Breaker DLRO testing

[aelectricalman]

We had a contact resistance (micro-ohm) test on an 800 Amp thermal magnetic breaker,with a test current of 10 amp. The results were


Phase

A = 79mOhms (Line to Load) through the closed breaker
B = 115mOhms (Line to Load) through the closed breaker
C = 95 mOhms (Line to Load) through the closed breaker

The manufacturer came back with a recommendation that the breaker be replaced due to the "large variation" in the phase to phase contact resistance. Those seem to be very good values to me and did not realize that variations within the specified "acceptable test range" it cause for concern. Can someone please elaborate?

 

[petersonra]

We had a contact resistance (micro-ohm) test on an 800 Amp thermal magnetic breaker,with a test current of 10 amp. The results were


Phase

A = 79mOhms (Line to Load) through the closed breaker
B = 115mOhms (Line to Load) through the closed breaker
C = 95 mOhms (Line to Load) through the closed breaker

The manufacturer came back with a recommendation that the breaker be replaced due to the "large variation" in the phase to phase contact resistance. Those seem to be very good values to me and did not realize that variations within the specified "acceptable test range" it cause for concern. Can someone please elaborate?

Seems rather high if these are milliOhms. That would be a 92V drop on B phase. Maybe microOhms (uOhms)?

It seems to me that you asked for the opinion of an expert and you do not like it. I think you would need to pay another expert to see if they will tell you what you want to hear.

I am not sure that a 50% variation phase to phase is bad or not but am inclined to believe the manufacturer when they say it is.

ETA: I found this.

http://ecmweb.com/test-amp-measurement/dc-testing-circuit-breakers

Manufacturers sometimes publish acceptable values, or they may be obtained from field experience. According to ANSI/NETA MTS-2011, contact resistance should not have greater than a 50% difference between the highest and lowest reading across all three phases. After performing maintenance, contact resistance should be as low and as even as possible across the three phases of the circuit breaker. The actual resistance will vary, depending on the circuit breaker?s continuous current rating. The smaller current ratings will have higher contact resistance. As a general rule-of-thumb, low-voltage circuit breakers should have contact resistances of 100 ?? or less, while readings above 300 ?? are an indication of trouble. Medium-voltage air-magnetic circuit breakers will have lower readings due to their larger current-carrying structures.

 

[templdl]

I was a breaker sales and applications engineer for as major breaker manufacturer for 18 years and we never considered a breaker contact resistance test as a valid indication of the health of a breaker. It is a test that makes it easy for the uninformed to pass/fail a breaker. Every time that a breaker is opened and closed again the contact resistance will change. As sduch I have always recommended that breakers be exercised when allowed because the moving and stationary contacs actually rub together when you do.
The best indication of the integrity of a breaker is under load checking the voltage drop across the line and load terminals and well as if there is any heating at each pole.
It is so simple to show a customer a measured contact resistance telling them that it is too high and it needs to be replaced. The customer having no clue trusts you even though the test is not a valid method of determining the integrity of a breaker.
I had the distinct privilege of being able to call and talk directly to those who designed breakers and held patents on them. I also knew the quality and warranty manager personally. I have had the opportunity to sit a a test bench and test breakers for their integrity when a breaker was returned for replacement under warranty.
I have always found the subject of breaker contact resistance to be most amusing.
Yes, contacts can fail because I have seen the damage. Heating often causes contact springs to weaken where the contacts can no longer be held firmly together. There are a number of causes for this.
Doing a simple infrared scan of a breaker underload will show this. Once contacts start to fail it isn' t pretty often cause by a compromised connection such as a line side terminal or a trip unit connection. Yes, I have seen a defective contact where it just fell off because it wasn't bonded to the contact arm correctly during manufacture.

 

[big john]

...The manufacturer came back with a recommendation that the breaker be replaced due to the "large variation" in the phase to phase contact resistance. Those seem to be very good values to me and did not realize that variations within the specified "acceptable test range" it cause for concern. Can someone please elaborate?

