Supplementary Equipotential Bonding

[Top Cat]

it's more if you can touch 2 extraneous conductive parts. if 1 were to become live, due to a fault, then as long as the 2 were bonded together, there would be no p.d. between them , therefore no shock.

Oh Tel please don't tempt me over this!

 

[seftonbarn]

it's more if you can touch 2 extraneous conductive parts. if 1 were to become live, due to a fault, then as long as the 2 were bonded together, there would be no p.d. between them , therefore no shock.

OK. Bear with me. I am just tring to learn. So the lighting circuit in the bathroom is not relevant yeah? Because one of the the extraneous conductive parts could have become live due to a fault anywhere in the property? As I say bear with me.

 

[telectrix]

not quite sure of your meaning, TC. you been on the fish heads again?

 

[telectrix]

the point of bonding is to ensure that there;s no potential difference between extraneous conductive parts. imagine an earth fault ...... any bonded pipework will rise to 240V for the duration of the fault, till the MCB trips. as long as other pipework is bonded, it's potential will be the same. no PD = no current flow through you if you are in contact with both simultaneously.

 

[Guitarist]

If there is no RCD, then bond all pipework that enters the bathroom with 4mm cable. You can do this in an airing cupboard or loft, just so long as the pipework is continuous.

 

[ackbarthestar]

Come on chaps is relatively straight forward...

There are two conditions that exist in a location where you are naked and wet and hanging onto two metal bathroom components

1/ they are exposed
2/ they are extraneous

If they are exposed then they will be connected to the MET through the cpcs and 411.3.2.2 applies with 415.1
If they are extraneous then two further conditions are available:

1/ If they are connected to the MET via MPB and all metal pipework then 411.3.1.2 applies
2/ If they are isolated from the MET because of , either non metal pipework then GN5 and GN8 applies, that is

rx<= Uo/0.01 - 1k ohms which equates to 22k Ohms

So as long as simultaneously touchable metalwork is either bonded to MET or 'isolated' then you have some form of protection.

The confusion arises at what point does isolation stop and bonding begin.

Bonding to the MET infers that the touch voltage will not rise above 50V.

example: A lighting circuit protected by a 6A MCB has a maximum Zs of 7.67 Ohms which complies with table 41.3 but when compared with the requirements of 415.2 must not rise beyond 50V, so an alternative Zs tied down to the surrounding metalwork must be 50/Ia which would be 30A for a 6A MCB. This gives us a maximum Zs of 1.67 Ohms.

Now when used in conjunction with a 30mA RCD in compliance with 411.3.1.2, 415.2, 701.415.2

then Zs < Uo/IAN = 230/30mA

= 7.67 k ohms

This appears to comply with the disconnection time of < 40ms

However, even at 40ms (2cycles of ac) may still be sufficient to provide a kill

So to comply with touch voltage we have

Zs < 50/30mA = 1.67 k ohms

This satisfies both compliance for disconnection times 411.3.2.2 and compliance with 411.3.1.2

Now the problem arises when the resistance between simultaneously touchable parts sits somewhere between being bonded (> 1.67 kOhms) and Isolation (< 22k Ohms)

At this level both RCD and supplementary bonding needs to be present... cheaper just to supp. bond everything

Argue amongst yourselves

 

[seftonbarn]

OK. Bear with me. I am just tring to learn. So the lighting circuit in the bathroom is not relevant yeah? Because one of the the extraneous conductive parts could have become live due to a fault anywhere in the property? As I say bear with me.

OK. Interesting stuff. But what is the answers to my question/s?

 

[seftonbarn]

the point of bonding is to ensure that there;s no potential difference between extraneous conductive parts. imagine an earth fault ...... any bonded pipework will rise to 240V for the duration of the fault, till the MCB trips. as long as other pipework is bonded, it's potential will be the same. no PD = no current flow through you if you are in contact with both simultaneously.

Yes I agree. An earth fault can give rise to PD between extraneous conductive parts. What is confusing me is 415.2.2 which explains how you determine the effectiveness of Supp Bonding between Extran parts and the calculation is based on Ia which is the current of the overcurrent protection device or the RCD in the bathroom. What has the circuit in the bathroom got to do with it when the earth fault could have been introduced to the extraneous conductive parts from any number of other circuits in the installation. Hyperthetically lets say there was no circuits in the bathroom at all; just a metal bath and a radiator with metal pipework. Some of the explanations above suggest that you could still get PD between the extraneous conductive parts and so it would be logical to apply Supp bonding. But how does 415.2.2 work in this case when for the formula is R</50 V/Ia there is no value for Ia.

 

[Des 56]

If there is no potential difference between pipes that can be touched simultaneously,that is what is sought

All the regs and formulas only provide information to make a decision on what is necessary to obtain that situation

 

[seftonbarn]

Come on chaps is relatively straight forward...

