strange intermittent power problem

[thomas harris]

anyone here ever heard of a three-phase main power switch on a machine giving intermittent power because of oxidized connections? this is 208 three phase on a 30 amp circuit. the machine was acting whacko on start-up, but would eventually keep running after it had its little "fit" when powered up repeatedly. i found some strange voltages on the line side of the switch. good voltage to the wire ends. good voltage to the crimmped on terminal, but zero voltage to two of the clamping screws , and 10 volts on the other. i loosened the clamping screws and sprayed some contact cleaner into the contact areas and retightened the lugs. then voltage on clamping screws and everything. now the question is with the srews snug, how could this be. i just have never heard of something that heavy-duty and carrying a large load exihibiting that problem. but then then i am no electrician or electronics guru either.

[hammermill]

not uncomomon. expecialy if it is some times switched under load. like turning off machine with the switch as it is operating.

another problem is spiders and grease bugs that get in the switch contacts,smashed and there bodies keep the contacts from closing properly, arcing follows and switchs die.

[steamin10]

Ya, and I would add mention of heat caused by resistance through the contacts, and mounting lugs. Taking it apart, wire tooth brushing and reassembling with anti oxidant paste can help. Many problems can mimic this situation, you have to winnow out what is here.

[thomas harris]

well this machine has not turn off in alarm mode since i cleaned the connections with spray cleaner. my guess is it may do it again and then i will remove the srews and wires and thouroughly clean with emery and apply anti-oxide paste. they are somewhat hard to access, so until/if any future problems this will be a done deal.

[jpfalt]

Another thing to consider is that it could be a wire going bad. Several years ago I had a similar occurrence. There was a blistered appearance partway down the lead. When cut open we found that the wire maker had hi the end of a copper conductor and standard practice was to twist a new conductor on to the end and continue extruding the wire insulator over the twist. What happened was that the twist heated and the connection went bad.

The nasty part of it was that an ohm meter showed continuity and the voltage drop through the wire only showed up under load. If it hadn't blistered, we never would have found it. Any more, I check both amperage and voltage through the circuit when things get weird.

[thomas harris]

the wires and terminal ends always have proper voltage to them. it's the connections on the switch itself that lose continuity. but then as i stated, it has been fine so far.

[spro]

Jpfalt. Are you saying one lead in Romex was twisted together and then insulated over?

[jpfalt]

Spro,

It wasn't Romex. I have seen this twice, once in stranded SO flex cord and once in stranded fixture wire. I don't know how they handle wire ends in Romex.

[hammermill]

i have a couple examples gathered from teaching. both times they were butt crimped.9romex)

[dly31]

I wonder if maybe those splices are only intended to keep the machinery running and are supposed to be cut out of the finished product? I am sure electrical codes do not normally permit splices in cables.

[thomas harris]

maybe they go on some type of rewinder for final packaging and are supposed to be pull at this point in production. when i worked in the the paper converting business we obtained huge rolls and cut them into sheeets. we would find what the trade called "pasters", which were basically splices in the roll most likely to keep the mill machines running. needed to pull them out somewhere in the finished sheets as some had the capacity to smash printing press blankets. prolly someone not paying attention on a boring production line. were the splices marked in any way?

[hammermill]

given the manafacturing process where rolls of material are fed thru extrusion dies for the final coating and then cut to the final legenth i am sure such repairs are on the fly and ment to be cut out but have sliped by.
the machine don't stop so quick hands are called for.


the other thought is dont take rough abrasives but use a pensel eraser etc to polish the contacts any thing more will lead to sticking and welding.