There is the NETA standard that petersona posted, but that's a very conservative number. I also assume you mean microohms and not milliohms?

If those are microohms, they are fine readings. This is a case where experience trumps NETA recommendations, and of the thousands of breakers I've tested those don't worry me.

Molded-case breakers often seem to have higher contact resistance, and relative to what I've seen in the field those are minor for a molded case breaker. To put it in perspective I like to calculate the heat dissipation of the contact: At 800A full load "B" phase would radiate 74 watts. It would definitely be warm, but that's nothing that would make me take that out of service.

If this is an old breaker out of warranty, you can make the judgment call to pop the front cover and burnish the contacts. You void the listing, but you could probably knock those values way down if it concerns you and it would save you a whole lot over purchasing a new breaker.

 

[templdl]

There is the NETA standard that petersona posted, but that's a very conservative number. I also assume you mean microohms and not milliohms?

If those are microohms, they are fine readings. This is a case where experience trumps NETA recommendations, and of the thousands of breakers I've tested those don't worry me.

Molded-case breakers often seem to have higher contact resistance, and relative to what I've seen in the field those are minor for a molded case breaker. To put it in perspective I like to calculate the heat dissipation of the contact: At 800A full load "B" phase would radiate 74 watts. It would definitely be warm, but that's nothing that would make me take that out of service.

If this is an old breaker out of warranty, you can make the judgment call to pop the front cover and burnish the contacts. You void the listing, but you could probably knock those values way down if it concerns you and it would save you a whole lot over purchasing a new breaker.

I doubt if any manufacture would confirm these techniques as a meas for a warranty issue or determining the health of a breaker. It would be interesting to learn if any manufacturer would recommend " burnishing" contacts. Unless a failure was obvious I would only issue an RMA and only issue credit based upon the breaker after being evaluated by my warranty people.
And NETA only uses those procedures as would be included in NEMA standards as I understand. NEMA AB3 and AB4 do not include anything such as this from what my references include. Poping the cover often violates UL listing.

 

[aelectricalman]

Seems rather high if these are milliOhms. That would be a 92V drop on B phase. Maybe microOhms (uOhms)?

It seems to me that you asked for the opinion of an expert and you do not like it. I think you would need to pay another expert to see if they will tell you what you want to hear.

I am not sure that a 50% variation phase to phase is bad or not but am inclined to believe the manufacturer when they say it is.

ETA: I found this.

http://ecmweb.com/test-amp-measurement/dc-testing-circuit-breakers

The results are in microohms. I stated that above, I just mistyped it below. Sorry.

 

[aelectricalman]

There is the NETA standard that petersona posted, but that's a very conservative number. I also assume you mean microohms and not milliohms?

If those are microohms, they are fine readings. This is a case where experience trumps NETA recommendations, and of the thousands of breakers I've tested those don't worry me.

Molded-case breakers often seem to have higher contact resistance, and relative to what I've seen in the field those are minor for a molded case breaker. To put it in perspective I like to calculate the heat dissipation of the contact: At 800A full load "B" phase would radiate 74 watts. It would definitely be warm, but that's nothing that would make me take that out of service.

If this is an old breaker out of warranty, you can make the judgment call to pop the front cover and burnish the contacts. You void the listing, but you could probably knock those values way down if it concerns you and it would save you a whole lot over purchasing a new breaker.

Yes, I did mean Microohms. Sorry.

 

[aelectricalman]

We are an engineering firm that specified a capacitor installation for a client. Their electrician mistakenly connected all phase A conductors in one metallic conduit with a ground, all phase B conductors in another metallic conduit with a ground, and the same for the C phase. You may recall my past post. Well, this caused issues where instead of the ground currents cancelling, they were added, thus heating up the ground conductors and melting the insulation on the conductors. In this process, there was an issue with said breaker in that it tripped due to the altered trip curve. As a result, the manufacturer required a series of tests. Mostly the tests were on the capacitors but they required a DLRO of both breakers. The breaker that was reported having tripped, is the one with the delta. Since there were these large deltas from phase to phase, they asked for it to be replaced. I just wondered if the bank manufacturer actually went to the breaker manufacturer or was this his own suggestion. Thats why Im asking this question. I want to better understand the process.

