Relay switching max voltage across contacts

[plugsandsparks]

Thought i would put this out there for any experience faced with this design issue.

TP Immersion heater 11KW, has built in contactor, coil voltage 400V
I want to bring in the immersion heater via control system and ideally not alter the immersion heater package.
The control system can operate a volt free contact which i could use to switch the 400V coil.
However, the control system manufacturer does not quote a max switching voltage, all its data is for 230V .
Now i know the current will be tiny but nethertheless there will be 400V across the relay contacts.
I can install another relay from RS that at least does quote a 450V max switching voltage but that would mean i have a control relay, switching a "special relay" that then brings in the immersion contactor.

Would you go with just the std relay and hope 400V will not harm it or would you put another relay in for comfort.
Yes i know the logical thing is to alter the coil voltage to 230V but that is not preferred ATM.

Any help / experience gratefully received

thx
P&S

 

[Lucien Nunes]

I would definitely add an interposer relay properly rated for 400V. Many small relays of the type used for volt-free outputs have contacts rated for 250V maximum, so in the absence of other information that is what I would expect. Although as you say the contactor coil power is small and it would probably work (possibly with the aid of a snubber or VDR) I would not consider it adequate unless sanctioned by the manufacturer. Plus you would be applying 400V across PCB creepage distances that might not be designed for it, increasing the risk of flashover or carbonisation if it gets damp etc.

 

[plugsandsparks]

I would definitely add an interposer relay properly rated for 400V. Many small relays of the type used for volt-free outputs have contacts rated for 250V maximum, so in the absence of other information that is what I would expect. Although as you say the contactor coil power is small and it would probably work (possibly with the aid of a snubber or VDR) I would not consider it adequate unless sanctioned by the manufacturer. Plus you would be applying 400V across PCB creepage distances that might not be designed for it, increasing the risk of flashover or carbonisation if it gets damp etc.

Thx LN, found a relay DIN rail , tiny and rated at 450V for £13, not worth the risk as your head is where my fears lay. Appreciate feedback

 

[SheffSimon]

Somebody is presumably paying for your professional expertise and you suggest that hoping 400V won't harm the contacts is a design option, despite the manufacturers data sheet telling you otherwise. Hard to believe when the correct solution is easy and cheap to implement.

 

[plugsandsparks]

@LN, well eventually i found the ideal solution. ABB had the answer. They were the only control system manufacturer that i could find that published the various voltages and loads across the industry standard of 250 and 440V along with currents for AC1 and AC3 and loads of data on capacitive loads. Its a shame that Gira and Theben do not give more design info around their switching capability.

I think i have now justified my colossal design fee...lol

One less component now in the system.

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