relay voltage withstanding

[Kardo22]

Hi
Could I use a normal relay for high voltage application if there is no switching under load?
For example: I need switch 2kV AC (max 20mA current) through the relay. The problem is these relays are really expensive (over 50$).
If I could make sure the switching doesn't happen under load, could I use a normal relay?
I think this migth have some connection with dielectric strength of the relay? But I'm not sure at all (and there are different dielectric strengths: between contacts, contacts and coil etc).
Could you help me please? can this be done?
Thank you.

 

[davenn]

hi there

No, I wouldnt recommend it, the relay is likely to fail

$50 or so is cheap for a decent relay

if you think that's expensive ya wanna try the $400++ for the RF relays us ham radio guys have to buy for switching 250Watts or so of RF at a couple of GHz

get the correctly rated relay for your needs

cheers
Dave

 

[shumifan50]

Strongly advise against.
2KV needs VERY different insulation from 220V, so it will always be dangerous.

 

[Kardo22]

Even if the dielectric strength is more than running voltage?
f.e. I found an electromechanical relay that has 380VAC switching voltage and 3kV dielectric strength for 1 minut between contacts. If I ran 2kV (max 20s time) through it, what would happen? Would it fail after 10-20 times?

 

[Harald Kapp]

The 3kV dielectric strength is a measure of the max. voltage that the open contacts can withstand for a short time.
When designing a circuit for switching high voltages, you cannot rely on the nominal operating voltage (2kV in your case). You'll have to take into account overvoltage due to effects like power surges, mains overvoltage etc.
You need to select components (relay) that are rated for at least the same nominal voltage as your operating voltage.

When considering relays, it is not suffficient to look at the open and closed state of the contacts. You have to look at the switching action, too. Assume your contacts are closed. When the relay opens, the voltage across the contacts will create an electric arc causing current to flow between the contacts (even if they are open!) The initial arc will inonize the air between the contacts, making it conductive and letting even more current flow.
If the voltage is high enough (2kV is high enough), the arc will persist and burn out the contacts. A high voltage relay is designed to overcome this by having widely spaced open contacts and fast switching speed. It may occasionally even incorporate a magnet to "blow" out the arc (by lenghtening the path of the arc via the electromagnetic force exerted on the arc by he magnetic field).

If I could make sure the switching doesn't happen under load

How would you ensure this? You'd need another switch to turn the 2kV on and off and you could use that switch in the first place instead of the relay you want to use.

 

[Kardo22]

Thanks Harald

About making sure switching doesn't happen under load:
I need to switch the circuit in several places, so I thougth about using 1 SPST high voltage relay that would turn off the current and use normal relays otherwise.

 

[Harald Kapp]

Please don't!

 

[Kardo22]

Ok, will do it in the traditional way with correnćt relays
thanks