PS. I appreciate the link.

 

[petersonra]

We are an engineering firm that specified a capacitor installation for a client. Their electrician mistakenly connected all phase A conductors in one metallic conduit with a ground, all phase B conductors in another metallic conduit with a ground, and the same for the C phase. You may recall my past post. Well, this caused issues where instead of the ground currents cancelling, they were added, thus heating up the ground conductors and melting the insulation on the conductors.

I doubt this had anything whatsoever to do with not cancelling ground currents. My guess is the electrician unintentionally created an inductive heater by not having all three phase conductors in the same conduit. It is a magnetic phenomena where the alternating magnetic field is not cancelled out as it mostly would be if all three phases were properly run together.

Was the breaker a regular MCCB with thermal trip? If so, it probably just tripped when heat radiated back to it from the overheated conductors.

I don't know what you mean by an "altered" trip curve either.

I don't know what "manufacturer" did the DLRO test on the breaker. Was this the breaker manufacturer or the capacitor bank manufacturer? I would ask for a copy of the test results and recommendation in writing. If it came from the breaker manufacturer, I think I would be inclined to believe it.

In any case, this is a problem for the contractor's insurance company to deal with and it is better for them to be involved since they are footing the bill.

 

[petersonra]

Poping the cover often violates UL listing.

Why would that be? It is the only way you can add stuff in the field in many cases.

Some breakers have a seal on the trip unit so if an attempt is made to replace the trip unit there is some evidence that occurred.

 

[Bugman1400]

The uOhm test values will be dramatically different based on humidity and harsh environment. They tend to be better during the winter. If the breakers are in a dusty plant then use an air compressor to blow them out and test again. Also, exercise the breaker a few times and reposition the test clamps and test again. NETA states that for low-voltage circuit breakers, the test values cannot deviate by more than 50% of the lowest value. I think your values barely make that requirement but, I would re-test.

 

[big john]

...It would be interesting to learn if any manufacturer would recommend " burnishing" contacts....

No molded case breaker manufacturer is going to recommend it when the alternative is selling a new breaker.

But when your choices are:
A) Perform some reasonable, empirically sound preventative maintenance.
B) Purchase a whole new breaker for thousands of dollars.
Then choice A) becomes a lot more attractive. But like I said, it's a judgment call on his part.

The uOhm test values will be dramatically different based on humidity and harsh environment....

Are you thinking of insulation resistance? Conductor resistance tests are not impacted by humidity. They are mildly impacted by temperature but not to the extent that it's gonna add or subtract dozens of microohms.

As far as the NETA value, that is the threshold for investigation, not the point at which a contact would be condemned. There's no good reason to take this breaker out of service.

 

[templdl]

No molded case breaker manufacturer is going to recommend it when the alternative is selling a new breaker.

But when your choices are:
A) Perform some reasonable, empirically sound preventative maintenance.
B) Purchase a whole new breaker for thousands of dollars.
Then choice A) becomes a lot more attractive. But like I said, it's a judgment call on his part. Are you thinking of insulation resistance? Conductor resistance tests are not impacted by humidity. They are mildly impacted by temperature but not to the extent that it's gonna add or subtract dozens of microohms.

As far as the NETA value, that is the threshold for investigation, not the point at which a contact would be condemned. There's no good reason to take this breaker out of service.

Then the question is who will assume the liability should there be a beaker failure as a result of a catastrophic failure, the person who took it upon himself to do an unauthorized procedure because in his opinion the "fix" was acceptable? The manufacturer that I was employed with would never condone this because of the cost of liability not that they can sell more breakers. They guy in the field that chooses to do this looks like a hero at the risk of a law suite shoud it result in a failure. There would also be a question whether or not this would be in violation of the NEC when a cover is removed on a factory sealed breaker in addition to doing a procedure that is not authorized by the manufacturer. Selling more breakers was never the goal of my company but maintaining their integrity of their was much more important.

