Lossy NC Contact Blocks on Pushbuttons

My plant has a lot of 20yo machines from Germany, which use pushbuttons by Schneider Electric. Everything mostly runs fine, but I'm noticing that none of the NC contact blocks are 0 ohms across the 2 pins. I'm getting between 500-3k ohms across just the button's contact block. Should I be concerned? Especially for the Estop buttons? Where should I draw the line for replacing "lossy" buttons?

My Estop circuit is 24Vdc going through 7 lossy NC estop buttons, accross maybe 200 feet of wire, to a contactor (probably 20V dropout voltage). A legitimate or illegitimate Estop trips out the contactor, and kills power to all motors. (Contactor turns on only after a special reset procedure is performed.)

I am getting only 22V by the time it goes back to the contactor coil. I accept that there will be some voltage loss, but from a make/break pushbutton? Should I replace all my estop buttons with another brand? (I was thinking automationdirect, since I have a more than 300 estop buttons in the plant.)

Like I said, everything runs right now, despite being lossy. The Estop buttons will open the estop circuit when pushed. The concern for me right now is preventing illegitimate estops. Just wondering what I should do? How severe is the need to replace these buttons? And in general, what is an acceptable amount of loss accross a NC pushbutton contact block, before you would replace it?
 
C

Curt Wuollet

Stop and think about that for a bit, the statements you made are contradictory. If indeed the contacts had even 300 ohms of resistance, you would be E-stopped. Are you measuring with the voltage applied? Or perhaps does your ohmmeter use so little voltage that you have a dry circuit? Your measurement is bogus somehow. And no, I wouldn't replace anything on that basis. If I was a betting man, I'd bet on the measuring ohms with the power applied.

Regards
cww
 
Have an assistant jumper across the contacts while you are measuring voltage at the coil. If you see a difference of 10% or better, I would say replace them. If your voltage drops below the coils holding voltage, then you will burn up your coils.

And I have always had good luck with AD.

> I am getting only 22V by the time it goes back to the contactor coil. I accept that
> there will be some voltage loss, but from a make/break pushbutton? Should I replace all
> my estop buttons with another brand? (I was thinking automationdirect, since
> I have a more than 300 estop buttons in the plant.)
 
Curt:

I think if you push the yellow button on a fluke meter, it can measure resistance across powered pins. I might be wrong.

I am also measuring dc voltage across the 2 pins while powered, and never getting a 0v reading (also measuring dc to ground for each pin and doing the math). Delta V varies per button but is around 100 mV plus minus 50 mV each. I replaced one button and it read 0.4 mega ohms. I then played with it, and maybe in 1 out of 3 tries I got 0 ohms.

Rootboy:

I am curious why this burns out the coils. Due to rapid flickering of contactor? I left out some details previously, but basically in my case, I actually have 3 contactors, wired in such a way that there is no flickering. It's more or less wired like a holding relay circuit. First contactor stays off most of the time and is for the user to reset the line. First sets 2nd. 2nd sets 3rd. 3rd, once set, passes E-stop's output (in my case a lossy 24V) to its own coil and 2nd. An Estop (maintained or momentary) will break the holding circuit. 2nd and 3rd will trip out. User needs to engage first one again to resume.

I assume 10% means 2.4V net loss is tolerable for the Estop circuit (not for each indiv button).

Good to know about AD buttons. Thanks.
 
Oh. I get what you're saying now. Nvm my comment re 10%. Smart idea!

> Have an assistant jumper across the contacts while you are measuring voltage at the coil.
> If you see a difference of 10% or better, I would say replace them.
 
G

Gerald Beaudoin

I'd bet right along with Curt....sounds like a measurement issue.

> If I was a betting man, I'd bet on the measuring ohms with the power applied.
 
C

Curt Wuollet

Not on my fluke, but the point still stands. Obviously they are making contact when wetted by the E-Stop string current. Measuring the voltage is a better bet and is telling something close to the truth. Now, set your Fluke to current and measure across an Open E-Stop. This will give you the current in the circuit. If you divide the measured voltage when closed, by that current you will get the actual resistance in situ. R = E/I. I'm fairly certain it won't be 300k, I'd bet on less than an ohm. The phenomena is quite common with contacts rated for amps of current. They sell special noble metal contacts for dry circuits. That's not to say that you don't have a bunch of crappy switches, you're just judging them the wrong way. A little oxide film may or may not interfere with the intended application.

But it appears these do make contact in a 24V circuit, or nothing would be working. By the way, I trust my Fluke, except when I know it's lying:^)

Regards
cww
 
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