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Electrical Rating of Single Phase Relay?

Electrical Rating of Single Phase Relay?

Electrical Rating of Single Phase Relay?


   I would like some opinions on this. A Mfg. rates a small single pole relay at 40A/250VAC. The applalication the relay is used in, the current the contact will normally see is vary low, on both make and break, maybe less that 1 amp. Its used to bypass a set of SCR's, which will be fully gated on, when ever the relay opens or closes.

   What is your opinion on using the relay on a 480 VAC circuit? I could see an abnormal case, where the SCR's have not gated on, before the relay opens. If that happens, then the relay could be interupting a motor load, with a FLA rating of 37 Amps. There are three of the relays, one for each phase, and they are not mechanicaly interlocked, as a motor contactor would be.

   When I ran across this applalication, my first response was that the relay is under rated because of the voltage. The company that used the relay in their equipment, told me that it is OK. Don't know what the Mfg. of the relay itself, would say.

Thanks for your opinion.

RE: Electrical Rating of Single Phase Relay?

Go with the manufacturer's ratings for the voltage. The amperage ratings are not associated with voltage, except in the case of an interrupting rating.

RE: Electrical Rating of Single Phase Relay?

The voltage rating is just about as important as the current rating. If the voltage is too high, arcing can occur causing all kinds of havoc with equipment operating that is not supposed to be. This rating is often determined by UL (UL recognized, backwards U, R) and is a safety concern. I would definately contact the relay manufacturer themselves and ask what the highest rating of your relay is or obtain a data sheet on it and find out for yourself. I am guessing the equipment you purchased is; one, not tested by any safety agency as a complete product; two, the product is UL recognized as opposed to listed (recognized parts have additional requirements when installing on the final product); or three, the relay itself is within is designed parameters.
It sounds like there may be other major problems with this design like with using 3 seperate relays to control three phase power (is it a motor control panel?). I would keep actively investigating until you know for sure.

RE: Electrical Rating of Single Phase Relay?

I have seen many manufacturers use these kinds of relays.
Bad news.

The rating on the relay is probably a resistive load rating.

Relays not connected?  Should be a formal motor contactor.

If the device is UL listed then I suggest contacting UL.
They will have a coniption, then they will ?

Richard Neff
Irrigation Craft

RE: Electrical Rating of Single Phase Relay?


    Thanks for your response to my post. I have contacted the manufacture of the relay, GRUNER AG, and this was their reply.

"As the relay is used as a bypass and is only carrying the voltage and current and does not switch it, there is no problem to use it for this application."

    The manufacture of the soft starter has also assured me that it is OK the way it is being used. The unit is UL approved.

     I have also chosen to use an additional 3 phase motor contactor to by pass the soft starter, after the motor is up to speed. The soft starter is now only used to start the motor, and then it is turned off. The additional bybass contactor is used to stop the motor.

   I did this for other reasons, unrelated to the GRUNER relay. But, as a side benifit, it also solves any conserns about the internal bypass relays not being mechanicaly tied together. Of course, I also had to add another set of motor over loads, with the additional contactor.

     This may be a little over kill, but from a process reliability issue, it will keep my production equipment up and running and making $$$.

    Case Closed.

RE: Electrical Rating of Single Phase Relay?

There reply would concern me as well. The voltage rating is important when the contact is open and the current rating is important when the contact is closed. If the panel is UL listed it must be ok. The above would still bother me and based on what I know about this panel and the Mfg, I would not buy from them again. Good luck.

RE: Electrical Rating of Single Phase Relay?

You were right to be concerned. Although it is a common practice to size the current rating of a bypass contactor as running current only, the voltage is another issue altogether. A relay rated for 240VAC is typically designed with spacing of live components suitable for up to 300VAC max.  480V will most likely flash over, especially if it creeps up to 500+ at night. It is possible that the UL labeling of that soft starter is for 240VAC only. If not, UL may have made a mistake.

In fairness though, it could be that the relay manufacturer knows that their design can tolerate 480V but has no market for 480V 1 pole relays, so did not UL list them as such (it is more costly). When the OEM (soft start mfgr) RE-listed the entire unit for 480V operation, UL would have evaluated it based on new tested results rather than previous listings. The soft starter manufacturer would have been assuming the risk of it failing the test for UL, but that would have been a negotiating point during component selection at the design stage.

UL listing a complete product is not like applying a UL508C label in a panel shop. If the product is going to have it's own label, everything gets tested together regardlewss of passing (or not passing) previous tests. I have seen medium voltage components that only show a 3300V rating on their own be used in OEM gear tested and rated at 7200V. As long as it passes. In that case, the component mfgr knew all along that the devices would pass at 7200V as part of another system and sold them to the OEM on that basis, but they had no market for them at 7200V stand-alone.

What mfgr. of soft starter is that by the way?

Quando Omni Flunkus Moritati

RE: Electrical Rating of Single Phase Relay?

Good post and good answers.
You guys are however way tooooo trusting on the UL thing.
There is something wrotten in Denmark I say.
Do us all a favor, call UL and tell them, they will be interested.  No work on your part, Make UL do all the work.  I believe that they would NEVER allow this (My company is inspected on this issue continuously).

The reason that this happens is that no one calls the bluff.
We are all responsible to call the bluffs when we can, and that makes the UL listing worth something.

Richard Neff
Irrigation Craft

RE: Electrical Rating of Single Phase Relay?

Added to Pumpdesigners post.

All too true. If you want to pursue this, there should be a number listed near the UL mark, starting with an E, for instance E123456a. That will be their UL file number. If it is not shown (and it is supposed to be), call the manufacturer and ask for it. You can then go to UL's web site and enter that file number. It will return a description. Here is a link to the site where you can do this.


Please post your findings.

Quando Omni Flunkus Moritati

RE: Electrical Rating of Single Phase Relay?

I obtained the UL number and looked it up on the UL web site. The Catalog number of the soft starter is listed on the UL web site, under the number given. All it tells you is that it is a Solid state motor controller.

RE: Electrical Rating of Single Phase Relay?

Thank you for checking that out.
If you could do just a little tiny bit more.
Go to:
Fill in the information and you are finished.

The UL engineer in charge of that account will examine.
If there is a variation from the engineering contract UL has with that client, UL will take action as they see fit.


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