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In my application the DC supply has rating of 1200V and 15A. I just want know the calculation for the trace clearance between power and ground.

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    \$\begingroup\$ Olin gave you a good answer but I just want to know what application uses a PCB that needs an 18 kilowatt power supply? \$\endgroup\$ – Barry May 6 '16 at 12:10
  • \$\begingroup\$ 25 HP VFD?... ? \$\endgroup\$ – Spehro Pefhany May 6 '16 at 12:57
  • \$\begingroup\$ Now 7.5HP later it will expand @Spehro \$\endgroup\$ – Sathyanarayanan G May 6 '16 at 13:06
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To pick one safety standard- 61800-5-1 (ref IEC 60664-1), for 1200VDC pollution degree 2 and insulation material group III, requires you to provide 12.6mm minimum creepage for reinforced insulation. Look up the relevant standard(s) for the precise definitions of the terms used.

It's your responsibility to determine what standard(s) your product must conform to and to make sure your design meets all of the constraints simultaneously.

At 1200VDC consider milling (unplated) slots in the PCB.

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    \$\begingroup\$ There's actually a minimum milling size before you're allowed to go from creepage to airgap. But damn if I can remember what it was. Some obscure part of IEC60950 et al. \$\endgroup\$ – Barleyman May 6 '16 at 13:04
  • \$\begingroup\$ @Barleyman That makes logical sense- consider debris rattling around that could bridge the slot. All the more reason to hunt down the exact standards and study them. It's also possible (and sometimes done) to press an insulating insert into the slot so it protrudes on both sides. \$\endgroup\$ – Spehro Pefhany May 6 '16 at 13:12
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Normal "air" can withstand about 1 kV per millimeter. That means at 1.2 mm spacing you are likely to get arcing. Obviously you want the spacing to be more than that, actually much more than that. I'd start with 5x more at least, but possibly even more depending on the usage and regulatory requirements.

Dirt coupled with humidity can greatly increase the leakage current across what would otherwise be a insulating surface. This is why there are often separate specs for creapage and clearance. Creapage is the minimum distance between two conductors along whatever insulating surfaces connect them. Clearance is the shortest direct-line path thru air between the two conductors.

Since the cost of failure can be very high (burn down house, kill people), there will be regulations you must follow, and they will be very conservative. These regulations will vary depending on intended use and how the overall circuit is protected. For example, something inside a grounded metal box that is not intended to be user-accessible will be more lax than a patient-touching medical device. Of course you shouldn't be anywhere near the latter if you have to ask here.

So the real answer is in whatever regulations you are required to design this device to in the jurisdictions you want to sell it, or in the requirements of the certifying agency your customers will require, whichever is more conservative.

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  • \$\begingroup\$ I wouldn't "start with 5x more". Unless your HV supply is inherently safe e.g. ES1 in IEC62368 you must use regulation limits. Spehro already referred to these. More recent IEC standards recognize that just because it's 2kV doesn't mean it's going to kill you but older ones have more mechanical definitions that can be a PITA. \$\endgroup\$ – Barleyman May 6 '16 at 13:01
  • \$\begingroup\$ @Barl: I think we're saying the same thing. As I said, ultimately the answer comes from the regulations. But 5x is a starting point if this is a one off you're not going to sell, expose others too, etc. It's still good to find what the regulations require, then expand the creapage and clearance distances to those if needed. \$\endgroup\$ – Olin Lathrop May 6 '16 at 14:35

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