Insulation Voltage for Relay

The (rms voltage) part implies it is an AC voltage rating. To get the DC rating multiply by 1.4.

You should be fine.

what is the brand name of the relay?

"The (rms voltage) part implies it is an AC voltage rating. To get the DC rating multiply by 1.4.

You should be fine."

Not necessarily so. If a relay does not specifically state that it has a DC switching rating, you cannot assume any DC rating. As the voltage goes up, switching DC becomes much more difficult that switching AC. The current ratings then go down drastically because of the additional arc clearing time. For example, I have seen switches with 400A AC ratings that are only rated for switching 2 or 3A at the same voltage DC.

And by the way, the "Insulation" rating has to do with the relay's ability to isolate from a voltage potential. That has little to do with the "Switching Capacity" of a relay.

Well put.

A misunderstanding that many make. Generally, most "good" manufacturers put the AC and the DC rating on their equipment.....

GA as well....

I think you got it right, but the rating for DC is based on the materials used in the contacts rather than the dielectric. Spark gap is 10KV per inch in air. Any wiring rated at 400v has probably been tested at 10KVDC on equipment I used to build (Hipotronics Inc.)

Arcs drawn by DC, need snuffing out, either by having much larger opening gaps, or magnetically forcing the arc to take a much longer path, so that it breaks. This usually a series coil....so the arcs current "blows itself out" so to speak.

AC arcs break themselves 50 or 60 times a second......

To #3 GA I would add that the 1500 V rating of the contacts indicates that the relay has a short throw (small space between open contacts) which supports a greater reduced rating than the typical 70% when going from resistive AC to DC.

Dear wahkao,

You need to investigate if there is a way to calculate the equivalent value in AC for a value in DC.

Might it have something to do with RMS perhaps?

Don't overlook that AC has a value of zero quite often. Does DC ever get switched off at zero?

You will find that we will be helpful here, but we won't do the work for you, it's better for you that way.

Its nothing (or should I say very little) to do with RMS.....

My last post explains it.

It does when he is asking about dielectric values. He also indicated 240 volts as a 'for instance' and at 10kV per inch of gap, I'd say he won't be noticing much trouble with the average relay at 240 vdc.

The fact that AC breaks and reverses and DC does not was the basis for my (leading) question to Wahkao about the possible difference between AC and DC when the switching occurs, but I chose not to be too specific since my take on this is that this is a homework question.

There is always a possibility I could have misjudged this, but you have to admit it is a pretty basic query. He has gotten quite a few specifics on the possibilities now, and given that he could possibly actually be shopping for a relay, my advice would be to call or email the seller and talk to the technical staff in sales. We are the wrong ones to be asking without actual part No. and other details.

The posted responses have been excellent well-rounded guesses about the general possibilities, but I hope a purchase decision is not made from what he has gotten so far.

Always ready to defer, but I notice we haven't heard from him lately. CJM