Relay Contacts Current Load

[Wp100]

Hi,

Recently tried but failed, I think, to get my point across about how to correctly use a relay.

The commercial temperature controller device, relay driven, is rated at 3kw @ 230vAC.

The users were connecting a 3kw heater to it and finding the relay would not last much more than a year ( perhaps being switched about 100 times a day)

I said how I typically derate power relays and such like by 30 to 50% to ensure good life and that for a 3kw load they would be better using a 20 or 30 Amp relay.
I used the comparison of driving the car at max revs all the time to highlight the point, but it fell on deaf ears.

Anyone suggest any evidence I can present to them to clearly emphasize the point in a non technical way ?

thanks.

 

[alec_t]

Not an answer to your request, but the relay switched ~365000 times in its one year life. That's more than the rated number for many relays .

 

[ericgibbs]

Not an answer to your request, but the relay switched ~365000 times in its one year life. That's more than the rated number for many relays .

I think your calculator batteries need changing 365 *100 .????

 

[alec_t]

Oops! Brain not in gear. 36500 is well within most relay specs!

 

[Reloadron]

Anyone suggest any evidence I can present to them to clearly emphasize the point in a non technical way ?

Yeah, my guess is the contacts are failing pitted and burned. Should this be the case try visually showing them the charred, pitted and burned contacts. If possible and watching safety have them observe the contacts opening and closing.

About 20 years ago I had a similar problem with some old large walk in ovens. I replaced the control systems including mechanical contactors with 75 amp zero crossing SSRs. IN 20 years only a single SSR failed and that was a result of a loose terminal causing excessive heat and burning the thing to death.

Ron

 

[shortbus=]

Mercury-wetted relays are normally used in high resistance type controlled temperature situations. Like kilns and heat treat ovens. The mercury relays last much much longer in this type of use. **broken link removed**

 

[crutschow]

I would suggest changing the system to solid-state relays, as Ron did. It's more reliable and creates much less electrical noise when being switched.