For many years we have used 240vac electromagnetic relays for all signalling and control in the water and waste water industry. Lately however as safety becomes more of a focus there are calls for 12/24 vdc to be used for all control.

The question is raised however about the effects of oxidation and the buildup of non conductive layers on the contact surface due to a lower switching voltage and low currents.

Will reliability go down?
Does voltage "punch through" dirty contacts?
Are there any studies on this?

 

It’s been a while since I have gone over this so I’m a bit rusty. But I think the main concern is the AC/DC and not as much the voltage (as long as the relay you are using is rated for the level of voltage and current in your circuit).

Check the label on the relay and see if it has an AC/DC rating or just AC or DC. If it does have a DC rating you should be fine for the 12/24DC control voltage. I think the contacts on most straight AC relays is meant to be self cleaning with an AC arc and my corrode up with DC.

Also be aware that a lot of the 24 volt control voltage is AC being switched out of a transformer, not DC. Check to see what your control voltage is.

I hope this helps.
Steve King
Plateau Controls

 

certainly nothing wrong with 240vac

seem to recall it was common in early pump station station designs pre 60's, when the starters and control circuits were 240vac as it avoided or minimized the use of interposing relays.

the higher control voltage may give you more pull-in force and may have been important consideration early on, but hardly an issue now with modern designs.

Look at your over all system design from the sensoring devices to final control elements, wiring run lengths, environment issues such as submersion etc.

At the same time it doesn't hurt to drop by the maintenance shack and have a chat with the shop foreman as the issues in the field some how don't usually make it to the front office as you already know,