my meter ramps up on 30mA range in 3mA steps).
I think your 10mA is probably the figure that many of us would start to feel more comfortable with as an indefinite upper figure absolute maximum sort of thing - 5mA being more like it.

50mA being more the "Oooh Ouch Keep Away!" sort of figure, hence a 30mA RCD for our own protection on a 95% basis of hoping we can survive it sort of feeling.

 

[bernardgreen]

Zt = Impedance of human body usually 1000Ω

I disagree with that,

The area of contact with the skin and the moisture content of the contact areas must be taken into account

Ear to ear can be as low as 100Ω
Hand to opposite foot can be as low as 500Ω

 

[ebee]

Yes agreed too. I think that the 1K figure might be OK to work out a ball park figure for common actual typical events it is not sacrosanct by any means.

 

[bernardgreen]

I think your 10mA is probably the figure that many of us would start to feel more comfortable with

Death can result from a 10mA current flowing through the body ( across the heart ) for several seconds, A domestic RCD will allow that,

 

[ebee]

Given that there might be a 5% fail to save a life figure on compliant RCDs and given that there might actually be a 7% worldwide failure risk on RCDs working correctly anyway I`d not be happy to calculate to allow 30mA indefinitely in any system if we can reduce it in actual practice.
Others probably disagree though.

 

[EFLImpudence]

Yes agreed too. I think that the 1K figure might be OK to work out a ball park figure for common actual typical events it is not sacrosanct by any means.

I also do not agree with it - or rather using it.

People's body resistance varies and the difference it makes to the value of a purely arbitrary figure - 23kΩ, 22kΩ, 46kΩ, 45kΩ - means it is not worth considering.

 

[EFLImpudence]

So - a pretty pointless article.

 

[ebee]

Well I found it online with some other stuff that looked OK on an Electrician`s website but this bit just made me look twice and think Hah! not sure about this here bit

 

[JohnW2]

JohnW2 has (to my satisfaction) previously proved that R<50/Ia is a fallacy and does not work. ... That no one else ever seems to have realised this is a mystery.

Indeed - and, in fact, ebee himself has just, in another thread, acknowledged the "fallacy of R<50/Ia", by recognising that the 'touch voltage' depends only upon R2/R1 (i.e. on relative CSAs of CPC and L conductor).

Kind Regards, John