Switching vs maximum voltage of a relay


Thread Starter

Michael Michalski

I am having a great deal of difficulty finding the maximum voltage I can subject a relay to. I need a relay that can carry 500mA at 400 volts.In my application however,the relay will never actually switch when the voltage is applied. THe controlling computer will set the relays to their proper configuration,then apply the power. The woltage wil be removed,then the relays will change over to the next configuration. Is it actually necessary to use a relay that can switch 400 volts or am I looking at the wrong specification?
you may well have a circuit that jacks up a lower voltage to reduce the amperage to not burn up your circuit, you need a computer to figure out the starting voltage, given the amperage?am ammeter to find the amperage?


John Waalkes

Wow, good question :)

We do the very same thing with our drive switching contactors (they allow us to use either VFD on either motor). We only switch them when de-energized.

Soo... since there is no danger of crossover because of carbon deposits, if it works once, it should theoretically work forever (Heavy on the "theoretically").

But I'm going to err on the side of caution and say that for 400v, you want the industry standard 600v relay. Otherwise, I would suggest that you contact a relay manufacturer and see what they suggest.

And just a word of caution, while our system is designed to never switch under load (not a real design consideration on our part, more of just a "normal mode" thing), that doesn't mean that it doesn't drop out upon occasion with spectacular results :)

You know, the bbbbiiiizzzzaaaapppppp sound and the big blue ball of fire :)

One biizzapp @400v on a 120v relay and that will be all she wrote.

(anyone else here ever work with the old selenium rectified 120v elevator controls? Talk about meltdown :)


Information about distribution and utilization voltages can be found in the following sources:

Standards covering the recommended nominal voltages can be found in ANSI C84.1-1989 covering 120 Volts to 230 kiloVolts.

ANSI C92.2-1987 covers AC Electrical Systems and Equipment Operating Voltages Above 230 kV.

In my experience 480 Vac distribution and 460 Vac utilization voltages, not 440 Vac, were established as the norm in the early 1960's.

The above standards also list the associated tolerances for single, two, and three-phase systems, having 2, 3, and 4-wire configurations.

Voltages in other countries differ from those in the US. For exact values refer to the IEC standards in Europe, and CSA standards in Canada.

Phil Corso, PE
(Boca Raton, FL)

Alan Rimmington

The relay must be rated for the voltage being carried. If the relay fails the contact will open breaking the full voltage. Many things can cause the relay to open including coil failures, wiring faults, process control faults etc etc


Jocko Harmet

The rating of a relay/contactor for voltage has to do with contact gap and contact spacing. if to low of a voltage relay is used, you could get arcing across the contacts when they are open. You aslo have to design for the possibility that a relay could open when voltage is applied (loss of control power etc.)which could cause contact welding - or in the case of a multipole relay Phase to phase shorting as most small relays do not have an arc chute to confine the arcing to an individual set of contacts. Your relay would therefore need to be rated for 400 volts.

Jocko Harmet
Phoenix, AZ

Jeffrey Eggenberger

You can never really guarantee that the relays won't switch during use. What if a coil should fail? You get the voltage rating from the manufacturer, it should be printed on the device.

Jeff Eggenberger

Curt Wuollet

You would still want the insulation and contact spacing for the 400V to prevent arcing across open contacts or breakdown. So for practical purposes I would say you should use a relay rated
for those voltages. You might be able to cheat a bit on current rating but almost any relay will handle .5 amp anyway.



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