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In my case I need a relay to switch continous 0.5A at 24V DC, but in time of closing the contacts there will occour an inrush current from the further circuit of 6A, but only for max. 100ms because there is a contactor (DC) in line of the switch.

I dont know the exact amount of damage of the contacts in that case, because its only for milliseconds. Do I need a Relay with max. switching current rating of 5A or 8A rating? Could there be a significant amount of damage in general?

I already checked the datasheets according to that, but the graphs are ending at the rated current.

I personally think I schould just take the overrated big one, but in my case a smaller component size would be better and it will be fully underrated wit 0.5A for the continous time

At a 5A type I would take the omron G6B-2214P-US or the Panasonic DSP2a-DC5V. They are DPST relays, the second pole will be used for a signal.

Thanks for your help!

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  • \$\begingroup\$ Welcome to EE.SE? What does the datasheet say? What does the manufacturers support say? \$\endgroup\$ – winny Apr 27 at 12:48
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    \$\begingroup\$ because there is a contactor (DC) in line of the switch - it's unclear why this causes the inrush current - draw out your schematic. \$\endgroup\$ – Andy aka Apr 27 at 13:22
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    \$\begingroup\$ Solid state relays might be worth considering depending on the application. \$\endgroup\$ – SomeoneSomewhereSupportsMonica Apr 27 at 13:44
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    \$\begingroup\$ Do you really need the isolation provided by the relay? If you do not, then no need to pay extra for a feature you won't use. MOSFET would be cheaper and would have no trouble with this inrush current. \$\endgroup\$ – peufeu Apr 27 at 16:20
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    \$\begingroup\$ In some cases, you might be able to apply an inrush current limiter. For example if the load is has large capacitors, you could use two small relays, with one having a series resistor to limit the initial current until the capacitors are charged. The smaller current spikes could benefit the reliability of the whole system. \$\endgroup\$ – jpa Apr 28 at 15:04
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In-rush current is a killer for contacts in relay (and elsewhere). A short current spike during operation, when the relay is properly closed might be tolerable (ask the manufacturer).

In-rush current on the other hand is much worse: It starts flowing at the very first instant the contacts barely close. Usually contacts bounce for a short moment, causing a current arc and extensive heating of the contacts. The same holds true for high currents when switching off the relay (i.e. when shutting down an inductive load).

As a result, the rating of relays are typically for resistive loads only. As soon as the load gets capacitive or inductive, the allowable limits are much lower.

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Contact rating is almost always limited by the switching current, not the steady state. It is at switching that the arcing and damage will occur.

enter image description here

Photo 1. (Left) Pristine contacts from a relay (Right) The nearly destroyed contacts from a relay operated under power for nearly 100,000 cycles. Image source: Arc suppression on Wikipedia.

While the damage in Photo 1 may be as a result of normal wear over a large number of cycles, the results due to over-current may be similar in a very low number of cycles and could lead to failure of the contacts to open. i.e., They may weld closed.

Don't think you know more than the guys who make the relays.

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This will end badly (probably with welded contacts, if not immediately, after some period of time).

If you can find a tungsten rated relay they use an appropriate contact metallurgy to handle surges of the order of 10x the continuous current. Expect the contact life (number of operations before it wears out) to be significantly lower than if you are switching the rated resistive load.

Compressor (motor) rated relays may also be appropriate but I doubt you'll find them in a relatively small size.

Another thing to look for is "TV" rated relays, such as TV-3 or TV-5 that are rated for generous amounts of inrush current. Cadmium is no longer acceptable in the contacts for RoHS reasons so they tend to use silver oxide and alloys.

Unfortunately, pretty much all such relays will be larger and more power-hungry than "signal" relays designed with contact forces and contact metallurgy to switch small currents.

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