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Thread: Insulation Ratings Additive??

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    Insulation Ratings Additive??

    So I'm asking what I believe to be a dumb question. We have a 600v insulation rating wire that is covered by a 300v insulation rated pvc tube. I say that the highest voltage potential for the combo to see is 600v. My boss is telling me that insulation ratings are additive?!?! Surely that is not correct! Anyone know or have info to back what he is saying.

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    Interesting question, but definitely not dumb.

    Electrically speaking,I'd say he's correct IF the rating of the PVC presumes a bare energized conductor.

    Otherwise, I'd have to say that the weakest link rules: 600v wire inside 300v conduit is limited to 300v.
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    Larry B. Fine
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    I agree that this is not a dumb question. My answer is that the boss is wrong.

    The voltage rating of any component is based on the ability of its insulation system to prevent current from leaking from an energized internal part to the outside world. Let's change the scenario by saying both the wire and the PVC tube have 300 volt insulation systems. Now impose a voltage of 480V on the wire. That will have enough "pushing ability" to force current through the wire's insulation. Now consider the wire is lying on the bottom of the tube, and the tube is resting on a metal bar. You still have 480V pushing current from the metal wire through whatever insulation it encounters. The resistance of the insulation systems is significant, but the current flow is small. So I don't expect any significant voltage drop through the wire's insulation system. Thus, you now have 480 volts pushing current through the tube towards the metal bar.

    I may have this all wrong. It wouldn't be the first time. But I will stand by my opinion that the insulation ratings are not additive.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Additive in what regard?

    By that I mean do you want to look at this from an electrical code standpoint? If so, I think you just have to go by the wires listing. Adding more insulation won't do anything to increase the listed voltage rating, so no, you would still have a 600V wire. Also note that the nylon jacket on most wire gets scuffed or torn when the wire is pulled into a conduit, so the nylon jacket on most wire is more for oil and gasoline resistance, and doesn't really help the voltage rating that much.

    Or from the physics standpoint? How much voltage can we really expect the combination of wire insulation and a PVC tube to withstand?

    From that standpoint I'm going to guess that maybe its even higher than the sum of the two voltage ratings. Like Charlie, I will admit I could be wrong.

    But look at this chart:

    https://www.anixter.com/content/dam/...-wisdom-en.pdf

    And notice that the insulation on a 15KV line is not even twice the thickness of the insulation on a 5KV line. So doubling the thickness gives 3 times the voltage insulation. Similar thing with 25KV. Five times the voltage of 5KV, but the insulation thickness is only about 3 times as thick.

    So its not a linear relationship. A layer of 600V insulation over a layer of 600V insulation would double the thickness, and might give you around a 1800V insulation level.

    However, there are also so notes about 0-2000 Volts, and that may be a different situation than the higher voltage KV rated cables.

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    Quote Originally Posted by charlie b View Post
    I agree that this is not a dumb question. My answer is that the boss is wrong.

    The voltage rating of any component is based on the ability of its insulation system to prevent current from leaking from an energized internal part to the outside world. Let's change the scenario by saying both the wire and the PVC tube have 300 volt insulation systems. Now impose a voltage of 480V on the wire. That will have enough "pushing ability" to force current through the wire's insulation. Now consider the wire is lying on the bottom of the tube, and the tube is resting on a metal bar. You still have 480V pushing current from the metal wire through whatever insulation it encounters. The resistance of the insulation systems is significant, but the current flow is small. So I don't expect any significant voltage drop through the wire's insulation system. Thus, you now have 480 volts pushing current through the tube towards the metal bar.

    I may have this all wrong. It wouldn't be the first time. But I will stand by my opinion that the insulation ratings are not additive.
    I'd say your assessment is likely correct. As a professor once told us, "There is no such thing as a perfect insulator. All materials are conductive, some just more so than others." How that would apply here is that if there is a resistance to the flow of current at 600V of the base insulation, that resistance has less of an effect if the voltage goes up to 900V (the level we can assume they were trying to get at with the 600 + 300 strategy). So if current STARTS to flow out of that 900V circuit because of the insulation only being good for 600V, it has by definition ALREADY exceeded what the 300V sleeve could block. Now technically, resistors in series are additive, which is likely where they are coming up with this idea. But in cable insulation it can be different because once you exceed what's called the "corona inception voltage" limit of the cable insulation and a partial corona discharge begins, it will very very quickly burn it's way through everything in its way. The 300V sleeve will not likely even slow it down. People often only associate this with MV and HV cables, but it technically occurs at LV cables too, it's just that it's more rare because the energy in the circuits is lower. But I've seen this happen with PVC insulation like THHN used on the output of VFDs where reflected wave spikes get into levels higher than the 600V rating, and in many cases the PD burns were phase-to-phase, where the total insulation was TWICE as thick because each conductor was insulated at 600V.
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    190516-1625 EDT

    Almost certainly they are additive. But not likely in a linearly fashion, and possibly in some negative fashion.

    Both thickness and voltage gradient within the material along with the material determines breakdown or other failure in an insulator.

    When you stack insulating materials both dielectric constant and leakage current along with material determine the electric field intensity. So many factors come into play. Just use the rating of the basic insulated wire.

    .

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    Quote Originally Posted by gar View Post
    190516-1625 EDT

    Almost certainly they are additive. But not likely in a linearly fashion, and possibly in some negative fashion.

    Both thickness and voltage gradient within the material along with the material determines breakdown or other failure in an insulator.

    When you stack insulating materials both dielectric constant and leakage current along with material determine the electric field intensity. So many factors come into play. Just use the rating of the basic insulated wire.

    .
    I agree that the biggest single wild card is the assumption that the electric field density (and the voltage drop that results) is uniform throughout the single insulation mass or the two insulation masses in series. Sort of like the way you cannot just put diodes in series to get a higher reverse breakdown voltage unless you add an impedance network to make sure the voltages are balanced both for steady state and for transients. Otherwise you might take out one at a time in a cascade.

    However, I would also definitely say that if all wires inside a 300V tube are rated for 600V, the tube does not lower that rating. if it did, putting insulated wire into metallic conduit would cause a dead short.

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    UK multi core cables have two voltage ratings. Industrial cable is usually 600/1000V, 600V Ph→E, 1000V Ph→Ph. Household cable used to be the same until the bean counters got involved, it’s now 300/500V.
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