Contact cleaning

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wiry

wiry

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I am self taught in the art of starter and contactor maintenance and I'm not really sure that I'm always doing things the best way. So if I may access the cumulative knowledge base here... what exactly is the best way to clean contacts? What cleaners are best? Is it ever proper to use an abrasive or other tool? Should one lubricate after cleaning? At what point should the call be made to replace the contacts? Is there any hazard to putting off replacement of badly eroded contacts? And what other maintenance should be done at the same time (e.g. tightening terminals)?
 
wiry said:
I am self taught in the art of starter and contactor maintenance and I'm not really sure that I'm always doing things the best way. So if I may access the cumulative knowledge base here... what exactly is the best way to clean contacts? What cleaners are best? Is it ever proper to use an abrasive or other tool? Should one lubricate after cleaning? At what point should the call be made to replace the contacts? Is there any hazard to putting off replacement of badly eroded contacts? And what other maintenance should be done at the same time (e.g. tightening terminals)?
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The best wy is not to clean them unless they need it, the brown layer you see is silver oxide and that not a problem at all. If you see any black marks from arcing or pitting you should clean them with scotchbrite pads, there are many different grades and you should only use the grade necessary to clean them, dont use a grade too abrasive. Never use files, emery cloth or sandpaper. Dont use cleaners, they are flammable and any residue may cause a hazard.

Contacts are silver plated because silver oxide is a much netter conductor than copper oxide (Silver and copper are about the same), any metal on the contact surface will oxidize so we want a good conductive oxide layer, if any copper is visible the contact should be replated or replaced. Dont use that paint on silver crap, it rubs off the first time the contacts close and wipe.


Contacts should be properly aligned and adjusted so that the contact surfaces bear with firm, uniform pressure. One of the best ways to check contact impression is to close the contacts on a piece of thin tissue paper with a piece of carbon paper next to the tissue. When the breaker is closed and opened, it is a simple matter to determine the amount of impression made on the paper. At least 75% of the contact surface area should make contact, <75% contact requires adjustment, resurfacing, or replacement.

Contact resistance should be measured with a microhmeter, values depend on the frame size and type but as a general rule they should all be within 50% of the lowest reading.

And finally, NEVER GREASE THE CONTACTS!!! (See pic)
 
certiain breakers say to use an electrically conductive grease (light film) on the contacts after cleaning them such as MOBILE 28. Its a red grease.
 
Zog's comments are excellent. I can only add a few more details.

One of the biggest mistakes people make is in thinking that contacts should be clean and smooth. They are only that way when unused out of the carton. After the first time they open an arc, they become rough and pitted and over time, that becomes more pronounced. But that roughness actually IMPROVES the contact area under normal circumstances. At a microscopic level, the "pits" on one side will match up to the "craters" on the other side, which effectively increases the contact surface area and keeps resistance as low as possible. Filing or otherwise abrading the contacts not only removes material, but also eliminates these matching surface structures, which makes the resistance higher again, which raises the contact temperature. But now with less material, the contacts run hotter all the time and the deterioration begins to accelerate at a much faster rate. So the first time you file or sand the contacts, you begin a process of shortening their useful life, usually resulting in cutting it at least in half from the studies I have seen. Cleaning is OK, but be as gentle as possible and learn to accept that pitted is good.

Filing (or otherwise abrading) is sometimes necessary in an emergency after a fault where you have to get back up and running, but replacement parts should be immediately ordered and swapped out at a convenient time if you want to maintain reliability. Filing should NEVER be considered routine maintenance. Most of the people who instruct you to file contacts are people trying to make money selling replacement contacts.

"Wiping action" of the contacts by the way refers to the fact that many are designed to "slide" ever so slightly when making, so that those pits and crates align better with each other.
 
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Some say less than 30 mV at rated I is good, more than 100 mV is bad.

You can measure contact resistance with a good digital voltmeter (at least 3-1/2 digits), an ammeter and an incandescent lamp; 25 watts to 500 watts, depending on the current you want. For 10 amps you could use a hair dryer or toaster. You could also use a step-down transformer for greater safety, but you still need a load to pull the rated current.

Put the lamp in series with 120 v and the closed contacts, so now the lamp acts somewhat like a constant current source.

Measure the voltage drop across the closed contacts and divide by the current flow, but you must use a four terminal (Kelvin) method. That is, the wires that bring in the current must not touch the wires that measure the voltage.

With a small contact it might be difficult. Some companies make Kelvin test clips for use with their microohmmeters, but you could also use the clips with the setup above.

Using this method, with enough current, you can measure the resistance of a quarter or nickel.

The hotter the contact gets, the faster it fails. The service lifetime of many things halves for every 10C rise in temp.

2 watts dissipated in a volume and surface area similar to closed contacts will the raise the temperature pretty far above ambient. So then, I guess a very quick way to check contact quality for many contacts is to use an IR temp meter; no physical contact, no shock hazard, no nothing.
 
zog said:
Relay contacts and motor starters/breakers are completely different animals, apples and oranges.
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But in my opinion they are what the OP asked about. :smile:


I am self taught in the art of starter and contactor maintenance and I'm not really sure that I'm always doing things the best way
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bth0mas20 said:
certiain breakers say to use an electrically conductive grease (light film) on the contacts after cleaning them such as MOBILE 28. Its a red grease.
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Please post that, I have to see that. Nearly all breakers recommend conductive grease (Mobile 28 is not a conductive grease, but you are right it is red) such as NO-OX-ID on the finger clusters, but never on the contacts, if you have a tech manual that says so please post it.
 
iwire said:
But in my opinion they are what the OP asked about. :smile:
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Right, the OP asked about starter contacts and a reference manual was posted for relay contacts, i wanted to be sure the OP wasnt confused about that.
 
Zog is correct - that RED GREASE is for the mating male/female connection on high voltage breakers.
If you file the contacts be sure you get replacements before they open more than once. Filing is nothing but trouble.
Use the paper test for contact pressure - you won't have a job for long if you spend the whole day checking ..... Best of all, get someone to splurge and buy a ductor, then you can really check connections!
 
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