Relay Contact Rating

Redfred said it all! GA.

'Relay' in the IEC countries can mean any one of (a) Contactor relay, which is a small contactor, (b) Protective relay, like overcurrent, earth-fault, ... any protective device which will have an output 'relay', smaller than (a) above, (c) the small snap-action switch in a thermal overload relay .... actually 'relay' is quite a vague name

A standalone small contactor will have a much higher rating than the snap switch of a thermal relay by construction alone. Then there is the Pandora's box of DC ratings, which often require relays to have permanent magnet blowouts....

Anyway, there is no formula .. you need to test for every rating you want to assign (sigh)

Read the manufacture's data sheet for the relay.

Yes I would expect the relay to be able to open more current at 120 volt than 240 volt.

I like the "AC-X" ratings the IEC uses for the type of load, very convenient.

Here is another point of view that I want to share with you guys. This came from one of our very senior engineer that I really respect.

"The contact rating depends on the wattage. So at 250VAC and 10Amp rating, the wattage would be 2500 Watts. So for 120VAC you divide the same wattage 2500 by 120 and you will get 20.83 Amps." So it will pretty much double the current rating of the contact at 120VAC.

Any thoughts!

With respect, it is true but only within some limits. Just think, would the relay be able to handle 100A at 24V AC? Not likely isn't it ? So there are many ratings of the relay contacts per se....

Carrying capacity, Making capacity and Breaking capacity. And, while it may be able to make and break double the current at half the voltage, it may not be able to carry it !

So you need to follow the manufacturer's recommendations.

Now we're into a completely different set of realms. First, there's the obvious engineering aspects of relay contacts. Second, there's the corporate pecking order between you, 6Sigma, and your respected senior engineer.

To the first point you will notice that this relay on page 3 specifies a contact rating of 10A @ 250VAC and 15A @ 125VAC. What is rare to find for many relays is that on page 4 there is an actual chart showing the various maximum approved currents for AC and DC voltages. Now I doubt that you happen to be using this particular relay. So your engineer maybe correct for the relay that has been chosen.

To the second point, it is the senior engineer's responsibility to oversee and approve all (appropriate) engineering criteria. If he/she is certain that this relay can handle the higher current, that is his/her call. (English really needs some gender neutral pronouns.) So tread carefully in questioning this conclusion. A safe method for you will be to obtain a print out of the manufacturer's data-sheet for your relay and the relay data-sheet I directed you to. At an appropriate time ask this engineer to educate you on how the relay you are using meets the higher current rating. You may find that this may just be a misunderstanding between the two of you in the actual circuit being controlled, which ratings you are concerned with, or a variety of other plausible scenarios. But do not push this to hard. You already have an authorized opinion.

Good Luck

Thanks redfred for valuable input. Thanks all .

regards

This reply is FIRST relative to gender neutral pronouns in English- the word "their".

SECOND- Relative to how much POWER (in amps) a contactor/relay can handle not only depends on the ability of the metallurgy to support the amount of energy moving through it without fusing together (welding itself shut)- there is also the ability of the contactor/relay to OPEN the circuit. As voltages increase, the potential for continued power transfer to occur as the device opens greatly increases.

Based on that issue, it IS reasonable that a relay rated for 10 amps at 240 VAC MIGHT be able to break a circuit drawing 15 amps at 120 VAC and its metallurgy could very well be able to handle that power flow. That said, unless I see it stated in the manufacturer's catalog or data/spec sheets, there is NO WAY I am going to put more amperage through any contactor/relay that is not rated for that load.

6Sigma writes :"This came from one of our very senior engineer that I really respect.

"The contact rating depends on the wattage. So at 250VAC and 10Amp rating, the wattage would be 2500 Watts. So for 120VAC you divide the same wattage 2500 by 120 and you will get 20.83 Amps." So it will pretty much double the current rating of the contact at 120VAC."

I would ask this 'very senior engineer' which manufacturers make such contactors?

The GE contactors, for instance, are all limited by current, not power. As a specific example of a bread-and-butter contactor, the CL02-series will switch 12KW at 220V, 27,5KW at 500V, Ith = 18A max. AFAIK, the other major manufacturers (Schneider, Siemens, Moller, ABB,...) all follow the same prescription.

I know from experience analysing fried contactors that if you attempt to switch even a gnat's whisker more current than the rating, the contactor will have a depressingly short life, the contacts will overheat due to I²R heating. If, as per 6Sigma's scenario above, you attempted to switch a 12KW load at 120V with a CL02, you would need to be quick in closing the switchboard door to avoid seeing it melt before your eyes.

Lets look at that from a slightly different angle.

Say you have a cable working at 1000V @ 1amp = 1Kw

and you have a cable working at 1000amp @ 1v = 1Kw

Do you suppose the cables would be the same size? I think not.

As others have said only within strict parameters, and as noted at lower voltages the current rating does not double also the voltage ratings are not directly interchangeable from a safety point of view as well as a construction requirement.

At the other end of the scale, is the minimum current, with it's own set of problems.

Dear CR4s,

Mathematically 10 x 250 = 20 x 125 , Fine. The relay contact are designed to carry say 10 A safely (with the given area of contact through which the 10 A flows ) Any mathematical jugglery will harm the relay by over loading the Contact

points ( Current density will double - contact heat dissipation will be a problem) Donot ask me whether 5A x 500 v will be safer !!

Manroop.Chennai

Hi Manroop,

I believe that it will be a problem. Heavy arking will take place at the time of switching. If you want to use the relay for that kind of application, you may use a triac in parallel with the contacts. Make sure that the triac's blocking voltage is not less than 800V when you are switching 500 VAC. The triac gate current must be maintained at least 50 ms after the power to the relay coil is removed. This way you can equalize the electrical life to the relay's mechanical life which at least ten times its full load life. A well designed relay can be used to carry up to ten times its rated current at highest rated voltage if protected by a triac.

If you are switching DC use a power MOSFET as protection devise.

Job Thykkoottathil