Lack of supplementary bonding - what’s the danger ?

[equitum]

Yes I get that my whole point was around the benefit it practice of what will already be a very low potential difference if bonding and CPCs are correctly connected. I’m going to measure this in my 2nd floor ensuite today out of interest (no supp bond in place, copper pipe work )

 

[trojanhawrs]

Yes I get that my whole point was around the benefit it practice of what will already be a very low potential difference if bonding and CPCs are correctly connected. I’m going to measure this in my 2nd floor ensuite today out of interest (no supp bond in place, copper pipe work )

Let's say you have 100V on your enclosure, and the resistance between your water pipe and earth is 2 ohms. If it's 10 metres long and you touched them together, the voltage at 5 metres will be 50V.
Now if you add a 1000 ohms for instance (like, a person) between the 100V and your water pipe, the voltage would basically be 0V at the 5 metre point because the resistance of your person took up the volt drop.
Add in a low resistance link between the 2 points and you're back to dropping over the entire circuit.

 

[EFLImpudence]

Still thinking about this !!! You have conveniently drawn the supp. bond at the point of touch, let’s say the tap.

I did - and I also drew the MET just five inches away - but as Trojanhawrs said:

Closer the better, but any link is going to reduce the potential between the 2 points.

Yes and if moved far enough from the tap it would be just connected to the MET and doing no good at all.

If the supp. bond was connected at say the floor level, electrically this is no different to without supp. bonding in place in your picture other than a negligible additional resistance of the pipe work /cpc/meb is it ?

But it is completely different; this is what you are missing.
Without the supplementary bond the pipe is just a way of you touching the MET.

Would it be better for you to understand if, instead of the pipe, it was just another piece of wire connected to the MET?
In this diagram, which would you rather be touching?

The one without a bond where you are touching points A and B - live and earth - or
the one with a bond (farther from the tap) where you are, in effect, touching points A and C on the same conductor.

 

[equitum]

Exactly that- you can see on your picture the bond is forming just a lower, very slightly lower resistance path than the strip of copper on the met - can you not see that ?

 

[EFLImpudence]

Exactly that- you can see on your picture the bond is forming just a lower, very slightly lower resistance path than the strip of copper on the met - can you not see that ?

Yes, but with the bond it is joined to the CPC with the voltage on it.

Looking at the top diagram Instead of touching point B: what if the person was touching another point on the live CPC.
Do you think that would be any different?

 

[equitum]

Let me put it another way for you - do you agree that the more you move that bond link nearer to the MET the less it becomes a benefit in terms of lowering any PD / Touch voltage ?

 

[EFLImpudence]

Yes. I said that in post 78.

But draw the diagram in scale and it would look better.

 

[JohnW2]

Still thinking about this !!! You have conveniently drawn the supp. bond at the point of touch, let’s say the tap. If the supp. bond was connected at say the floor level, electrically this is no different to without supp. bonding in place in your picture other than a negligible additional resistance of the pipe work /cpc/meb is it ?

Since (other than in extraordinary circumstances) essentially no current will be flowing through the pipe downstream of the point of bonding, it doesn't matter where the pipe is bonded.

Even if the pipe were bonded, hypothetically, 50 metres from where it was connected to the tap, the tap would still be at the same potential as at the point of bonding since, in the absence of current flowing through that 50 metres of pipe, there would be no 'voltage drop' (i.e. difference of potential between its ends).

 

[JohnW2]

Let me put it another way for you - do you agree that the more you move that bond link nearer to the MET the less it becomes a benefit in terms of lowering any PD / Touch voltage ?

No. See what I've just written

 

[JohnW2]

Closer the better, but any link is going to reduce the potential between the 2 points.

As I've just written, and assuming that everything in the room has supplementary bonding (so the entire room is a true equipotential zone), it surely doesn't matter where one bonds things, does it?

