NETA MTS-2011 Microhm Contact Resistance Measurement

[mmt019]

NETA MTS-2011, Part 7.6.1.1, Section 3.2.3 indicates that the contact resistance, in this case on a molded case circuit breaker, that in absence of manufacturer's data, the standard suggests to investigate the problem if a value deviates from adjacent poles or similar breakers by more than 50% of the lowest value. However, at what point is it considered bad enough for replacement? Generally, the techs operate the breaker 3-4 times prior to taking a final reading. However, when all three poles are reading in the micro-ohm range anyway, and there are not any tripping problems occurring, when is a breaker considered bad? Molded case circuit breakers are not intended to be worked on and usually that type of breaker is considered "a throw-away"?

 

[collies99]

What does the manufacturer has to say about it ?

For LV breaker where the contacts are readily available by the removal of the arc chute assemblies, I will remove it it and attemp contact cleaning to bring the values below 60 micro.

For non-accesible contacts and due to time contraints, perhaps a dozen trip/close may bring the numbers closer in line and lower. When that does not work, I work out the I2R losses based on the actual jlosd if known, or based on the breaker rating if unknown. You then have to quantify if ie. 100W, 50W, or even 25W per contact heating effect is detrimental for the respective breaker based upon size/rating, that one is a judgement call.

Else the best you can do is make a note that contact values does note fall eithin NETA guideline of falling within 50% of the lowest. If you are brave enough, make a suggestikn to replace kr repair the breaker.

Welcome to the world of testing.

 

[oldfieldguy]

In the hundreds of breakers my crew tested, I t was not uncommon to find one that tested poorly withe the DLRO. We found that connecting the questionable phase in series with a hundred watt light bulb and cycling the breaker on and off several times, letting it interrupt a little current quite often cleared the issue.



old field guy

 

[Zogzog]

NETA is now an ANSI standard so if you are outside those specs then you investigate and try to resolve, if you can't then it fails and it is up to the owner of the equipment to replace or not.

 

[dpmac]

IMHO if you find new molded case breakers with contact resistance variations greater then 50% (as per NETA/ANSI) and you attempt to "reject" these breakers you will be politely instructed to pound sand by any molded case breaker manufacturer. The breakers are manufactured to certain standards. 50% deviation in contact resistance is not in those standards. Testing of molded case breakers is not a simple matter.