# What does CPM unit mean in relay specification?

I have a power relay, where in specification it states that "Max operating speed (at rated load)" is "6 cpm". What does that mean?

C.... per minute? Does that mean, that relay can change state every 10seconds (60s/6cpm)?

Other questions is, how to explain "Expected life (min. ope.)" under "Contact specifications", where cpm unit is used again?

Here is the relay datasheet: farnell.com/datasheets/55170.pdf

• Yes, That's the cycle rate at which it is rated. So expect it to last longer if your going slower but shorter if your going faster than that. Commented Nov 15, 2015 at 9:30
• @Spoon: so you are saying that I can go faster than that? Could I switch state for example like on 100ms? Commented Nov 15, 2015 at 9:43
• You can but there are a few related issues. First how fast the contacts can close? This depends on the mechanics. Second the contacts can weld shut due to heat build up during every arc..so delay helps extend the life by allowing cooling. Then there is contact bounce... which in bad cases could be 100ms long.. Commented Nov 15, 2015 at 9:52

The 6cpm is indeed one switch over per 10 seconds, this is the effective advised maximum operating speed under a load, because making and breaking an active current will create a thermal (and even ionic in quite a few cases) effects in and around the contacts.

Going faster under load will seriously decrease the life expectancy, because the contacts are not made for those stresses.

If your load is lighter, or generally more friendly than the rated load, you may be able to go faster and still get a close to similar life time, in relays they generally only specify no load and rated load conditions, sometimes a middle figure, and leave the rest to maths and experience on the engineer using them.

The second table underlines the difference between the life time of the mechanics and its capabilities and the life time of the contacts themselves. If you put a rated load on, it will last "only" 100000 operations at maximum 6cpm, less long at a higher speed, somewhat longer at lower speeds. Same goes for the load, if the instantaneous turn-on and turn-off power on the contacts are halved, you may get a statistical average of 150000 operations at 6cpm, for example.

But, you can see, that with no load to damage the contacts, suddenly the thing can be expected to operate for up to 5000000 operations (50 times more) at a speed 30 times faster, because that's what the mechanical system supports.

As to your extra question in the comments to your question: Can you operate it at 100ms switch time? If you look at the datasheet, you see it's operating time is 15ms and its release time is 5ms, so yes, you probably could use it for switching over at 10Hz. You have to be aware though, that 20ms of every 100ms period the contacts will be "floating in air" between positions.

You also need to be aware that the relay is not in any way rated or tested for that. The 180cpm is pretty much the limiting factor they assumed in testing, because that's the upper limit they envisioned, so you may be putting on stresses it cannot handle in the long term. But it is likely to work for at least a while.

• Very nice detailed answer. Thank you. As it turns out, based on your answer, this relay will not be a good match for my project, where I need 16 relays to supply lights from 30W to 400W. I was originally looking for SSR which can operate with 2A output/contact (230V), but didn't find any, so I tried this relay, where given questions arise. Will have to look better for some cost/efficient SSR's. Commented Nov 15, 2015 at 17:29

It's cycles per minute and is used when specifying the expected life expectancy of the relay over a period of time, because as you'd expect for a mechanical relay the life depends on the total number of actuations. Here's a detailed Teledyne Relays Application Note on MTBF calculations for relays.

As Spoon mentioned in a comment you'd expect it to last longer if you're going slower but a shorter amount of time if you're going faster. The maximum actual operating speed will be determined by the contact opening and closing time which doesn't seem to be specified for most relays and also there may be thermal issues. You might be better looking for a solid state alternative if both fast switching and a long lifetime are required.

• Thanks for the answer and link, but I was more satisfied with Asmyldof answer. Vote up nevertheless. Commented Nov 15, 2015 at 17:31