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Regs: System Impulse Overvoltage Uw

Willie Fraser

A requirement of equipment installed in an Overvoltage Category IV system, for 400V, is that it should have a rated Impulse withstand voltage Uw of 6kV (table 443.2). I can see from manufacturers data sheet the rated Impulse value for each bit of kit, but cannot see an Impulse value on the cable data sheets, can someone please advise how the cable is verified for the Impulse Overvoltage.

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    The glib answer is to say contact the cable makers. But more practically, for all but the thinnest plastic insulation, the break-over voltage is determined by the quality of the ends and terminations, and not by the insulation of the cable. PVC or polyethylene will hold off many tens of kV across a mm thickness of dielectric.

    Air will only hold off about 1kV/mm and any surface which is open to air will start to support tracking at a few hundred volts per mm, depending on surface finish, humidity etc.

    Mike.

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  • 0

    The glib answer is to say contact the cable makers. But more practically, for all but the thinnest plastic insulation, the break-over voltage is determined by the quality of the ends and terminations, and not by the insulation of the cable. PVC or polyethylene will hold off many tens of kV across a mm thickness of dielectric.

    Air will only hold off about 1kV/mm and any surface which is open to air will start to support tracking at a few hundred volts per mm, depending on surface finish, humidity etc.

    Mike.

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