Relay contact resistance spec off?

Question

This Finder relay series specifies

Power lost to the environment

• without contact current 0.4 W
• with rated current 1.4 W

And a rated current of:

Rated current/Maximum peak current 10A / 15A

If I am getting everything right, this would translate to a contact resistance $$ R = \frac{P}{I^2} = \frac {1 {\rm\,W}} {{10}^2 {\rm\,A}} = 10 {\rm\,m\Omega} $$

I find it strange since other similar relays specify 100 mΩ for contact resistance, also made of AgSnO2. On the other hand, maybe they have an extra powerful spring / more material - This paper mentions:

With the SnO2 content increase from 15 wt.% to 30 wt.%, it was found that contact resistance of Ag–SnO2 materials increased by 33% and stabilize in 5 mΩ when contact force over than 70 N

Is there a mistake in my judgement / datasheet?

FWIW I am interested in the contact resistance when the coil is not energized.

Answer 1

The OP mentioned two statements:

Rated current/Maximum peak current A 10/15

But also mentioned:

I am interested in the contact resistance when the coil is not energized.

These two statements are not congruent, because statement 1 relates to the current rating of the NO contact, whereas statement 2 relates to the NC contact. The current rating of the NC contact is much less than that of the NO contact, 5A compared to 10A ("rated" current, ignoring "maximum peak" current), refer image of datasheet below:-

It may be possible to calculate the NC contact resistance, but one of the following assumptions would have to be made:

The power dissipation of the NC contact when it is conducting rated current (5A) is either:
(a) the same as that for the NO contact at its rated current (10A), excluding the power of the coil, ie: 1.4W - 0.4W = 1W.
(b) the same as the total power dissipation of the relay (coil plus NO contact), ie: 1.4W.
(c) something else, ie: neither (a) nor (b) is true.

If (a) is true, then that would put NC contact resistance at about four times that of the NO contact. If (b) is true then the contact resistance would be higher than that.

I suspect it's best to contact the manufacturer to get the definitive answer.

Answer 2

Yourlast line should be near the top.

I am interested in the contact resistance when the coil is not energized.

That totally changes the nature of the question.
The data sheet gives energy loss with and without contact current.

Even if this includes contact energy loss for Normally Open contacts it says nothing about the resistance of Normally Closed contacts when the coil is not energised - which is what you want to know.

Normally Closed (unoperated) contact resistance relies solely on spring pressure.

I can see no mention of contact resistance anywhere.
Even if your calculation is relevant the best you can do is assume that Normally Closed contact resistance is the same as for Normally Open contacts - or ask the manufacturer.

Answer 3

Most relay's datasheet measure contact resistance at low current 1-2A. But at higher current, relay contact will be reduced from electric pulling. For example as Song Chuan Relay 118-2A datasheet as below link. Contact resistance at 1A is 100mOhm, but at 20A is 8mOhm. http://www.songchuan.com/sc/wp-content/uploads/2023/09/118-1_23.05.24.pdf