Solved: Relay lifespan different for NO vs NC contact? - Page 1
Forum section: electronics / beginners

author: incf
author_rank: Frequent Contributor
timestamp: April 16, 2025, 02:51:23 pm
content: See attached.Why would a relay have different lifespans for the NO vs the NC contact?And what exactly does the lifespan measure anyways? (does it include both applying and removing coil power? Or is it just one or the other?)

author: calzap
author_rank: Frequent Contributor
timestamp: April 16, 2025, 03:30:25 pm
content: I don’t really know. But would guess it has to do with the duration of the spark as the contacts move apart or together. Or for a particular model of relay, the contact composition might be different for NO versus NC.Mike

author: jwet
author_rank: Frequent Contributor
timestamp: April 16, 2025, 03:48:06 pm
content: The physics of opening and closing are very different if you think about it.On an NO contact, you're pulling in the armature pretty fast and then potentially get a brief spark right before hard contact that extinguishes it.On an NC contact, you have current flowing and have sort of a jacob's ladder effect while opening, spark duration is much longer and no hard stop just distance. The force is that of the return spring which is by definition less powerful than the activation force.Those are pretty stout relays, signal relays used in ATE, etc can have contact lives of 10^8 operations, more if switched "cold". COTO makes these things and has written tons of what happens when you open and close relays- the metallurgy of the contacts is real science.

author: incf
author_rank: Frequent Contributor
timestamp: April 16, 2025, 04:08:14 pm
content: maybe the geometry of the relay produces more sliding action/wear on one contact vs the other?

author: SteveThackery
author_rank: Super Contributor
timestamp: April 16, 2025, 04:18:09 pm
content: Does the relay contact "know" whether it is NO or NC? Of course not.In both types of relay the contacts will experience the same number of "makes" as "breaks".Therefore only if there are systematic differences in the types of load switched by NO and NC contacts will we see any difference in lifespan.FWIW, my first job was in a telephone exchange which had thousands of relays, almost all of which had make, break and changeover contacts in "stacks", and some even had make-before-break contacts. The switched loads were very often inductive, sometimes resistive. Even after decades of use the relays showed no signs of wear in the mechanism, and precious little wear or erosion of the contacts themselves, except for a few in special roles. This is solely due to the durability specifications they were manufactured to.With an important exception there was never any correlation between contact wear/erosion and whether it was a make or break contact. Where contact wear/erosion did take place the contacts could be dressed using special contact files (which were made with blunt teeth).This is the exception: oil contamination. If lubricating oil got on the contacts, the tiny spark as the contacts separated turned some of the oil into carbon. As time went by the amount of carbon formed on the contacts increased, such that the minute, usually invisible spark turned into a visible arc. This burnt even more of the oil in a vicious circle such that prolonged and bright arcs would be drawn out. I never saw a contact set bad enough to sustain a continuous arc, though. Anyway, this prolonged arc would quickly destroy the contacts, requiring replacement.

author: BradC
author_rank: Super Contributor
timestamp: April 16, 2025, 04:23:39 pm
content: Quote from: SteveThackery on April 16, 2025, 04:18:09 pmWhile that may be true, there may well be a quantifiable speed difference between open and close. Spring vs Solenoid will alter the speed of the armature, and therefore as described above, arcing could well have a different duration.

author: SteveThackery
author_rank: Super Contributor
timestamp: April 16, 2025, 04:29:13 pm
content: Quote from: BradC on April 16, 2025, 04:23:39 pmYes, fair point. See my previous post - there was no evidence of unequal wear or erosion with the thousands of relays I worked with. Not to say it can't happen, just saying it didn’t with them.You might find this interesting:https://www.britishtelephones.com/relay3000.htmYou will see that the article uses the much better "make" and "break" terminology instead of the more modern "normally open" and "normally closed", which presumably was invented by someone unfamiliar with relays because it doesn't extend to change-over contacts, which were originally called - yes - "changeover". Sorry - this paragraph is off topic and merely a bee in my bonnet! 😅

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 16, 2025, 04:46:27 pm
content: I didn't find the OPs curves for context, but in some data sheets I did find there may be a clue.https://xonstorage.blob.core.windows.net/pdf/songlerelay_sla5vdcslc_xonjuly20_20_nocatlink.pdfIt shows an electrical lifespan as 10^5 operations, and a mechanical lifespan of 10^7 operations. The curves shown are consistent with the mechanical lifespan numbers so I suspect the curves have nothing to do with the contacts themselves, but reflect differences in the mechanisms for the two contact types. I have no idea what constitutes a mechanical failure. Failure to change the contact state? Drop below specification on contact spacing in the open position? Number of bounces on closure? Any of the above? Lots of digging if you really care.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 16, 2025, 06:22:36 pm
content: Quote from: SteveThackery on April 16, 2025, 04:18:09 pmRelay does not "know" anything, but the difference between NO and NC contact is that the directions where the contact is made or broken by electromagnetic force, or spring force, are reversed.Most relay datasheets show the same difference in lifetime, or even maximum rated current.

author: incf
author_rank: Frequent Contributor
timestamp: April 16, 2025, 07:02:38 pm
content: The difference in speed and force explain it. Solved

author: mikerj
author_rank: Super Contributor
timestamp: April 16, 2025, 11:53:10 pm
content: Quote from: SteveThackery on April 16, 2025, 04:29:13 pmA single pole changeover will have one NC, one NO and one common contact.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 17, 2025, 02:20:45 am
content: Quote from: Siwastaja on April 16, 2025, 06:22:36 pmYou do realize that as SteveThackert point out unless the relay is a one shot device, both NO and NC open and close circuit exactly half of the total state changes. Potentially current flow in the closed position lasts longer ( on off duty cycle is different) but if that matters exact conditions for the stated results should matter. You could argue that arcing issues are evaluated at maximum rates current and voltage, but the you would imply that for applications in lower current or voltage regimes these ratings are perhaps wildly conservative.If you really need to know you will have to run down an application engineer at a company that builds for the reliability market. The low cost venders won't know and will probably just Google a typical answer.