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How to choose a high voltage relay

Guest bosskardo

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Guest bosskardo

I have a high voltage withstand tester and I'd like to use a relay to switch its path. That way I can do different tests without moving wires.
The problem is that the tests use as high as 2kV voltage. Allthough the current is low, max 10 mA. Tests run with AC nad 50 or 60Hz.
Test  environment is indoors but the relay will get a lot of use.

I'm at a loss how to choose a relay, all high voltage relays are also high current but I don't need high current.
I don't know how big the safety factor should be. The tester monitors the voltage and current and cuts of if either goes over the limit but there is bound to be some delay. I think the safety factor can be a lot lower than motors(as I don't think I'll have problems with big transients) but how much. Relays are usually overdimensioned by a factor of 5 to 20. Could I use 5?
How much does hot switching damage a relay thats not meant for it? If I'm planning on avoiding it but there could be an event when it happens, should I still use a relay thats meant for hot switching? I'm tring to avoid sealed inert gas or vacum relays and thats why I'd like to use one with as low voltage as possible( if I'm not wrong then most relays from 5kV up are sealed).

How would I go abaout choosing a correct relay. I have read several guides but they weren't much help. Can someone help me?

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Guest bosskardo

The voltage shouldn't go above 2kV (the tester has failsafes but I think it can go a little over it until the tester can shut the current down). The voltage (depending on the test from 1kV to 2kV is ramped up to its max and held there for 5 to 15 seconds).

Thank for the links Looked at them, these seem good but is isn't it a waist to use a 2A relay when my current is well below that. Could I perhaps use a lower voltage relay that has the power rating well above my need (fe. if I have 2kV, 10mA current, then I use 1kV,1A relay?)? Or will that completely kill the relay?
About the price, they are a little over my limit. I set it at 25 euros, not the end of the world if it goes a little over but I'd like to get sth cheaper.

Also, for future reference, I'd like to know how much should I overdimension my relay? I've heard that you should use 10-50% of relays max but how exatly would I determine a more exact percentage how much could I use?

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The current rating is not a problem. It's better to go higher than lower. Using a power relay to switch low current signals is only a problem at very low voltages.

The problem with using a lower voltage relay is the contacts can arc over and what's worse it it could be unpredictable.The relay may seem fine most of the time but it could suddenly arc over due to ambient ionising radiation.

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Guest bosskardo

These relays are all SPST but I need to switch between 2 states.
Searched for SPDT and it seems their price starts at 100 euros.

Would it be possible to use a regular relay for switching between states and then use a SPST high voltage relay to break the circuit whenever I need to switch?
If I have understood correctly then it should work. Am I rigth that high voltage relays need to prevent current arcs when switching? Then it should be ok to use a normal relay if I can make sure there is no current when switching?

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No, an arc can form when the relay is not switching, if there isn't enough clearance between the contacts. The breakdown voltage in air is typically 1kV/1mm, so for 2kV the gap between the contacts needs to be at least 2mm. A high voltage relay may be filled with a gas or vacuum which has superior dielectric properties to air so a smaller gap can be used.

Can't you just invert the logic signals for controlling the relays?

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