Clearance of Exposed Nodes on High Voltage Relay

Question

I'm trying to use this high voltage relay for a machine that will assist in HI-POT tests. The relay can handle about 10kVpeak according to the datasheet and I need it to handle 3.5kVrms maximum. I've run 3.5kV across the high voltage nodes in a HI-POT test on the relay and everything works as intended. While designing the PCB that will mount the relay, I looked online to see what the clearance rules are to prevent arcing elsewhere; but according to the IPC-2221B standard, the the exposed nodes need to be about 25mm apart for 3.5kV, which is already farther than the nodes on the relay are.

Can I ignore this standard or is there a different standard I should be using more suitable for HI-POT tests? How close can I actually make two exposed nodes on the PCB at 3.5kVrms?

Answer 1

You should distinguish between safety specifications and operating specifications.

25 mm clearance or creepage for 3.5 kV is a safety specification. For comparison, IEC-61010 (a safety standard I happen to have handy) requires 10 mm clearance and 12 mm creepage for 3.2 kV AC (under certain specific material and "pollution" conditions). You can see the IPC spec is much more conservative than the IEC one. IPC is also not a legal requirement in any jurisdiction I know of.

The dielectric strength of air is about 3 kV/mm, so you are not going to get arcs between pins until you bring them closer than ~2 mm from each other.

You might see leakage current through contaminants on the PCB surface if you cannot clean your PCB and keep it clean in operation. How much this affects your circuit depends on the exact nature of your circuit and how much contamination gets on your board.

If you're going to sell your design, it would be a good idea to find out exactly what regulations you are subject to and get hold of the relevant documents. Or hire a PE with access to the appropriate documents to advise you on the required creepages and clearances for your specific application and market.

Answer 2

According to this page, you should have about 25.3mm for 3500V. However, this is a pretty conservative spec, and takes into account creepage, corrosion, impurities, etc. If you're designing a one-off piece of test equipment which will spend its life in a clean, climate controlled environment, I'd say you're pretty safe (or as safe as you can be with 3KV floating around).

If you want more assurance, you could use epoxy or silicone to cover the terminals and fill the air gap.