There are two conditions that exist in a location where you are naked and wet and hanging onto two metal bathroom components

1/ they are exposed
2/ they are extraneous

If they are exposed then they will be connected to the MET through the cpcs and 411.3.2.2 applies with 415.1
If they are extraneous then two further conditions are available:

1/ If they are connected to the MET via MPB and all metal pipework then 411.3.1.2 applies
2/ If they are isolated from the MET because of , either non metal pipework then GN5 and GN8 applies, that is

rx<= Uo/0.01 - 1k ohms which equates to 22k Ohms

So as long as simultaneously touchable metalwork is either bonded to MET or 'isolated' then you have some form of protection.

The confusion arises at what point does isolation stop and bonding begin.

Bonding to the MET infers that the touch voltage will not rise above 50V.

example: A lighting circuit protected by a 6A MCB has a maximum Zs of 7.67 Ohms which complies with table 41.3 but when compared with the requirements of 415.2 must not rise beyond 50V, so an alternative Zs tied down to the surrounding metalwork must be 50/Ia which would be 30A for a 6A MCB. This gives us a maximum Zs of 1.67 Ohms.

Now when used in conjunction with a 30mA RCD in compliance with 411.3.1.2, 415.2, 701.415.2

then Zs < Uo/IAN = 230/30mA

= 7.67 k ohms

This appears to comply with the disconnection time of < 40ms

However, even at 40ms (2cycles of ac) may still be sufficient to provide a kill

So to comply with touch voltage we have

Zs < 50/30mA = 1.67 k ohms

This satisfies both compliance for disconnection times 411.3.2.2 and compliance with 411.3.1.2

Now the problem arises when the resistance between simultaneously touchable parts sits somewhere between being bonded (> 1.67 kOhms) and Isolation (< 22k Ohms)

At this level both RCD and supplementary bonding needs to be present... cheaper just to supp. bond everything

Argue amongst yourselves

Yes - so from the red text above I need to add supp bonding if my Zs is exceeding 1.67ohms (no RCD). But if I can't touch anything on the lighting circuit simultaneously with extran parts why use Ia of the lighting circuit. If the earth fault was introduced, lets say by an immersion heater protected by a 40A MCB and no RCD. Ia would then be 200A (from Fig 3A4) and so your calculation would be 50/200 = 0.25ohms. In other words shouldn't we be using the largest value of Ia in the installation rather than the largest value of Ia in the room containing the bath or shower.

 

[ackbarthestar]

Yes - so from the red text above I need to add supp bonding if my Zs is exceeding 1.67ohms (no RCD). But if I can't touch anything on the lighting circuit simultaneously with extran parts why use Ia of the lighting circuit. If the earth fault was introduced, lets say by an immersion heater protected by a 40A MCB and no RCD. Ia would then be 200A (from Fig 3A4) and so your calculation would be 50/200 = 0.25ohms. In other words shouldn't we be using the largest value of Ia in the installation rather than the largest value of Ia in the room containing the bath or shower.

Yes I agree, If the lights or shaver sockets in the bathroom are class II or out of simultaneous reach. I was thinking along the lines of a snagged cable loose above the bathroom making fortuitous contact with the plumbing pipes as stuff gets moved about in loft spaces.

I chose a lighting circuit really to demonstrate that an earth fault voltage sitting on extraneous metalwork does not have to be 0.05 ohms, the largest Zs allowable which would keep the touch voltage to a maximum of 50V. If you had a 10.8kW instantaneous water heater which had caused an earth fault right above the cast iron bath you were standing in while having a shower then you would need a theoretical resistance path to the MET and around the bathroom to be no higher than 50/250 = 0.2 ohms. (personally I would prefer in this case the bath to be isolated from the MET and surrounding metalwork)

Now that RCDs exist and a global 0.4s disconnection time on final circuits, the importance of 'front line' protection by the use of supplementary bonding has lost its importance.

But we still have uncontrolled conditions out side of the installation such as network fluctuations in voltages, lost, damaged, or high resistance supply neutrals, which can lead to voltages appearing on simultaneously touchable metalwork for longer periods than 0.04, 0.4s, 5s. Voltages from PME systems could be there indefinitely until someone with wet hands and feet comes into contact.

 

[seftonbarn]

Yes I agree, If the lights or shaver sockets in the bathroom are class II or out of simultaneous reach. I was thinking along the lines of a snagged cable loose above the bathroom making fortuitous contact with the plumbing pipes as stuff gets moved about in loft spaces.

I chose a lighting circuit really to demonstrate that an earth fault voltage sitting on extraneous metalwork does not have to be 0.05 ohms, the largest Zs allowable which would keep the touch voltage to a maximum of 50V. If you had a 10.8kW instantaneous water heater which had caused an earth fault right above the cast iron bath you were standing in while having a shower then you would need a theoretical resistance path to the MET and around the bathroom to be no higher than 50/250 = 0.2 ohms. (personally I would prefer in this case the bath to be isolated from the MET and surrounding metalwork)

Now that RCDs exist and a global 0.4s disconnection time on final circuits, the importance of 'front line' protection by the use of supplementary bonding has lost its importance.

