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I have been using DPDT relays to control high-current loads (~15 A at 240 Vrms) though I am only using them in a SPST, NO fashion; that is, the load is disconnected when the coil is de-energized, and vice versa. A supplier has offered DPST parts that basically remove the unused NC contacts (top two on the image below).

The copper pour (blue) is on the top and bottom layers in 2 oz copper as well as two inner layers in 1 oz copper.

Is there a benefit of keeping these DPDT relays around despite the unused contacts, or would thermal vias do just as well to keep the PCB trace temp rise down?

Relay footprint

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The copper pour (blue) is on the top and bottom layers in 2 oz copper as well as two inner layers in 1 oz copper.

The heat dissipation is well taken care according to the copper thickness you have mentioned. Considering the 330 mils trace width, the temperature rise for a 1 inch trace is not more than 10 degrees which is really acceptable. The PCB trace width is perfectly fine.
I would only suggest to use more number of vias on either end of the traces where you connect the traces together from all the layers.

Is there a benefit of keeping these DPDT relays around despite the unused contacts?

Yes. If you have no problem retaining them, keep it as is. One fine day,itmay be helpful to you. Think of the present supplier not supplying the items any more or chance of using a cheaper DPDT switch from another project.

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Is there a benefit of keeping these DPDT relays around despite the unused contacts, or would thermal vias do just as well?

They would probably dissipate some heat, however, most of the heat is generated on the other side of the relay in the coil. At 240V one has to start worrying about spacing between the pins and traces (need 1.8mm per IPC specs).

If they are no connect, then you should not connect anything to them without checking with the manufacturer first. (you don't know what the relay looks like on the inside)

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  • \$\begingroup\$ Apologies, should have specified: I'm concerned with the temp rise of the PCB traces due to the high currents, not the relay temp rise. And yes, I have 200 mil (~5 mm) design rules in place, but luckily with these big 20 A jobbies I have comfortably given 330 mil (~8.4 mm). \$\endgroup\$ – calcium3000 Aug 22 at 16:56
  • \$\begingroup\$ @calcium3000 - The heat generated in the relay (coil) is where most of the heat in the PCB will come from. \$\endgroup\$ – Bort Aug 22 at 17:55
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For future flexibility (as Umar suggests), probably yes.

1st answer: Electrically, probably not.
2nd answer: Needs checking - see below:

Look at the relay contact resistance and/or voltage drop specification.
At 10A, I^2R heating of

  • 100 mW requires 1 milliohm contact resistance.
  • 1 W requires 10 milliohm contact resistance.

1st answer:

Odds are the actual spec is somewhere in that range.
100 mW is very unlikely to bother you and 1 watt is unlikely to.
So, relay specifications should establish an upper limit of the likely problem, before other thermal dissipation sources are considered.

2nd answer:

THEN I looked up a datasheet.
I could have removed the above optimistic conclusion, but will leave it in as a lesson to others (as well :-) ).

It's reasonable to assume that an Omron part will be "as good as most".
Low cost little known source parts MAY be as good, but also may realistically be significantly less good.

Here is the data sheet for an Omron G2R 230VAC 10A PCB Power relay

On page 6 it gives contact resistance as 30 milliOhm max/1 pole and 50 milliOhm max/2 pole worst case. A note states the measurement is made at 5V, 1A. It is not certain whether the 5V refers to a coil voltage or ??? 1A will be the contact current.
So - worse than I assumed.

At 10A and 30 milliOhms power dissipation in a single contact is 3 Watts.
I think it may be worth asking the manufacturer for specifications and even doing some measurements.

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