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In this data sheet for a CUI Devices PR23-5V-900-1A "1 Form A" mechanical relay, it specifies:

Max switching voltage: 250VAC
Max switching current: 30A
Max switching power: 7500VA

...but also:

contact rating: 1 Form A: 30 A @ 125 Vac

...which seems to imply that it can't actually handle switching the higher voltage/current. It seems like this relay really can switch 30A at 250V (see e.g. the "Life Curve" in the spec sheet), so what confusion explains this apparently contradictory contact rating spec?

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  • \$\begingroup\$ Actually the relay can't switch 30A and 250VAC, it can only switch 30A or 250VAC, but not simultaneously. There are 560W and 7500VA limits too. Also, it is a different thing to switch a 30A load on/off with a relay, than to just pass it through contacts without switching. \$\endgroup\$
    – Justme
    Commented May 29, 2023 at 20:06
  • \$\begingroup\$ the voltage rating says that 250 Vac can be switched off without arcing ... higher voltage may cause an electrical arc to develop between the contacts ... that voltage can be affected by the amount of current passing through the contacts because excessive heating in the contact area lowers the breakdown voltage of air \$\endgroup\$
    – jsotola
    Commented May 29, 2023 at 20:24
  • \$\begingroup\$ @Justme - Thanks; given the rest of the spec sheet, it seems like it is intended for 250VAC/30A switching? See the "Life Curve" graph for example. Re: the 7500VA/560W, my research seemed to indicate that the implication there was AC (7500VA, which is the 250VAC*30A) vs DC rating (560W) -- other relays spec it that way using those units, and anyway why would the relay care about the distinction between true and apparent power? My impression was that they were just sloppy in the spec sheet, but if I'm wrong I'd appreciate being corrected. \$\endgroup\$
    – user272901
    Commented May 30, 2023 at 19:54
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    \$\begingroup\$ The relay contacts, when connected, may not care if the AC load is reactive. The relay contacts do care when switching. Capacitive load takes an inrush current spike when contacts are closed, and inductive loads try to keep the current running when opening the contacts so there may be high voltage spikes or arcing. \$\endgroup\$
    – Justme
    Commented May 30, 2023 at 20:02

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The datasheet is a bit dubious, and I'd be suspicious of using this to switch mains voltage in a serious application without more information and perhaps without accessing the UL file.

I suspect the 30A@125VAC / 30A@24VDC is a UL rating, and perhaps the only UL rating, the relay has.

It looks like the manufacturer thinks that it can switch 30A resistive at 240VAC and maybe 15A at 250VAC (and perhaps even 10A at 277VAC) but that does not mean that UL concurs, or that it was even tested for those ratings.

Here is what a proper datasheet for a similar product looks like (from the current owner of the original manufacturer of this style of relays- which date from the 1990s, if memory serves). There are many imitations of this style of relay, from well-known makers such as Omron to lesser known sources.

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