EBen63060

Hello everyone. I am looking for some kind of specs for troubleshooting voltage loss between contacts on 110VAC contacts. For the last 7 years I have replaced anything reading 500 mV or more per an electrician I worked with at a foundry in Tacoma. I have found this to be pretty good preventative medicine. Our programmer is putting together a training class for our maintenance group ( It is a small group.) and I have not been able to find a documented practice for replacing relays. We only have two other electricians whose methods of troubleshooting are antiquated at best and will probably disregard this training anyway. This class has been mandated by upper management, so an attempt has to be made. Thanks for any input.

miller_elex

Upgrade what you can to solid-state relays.

I don't know much about PM's, being an installer.

LJSMITH1

Solid state is a good choice if it fits in your amperage requirements.

If that doesn't interest you, try mercury contactors. I have used them extensively in process heating applications where the high number of cycles typically kill most standard contactors. They also work great for motor control, where some motors have a higher start current draw that arcs the crap out of standard contactors. Many of them are direct replacements for standard form contactors.

http://aecsensors.com/html/mercury_r...ontactors.html

__________________
Larry

ATSMAN

I have never heard of measuring mv across contacts of a control relay (10A rated) to determine if the contact is good or not. First of all, using ohms law, the mv reading will vary greatly depending on the current passing through the contact.
This type of test is usually used on higher current devices (i.e. circuit breakers, contactors, knife switches) when the circuit can't be shut down
or a Ductor is not available. We then divide the mv by the current to get the contact resistance. It coms pretty close to a Ductor reading.

If the contact is exposed, clean it with Scotchbrite. If it is a plug in relay, change it out.

Tony

John Valdes

Sounds like nonsense to me. Who got management on board? You do not check contacts or PM contacts. It's silly. Contacts are either NO or NC. How do you go through a plant full of relays and perform this PM. Do you really think this will save downtime? Good troubleshooting skills are whats in order, not measuring MV's across a contact. Why MV's? Where do you get the MV supply to do the testing. Your multimeter can only measure MV's, not produce them.
Those electricians that will not participate. They most likely realize you are wasting time and money as I do.

EBen63060

I apologize for the misunderstanding. We do not P.M. contacts and have never been with an outfit that has.
I spent almost three years at a foundry in Tacoma( Atlas Foundry,the only one in Tacoma) and the head electrician there was the first person who has ever told me about measuring the drop across contacts. He had been sent to classes over the years for overhead cranes, so I am not sure where he acquired this technique. I have almost 7 years in overhead cranes myself and can attest to this method. If I am troubleshooting a circuit and come across contacts that have more than a half volt drop across them I will go back and replace them when I can. I have used it on 480 v contactors and 120VAC control circuits. I never asked Glenn where he learned this, but this guy is one of the sharpest guys I have ever had the pleasure to work with. I was in a hurry to try and get some info on this and that is why I posted the question here.
Solid state relays will be used down the road, but our senior electrician has some very old school ways about him and does not approve of modern methods. This guy will run jumpers on a cube relay to utilize the other set of contacts instead of replacing the relay. 10 minute repair instead of 30 seconds to swap out a relay. I was looking for something written to show him in hopes of saving our mill some down time. We lose about a grand a minute when the mill is down. Sorry about my babbling, but I don't want you folks to think I am a complete idiot, although I do have my moments. I hope this makes some sense. I just got off work and think I may be coming down with that bug that is going around. Gotta say I am proud of myself, one of the last in the mill to succumb to it!!!

LJSMITH1

Voltage drop is measured across the contacts with the meter set on mV scale. This needs to be done on a live circuit with voltage/current flowing across the contacts. A lower voltage drop indicates low contact resistance. The higher the contact resistance, the hotter the contact surfaces get and the more likely they are to arc out. For example, we measure voltage drop all the time on our cable and conduit fittings. The UL 514B specification requires a fitting to be connected to a 30A DC load and have no more than 15 mV drop across the fitting when the power supply is connected to the box and to the cable or conduit.

It seems that in the OP's case, his equipment is sensitive enough to notice voltage drop, which would be a great application for an SSR or Mercury Relay. I am also assuming that proper troubleshooting is being done when a failure of a relay is analyzed. More often than not, the problem has little to do with a standard cube relay and more with the signal (limit or proximity) switches or other circuit components.

Personally, I find it unusual for control relays to have contact problems unless they have millions of cycles at the rated load (typically 10A). However, Contactors are notorious for burning through contacts depending on the type of motor load and initial draw amps.

__________________
Larry