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# Relay contact DC voltage rating

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#### udhay_cit

##### Full Member level 6
Hi,
I have a relay with 30V@5A DC rating. Assume its a resistive load. What will happen if I used it for 120VDC@1A?
Will it damage or arc between contacts or degrade over the time? What will happen? where do I find the calculations or study material to understand this better?

Hi,

In the worst case, an arc can occur when the contact opens. If the distance between the open contacts is too small for the applied voltage, the arc may never go out. -> fire!

There will be no calculator as it depends on many parameters. (Contact gap, opening speed, magnetic arc extinguisher, applied voltage, current, inductance....As soon as you use an electrical device beyond the specified limits, you do so at your own risk. Insurance doesn't usually cover this.

Klaus

Hi KlausST,
Thanks for the replay. So the major deciding factor for voltage reating is the creepage distance between contacts and the conductor thickness is responsible for the current carrying capacity, right?

The ”creepage” distance between contacts is zero.

As Klaus said, the problem is arcing, which may cause fire, or, may weld the contacts together.

Short answer: Use a part that meets your requirements. Unless you like fire.

udhay_cit

### udhay_cit

Points: 2
Hi,

Apparently you did not read my post.
--> For voltage rating it´s more than the bare contact (air) gap (which is not the same as creepage distance)

The same is true for the current rating: It´s more than the pure contact thickness.

Klaus

udhay_cit

### udhay_cit

Points: 2
Seems choosing relay for DC operation becomes empirical unless already specified by manufacturer tests.

Seems choosing relay for DC operation becomes empirical unless already specified by manufacturer tests.
You could say that about ANYTHING. ”unless specified by manufacturer test”. Manufacturers don’t don’t just sell you a relay and say “here’s a DC relay, you figure it out.” They have specifications like any other device.

Hi,

Thanks Barry. This exactly is what I wanted to say.

Chose the relay so that the relay manufacturer specifications meet your application requirements.
Everything else will be on your own risk (which I don't recommend )

Klaus

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