Supplementary Equipotential Bonding

[seftonbarn]

I recall being taught that if you measure continuity between the MET and extraneous-conductive parts in a bathroom and, as a rule of thumb, it is either less than 1667 ohms or more than 1 M ohm, then you do not need supplementary equipotential bonding. I think I understand that the 1667 ohm figure comes from Reg 415.2.2 (R</ 50 VIa =50/0.03). Not really sure where the 1 M ohm figure comes from. Reading 415.2.2 though, I see that the 0.03 is if you have RCD protection. Well the bathroom does not have RCD protection yet (justifying a C3 I guess as it is compliant under older regs(or is it a C2)). Using the same formula R</ 50 VIa I now use App 3 to determine the current from a 6A BS60898 (MCB) to trip in 5s so Ia is 30A. So now we have 50/30=1.66 ohms. Testing between extraneous conductive parts in the bathroom and the MET I get greater than 1.66 ohms. So, according to 701.415.2 (vi) I need to apply supplementary equipotential bonding. Am I right so far?

Reading 415.2.2 it explains how to determine the effectivenessss of supplementary equipotential bonding but states that it is only required between simultaneously accessible exposed conductive parts and extraneous conductive parts. But I have only got a lighting circuit with a pull chord switch. So I have no simultaneously accessible exposed conductive parts and extraneous conductive parts, so I don't need supplementary equipotential bonding. Is that right?

I will add that I intend to fit an RCD but I'm just trying to understand the Regs.

 

[Jabbajaws]

I beleive that supplimentary bonding can be omitted now, as long as main equipotential bonding is in place with 30mA RCD protection.

And if anyone has already told you this in the time it takes to write this:

im a slower typer...

 

[seftonbarn]

I beleive that supplimentary bonding can be omitted now, as long as main equipotential bonding is in place with 30mA RCD protection.

And if anyone has already told you this in the time it takes to write this:

im a slower typer...

Thanks Jabbajaws. Happy with that but the bathroom has no RCD.

 

[Sparky Ninja]

Can you maybe break this down easier to understand.

Why are you suggesting a required disconnection time of 5 seconds?

Is this an inspection only or are you carrying out work?

 

[telectrix]

So now we have 50/30=1.66 ohms.

no, it's 230/30 = 7.667ohms.

 

[sedgy34]

you will have to upgrade the bonding, before carrying out any works!! also most on here will say rcd protection for the bathroom i say unless your putting a switch wire in the wall dont bother. tin hat on lol

 

[seftonbarn]

It's my own house. I'm just trying to understand the Regs.

In order to determine the effectiveness of supplementary equipotential bonding 415.2.2 tells you to use the formula R</ 50 V/Ia (where Ia for an RCD is the tripping current (hence the 0.3 in the first calc (1667 ohms)). But if you have no RCD then 415.2.2 says Ia is the current causing automatic operation in 5 secs of the overcurrent device. From App 3, for a 6A BS60898 MCB this is 30A. So 50/30 = 1.66 ohms which I am exceeding. So I guess that means I need supplementary equipotential bonding. But if I only have a lighting circuit with a pull chord there is no simultaneously accessible exposed conductive parts and extraneous conductive parts and so there is no need for supplementary equipotential bonding. Are my assumptions correct?

 

[telectrix]

as i said before. max. Zs is 230/In. so for a 6A 60898 type B that 7.67 ohms.

 

[ackbarthestar]

So now we have 50/30=1.66 ohms.

no, it's 230/30 = 7.667ohms.

Touch voltage surely ?

However the upper limit is regarded as being 10mA maximum current flow from GN5 and GN8 for wet body conditions

which makes it

Rx > Uo /0.01 -1k ohms (body resistance 95% of population) = 22k ohms

measured in Zooilliamps :laugh:

 

[sedgy34]

use tels formula and refer to BGB

 

[seftonbarn]

So now we have 50/30=1.66 ohms.

no, it's 230/30 = 7.667ohms.

Oh, me getting even more confused now, me thought that nominal voltage (230 in a domestic) was refered to as Uo for calcs in the regs. So whats the 50V ac bit mean then in 415.2.2? Also there are a lot of references to the magical figure of 1667 ohms on the forum which I thought came from 50/0.03 and the formula in 415.2.2?

 

[telectrix]

Touch voltage surely ?

However the upper limit is regarded as being 10mA maximum current flow from GN5 and GN8 for wet body conditions

which makes it

Rx > Uo /0.01 -1k ohms (body resistance 95% of population) = 22k ohms

measured in Zooilliamps :laugh:

it's 2 different situations. max. Zs is to ensure disconnection of supply in the event of a fault of negligible impedance. supplementary bonding is to reduce the touch voltage between simultaneously touchable extraneous parts.

 

[seftonbarn]

it's 2 different situations. max. Zs is to ensure disconnection of supply in the event of a fault of negligible impedance. supplementary bonding is to reduce the touch voltage between simultaneously touchable extraneous parts.

Thanks Tel. Thought I may be getting mixed up between different formula. And sorry about the overlap of replies earlier. I am happy with all the formula now and whichever one I use it points at the need for SEB (when there is no RCD); but if there are no simultaneously accessible exposed conductive parts within touching distance of extraneous conductive parts, I assume I don't need SEB. Is that right?

 

[ackbarthestar]

it's 2 different situations. max. Zs is to ensure disconnection of supply in the event of a fault of negligible impedance. supplementary bonding is to reduce the touch voltage between simultaneously touchable extraneous parts.

Yes, but the 3 conditions are required if Supp bonding is to be eliminated. from 701.415.2

1/ Disconnection time complying with 411.3.2.2 (this is your bit - max Zs)
2/ MPB in place to 411.3.1.2 (This is my bit - touch voltage)
3/ RCD to 415.1 (this provides compliance with 701.411.3.3)

 

[tony mc]

If you have accessible extraneous conductive parts within a room containing a bath shower etc then you need supplementary bonding unless a RCD is fitted protecting all circuits and the 3 conditions that go with the RCD are met.