But we still have uncontrolled conditions out side of the installation such as network fluctuations in voltages, lost, damaged, or high resistance supply neutrals, which can lead to voltages appearing on simultaneously touchable metalwork for longer periods than 0.04, 0.4s, 5s. Voltages from PME systems could be there indefinitely until someone with wet hands and feet comes into contact.

Thanks for this reply and your previous one which was most informative. I now have a much better understanding. Maybe I have not articulated myself very well throughout this thread but it is 415.2.2 (using R</ 50 V/Ia to determine the effectiveness of SEB) that is causing me all the confusion. And which value of Ia to use. This and other forums suggest that you should be using the highest value for Ia that exists in the circuits actually in the room containing the bath or shower. I think that you should be using the highest value of Ia in the whole installation. Any thoughts on that interpretation?

 

[ackbarthestar]

Thanks for this reply and your previous one which was most informative. I now have a much better understanding. Maybe I have not articulated myself very well throughout this thread but it is 415.2.2 (using R</ 50 V/Ia to determine the effectiveness of SEB) that is causing me all the confusion. And which value of Ia to use. This and other forums suggest that you should be using the highest value for Ia that exists in the circuits actually in the room containing the bath or shower. I think that you should be using the highest value of Ia in the whole installation. Any thoughts on that interpretation?

The value of Ia will depend on the size of protective device used, that's precisely why 0.05 ohms is mentioned in GN3 as a rule of thumb. A coverall situation/application. I only use lighting as an example of one type of a circuit that will be in use while electric immersion,bath/towel/shower/convector/heaters are being removed for other forms of heating and is common to all bathrooms.

My interest is in knowing the limitations. We know the minimum desirable value of supp.bonding (< 0.01 ohm) But do we know what the maximum R would be or to see it the other way around, the minimum IR value to achieve an 'effective' isolation in the case of a fault or wayward external voltages affecting the potential on internal metalwork .

 

[seftonbarn]

OK got it. I'm off to Screwfix for an RCd.

 

[Mark Wright]

Come on chaps is relatively straight forward...

There are two conditions that exist in a location where you are naked and wet and hanging onto two metal bathroom components

1/ they are exposed
2/ they are extraneous

If they are exposed then they will be connected to the MET through the cpcs and 411.3.2.2 applies with 415.1
If they are extraneous then two further conditions are available:

1/ If they are connected to the MET via MPB and all metal pipework then 411.3.1.2 applies
2/ If they are isolated from the MET because of , either non metal pipework then GN5 and GN8 applies, that is

rx<= Uo/0.01 - 1k ohms which equates to 22k Ohms

So as long as simultaneously touchable metalwork is either bonded to MET or 'isolated' then you have some form of protection.

The confusion arises at what point does isolation stop and bonding begin.

Bonding to the MET infers that the touch voltage will not rise above 50V.

example: A lighting circuit protected by a 6A MCB has a maximum Zs of 7.67 Ohms which complies with table 41.3 but when compared with the requirements of 415.2 must not rise beyond 50V, so an alternative Zs tied down to the surrounding metalwork must be 50/Ia which would be 30A for a 6A MCB. This gives us a maximum Zs of 1.67 Ohms.

Now when used in conjunction with a 30mA RCD in compliance with 411.3.1.2, 415.2, 701.415.2

then Zs < Uo/IAN = 230/30mA

= 7.67 k ohms

This appears to comply with the disconnection time of < 40ms

However, even at 40ms (2cycles of ac) may still be sufficient to provide a kill

So to comply with touch voltage we have

Zs < 50/30mA = 1.67 k ohms

This satisfies both compliance for disconnection times 411.3.2.2 and compliance with 411.3.1.2

Now the problem arises when the resistance between simultaneously touchable parts sits somewhere between being bonded (> 1.67 kOhms) and Isolation (< 22k Ohms)

At this level both RCD and supplementary bonding needs to be present... cheaper just to supp. bond everything

Argue amongst yourselves

Looking for some kind of official source for meeting disconnection times/Zs and keeping touch voltage below 50v on both TN systems not just TT -Pre RCD or just without RCD with Fuses and MCB.

Maybe some old BS7671 or old Book, Workshheet etc? not sure I've got anything before 16th but if you point me in the right direction that would be great, Thanks

 

[Mark Wright]

I've seen this and it's good but it does say

Supplementary Equipotential Bonding Requirements

Questions relating to the requirements for supplementary equipotential bonding are frequently asked; a very common one is where should it be installed. This article looks at the requirements for supplementary equipotential bonding in BS...

www.electriciansforums.net

"Note that the touch voltage
may rise above 50 v; the
value of 50 v is used as a
constant in the formula to
ensure sufficient current is
flowing to operate the RCD."

so still a bit confused???
is 50v touch voltage really a thing?