Contactor Electrical Maintenance in Practice

improper wear or discoloration

are generally the best indicators of increased resistance.

I can state from the old days of auto repair - points that show signs of arcing (material deposition, discoloration, etc) are signs of problems in the electrical system, but are problems in themselves as the higher resistance will cause it's own problems.

Such is the stuff that Industrial Engineers do -- determine why failures occur and what to do to minimize them. Often, it takes the help of other engineers (electrical, mechanical, etc.) to determine what causes failures and what can be done, and an inspection procedure should be devised for each type of device, machine, equipment, etc.

Thereafter, maintenance contractors should be expected to follow procedures for inspection and reporting on conditions of equipment.

A word of advice: Don't generalize inspection or report forms -- make them specific to the equipment being inspected.

Depending on the size of the contactor there is a point where one is simply better off replacing it. A 60amp device is cheaper to replace than the time it takes a union electrician to service it properly. It takes a particular skill to clean and dress bigger contacts without actually accelerating failure. It looks and sounds so simple... but you have no idea how many messes I have cleaned up just because of that.

Contact resistance is measured in micro and milliohms and needs to be done in one of two ways and not simply checked by sticking your Fluke across them. The preferred test is a digital low resistance ohmmeter (DLRO), commonly known as a Ductor. If you have a lot of units to test on a regular basis drop the $2500 or so into a good ductor. The other method is to do a millivolt drop test. You have to be careful with this test though because if the ac inductive reactance in the circuit is big enough (relative to dc resistance) it can mask problems in the contacts. If you do millivolt tests do them in conjunction with a good IR scan. The two tests together ... heat and resistance.. go hand in hand and you can find a lot of problems this way.

http://www.netaworld.org/files/ItemFileA1335.pdf

Regardless of which way you go, establish a record so you can monitor trends.

For standards, go to NETA. You'll find what you need there.

And lastly...since you asked about documented proof of being worth the expense...

Hi Switchman,

Our 600amp chiller's contactor burnt recently. And I agree with you totally as it took a long time to replace those burnt contact points.

I'm interested to download NETA's material from the link that you send but the file is no longer available. I really need to improve the contactor's maintenance program. Can you help to provide me a copy?

Thanks

As mentioned by Switchman, IR or Thermal Imaging is one of the best thing for electrical maintenance. With contactors we were able to pick up smaller and non serviceable contactors that needed to be replaced, along with termination issues. With the larger serviceable contactors, we would only do a contact wear check and maybe clean off the carbon around the arc chutes. Never file a contact, which was harder to get the old school electricians to understand.

We are now in a more disposeable time and rarely change contacts, again costs make it easier to chane the contactor along with it being quicker most of the time.

MCCB's are a different story, and again IR is helpful here.

Trevor.