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I'm looking for PCB EM-relay for switching 25A 380V load, and there is a lack of models on the market.

I have find one of them. The T9G relay has MSV = 480V, but rated voltage = 277V. Also in header of the datasheet - "UL approved for 480VAC switching".

What's the true difference between them? Can I use it for 380V delta load switching?

PS switching-arc problem is already solved by parallel TRIAC, so don't touch it.

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And there is lack of models on the market.

Hardly surprising. At this power level (16.5 kVA) you'd conventionally expect to use a three phase contactor mounted on a DIN rail, not a PCB-mounted relay. Your suggested relay is also a single pole device, so you would need 3 of them for your delta-connected three phase load.

Can i use it for 380V delta load switching?

At 25 A? Not anywhere according to the data sheet. Which region are you in? You mentioned UL approval (USA) but 380 V is a typical continental European supply. If you're working in Europe note the IEC 61810-1 ratings on page 1 of the data sheet only allow a maximum of 250 V working so no 380 V operation at any current over here.

Wherever you're located you should make sure the devices you use meet your local certification requirements.

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The rated voltage indicated in the datasheet is the voltage that the rated contact currents are measured at. Or in other words, it's the voltage that the relay's contacts are characterized at.

For example, if you look at the Contact Data table at the 1st page, you'll see the contact currents of 20A or 30A, depending on the contact configuration (i.e. 1 Form-A, or 1 Form-C). These rated contact currents are valid for rated switching voltage i.e. 250V. This is a method of saying "My relays are capable of switching 20A/30A load currents, but under specific conditions". So the manufacturer says that "Our relay is capable of switching load currents up to 20 (or 30) Amperes, but at 250V. So these allowed contact currents may be lower at higher voltages".

Then if you look at the Contact Ratings table at the same page, you'll observe that the contact currents at 480V are much lower than rated values.

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