Discuss Disconnection Time Requirements? in the UK Electrical Forum area at ElectriciansForums.net
Another thing that used to be a more prominent requirement on the earth impedance, but is now covered under 415.2.2 for supplementary bonding, was to have no more than 50V AC fault potential at that fault current.
The requirement is for supplementary bonding of conductive parts in an installation, so supply/DB earth to steelwork, pipes, etc. It is not a requirement on the total resistance to "true Earth".Can you further elucidate on this? Did this mean that the system CPC between boards or the DNO PME was required to be a lower Z than the live phases?
There are additional requirements for both of those areas!I can still picture a remote 100 amp distribution board feed to an outbuilding, caravan or farm coop where the source is very strong but the run such that 4 seconds would be realistic. The caravan and barn are the most dangerous in that you have remote earth everywhere around the metal object.
Hi - I think the short disconnection times are intended for (most) final circuits with current using equipment or outlets. The longer disconnection times are permitted for distribution circuits as per @Strima .Why do circuits 32 amps and under require a 0.4 second disconnection time at 230 volts, but circuits over 32 amps are legally allowed 5 seconds?
The requirement is for supplementary bonding of conductive parts in an installation, so supply/DB earth to steelwork, pipes, etc. It is not a requirement on the total resistance to "true Earth". So it won't guarantee you don't exceed 50V AC absolute, only that you would be unable to touch parts within an installation that had more than 50V between them. Essentially it is establishing the degree of equipotential in an equipotential region!
Hi - I think the short disconnection times are intended for (most) final circuits with current using equipment or outlets. The longer disconnection times are permitted for distribution circuits as per @Strima .
See wording of Reg 411.3.2
That is the idea. Also with TN-C-S (i.e. the PME case) the bonding has to be at least 10mm copper equivalent (10mm Cu, 16mm Al, 80mm Fe, etc) as it could carry high currents under fault conditions (tens or hundreds of amps, depending on how low the Ra of the extraneous conductive parts are) so you have a fire risk if thin wire is used.For PE or PME voltage drop between the DNO transformer and MET, bonding all extraneous conductive parts within a building will reduce voltage potentials.
The 50V limit is planned for under the fault-disconnection currents, and it is for an "installation" which of course varies depending on the context. Could be a single floor flat, up to a large factory building, etc.However, there is still the issue of voltage drop between the main distribution board and sub-mains, especially for strong sources (11kv in the basement) with long SWA runs in large buildings. It is possible to legally exceed 50 volts, correct?
I guess traditionally most ring final circuits feeding our 13A sockets were fed from 30A fuse / 32A MCB. But checking the 18th regulations it now has 411.3.2.2 stating the 0.4s TN case applies to circuits not exceeding a rated current of:What is driving the 32 amp limit? In the Indian Electrical Code for example 0.4 seconds is required for all final circuits, even those over 32 amps. An excerpt from India's code:
Also remember you probably would not run the a single feed up a building, probably you would spilt it at a panel near the transformer and each area in the building would get a smaller feed via a 100A/200A fuse/MCCB or whatever at that panel. So the disconnection current for that area would be based on its feed fuse, not an 1600A ACB or whatever on the transformer secondary.If you have a lot of current (e.g. your own HV/LV transformer) you would be looking at conductors to match. For large SWA the armour is not always sufficient so you might have an internal copper core for the CPC, or an additional earth cable run along side the SWA.
Hi - I don’t know why the UK has put the 63A and 32A limit in place for the shorter disconnect times in final circuits. My guess it’s it’s a matter of practicality. Keep in mind this is the minimum standard and we can always exceed the requirements of the reg and provide shorter disconnection times.What is driving the 32 amp limit? In the Indian Electrical Code for example 0.4 seconds is required for all final circuits, even those over 32 amps.
But you would only see the local potential differences, and that would be kept reasonably low. Yes, you might be very unlucky to see more than 50V but it would be for less than 5s and you would have to be in contact with the oven and related ground potential at the same time. Also remember the 5s limit is not a goal, it is the upper case, and when designing this you are using worst-case assumptions on supply voltage, etc.All true. But I'm still thinking of a piece of equipment (like a pizza oven with metal handles) connected to a circuit in excess of 32 amps.
Well the sub-board or bus-bar chamber in that case would be at least 1600A internally, only the sub-main leaving it would be 200A rated! That would be the sort of situation where fault currents in the 50kA region would be likely, and a good case for HRC fuses to limit the peak fault current stress.Also the voltage drop along a feed between a 1,600 ACB and and a 200amp sub-board with the fault inside the sub-board.
What happens to the phase voltage would depend very much on the relative impedances of the supply transformer and wires, etc:
If you have a hard three-phase fault (L1-L2-L3) and a symmetric system you get essentially 0V = true earth (at hypothetical fault point centre), though in reality you would see some volts on the conductors if imbalance, or tens/hundred volts you are sustaining an arc-fault (even if the centre of the arc is hypothetically at ground potential).
If you are at the end of a long supply cable with similar L & N/E conductors then your phase impedance (Zxfmr + Zdist + R1 of installation) and earth impedance (Zpen + R2) are very similar so under a hard single phase fault (L-N or L-E) you would get roughly Uo/2 w.r.t. true earth at the fault point.
Close to the transformer, and assuming the fault was really hard (not an arc, a "bolted short"), then you would probably see tens of volts as the transformer impedance might dominate the short run of L & N/E cables to the distribution panel. But it is going to be a serious amount of current flowing so the volts that happen to be there are the least of your worries if you have the misfortune to be present...
However, if you have a system with a higher earth return impedance, worst case a TT earth rod, then you would see practically the whole supply as the VD on the phase cable would be a fraction of that on the circuit to rod and back via the Earth.
Very non-linear for voltage, less so for current waveform.Correct, but an arc is not a linear sine wave current draw. It will produce some neutral/ground current.
In all cases bonding is used to maintain equipotential regions, and so no matter what you should be safe if CPC integrity is maintained.The thing about TT, is that is behaves like a high impedance earthed (IT) system from my understanding. Everything in the building that is bonded (has a CPC or bond wire) will be near zero relative to each other and the presence of limited fault current will actually
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