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I am working through the creepage and clearance in IEC 61010-1, and I find that transient voltage (1.2 x 50 µs) is taken into account for clearance, but in creepage, only the working voltage is taken into account (with altitude, material group and pollution degree).

Why is transient overvoltage not taken into account (unless clearance is bigger than creepage)? I can have a working voltage of 1000 VDC, and if my transient is 500 V or 10 kV, the standard does not take it into account. Why is this?

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  • \$\begingroup\$ Which specific sections/clauses are you having concern with? \$\endgroup\$ Commented Jun 23 at 20:12

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Creepage problems takes time. If you take a look at any fractal pattern using high voltage across wet wood (link), the process takes several seconds to develop. What you're seeing here is tracking. Your typical lightning impulse is in the order of 10-100 µs. Same thing with partial discharge, the process can take days to years before it becomes a problem so it's related to the working voltage of the circuit, not transients.

Image stolen from YouTube (johni1084) link above: enter image description here

Clearance problems will however show themselves immediately as flashover when the transient comes, hence it's related to impulse voltage, not working.

Image stolen from https://encyclopedia.pub/entry/28485 enter image description here

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This is my take on it. Probably because if there is a transient, the section will arc and the arc will not maintain because of the distance. So you may get arcs only for a short time but then they will self extinguish.

But if you have arcing and transients like this in the design, there are probably bigger problems. It's easy to add a TVS\GDT to keep the transients tamped down. In AC mains products you need to design for transients anyway and these extreme transients are unlikely to pass the other requirements of IEC 61010.

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