Multicore cables in parallel - hysterisis (eddy currents)

Problems can occur with multicore cables, but only if there's an imbalance in the "going" and "return" currents in each cable. Normally such arrangements aren't permitted (521.8.1) and where cables are used in parallel things should be arranged so that there's a more-of-less equal division of currents - so again there should be a pretty good balance within each cable. I can see there may still be some small imbalance in practice - but as long as the imbalance is relatively small (<100A say) and we're talking thin steel sheet enclosures rather than 1/2" cast iron, the result is likely negligible (there is a suggestion that the whole 521.5.1 regulation can be safely ignored for DB where the imblance per hole is <100A (from memory).

   - Andy.

Not really - the wires in the SW armour are neither  magentically in good contact nor forming a closed magnetic path enclosing a net current.. Had you said welded conduit, I'd have no problem with the statement.

I'm still not seeing the problem, so long as the cores in each cable form a complete circuit, that is to say that all 4 current carrying cores that carry associated current, are together.

Actually, for sensiible thicknesses of steel box, and sensible hole sizes you need many tens of amps going out of one hole and coming back via another to raise things by a few degrees.

Far more common, with unbalanced currents and it does apply to SWA or AWA as well, is the transformer-like problem of armours earthed at both ends forming a one turn loop for voltage pick up. Dog bone slots don't help much there, as that really is an eddy current effect .

Its why single core SWA does not exist....

Strictly, the problem we are discussing is not eddy current, rather it is magnetic hysteresis, though i appreciate even some publications that really should know better get confuse the two.

Mike.

Hi,

I'm not disputing the physics here. The question is compliance with BS7671.

Gareth

The question is compliance with BS7671.

The issue then is the word "collectively" - does it refer to all conductors of the circuit or merely some suitable collection of them. Certainly ambiguous (online dictionaries suggest both "as a group; as a whole" as a definition - so could be either way). Given the physics, "a suitable group" might not be an unreasonable interpretation.

Also consider how the reg could apply to other situations - say a steel adaptable box used as a JB for lighting - it would be quite normal to take just the L and SL (without N) through one hole for a switch drop, SL and N through another to the light - so neither contain "all" the conductors of the circuit - but do contain a suitable (balancing) group.

   - Andy.

AJJewsbury said:

Also consider how the reg could apply to other situations - say a steel adaptable box used as a JB for lighting - it would be quite normal to take just the L and SL (without N) through one hole for a switch drop, SL and N through another to the light - so neither contain "all" the conductors of the circuit - but do contain a suitable (balancing) group.

But 521.5.1 does not say that all of the conductors must enter (or leave) together. It simply says that the ones which do must go through the same hole.

Hi All,

Thanks all for your comments!

I have just spoken to the technical helpline at the NICEIC who have confirmed that all conductors of the circuit (even consisting of multicore cables) must enter the same aperture of a ferrous enclosure in order to comply with BS7671.

Cheers

Gareth

Again  this falls back to definitions, in this case 'circuit' which seems to mean something different in standards land, to the meaning for the broader world of electron herding. Current flowing out to a light switch and back does form a circuit in an electrical sense. - but in standards speak is only part of a circuit.

AJJewsbury said:

The issue then is the word "collectively"

As opposed to "individually". I think that there is no doubt about that.

I am leaning towards all of the conductors collected together. I accept that a number of conductors could be collected together in bundles, but then you would have to decide what to put in each bundle. The natural inclination might be to collect items of the same type, so all the L1s, all the L2s, etc., which would not be very helpful.

But 521.5.1 does not say that all of the conductors must enter (or leave) together. It simply says that the ones which do must go through the same hole.

So how does that work with L & SL through one hole and SL & N through a different one?

   - Andy.

Expanding on that thinking, say we had a 3-phase supply and L1 & N left through one hole and L2 & L3 left through another ... good or bad? I suggest "it depends" - if all four fed a 3-phase load then bad, but if L1 & N fed a 230V single phase load and L" & L3 fed a 400V single phase load then I reckon all is well.

So it's down to whether what goes through a single hole is balanced  - i.e. what we'd traditionally call a circuit (but doesn't match BS 7671's definition of a circuit which encompassed all conductors downstream of a protective device).

The next question is whether we'd consider two multicore cables in parallel to be one "circuit" or two. From an overload perspective we assume two - since each cable needs only to be rated for its theoretical fraction of the whole current - without any margin for error - so it would be consistent to assume tha same principle applies and each cable carries only its intended currents - and hence would remain in balance - so treat each cable as its own "circuit".

   - Andy.

AJJewsbury said:

The next question is whether we'd consider two multicore cables in parallel to be one "circuit" or two.

AJJewsbury said:

so treat each cable as its own "circuit".

That solves one problem, but creates another. Now you would have to have two overcurrent protective devices.

AJJewsbury said:

The next question is whether we'd consider two multicore cables in parallel to be one "circuit" or two.

AJJewsbury said:

so treat each cable as its own "circuit".

That solves one problem, but creates another. Now you would have to have two overcurrent protective devices.