 

[don_resqcapt19]

Why would that be? It is the only way you can add stuff in the field in many cases.

Some breakers have a seal on the trip unit so if an attempt is made to replace the trip unit there is some evidence that occurred.

The instructions for the field installed auxiliary contacts that I have added to breakers all had a statement in the instructions that said adding the field installed aux switch kit supplied by the breaker manufacture would void the listing of the breaker.

 

[petersonra]

The instructions for the field installed auxiliary contacts that I have added to breakers all had a statement in the instructions that said adding the field installed aux switch kit supplied by the breaker manufacture would void the listing of the breaker.

I have never seen such a statement on any product. Can you scan that? My understanding is that UL specifically allows certain field changes to breakers without it affecting the listing, and even changing the trip unit on units with interchangeable trip units is permitted.

I agree you cannot open up a sealed one without some kind of consequence but do all MCCB have seals?

 

[Bugman1400]

No molded case breaker manufacturer is going to recommend it when the alternative is selling a new breaker.

But when your choices are:
A) Perform some reasonable, empirically sound preventative maintenance.
B) Purchase a whole new breaker for thousands of dollars.
Then choice A) becomes a lot more attractive. But like I said, it's a judgment call on his part. Are you thinking of insulation resistance? Conductor resistance tests are not impacted by humidity. They are mildly impacted by temperature but not to the extent that it's gonna add or subtract dozens of microohms.

As far as the NETA value, that is the threshold for investigation, not the point at which a contact would be condemned. There's no good reason to take this breaker out of service.

Yes, I was thinking of insulation resistance. Thanks for the correction. However, if the contact is subject to the open-air in some way (and not in a vacuum bottle), I would expect dusty environments to have a substantial influence.

As far as I know, there is no official value at which a contact would be condemned.....right? So, for such a subjective test, I would start with the NETA value. Or, do you know of an IEEE or IEC std that defines a condemned contact.

 

[big john]

Then the question is who will assume the liability should there be a beaker failure as a result of a catastrophic failure...?

As the people who disassemble the breaker, we would assume the liability. It's literally no different than any other piece of electrical equipment we touch, and there needs to be a certain amount of expertise involved to effectively evaluate the suitability of the repairs. Liability always exists and the people performing the work need to act accordingly.

Petersona, here's the text of what Eaton says about field installed devices on some of their MCCBs it does void the listing:

...For sealed circuit breakers (LDB), Underwriters Laboratories Inc. UL 489 requires that internal accessories be installed at the factory. The shunt trip is listed for factory installation under UL File E7819.

Where local codes and standards permit and UL listing is not required, internal accessories can be field installed in sealed circuit breakers. In this case, UL listing becomes invalid and the label should be
removed.

 

[templdl]

The instructions for the field installed auxiliary contacts that I have added to breakers all had a statement in the instructions that said adding the field installed aux switch kit supplied by the breaker manufacture would void the listing of the breaker.

You are correct especially on smaller frame breakers, 100, 125, and 225a frames. These breakers most likely come with a factory seal of some type that must be broken in order to remove the cover to install the accessory.
This is also true with larger frames that have NIT non interchangeable trip units where covers must be removed Those with IT interchangable trip units you can add accessories w/o voiding the factory seal.
In order to follow the letter of the law of NEC art 110 factory instructions must be observed. However, how many licensed electrician would observe this and then be gigged by the AHJ.

 

[don_resqcapt19]

Yes, these would have been smaller breakers in starter buckets where they wanted an "interlock" for external control power.

 

[templdl]

Yes, these would have been smaller breakers in starter buckets where they wanted an "interlock" for external control power.

Typical for MCPs. There may be also and option to add an electrical interlock on the handle mech though.