Braided PVC Cable

[PJC58-Hythe]

I apologize if this is in the wrong section...

I have ordered some modern cotton braided PVC cable from here as they stock a vast array of colours. While I am happy to use this for low voltage and heater wiring I am less happy to use it for HT wiring, for the following reason; they cannot tell me what the safe working, or breakdown voltage of the insulation is.

The questions I am hoping someone can answer is;
a) how do I test it.
b) what is the insulation breakdown voltage / mm of PVC insulation?

[frankmcvey]

Hi, Peter,

This page gives the dielectric strength of PVC as 25kV/mm.

http://myelectrical.com/notes/entryid/178/cable-insulation-properties

This page suggests that for max 600V AC working you'd want to see at least 100MOhms insulation resistance using a 1000V Megger. "provides representative acceptance values for IR test measurements, which should be used in the absence of any other guidance (from the manufacturer or other standards)"

http://www.openelectrical.org/wiki/i...esistance_Test

Cheers,

Frank

[PJC58-Hythe]

Thank you Frank, that's most helpful.

My guess is that the insulation will be up to the job, but a test with a Megger seems the way forward. So I will probably wrap a sample of cable in foil and measure between the inner conductor and the foil. Just got to get a friend of mine to lend me his Megger...

[Radio Wrangler]

Dip it in salted water? More intimate than foil.

David

[David G4EBT]

regarding insulation testing, I use a 'Victor VC60B' Insulation Tester - I'll resist calling it a 'megger'! ('Megger' is of course a trade name, but has tended to become a generic term for an insulation tester, just as 'Hoover' and 'Thermos' have become for vacuum cleaners and vacuum flasks). I've had it about two years and have found it to be excellent. It's powered from a 9v battery, and tests at 250V, 500V and 1,000 V DC, and 750V AC. Simple to use, widely available and not too expensive - expect to pay £35 or so 'Buy It Now' on e-bay, which is where mine came from, or £60 or so from the likes of Amazon.

I've attached a picture of the VC60B, which I should perhaps explain.

For all sorts of repairs I use a two-part epoxy resin product known as 'Super Steel'. (Excellent for Bakelite cabinet repairs etc). I thought it would be ideal for sealing the ends of re-stuffed vintage capacitors on a pre-war set, as indeed it has proved to be. The term 'steel' rather suggests that it would be conductive, so before I used it, I put a strip of the Super Steel onto a piece of glass about 5cms long, and tested it at 750V AC and 1000V DC with the VC60B. The display readout range is from 0.1 M Ohm to 2,000 M Ohm. No sign of any conductivity with the Super Steel test.

The test clips can be seen clamped onto the 'Super Steel' at the bottom of the first picture. The second picture shows some re-stuffed capacitors.

I've also tested waxy paper caps, with interesting results. Some - at three times their rated WV - showed infinite resistance, (in excess of 2,000 M Ohms at any rate). Others, at 250V, acted like a 5k resistor! Quite enough to confirm to me that waxy caps can't be relied upon and are best routinely replaced, which I think most of us do anyway.

(The figure '1' on the display of the VC60B means infinite resistance - not 1 M Ohm).

It is very hard to make an insulated wire that isn't capable of a thousand volts or so, I wouldn't worry at all. For external cables I would use 'rated' stuff if not for safety but for culpability.

[Herald1360]

If you measure the O/D of the PVC over the wires and you know the wire size (AWG/SWG or 7/0.2, 16/0.2 etc) then a quick google should tell you what it's likely to be rated at.

[PJC58-Hythe]

David that looks like a good (cheap) device, I may well invest in one, thank you. Chris I did think I might be able to do that, but I would be rather be completely sure especially as some of the wiring goes at right angles through a fairly tight hole on a metal chassis!

[ColinTheAmpMan1]

David, I too have a Victor VC60B+ and find it excellent. But I think you may have misread the paperwork (unless I have) in that the 750VAC range is for measuring AC voltage, not resistance. Admittedly, the "Japlish" instructions are not that clear.
Rather OT, but I too get wound up when people use trade names as generic. The ones that really get my goat are "biro" and "hoover", but I was rather surprised to learn that "Portakabin" (and presumably "Portaloo") and "Granary" are trade names too.
Judging by the supplier, this wire is primarily for classic vehicle wiring and so is capable of insulating about 14 Volts minimum; it will, of course, have a much better insulation breakdown voltage than that. The OP mentions it going through holes in a metal chassis; it is well worth pointing out that grommets must be used in that situation, even with low voltages, as chafing will eventually cause problems.
Colin.