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I have a single AC-DC supply (42V @ 600W) which supplies three motive loads. Each motive load is physically separate and has its own cover which prevents the operator from gaining access to the moving parts.

Opening one of the three covers should result in the motive power being removed from the exposed moving parts, but the remaining covered loads can continue to operate.

Functionally, this is not a problem. Three safety approved interlock switches used with each cover which in turn control one of three 30A automtive style PCB mounted relays. All done. The single 'bulk' DC rail goes to all three relays and then each of the three loads is powered according to whether its cover is open or not.

But...doing this in an approved manner is tricky. I'm talking IEC 62368-1:2018 (3rd edition) here, the replacement for IEC 60950. This demands that any relay used for interlocks is subject to its own approvals (IEC 601810-1). I have yet to find a relay that can break this level of DC current (~18A) which carries this approval.

So, I looked at the problem from a different perspective: choose a modular AC-DC supply for the 42V@600W (notionally three off 200W 42V outputs) which can then have their ENABLE signals controlled by the state of the cover interlock switches. Nice work, you say....and I would agree until I start to speak to PSU manufacturers who then point out that their enable inputs are not rated (approved) for use as interlock inputs. Doh! (it's a single point of failure issue; an opto-isolated enable input can have its optotransistor outputs shorted together, thus rendering the enable input disabled!)

So I really don't want to have three separate AC-DC PSUs with a safety interlock on the mains input side, but this is the solution I appear to be converging towards....really? What have I missed?

I cannot be the first person to come across this issue - so why the blank faces everywhere?

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  • \$\begingroup\$ use a 30A diode clamp on the contacts? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 23 at 18:25
  • \$\begingroup\$ @jsotola - fair comment about the confusion. Sadly capacitive load inrush will take all a PSU has to offer, so if I split a 600W 42V output across three loads via three separately controlled relays, each relay needs to be able to break the complete short circuit curent of the PSU - ~18A. Anyway, the issue here is the lack of reasonably sized relays that can break this current within the context of IEC 61810 compliance and/or PSU enable inputs being used as part of an interlock systek without being subject to single point of failure analysis as per IEC 62368-1. But thanks for stopping by! \$\endgroup\$ – buttercup Jul 24 at 16:08

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