Lack of supplementary bonding - what’s the danger ?
- Thread starter equitum
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If supplementary bonding were present (between A and B), the resistance between A & B would be nowhere as great as 0.25Ω - as I implied, it would be pretty negligible.
As above, the resistance of the SB conductor would be very small so, even if 200A flowed through it, the voltage across it would be far less than 50V - e.g. 3 metres of a 4mm² conductor would only have a resistance of about 0.0165Ω (hence a pd of only about 3.3 V if 200A were flowing through it. Furthermore, only part of the fault current would go through the SB conductor, the rest (maybe even most) of it going through the CPC.
I agree with all of the above, I used 0.25ohm to show how big the resistance would need to be before a safe level of 50V was exceeded even with a large fault current and ignoring parallel path of the ECP. Back to my initial post - supplementary bonding where main bonding is sound is like putting nuts on your granola.
So you are now agreeing it wouldn’t be 144V ??
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Fair enough, but I thought you were questioning the value of supplementary bonding. You now seem to be agreeing that, in practice, it will always limit touch voltages to 'very safe levels'?
As I've explained, even if the main bonding is 'totally sound', in the absence of supplementary bonding there is still a risk of 'dangerously high' touch voltages (until the fault is cleared by a device).
Conversely, if supplementary bonding is present, such that the entire room is an equipotential zone, the room is then essentially 'totally safe', even if no main bonding is present at all.
Fair enough, but I thought you were questioning the value of supplementary bonding. You now seem to be agreeing that, in practice, it will always limit touch voltages to 'very safe levels'?
No - I’m saying that without supplementary bonding the resistance of the ECP and the CPC from the diagram without Supp. Bonding is likely to be much less than 0.25ohm just through main bonding and common CPC’s
I think you are probably missing the point ...
In the scenario illustrated, the resistance (to earth and/or MET) of the extraneous-c-p is essentially irrelevant, since only a tiny current will flow through it - the tiny (but still potentially lethal) current through the body of the victim.
Even if the path from extraneous-c-p to earth/MET had a resistance of "hundreds of Ohms", there would still be a very high (and potentially lethal) potential difference (more than half the supply voltage) between it and the exposed-c-p of a circuit with an L-CPC fault,m hence with the possibility of a dangerously high current flowing through a victim (until the fault were cleared),
As I've said, the only thing that will remove that risk is supplementary bonding within the location.
Let me put it another way. The supp. Bond cable is to make the resistance between the things you can touch that are extraneous as low as possible. A copper pipe in the bathroom that is connected to the MET by a 10mm2 wire is likely to be in the 0.05 ohm bracket, is it not ?
Of course - but I'm getting confused about whether you are talking about the situation with, or without, supplementary bonding.
As I've said, in the absence of supplementary bonding, the resistance between extraneous-c-ps to the MET is essentially irrelevant.
Conversely, if everything touchable within the room is connected together via (very low resistance) supplementary bonding, then it doesn't matter a jot as to what is, or is not, connected to MET/earth, nor does the resistance of any such connections matter a jot.
But without SB, point A is live and point B is connected to earth.
Do you have in mind 415.2.2 which you are trying to satisfy?
John has previously shown the requirements in this regulation to be wrong?
John has previously shown the requirements in this regulation to be wrong?
You're missing the point that (without) SB), during the duration of the fault, a very high current is flowing through the path from A to MET, whereas almost no current (only the current through the victim) is flowing from B to MET.
Hence, whilst the voltage of A rises to very high relative to MET (about 144V in my example), the voltage at B remains very close to MET potential - hence a very large potential difference between A and B.
As an example, if both A and B had path resistances of 0.24Ω to MET, with a fault current of 600 A and with 144 mA flowing through the victim (hence assuming a body resistance of 1kΩ ), then (with 2.5mm² T+E), the voltage at A would be about 144V above MET potential, and the voltage at B would be only about 0.035 V above MET potential
Yes penny has dropped . The supp bond closes the loop in the earthing arrangement. I see it now. Thanks for being patient. How is an RCD going to help in that scenario? (Without supp bonding) It isn’t is it ? Which begs the question why is it (sup. Bonding) permitted to be omitted if it’s RCD protected and the other criteria are met ?
They detect tiny currents flowing to earth - including through a person.
Although the MCB will likely be adequate in the situation we have been talking about.
.Because the R in the equation R≤50V/Ia can be 1666Ω because Ia is 0.03A.
If it were to be higher than that which is highly unlikely the SB would still be needed.
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