1
\$\begingroup\$

I have a relay with following contact rating: enter image description here

Since the maximum switching voltage is 250VAC/25A, can I use this relay to switch 380VAC/1A load? What is the consequences?

\$\endgroup\$
4
  • \$\begingroup\$ Possibility of shoot through when contacts are open. The AC voltage rating tends to be due to clearance between components. Shorter contact life due to arcing. \$\endgroup\$
    – vini_i
    Mar 2, 2020 at 13:32
  • 30
    \$\begingroup\$ Why do so many posters ask a question like 'the data sheet tells me the max is X, can I run it at Y, where Y>X?' You are lucky to get a data sheet! Respect it. Or do your own tests and buy some spares. Not only contact clearance, but also speed of opening to that clearance, are important for the voltage it can break. Expect the life to be shorter. Whether you get 10% of the rated operations, or only one operation, is down to your tests. \$\endgroup\$
    – Neil_UK
    Mar 2, 2020 at 13:55
  • 2
    \$\begingroup\$ @Neil_UK Why is the person lucky to get a data sheet? Are people in some cultures doomed to use shoddy product from certain shabby countries? Why not use a Siemens or Omron unit if it's the right thing for the job? \$\endgroup\$ Mar 2, 2020 at 16:01
  • \$\begingroup\$ I've been an Avionics tech for over 35 years. Don't do it all your gonna do is have a fire! \$\endgroup\$
    – user244262
    Mar 3, 2020 at 23:07

5 Answers 5

19
\$\begingroup\$

Since the maximum switching voltage is 250VAC/25A, can I use this relay to switch 380VAC/1A load?

You can't horse-trade current rating and voltage rating on a relay contact.

What is the consequences?

  • Early failure of the contact (most benign first)
  • Inability to perform an open circuit due to arcing
  • Excessive heat due to open circuit arcing (leading to.... fire)
\$\endgroup\$
3
  • \$\begingroup\$ Sometimes, you can. It depends on how important is the project and how bad can be a failure thereof. If this is a hobby project with no potential to set the house on fire, I would try. \$\endgroup\$
    – fraxinus
    Mar 3, 2020 at 10:23
  • 6
    \$\begingroup\$ @fraxinus One of the failure modes is that a circuit handling a lethal voltage remains energised "unexpectedly" - I'd say that is a rather high risk for most hobby projects, and not really a "suck it and see" option. \$\endgroup\$
    – MikeB
    Mar 3, 2020 at 11:54
  • \$\begingroup\$ @MikeBrockington sure, but a relay cannot be considered a safety disconnect. And if you are working at 380V AC (3-phase, that is) then both relay contacts are live and you shouldn't touch anything that is not visibly unplugged anyway. \$\endgroup\$
    – fraxinus
    Mar 3, 2020 at 12:30
14
\$\begingroup\$

When you're dealing with voltages that high, the biggest challenge for a relay is snuffing the arc. The arc is proportional to voltage.

As a relay opens, voltage will arc across the contacts. That's a sure thing. It can't be stopped. Further, once the arc ionizes the air across the gap, that makes it much easier for the arc to continue. However the relay stops the arc one of several ways:

  • Opening the contact too far for the arc to leap. This is greatly helped by AC power; since AC voltage and current go to zero twice per cycle. This is why DC ratings of relays are very, very much lower than AC ratings.
  • Magnetic blowouts. This uses a magnetic field (typically a collapsing one from the interruption of power) to suck the arc into an arc chute, where it is snuffed.
  • Air blowouts. This is a puff of high pressure air blasted across the contacts to blow the ionized gases away into an arc chute, replacing it with non-ionized air.
  • Oil immersion. The contact swims in an insulating fluid, with a much higher dielectric strength than air, and will not ionize or support formation of an arc.

So when you use a relay that is under-rated for your voltage, the very obvious failure mode is that it will simply arc across the contacts anyway, either welding the contacts together, or setting the relay on fire. When post-failure forensics are done, this failure will be a surprise to no one.

\$\endgroup\$
0
\$\begingroup\$

Manufacturers data should always be followed if for no other reason than of liability. No mention of type of load is considered, reactive load would significantly reduce contact capability. Multiple poles can help only providing pole isolation is acceptable, otherwise there is one big mess. The relay listed is a low voltage type which is 240v absolute max and only under special conditions. To expect it to switch 300+v is totally impractical and will result in (spectacular) failure.

\$\endgroup\$
-3
\$\begingroup\$

Not recommend for 1 contact. You can series 2 normally open contact to get higher breakdown voltage. Just incase that both contact using same mechanism (DPDT or more).It could be better if you have 4PST then you can series all 4 contacts.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ This does not help. Why would it help? The first of the series coupled contacts would have to break the whole voltage and current anyway, and the second third etc only switches on 0V, when the circuit is already broken. \$\endgroup\$
    – MrGerber
    Mar 4, 2020 at 9:57
-3
\$\begingroup\$

I agree with the Four pole single throw relay. To safely pull it off without question, you use the 240 volts to energize the coil of the relay and that allows you to pass whatever voltage you want to send through it. It can be AC, DC, single phase or 3 phase it doesn't matter.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ What? You're basically describing how a relay works. \$\endgroup\$
    – MrGerber
    Mar 4, 2020 at 9:59