Multicore cables in parallel - hysterisis (eddy currents)

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:

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

Non-compliant! 

Here's another one. Does a ring final whose cables enter at opposite ends of a back-box comply with 521.5.1?

Non-compliant! 

It's that careless use/ definition of the word "circuit" that's makes these regs a problem isn't it ? - we see a similar problem in reverse where the literal wording for grouping factors don't seem to apply to multiple cables from the same circuit.

However two currents are  flowing in two cables adjacent, - say two lengths of T & E going out & back to a socket on a ring final in trunking, they heat each other just as surely as if they were supplying two radials on different fuses  carrying the same two currents.

Then there is the non-matter of neutrals in 3 phase sub-mains that feed 3 single phase loads, as this is not a shared neutral, even though the 3 phases may have one fuse each ;-) 

This is probably just another one on the mercifully short list of where the letter of the regs can be read to either require things that are not always necessary, and in other cases not actually be sufficient for some corner cases. They are being squeezed out over successive revisions, but probably not as fast as folk are getting less keen to make a technically informed local interpretation.

Mike

I wonder how the brass glands are secured ... steel locknuts perhaps?

   - Andy.