The "ice cube" relay in question is allen bradley's 700-HC series. I was browsing through the spec sheet looking at the electrical data to make sure that the relay has the proper voltage/current rating for my application, but found one too many ratings. See attached snip. I'd think that the two ratings that matter are "rated thermal current" and "rated insulation voltage" which tells the user the maximum voltage/current the relay can withstand.
What is "Min. low energy permissible load" telling me? Why is there an HP column? You would never use an ice cube relay to drive a motor. There are ratings for "general purpose" and "resistive"; which one do I take into account?
Best Answer
These tell you how much the relay can carry when closed, not how much it can break when opening. You need to look at the 'contacts' section of the data sheet to see what it can break.
Needless to say, you have to stay within the tightest set of restrictions for the relay for it to give you a long a trouble-free life.
Relay contacts need a certain minimum voltage across them, and current through them, when they close to make proper electrical contact. This is known as 'wetting'. Without this minimum voltage, they might not be able to break down any film that forms on the contacts, and so remain high resistance. This minimum voltage is easily met when switching power circuits, but if you want to switch audio signals or measurement circuits, then go for the gold contacts, or even a relay with liquid-wetted contacts.
You might never use an ice cube relay to drive small motors, but the designers of this one clearly think people would, and have tested it to demonstrate that it does.
The general purpose rating is for AC, and it will break as much as 240V AC. However, if you want to use DC, then you must limit yourself to 30v maximum, and only resistive (so nothing inductive) loads. I feel this section could have been phrased a bit differently, to bring out the meaning I've given it.
You will often see questions on here saying 'can I use this AC relay for DC?' This one is saying you can, but with very much reduced breaking voltage. The reason for this is that mains AC goes through zero current 100 or 120 times per second, making the arc that forms between opening contacts easier to extinguish. With DC, this doesn't happen, so the breaking capacity is less.
This is a quality relay, tested and qualified at multiple contact voltages. You'll probably find a lifetime specification somewhere in the data sheet as well. You don't get this sot of information with the typical cheap relays you find on fleaBay and the like. Caveat emptor.