Digital Inputs

[Lc85]

Our customer has asked us to provide a potential free contacts rated 8A-220vac/24dc for digital inputs into their system. however our instrument suppliers are saying that this rating is really high and that they have never came across it before, they have only supplied pressure switches rated 5A and the closest they can find for our need is 15A. Has anyone got an idea as to why the switch contacts need to be rated so high?

I have found a relay that i think would meet the specs weidmuller DRM270024LT. Would we need to run the digital inputs through this relay to input into the clients system?

Thanks

 

[dpc]

I'm confused by your OP - is there a typo. The spec is for 8 A and you found a 15 A? What's the problem? 8 A @ 240 V is not that high of a requirement in an industrial control setting, in my experience.

 

[Skogsgurra]

And I am confused by your customer's request.

Why would he need that? Are you sure he knows what he's talking about?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Just a guess: if these devices are being used as hardwired interlocks in, say, a motor starter then the 8A requirement may be related to the rating of the control fuse. 6A is a very typical control fuse rating. 8A is the next 'standard' relay contact rating above 5A.

If these contacts are driving PLC or DCS I/O then the 8A requirement seems somewhat overkill and may be counterproductive. A gold-flashed signal contact will usually perform better in the long term than a Ag-Cd power contact when switching very low currents at low voltage.

 

[dpc]

That sort of contact rating used to be fairly common when pilot devices were connected directly to motor starter control circuits, large solenoid valves, etc. This was done to avoid the requirement for interposing relays. Certainly not needed if the only function is a PLC input as is common these days.

A lot of these old specifications have been recycled for the past 40 years or so.