Counterfeit power cords

[jrshabs]

HEY BROS

noob question here. is it safe for my CPU to use the power cord KKCK rated 10A -250V

[LongRunner]

If it's the same one I mentioned, using it is playing Russian roulette, so don't.

What does the resistance of each conductor measure?

[Behemot]

You need high-precision meter, or maybe even ESR meter to measure such low resistance values, most cheap multimeters can have resistance of several ohms just on their own probes so you won't get any reasonable numbers.

[LongRunner]

The probes I used are OK (the meter showing 0.0–0.1Ω when they are touched together), although I'm aware that 0.1Ω precision is a bit low for the job (can't use it to determine the conductor size of an unlabelled cable, for example, unless it's very long).

Perhaps a better method would be to load the conductors with a constant current source and measure the voltage across them, then work out the resistance from there (R = V / I). Of course, you'd have to be prepared to switch off the power if the cord under test overheats.

[Behemot]

Yeap, that's possible. Should be higher voltage though, the resistance can vary with voltage. I guess you are aware of the affects of high-voltage conduction (electromagnetics comes to play forcing electrons on the rim, that's why sometimes worse conductor material with a layer of copper may be used and it is not that bad).

[LongRunner]

I'm confused, but I assume you're referring to skin effect, which is a function of frequency. The diameter of the wires in the cords in question is small compared to the skin depth in copper at 50/60Hz, so the resistance wouldn't be significantly higher than at DC. PSUs without APFC will produce harmonic distortion but this still shouldn't make the skin effect noticeable.

For what it's worth, the official current ratings for flexible cables could be said to be somewhat marginal, given the rather low ambient temperatures assumed (25–30°C). (The maximum conductor temperature allowed for flexing PVC-insulated cables is 60°C even though the rated temperature may be higher, because PVC gets soft when hot.)

Official ratings (correct me if I'm wrong):

0.5mm²: 3A
0.75mm²: 10A (up to 2m)*/7.5A (Australian standard)
1.0mm²: 10A
1.5mm²: 16A
2.5mm²: 25A

These are for single-phase, de-rate by 18% for three-phase circuits.

*That's a somewhat problematic specification as the IEC-type cords can be chained, enabling the 2m limit to be exceeded. And the cord that came with my current monitor has 0.75mm² conductors despite being longer than 2m. So the enforcement is rather lacking.

I would prefer to stay within these limits (which I admit to making up, but should be reasonably accurate assuming an ambient temperature of 40°C):

0.75mm²: 6.7A 1-phase/5.4A 3-phase
1.0mm²: 8A 1-phase/6.7A 3-phase
1.5mm²: 10A 1-phase/8A 3-phase
2.5mm²: 16A 1-phase/13A 3-phase

I dunno about 0.5mm². It seems to be only barely acceptable (assuming a C16 circuit breaker upstream) for small loads. I've never seen a detachable cord legitimately using it, anyway (even though it would theoretically be adequate for the C7/C8 couple).

This doesn't mean I would actually enforce my standard any time soon, as until we end the tradition of bundling the cords with each new appliance, it would mean more wasted copper.

I don't know if the new-fangled mPPE (modified polyphenylene ether) insulation (not yet used for mains cords, but available for hook-up wire and communication and control cables from one manufacturer) softens when hot like PVC does. But I guess it would be a bad idea to let the cords get too hot to touch even with insulation that can take the heat.

So I would suggest to award bonus points to PSUs that make the cord optional (or simply don't include one). That latter version may be a tad controversial though…

[Behemot]

There is similar effect with high-voltage DC too. Maybe not that signifficant though. But the electrons themselfs push the other ones away which than also results in higher concentration by the surface as it is further from the center. It's like reversed gravity

As for the cable lenghts, well, your monitor has detachable cord, or fixed? If fixed that it is never going to pull more than aprox. 1 A so no problem here.

[LongRunner]

I wouldn't have mentioned the monitor cord there if it was fixed (or a C5 or C7 type – not that I've seen C6 or C8 inlet on a monitor).

How high of a DC voltage does it take for the effect you described to become significant?

Incidentally, I have actually seen a 2.4kW (10A at 240V) oil-filled column heater (made by DeLongHi IIRC) with a 1.5mm² cord. So there are some manufacturers that use more than the minimum required. Use an extension cord (the standard plug/socket here in Australia is rated for 10A, not the 16A of most European types, although there is a 15A version with a wider earth pin) and you're back to the minimum requirement, but it'd realistically be OK (you aren't using a heater in a 40°C summer, are you?) under normal circumstances.

[Behemot]

Oh man, look at this crap The chinese fools are really retarded or what?

1.jpg (17.22 KiB) Viewed 48991 times

[LongRunner]

I've seen (pictures of) comparable arrangements before. Here is an example with a "Europlug" (broken) and C13, which is even worse… (Don't complain to me about the dreadful lighting there.)

Given how horrible many of these are, you'd think there'd be a few PSAs warning of them (not just individual case reports).