 

[equitum]

No. See what I've just written

Ok so please draw the fault current flow on the diagram with the bond in place somewhere between the tap and the met

 

[JohnW2]

Ok so please draw the fault current flow on the diagram with the bond in place somewhere between the tap and the met

Fault current shown in blue, as I see it. Dashed and dotted blue lines showing split into three routes:

via CPC and MET to installations earth​

through pipe to earth​

through pipe then via MEB and MET to installation's earth​

Since negligible current will go though the pipe from B to the point of bonding (which I have labelled "C") (only the tiny current through the victim), points B and C will always be at essentially the same potential.

As for the potential at A (which determines the 'touch voltage A-B), that will, be dependent on the resistance of the path between A and C (i.e. a bit of the CPC and the supplementary bonding conductor). That will usually be pretty negligible, even with very high fault currents - BUT, whatever it is, as I see it, it is determined by the length (and conductor CSAs) of that path from A to C, NOT on how close C (the place of SB) is to B (the tap/'point of contact.

In some (maybe many) situations, the resistance of that path (hence potential at A, hence touch voltage) could be lower when point C NOT at point B (i.e. the tap) - it would seem that it depends entirely on the 'geography' of the pipes and conductors.

,

 

[equitum]

Thanks excellent diagram. So that contradicts what you said earlier

“Since (other than in extraordinary circumstances) essentially no current will be flowing through the pipe downstream of the point of bonding, it doesn't matter where the pipe is bonded.”

A-C will in practice be pretty close to A-B since both in the same relatively small room.

So I stand by my statements that provided the main bonding is in place the level of shock risk of supp. bonded and non supp bonded is dependent on the resistance of the cpc of the faulty appliance versus the proximity of the bonding to the touchable metallic parts the bathroom and the resistance of the ECP to the MET. I don’t believe I’m “missing” anything

 

[trojanhawrs]

Your statement was that

Yes I get that my whole point was around the benefit it practice of what will already be a very low potential difference if bonding and CPCs are correctly connected.

Which is wrong. Without supplementary bonding, you can consider all cpc/bonding resistances to be negligible - the only load that matters is you (because your resistance will be exponentially higher than any of them). So if there's 100V between A and the MET, that 100 volts is going through you.

If there's a parallel path (the A-C) then yes the resistances matter, the lower A-C is as a percentage of the whole fault path (A-MET) will determine the potential difference between A and B.
John was also wrong to suggest it doesn't matter where the pipe is bonded.

 

[JohnW2]

Thanks excellent diagram. So that contradicts what you said earlier

Since (other than in extraordinary circumstances) essentially no current will be flowing through the pipe downstream of the point of bonding, it doesn't matter where the pipe is bonded.

I see no contradiction. As I said, since virtually no current (other than the tiny amount flowing through the victim) will flow through the pipe between B and C,m the potentials at B & C will be almost identical - and that remains true no matter where "C" (the point of bonding) is.

A-C will in practice be pretty close to A-B since both in the same relatively small room.

As above, since there will always be virtually no difference between potentials at B and C, the A-C and A-B potential differences will always be virtually the same, in a room of any size.

So I stand by my statements that provided the main bonding is in place the level of shock risk of supp. bonded and non supp bonded is dependent on the resistance of the cpc of the faulty appliance versus the proximity of the bonding to the touchable metallic parts the bathroom and the resistance of the ECP to the MET. I don’t believe I’m “missing” anything

I disagree.

As I've said, if complete supplementary bonding is in place, such that the entire room is an equipotential zone, then it doesn't matter a jot whether or not main bonding is in place.

As for the rest of what you suggest, as I've written an illustrated, "the proximity of the bonding to the touchable metallic parts" is essentially irrelevant to the 'touch voltage". In the diagram I presented in my last post, the touch voltage (A-B) would be essentially identical, whether the if the length of pipe from B (the tap) to C (the point of supplementary bonding) were 2 metres or 50 metres. As I wrote, all that matters is the lengths and CSA's of the 'wired' path between A and C (i.e. a bit of CPC and the SB conductor).