how does voltage affect wire insulation?

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Old 01-12-16, 03:11 PM
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how does voltage affect wire insulation?

I'm interested in how voltage can affect wire insulation?
For example, to discharge a capacitor you can run a resistor across it.
However, some capacitors can hold 450V +
What happens if you use incorrect wire insulation rated to 300V?
The current is minimal so the wire won;t overheat.
 
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Old 01-12-16, 03:37 PM
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If you exceed the wires insulation rating, you can get an arc right through the insulation. For example, if you had a 300 volt rated wire connected to one side of a 500 volt source and the wires insulation also touched the other terminal or side, you could get an arc through the insulation (called breakdown) It's pretty much like there was no insulation on that part of the wire.

As a practical matter, insulation voltage ratings tend to be conservative, so if the rating is exceeded only slightly, it probably won't breakdown. But it would be a bad idea, for example, to use automotive primary wire rated for 25 volts to wire a 120 volt ac circuit.
 
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Old 01-12-16, 04:34 PM
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Another factor is temperature rating of the wire's insulation. You will often find internal wiring in devices that is smaller gauge then would be permitted for house wiring but it is okay because the higher temperature rating of the insulation allows the wire to handle the extra heat generated by the higher resistance of the smaller wires without the insulation breaking down. In fact copper wire can withstand far higher currents than their listings in charts. It is their insulation that limits the current they can be used for.
 
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Old 01-12-16, 06:59 PM
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I need to discharge a capacitor in an amplifier.
The capacitor is rated to 450V so I shouldn;t use 300V 12AWG.
Guessing test leads for a multimeter will be enough but it just seems strange as the insulation on test leads is the same if not thinner than that on 12AWG NMD 90 wire.
 
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Old 01-12-16, 07:23 PM
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I would use a screwdriver with a plastic handle. If you use wire the voltage is momentary and the leads should handle it.
 
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Old 01-12-16, 08:06 PM
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A couple of things going on.

Different insulation materials have different insulating properties. So you can't go by thickness alone unless both materials are identical. Test leads need to be flexible so they use a thin insulation with high dielectric strength. Wire intended for AC use needs to be tough and abrasion resistant to withstand the rigors of being pulled through conduit, twisted and jammed into boxes, etc. And as Ray pointed out, the operating temperature range is another factor.

Also, the insulation used on a particular wire may be capable of withstanding higher voltage, but is simply not tested and certified (By UL/ETL/CSA etc) for the higher voltage because it isn't necessary for the intended application.

Personally, I wouldn't hesitate to use 300 volt AC wire to discharge a 450 volt cap. For one thing, the peak voltage on 300 VAC RMS is about 425 volts, so it can withstand that all day. I'm certain the safety factor is more than enough to get you to 450. Now 1KV? Probably not.
 
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Old 01-12-16, 08:48 PM
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It's some electrolytic capacitors that I need to check if they are discharged. There are bleeder capacitors in the circuit but it's quite old, maybe 50 years+ so if everything is still working I'm not sure.
However, there are some 3300pF, 1000V, 0.001uF, 1000V disc/mylar caps in there that although I'm doing nothing with, I guess I have to be careful of or is it only electrolytic capacitors that are dangerous?
 
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Old 01-12-16, 09:14 PM
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google "chicken stick discharge". Or, if the threat is under 600V or so, use a voltmeter to see what the cap holds. Or, fashion a longish screwdriver and a jumper wire to ground. Worked for me when I had to discharge 30kV caps. All this needs to be done with active power OFF.
 
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Old 01-13-16, 07:16 AM
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How long has it been since power was applied?

It would be very unlikely for those smaller caps to be used in a way that allows them to maintain a charge after power is turned off, and in any case, they don't store enough energy to be a threat.

The big electrolytics can hold a wallop so it's smart to discharge them before working on the circuit.

If this is an old piece of equipment that hasn't been powered up for years, then you should read about "reforming electrolytic capacitors" before you power it up. Big old electrolytics tend to dry out over the years. Sometimes they can be salvaged with the reforming procedure, often they need to be replaced.
 
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Old 01-13-16, 08:05 AM
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Virtually all building wire types (THHN, XHHW, NM, etc) are rated to 600V, not 300V. The only common wiring you'll run into that is rated at 300V is service junior (SJ) flexible cord. That's what the junior means -- just service rated cord is good at 600V.

The insulating properties of the overall insulation system have as much to do with the thickness as with the materials, the purity of the resins, the intended operating environment and the manufacturing process used to lay them down. Modern insulations are often layered with different polymers applied in layers of various thicknesses. In other words, you can't tell by looking.

Edit: I thought I should also mention type SV (vacuum cleaner cord) and SPT (various lamp cord / light duty extension cords) which are also rated only at 150V or 300V.
 

Last edited by ibpooks; 01-13-16 at 08:32 AM. Reason: add info
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Old 01-13-16, 05:55 PM
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How long has it been since power was applied?

It would be very unlikely for those smaller caps to be used in a way that allows them to maintain a charge after power is turned off, and in any case, they don't store enough energy to be a threat.

The big electrolytics can hold a wallop so it's smart to discharge them before working on the circuit.

If this is an old piece of equipment that hasn't been powered up for years, then you should read about "reforming electrolytic capacitors" before you power it up. Big old electrolytics tend to dry out over the years. Sometimes they can be salvaged with the reforming procedure, often they need to be replaced.
I'd actually already powered it up a few times before I'd started looking into this. Very lucky apparently that it's still ok. It needs a fair bit of work including replacing some tubes.
The electrolytic capacitors should be ok, I'm worried about the screwdriver method as it might damage the equipment?
I alreday have a resistor so planned to use that.

However, I also saw some ceramic/mylar caps rated at 300pF (rather than uF) but 1000V.
Are these more dangerous than the elctrolytics or becaus they're "dry", they're ok.

Virtually all building wire types (THHN, XHHW, NM, etc) are rated to 600V, not 300V. The only common wiring you'll run into that is rated at 300V is service junior (SJ) flexible cord. That's what the junior means -- just service rated cord is good at 600V.
Maybe it's a CEC vs NEC thing but almost every NMD cable I has only has 300V on the outer sheath.
 
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Old 01-13-16, 09:01 PM
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Discharging with a screwdriver makes a bit of flash sometimes, and a loud pop sound sometimes, but won't damage anything. But use the resistor if you like, it's less "exciting".

The lower value caps are not generally dangerous even if they are rated at 1000 volts. A 300pf cap stores 1 millionth as much energy as a 300 microfarad cap, at the same voltage. It's just not enough energy to hurt.

Whether a cap is "dry" or "wet" has no bearing on whether it is dangerous or not.

Many older ceramic caps were rated for 1000 volts because it was easy to make one handle that voltage. That doesn't mean it ever sees that much voltage in operation.
 
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