Single and Multi-Core Cable

"Single-core cables are become extinct?"

Where on earth did you get that idea?

Single core cables are definitely NOT becoming extinct. Now, technically, single core cabler would carry more current fore the same cross section, for, their net impeance will be lesser, as they do not have the effect mutual inductance from neighbouring cores, as in multi-core cables. Comparison, best, is obtained from cable manufacturer's rating charts.

Because after digging in the cable maker land, what I see are only multi-core cable catalogs. Hope that this will not annoy Tony. Yet, you can also see that most of the impedance information is for multi-core application.

It may not be an exact definition, but I regard "cable" as being more than just a wire or an insulated wire -as having a protective sheath over the insulation, which may be insulating and or metallic - and being multicore generally. Most of the small single core insulated conductors are called "equipment wire" or "panel wire" - if you search for "cable", you will find multicore! If you look it up in Wikipedia http://en.wikipedia.org/wiki/Cable you find it is "Two or more wires.....together". But most of the specifications for insulated wires call them single core cable! Two single cores have more air space around them than a multicore assembly, hence the higher ratings. But this can be a deception in 3 phase circuits or passing through metallic glands.

Ah, impedance of single-core cables. Yes, it's trivial to come up with DC resistance for a single wire or cable, and even AC resistance at 50 or 60Hz due to skin effect, but impedance, whoa, that's another matter. This is because impedance is around a loop, and the return current path can differ from installation to installation. "For single core cables systems, the overall impedance must be calculated from the geometric arrangement of the conductors, as is the requirement for overhead lines." [DIgSILENT Pacific User Conference 2007, discussing their PowerFactory software.]

Also, if a single-core cable has a screen, it should be grounded at one end only. "If the screens are not bonded at both ends, induced currents do not flow, however screen voltages become elevated." Transposing screens, trifoil formation, non-magnetic steel? Aha, we have now uncovered a potentially-complex scene beyond the scope of this post (and the knowledge of this writer).

Well! This panel is installed without the heater.

The photo shows single-core cables with screens. The screens were presumably connected at both ends. High screen current caused excess heating at two of the screen connections, and damage to the cables.

Thanks Win (for A of E, the best electronics book I know!). But :-

1. Referring to your image, high screen current alone may not be the cause of failure, but currents (induced by the cables magnetic field) heating the metallic gland plate through which the cables pass and/or circulating currents in the screens of the two failed cables passing across the gland plate. Since stainless steel has high resistance this causes a lot of heat. The result is local overheating of the cable at the gland plate [and eventual failure]. Notice there is melted insulation right at the gland of the failed cables. It appears that the power comes in below - fault arcs always travel away from the source!
2. I have to take issue with Electricalexpert65, on the idea that single core cables inherently have less impedance. Inductance depends on geometry, whether the wires are in a cable or independent. the more apart the wires carrying the "go" and "return" currents, the higher the inductance. With single core cables it is easy to run "go" and "return" apart by a big distance, in extreme making a real "inductive loop". With small cables, the inductive reactance is negligible compared to the resistance [at 50/60 Hz], but beyond about 50 sq mm copper the inductance begins to dominate. Since the inductance is a result of the magnetic field per amp, the bad effects with magnetic shields [including conduits] and gland plate materials emerge with high cable currents.
   

No keep looking, almost all the applications I work with use single core screened and unscreened cables, standard and custom manufactured (up to 33kV AC). You don't mention what voltage or type of cable.

The data is available and more can be found in the application sheets available off the cable manufacturer' website and in the electrical standards.

It doesn't annoy me, but you need to get your spade out and do a bit more digging around cable manufacturers.

British Standard 7671.

Single core cables are not extinct otherwise all the house wirings would be lost.

As to the ampere ratings of single vs multicore, I agree impedence is a factor but also the manufacture of the cable, so go to the cable manufacturer to get the data

House wiring is also multi-strand cable. What I mean here is a cable which has only one solid and hard copper conductor.

And it is very useful when some people here think of the impedance. It is a good point. I think.

Ah, you meant single strand, not single core! Example below of single strand, at least up to 2.5mm². You will need to download the specifications on these pages to see details. They contain details of cable ratings for different arrangements according to BS7671, the British Standard for building electrical installations, mentioned by PW Slack.

http://www.eland.co.uk/bs-standards/bs6004/cable68/6181y-double-insulated-wiring-cable-to-bs6004.html

http://www.eland.co.uk/electricalcable/fixedwiringcable/cable66/6491x-cable-h07v-rh07v-u-to-bs6004.html

http://www.eland.co.uk/electricalcable/fireintegritycable/cable74/fireforce-fire-performance-cable-ph30-bs6387.html

The third actually pictures a single core, real evidence!!

The following looks a good source of cable current ratings to BS7671 and other data :-

http://www.batt.co.uk/products/category/48/Technical-Information

Regards,

67model