Relays ratings

[perky]

Hi All,

I've been looking at using a relay to switch a 240VAC circuit. I've noticed that the vast majority of these relays are specified, and certified, at 250VAC for the maximum current rating.

If you assume the UK mains is 240VAC +6%/-10% (shortly to become harmonised to 230VAC +/-10%), this would mean that at worst case the voltage could be 240VAC + 6%, i.e. 254.4VAC. This would preclude the vast majority of these relays from ever being used on UK mains (or anywhere that uses 230VAC +/-10%)!!

Now, let's say the relay current is significantly over-rated, e.g. switching 16A with a 30A rated relay. Would that be an argument to allow the extra 4.4VAC and allow the component to be used? Does the certification allow, or justify, such derating?

It seems crazy to me that they certify these components to 250VAC rather than 254VAC. Any thoughts anyone?

 

[crutschow]

The voltage rating is basically concerned with arc-over when the contacts open. If the voltage is too high, the arc may not extinguish and cause a short-circuit.

But I don't believe 4.4V above the rating is anything to worry about. I would expect that the 250VAC relay rating has ample margin to tolerate an extra 4.4V (1.8%) without excessive arcing.

 

[perky]

The voltage rating is basically concerned with arc-over when the contacts open. If the voltage is too high, the arc may not extinguish and cause a short-circuit.

But I don't believe 4.4V above the rating is anything to worry about. I would expect that the 250VAC relay rating has ample margin to tolerate an extra 4.4V (1.8%) without excessive arcing.

Thanks. I agree, there are various effects to the contacts like migration that effect the lifetime. I think the arc is going to extinguish though at some point (at zero crossing for AC, and when the contacts get far enough apart not to cause breakdown). I think over-rating the current will help a lot in this.

My concern is the certification, yes it will almost certainly work OK but do I need to technically re-certify the component if it's certified to 250VAC and it's used at 254.4VAC? That's an expensive job.

Mark.

 

[Willbe]

Thanks. I agree, there are various effects to the contacts like migration that effect the lifetime. I think the arc is going to extinguish though at some point (at zero crossing for AC, and when the contacts get far enough apart not to cause breakdown). I think over-rating the current will help a lot in this.

My concern is the certification, yes it will almost certainly work OK but do I need to technically re-certify the component if it's certified to 250VAC and it's used at 254.4VAC? That's an expensive job.

Mark.

That depends if 250 means the interval 249.4 to 250.4
or the interval 245 to 254.

250 could have two significant figures or three.

250 in this context probably means 250 +/- 20%.

 

[perky]

That depends if 250 means the interval 249.4 to 250.4
or the interval 245 to 254.

250 could have two significant figures or three.

250 in this context probably means 250 +/- 20%.

Well I was under the impression the 'rated voltage' was a maximum for approval purposes not a nominal. I guess I need to find out exactly what this figure is..

 

[Willbe]

Fwiw

define: "rated voltage" - Google Search

Different organizations might mean different things.

There is also the idea of a Safety Factor. For a factor of 4, half of 250v relays would fail at 4x250 = 1000v.
And, does "fail" mean fail spectacularly and dangerously
or just have an impractically short life?

 

[GooseElectro]

Ok then so if i had a 100w light bulb at 240v (UK mains) (this would be around .5 A?) would this relay be ok for the job ?:

**broken link removed**?

Could you also tell me how much current it will take to switch the relay and will this be to hold it on or only to switch?

thank you for any replies

 

[dknguyen]

Just use V=IR with the coil specifications given.

I = V/R = Coil Voltage/Coil Resistance

 

[GooseElectro]

Just use V=IR with the coil specifications given.

I = V/R = Coil Voltage/Coil Resistance

Right yes. so 6v and a 80ohm coil would draw 75mA?

and is the relay in the link ok for switching a mains 240v 100w light bulb ?
and am i correct that this would be just under 500mA to switch ?
and that the 6v relay in the link could switch up yo 5A at 240vots ac (i.e. UK mains)?

I am trying to be sure as it seems a be small for switching mains but i think the spec is ok, could you tell me if i am right .

thank you for you reply

 

[dknguyen]

Yes, 75mA.

It says right on the page that the relays works for 5A@240VAC. The relays just have different coils to activate them using different voltages- the primary terminals are still the same.

You know the lightbulb is 100W and works at 240V. It's current can be calculated from:

Power = Voltage * Current

You don't even have to punch it into a calculator. It should be fairly obvious the relay is oversized if anything.

 

[GooseElectro]

Yes, 75mA.

It says right on the page that the relays works for 5A@240VAC. The relays just have different coils to activate them using different voltages- the primary terminals are still the same.

You know the lightbulb is 100W and works at 240V. It's current can be calculated from:

Power = Voltage * Current

You don't even have to punch it into a calculator. It should be fairly obvious the relay is oversized if anything.

Right ok thanks alot, very quick reply,

I was farly sure that i was correct but having not really worked with mains much i
wanted to be sure.

Are there any other things i should think about other than obviously insulating well, i wouldn't need to fuse it any further because I am switching it or any thing like that?

I also saw in a previous post some one mentioned about zero crossing, i understand that this is when you switch at the split second where there is actually no (or very little) current flowing. I suppose this is only needed if you wanted to switch using components with very low ratings?

thanks allot for all your help much appreciated

 

[dknguyen]

I think relays are too switch too slowly for zero crossing to matter. For triacs though, switching on at the zero crossing reduces noise. Switching off would also reduce noise but for triacs this is a non-issue because of the way peculiar triacs work. Once turned on, they stay on. The only way they can turn off is if power is cut externally (ie. the zero crossing). So triacs only turn off if the control signal is removed when a zero crossing happens. THis is why they aren't used with DC. You can turn it on, but not off unless you have a switch somewhere else that can actually break the current flow.

For relays it definately would reduce noise and prolong relay life by reducing sparking when the contacts open. It would also reduce noise when the contacts close just like the triac.But as I said, I think relays are too slow to reliably switch on the zero crossing.

Incidentally for a transformer turning on at the zero crossing is the worst time to turn it on. It basically causes a massive inrush current spike on startup which might saturate and overheat the transformer because of the way transformer magnetizing or leakage currents work. I don't fully understand it myself.

I'd fuse something connected to the mains because if something goes wrong (like a short) you might destroy your board because it might melt or light on fire and you would instead trip the breaker.

 

[GooseElectro]

I think relays are too switch too slowly for zero crossing to matter. For triacs though, switching on at the zero crossing reduces noise. Switching off would also reduce noise but for triacs this is a non-issue because of the way peculiar triacs work. Once turned on, they stay on. The only way they can turn off is if power is cut externally (ie. the zero crossing). So triacs only turn off if the control signal is removed when a zero crossing happens. THis is why they aren't used with DC. You can turn it on, but not off unless you have a switch somewhere else that can actually break the current flow.

For relays it definately would reduce noise and prolong relay life by reducing sparking when the contacts open. It would also reduce noise when the contacts close just like the triac.But as I said, I think relays are too slow to reliably switch on the zero crossing.

Incidentally for a transformer turning on at the zero crossing is the worst time to turn it on. It basically causes a massive inrush current spike on startup which might saturate and overheat the transformer because of the way transformer magnetizing or leakage currents work. I don't fully understand it myself.

I'd fuse something connected to the mains because if something goes wrong (like a short) you might destroy your board because it might melt or light on fire and you would instead trip the breaker.

Thanks, some very useful infomation, that will help me alot

Thanks alot for your help