AIR circuit breaker 2

[golftennis]

Hi All,
Air circuit breaker How to check contact resistance using current injection method feed back appriciated.

 

[oldfieldguy]

Inject a measured value of DC current through the contact in question. Measure the voltage developed across the contacts and other components in the primary power path. The more current, the higher the voltage you measure. It's that Ohm's Law thing. Typical measurement currents are 10 and 100 amps.

The "DC" thing is to make sure you're measuring just the resistive component of the circuit impedance. Using AC, the voltage drop contains both resistive and reactive components, and a significant change in resistance could be masked by a large reactive component.

Or use a DLRO (digital low resistance ohmmeter). Several companies manufacture them and they're made for this type of measurement.

Or call your friendly local testing company and have them drag their equipment out and test the circuit for you.

I'm not being flippant here, but there are a few tricks to determining exactly what's going on with a questionable reading.

old field guy

 

[Zogzog]

I agree with the old field guy, lots of little tricks to know if the readings are good or not, the problem is the value you call good depends on the breaker ratings and type, is 100 microohms good? Could be very good or very bad depending on the breaker.

 

[ScottyUK]

At 50Hz and passing current through a breaker pole I doubt you would see any reactive effects to worry about so an AC source would be fine to use. A large CT with the secondary driven from a variac is a reasonable way to obtain high AC current. The caution about reactive effects is a valid caveat to be aware of though - part of choosing the right technique for the job.


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If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

OK, I will make it easier, buy one of these:

http://www.aemc.com/products/pdf/2129.80.pdf

Readings depend on frame size but should never be higher than 50% of the lowest phase.

Or go here to find a tetsing company in your area

http://www.netaworld.org

 

[oldfieldguy]

Scotty--

There was a pretty extensive article a few issues back in NETA World, the technical magazine of the interNational Electrical Testing Association concerning the use of AC in testing contact resistance.

It's an interesting read. The basic thrust of the article is the errors to which I referred.

That said, the AC millivolt drop test is better than no test at all.

DC high current sources are actually fairly common around industrial facilities, cleverly disguised as arc welding machines.





old field guy

 

[ScottyUK]

NETA magazine ain't something we see over here, but I'll see if I can find a copy. Still unconvinced that a contact set and associated copper will have any appreciable reactive effects at 50Hz but certainly willing to learn - thanks. If you have a copy then would you mind scanning it?


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If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

Here it is, I work with Dan if you have any further questions but the article explains it all

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

 

[ScottyUK]

Thanks guys. I think my first question for the author, if he doesn't mind explaining, is where any appreciable inductance exists in such a short piece of bus to give the effect described at 50Hz. I understand the theory behind what he is saying about impedance v's resistance very clearly and I have no argument with the basic premise of the article but I just can't reconcile the results he obtained with the circuit parameters required to produce them. Possibly some other effect is upsetting the results such as the presence of a CT core on the bar. If such cores are present then it may prove informative to inject at the breaker stabs and right at the poles for comparison purposes.

As a documented test the article has some large gaps and certainly doesn't "say it all", but it raises some questions which are worth pursuing. I have access to a high current DC source (>500A), a PCITS and an accurate voltmeter but am somewhat pressed for time at present because of generator outages. I will try to carry out a more clearly documented test, including equipment types etc and connection details and note of any additional components such as CTs one the bar, and will post the results. Perhaps your colleague could do the same and we can compare results?


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If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

Yes there are CT's. Dan wont be doing any other tests for awhile, he is about 6 months behind on stuff the way it is, but please post your results and I will pass them on to him.