High currents in Bonding conductors
- Thread starter bernardgreen
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I'm not sure that is much more than a 'statement of the obvious' (and a topic we often discuss), is it?
NO conductor should be used as a main protective bonding conductor unless the designer has determined (either by calculation or by adherence to 'deemed to satisfy' regulations) that it is adequate for the job (i.e. will not result in unacceptable 'overheating'). That applies to a length of G/Y-insulated cable just as much as it does to the armour/cores of SWA.
In practice, it is probably rare for SWA armour or cores to be used as a MPB conductor in situations in which very high sustained currents could flow. It is only likely to be used as a main binding conductor when, for example, supplying an outbuilding. In that situation, high sustained currents in it (due to supply-side faults) could only arise if it were bonding extraneous-c-ps in the outhouse which had a very low impedance path to earth - and I think that would be a pretty rare situation (and, if it did arose, probably a reason for TTing the outbuilding).
However, as I've said, this is nothing new. When discussing supplies to outbuildings we very often talk about, and consider, the adequacy of the SWA armour as a main bonding conductor in TN-C-S installations. I think it is not unusual for it to transpire that the armour is adequate (in terms of the regs' requirements for main bonding effective CSA) but, if not, it can be an argument for TTing the outbuilding, or even running a separate bonding conductor. We often discuss these issues.
Kind Regards, John
What is your point, Bernard?
That equally applies to any conductor used as an MPB.
The article is not actually about "earthing" the armour.
That equally applies to any conductor used as an MPB.
The article is not actually about "earthing" the armour.
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I remember working on one site where the earthing within the site was massive I spent 3 months fitting earth rods each one 8Ω or less, idea was to stop sparks which could ignite gas.
There was a stand by generator on site with earth pit I remember it was a gas turbine so not a small generator, where the DNO supply came in there was a massive resistor around 1.5 meters square. So it the DNO earth was not at same voltage as site earth then there was a limit to current which could flow.
There was a stand by generator on site with earth pit I remember it was a gas turbine so not a small generator, where the DNO supply came in there was a massive resistor around 1.5 meters square. So it the DNO earth was not at same voltage as site earth then there was a limit to current which could flow.
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Yes but some "designers" do not take into account the possibilty of extremely high currents in bonding wires when there is a fault. They seem to assume that the MCB will limit the fault current to a lafe level or the RCD will operate when there is more than 30 mA of current in the bonding conductors.
During recent excavations in the village ( for gas pipe repairs / replacement ) several lead water supply service pipes were found and it is believed that the mains were also metal. These will present very low impedance extranuous conductors if they are bonded to the MET. One service pipe was damaged and the water company repaired it with the longest length of plastic they could squeeze between the sdes of the excavation.
That is undoubtedly true. However, as both EFLI and myself has said, that is true of any bonding conductor - there is absolutely nothing 'special' about SWA armour/cores.
Indeed - but, again, that establishes the requirement for 'adequate' bonding conductors, whatever they consist of (nothing special about SWA). Also, as I said, it would probably be pretty unusual for SWA to be providing main bonding for very low impedance extraneous-c-ps.
I don't know whether you regard the requirements of BS7671 as being adequate/safe, but they will be satisfied by main bonding conductors of 10mm² copper (or equivalent) for virtually all domestic PME installations.
So, as EFLI has also asked, I wonder what (new) point you are trying to make?
Kind Regards, John
While we are here -
Could we dispel the often stated misconception that the c.s.a. of steel armour used for a main bonding conductor has to be eight or nine times that of copper?
The regulation does say that material other than copper shall have the same 'conductance' but is that correct or what it means?
There is no mention of the resistance of a main bonding conductor, but only that it must withstand without damage the likely current it may carry, so, in PME installations it must be 10mm² copper.
Therefore the relative sizes are determined by k2/k1×s; not the resistance ratio.
The armour of 2-core 6mm² or 3-core 4mm² will satisfy this.
For example: a detached garage may require a 10m. main bond which in 10mm² copper would have a resistance of 18.3mΩ. This does not mean the armour has to be 80mm² and have the same resistance to satisfy the requirement of the main bonding.
Could we dispel the often stated misconception that the c.s.a. of steel armour used for a main bonding conductor has to be eight or nine times that of copper?
The regulation does say that material other than copper shall have the same 'conductance' but is that correct or what it means?
There is no mention of the resistance of a main bonding conductor, but only that it must withstand without damage the likely current it may carry, so, in PME installations it must be 10mm² copper.
Therefore the relative sizes are determined by k2/k1×s; not the resistance ratio.
The armour of 2-core 6mm² or 3-core 4mm² will satisfy this.
For example: a detached garage may require a 10m. main bond which in 10mm² copper would have a resistance of 18.3mΩ. This does not mean the armour has to be 80mm² and have the same resistance to satisfy the requirement of the main bonding.
The bonding cable between a lost ( broken ) Neutral and an extraneous conductor that has a 1 ohm inpedance to ground may have to carry up to 230 amps depending on the loading of properties connected to that Neutral, With a well balanced 3 phase distribution between properties downstream from the break in the Neutral the current through the bond to Ground would be small. But with a single phase distribution to properties downstream from the break the current in the bond would be the total load of the properties supplied.
We've been through this before, and I think we have agreed that there is an anomaly/inconsistency in the regs.
As you imply, the 'k-values' are not just about resistivity (hence temp rise for a given current) - they presumably also take into account how the material behaves at high temperatures (e.g. how easily it melts, and at what temperature, and how it behaves at temps below melting point - if I recall correctly, steel melts at about 1500°C, but copper at around 1000°C and aluminium only around 600°C), and also what the thermal conductivity is.
Kind Regards, John
We know that, albeit extraneous-c-ps do not usually have impedances anything like that low, but what is your point?
Are you perhaps suggesting that the regs are dangerous in being satisfied by main bonding conductors that could probably not cope with such currents for appreciable periods of time?
Kind Regards, John
I wasn't trying to make a point.
I came across the article a few days after trying to explain to an "electrician" that the currents in bonding cables were not limited by any of the protective devices in the Consumer Unit. The limiting factors are the impedances and worse case ( zero impedance ) the fuse in the sub-station.
Underground steel tanks can produce a very low impedance path to Ground,
I came across the article a few days after trying to explain to an "electrician" that the currents in bonding cables were not limited by any of the protective devices in the Consumer Unit. The limiting factors are the impedances and worse case ( zero impedance ) the fuse in the sub-station.
Underground steel tanks can produce a very low impedance path to Ground,
It was really the point of your most recent post I was asking about - given that it is not 'news' that, if there are extraneous-c-ps with very low impedance to ground, then very high currents can flow in bonding conductors under certain, rare, fault condition s.
It sounds as if he also may have been one of those 'electricians' who thinks that protective devices can 'limit' currents in any situation!
I'm sure they can but, apart from a few LPG tanks (and maybe oil ones, although I've never heard of anyone burying one of those domestically), one is unlikely to come across them in domestic environments!
Kind Regards, John
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