# Why the insulation rating of a wire depends on voltage?

I read online that insulation rating of the conductor depends on the voltage rating and not on the current. Can some one explain me how current doesn't contribute to insulation rating?
Am i missing something?
Kindly explain me

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Tom.G
Hi srinaath,

Insulation has two ratings. One is Voltage rating, how high a voltage it can block without breaking down. The other is Temperature rating, the highest long term temperature it can withstand without damage.

Of course the Conductor is rated by the maximum Current it can conduct without overheating. For a bare, uninsulated, conductor, depending on the use, this maximum temperature may be where the conductor starts to melt, where it corrodes by reacting with the atmosphere, or just gets hot enough to start a fire. For a wire with insulation, it is usually the insulation that limits the maximum temperature.

Tom

srinaath and berkeman
CWatters
Homework Helper
Gold Member
In addition to the rated voltage you also need to ensure the operating temperature is within spec. The operating current effects the temperature of the wire (I^2R heating) so current does effect the insulation.

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srinaath
Another way to approach this is to consider an electrical/hydraulic analogy, even though it doesn't hold up very well in the details.

For a given pressure, pipe inner diameter (wire cross-sectional area) determines how much current flows. Pipe material and wall thickness (insulation material and thickness) determines how much pressure (voltage) can be contained. If pressure (voltage) rises much above burst strength (dielectric breakdown) rating, water is no longer contained solely within the pipe walls - it develops a leak, or may burst catastrophically (insulation system develops leakage, or breaks down completely).

srinaath
Am i missing something?
The purpose of 'insulation' is to prevent (unwanted) current. So, while it is still insulation there is no current through it (regardless of the current carried inside the cable): when it broken down and current is flowing through it, then it is no longer insulation.

srinaath
Thanks Tom, cwatters, asymptotic and Rive for your valuable explanation.