contact arcing

[bmbihntr]

I have multiple dry contacts which are being degraded by arcing, what is the best way to reduce the damage. would a varistor do the trick???

 

[Sceadwian]

We can't offer any suggestions until you provide us with some detail.
AC/DC what voltage, what current, what is the load?

 

[bmbihntr]

24 vdc from alan bradley disceet input board, monitoring position of contacts

 

[dknguyen]

24 vdc from alan bradley disceet input board, monitoring position of contacts

Seriously, more information please. What is the *LOAD* and load current? If I read your reply correctly, you only told us the device monitoring the contacts, not what the contacts are actually controlling.

 

[Sceadwian]

monitoring positon of contacts of what? You're making us guess here. Please post a paragraph or two of your exact situation details about the entire setup in general the failure rate you're having.

 

[bmbihntr]

Contacts are used for position indication which should draw about 4.1 ma at 24 vdc during its on state, and an input impedence of 6.0 kohms

 

[dknguyen]

THe input impedance of what? WHat is the load (what is the device that is the load)? What do you mean the contacts are used for position indication? More info! Stop being lazy and type out a description of the entire system.

 

[bmbihntr]

We are utilizing small reed switches located on a positioner which are actuated by a small magnet as it comes into proximity. the position switches are powered by 24 volts, dc from an Alan Bradley Flex I/O module. The maximum onstate voltage from the module is 31.2 vdc, with a maximum current of 6ma on an input impedence rating of 6 Kohms.
There is no load, the action of making and breaking the contact appears to degrade the integrity of the contacts vrey quickly. the plc input board is simply monitoring the state of the contacts.

 

[Sceadwian]

So it's a rotary switch fed from 24V to a 24V Allen bradley I/O card? You must have your impedance wrong because with resistance that high there's no way you could be getting damanging arcing on the position indicator. Something else has to be going on. How fast are these reed relays being triggered? You should probably be using hall effect or inductive proximity detection sensors not reed relays and a magnet.

 

[bmbihntr]

My bad the 6K impedance is the rating of theallen bradley module, I am simply monitoring the status of the contacts, whether it is open or closed.

 

[Sceadwian]

Do we have to drag the information out of you or what bmbihntr? What's the switching rate of the relays?

 

[dknguyen]

Okay, I get it now. A reed switch is not the same thing as a reed relay. A reed relay is a reed switch that has an integrated electromagnet so you can control it by applying a current to generate the magnetic field. A reed switch does not have this electromagnet, and an external magnetic field must be applied to control it, like from a magnet.

But yeah...I don't see a source of arcing otherwise since it sounds like the reed switch is just connecting the input terminal of the Allen Bradley I/O module directly to +V or ground with a very small current flowing. Are you sure the reed switches aren't just wearing out from you exceeding their switching lifetime because you are switching them at high speed?

 

[bmbihntr]

sorry kind of new to this whole thing. The switches are located on valves, which may stay in one position or another for days at a time, there should be no chatering of the contacts, voltage is very clean, as the magnet turns with the stem of the valve, it releases the switch, over time the impedence of the contact increases to a point which is no longer recognised by the input module therfore giving a false position of the valve.

 

[dknguyen]

Huh, weird...*scratches head*.

So the switching frequency is very low (days at a time?) and the current is very low and, so far, there is no inductance in the circuit (so no inductive kickback to produce the arcing). The contacts aren't chattering, but the impedance increases slowly rather than suddenly meaning the contacts are electrically failing, not mechanically failing.

I suppose...you could put a varistor in parallel with the reed switch and see if that makes a difference, but to be honest without knowing what is causing the arcing (if it even is arcing that is causing the failure), the varistor might be in the wrong place to prevent the arcing.

I'm going to take a shot in the dark here...how long and how thin/thick are the wires between the reed switch and IO module?

 

[Sceadwian]

Steam valve.. heat... moisture are these reed switches properly sealed and rated for the environment you have them in? It sounds like they're corroding. Have you opened one up to see what's going on inside for real? Picutres of a new and 'failed' switches internals might be useful (just make sure they're well lit and in focus)

Also why do you think it's arcing that's causing the contact failure, at the current and repetition rates involved there should be no chance of that occurring let alone to the point of failure.

 

[bmbihntr]

these valves are mostly located out side in new england, all positioners are rated class 1 div. 1 and sealed appropriately, most actuation of these valves, happens in the winter, and therefore most failures also occure then. However, this is not always the case. We have had failures in the summer months as well. I've measured resistance across a new switch 0.0 Ohms, and a failed switch 20 ohms, on the normally closed contact. I can watch the contact make and break with my meter, but the resistance of the contacts is too high. The contacts them selves are in a hermetically sealed housing, located in a gas filled, vac tube. so moisture is not a factor

 

[Sceadwian]

By positioner you mean the reed switches? Can you provide the part number / maker or better yet the datasheet for those switches?

 

[gerty]

What about vibration? Reed switches are not very forgiving when it comes to impact or vibration. Are the switches located on a piece of machinery that has either?

 

[bmbihntr]

there could be some high frequency harmonics but no real shaking going on. these are mounted on valves pretty far from any machinery

 

[Sceadwian]

Can you link the part number or data sheet of the reed switches you're using?