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Archive for June, 2010

Disaster Replacement Circuit Breakers

June 25th, 2010 No comments

An engineer called MIDWEST and asked how he could make sure a replacement for one of his old obsolete circuit breakers was not something out of one of the US disaster areas. He said it was a used replacement circuit breaker, but it looked shiny as new and seemed to operate okay. But he just had a bad feeling. To MIDWEST, that means he knows something he isn’t telling us. Regardless, here’s the short answer to his question. If you are qualified, remove the cover from the circuit breaker. Remove the arc chutes. On old circuit breakers, be careful, because the arc chutes may fall out in a dozen pieces. Remove the lugs.  Carefully remove the trip device.  Be sure not to lose any barriers or arc dividers. Then a close visual inspection must be made to detect any signs of corrosion or contamination on the breaker operating mechanism or inside the circuit breaker trip device or under the current carrying components. If the breaker got wet, corrosion should be easy to spot, but be sure to make a hard focused inspection.  Something more than a cosmetic look. During this process, look for signs that someone else has already tried to recondition the circuit breaker. Just cleaning a used or even a new circuit breaker, after it has been immersed in water, can be nearly impossible. There will be corrosion and dirt, even mud, in areas that are inaccessible. The trip device must be taken apart. MIDWEST has experience repairing old trip devices. It usually just isn’t worth the time. And if you mess with the trip element, there is a whole sequence of over current tests that must be performed afterwards. You can’t take a trip device apart, put it back together, and not completely test the circuit breaker afterwards. So, if you suspect an old, obsolete, or new circuit breaker has been exposed to water or mud, your best decision might be to destroy it and then throw it in the trash. Note, we strongly encourage destruction of any circuit breaker that is defective, so no one tries to reuse it. If you find nothing to indicate the breaker was damaged, then you have to put it all properly back together and test it.   

How Circuit Breaker Arc Chutes Work

June 16th, 2010 1 comment

Buy EHD3070 Westinghouse Molded Case Circuit Breaker

Buy EHD3070 Westinghouse Molded Case Circuit Breaker

The arc chutes for molded case circuit breakers and old power breakers look simple enough, but they actually are a complex design and perform extremely important functions.  For excitement, read the patent application for an old circuit breaker arc chute, if you can stay awake.  MIDWEST sometimes is asked to explain what an arc chute does. We always go for the short version.  Basically an old or new circuit breaker arc chute stretches the arcing that takes place when a circuit breaker opens, such that the arc is too long for the voltage to keep it going.  Arc chutes have arc dividers in the form of flat segments stacked one above the other, with an air gap between them.  When the arc occurs, it is expelled into the arc chute and into the arc dividers, such that it wraps back and further between the arc dividers. The wrapping back and forth around the arc dividers effectively stretches the length of the arc until it is just too long for the voltage to keep it going.  When this happens, the arcing stops.  The arc has been extinguished.  When the circuit breaker opens, the main current carrying contacts open first and a different set of contacts, the arcing contacts, open second, such that the arcing contacts endure limited damage from the arcing, until the arc chutes interrupt the arc. So the combination of the arc chutes and the arcing contacts protect the main contacts from arcing damage when the circuit breaker opens and when it closes.  When a circuit breaker is closed, the arcing contacts close first, again taking on the arc such that the main contacts are protected from arcing damage when closed.  This is especially important when the circuit breaker interrupts a high current fault and there is a real blast in the arc chutes.  Each phase, ie pole, of a circuit breaker has a separate arc chute. This is pretty much how the arc chutes of obsolete, old, and new replacement circuit breakers operate.  So the arc chutes perform an extremely important function.

 

Vacuum Technology Obsoletes Air Circuit Breakers

June 11th, 2010 No comments

MIDWEST recently sold a 2500 amp 15 Kv old used air circuit breaker for $5000.00.  It was in excellent condition and years ago it would have sold for $25,000.00.  One might think the reason for the big drop in price is because of the age.  But the biggest reason is the use of vacuum technology in 15 Kv class circuit breakers.  Vacuum breakers are used to replace air circuit breakers in new 15 Kv switchgear. And many old medium voltage air circuit breakers have been replaced with new vacuum breakers or retrofitted with vacuum technology. 

 

Maintaining and Testing 5 Kv and 15 Kv vacuum circuit breakers is a lot easier than the old air circuit breakers.  And the vacuum breakers are a lot easier to handle.  An old or obsolete air circuit breaker could weigh 1500 pounds.  The new vacuum equipment is half that.  The biggest difference is the simple replacement of the old heavy arc chutes with the simple vacuum bottles. For an historical perspective, the early 1900s saw the use of big old oil circuit breakers.  The mid 1900s began the use of air circuit breakers. And the late 1900s began the use of vacuum breakers.  MIDWEST has worked on all of them and the vacuum breakers are just so much simpler to service.  Some older electricians and switchgear service technicians do not trust the vacuum circuit breakers as a safe circuit open device.  I wouldn’t trust vacuum circuit breakers or old air circuit breakers or obsolete oil circuit breakers as open circuit protection. The circuit breaker must be racked out and, of course, the circuit grounded before any circuit work or equipment maintenance. By racking the breaker out, one has a “visible open.”  Medium voltage air circuit breakers are an example of equipment becoming obsolete long before they actually wear out, all due to new technology. 

Replacement Circuit Breaker Tight but Loose

June 4th, 2010 No comments

During our annual Thermographic Scan of a large manufacturer’s electrical system, MIDWEST found a serious problem with a new circuit breaker. The middle line side bolted connection was extremely hot. They had recently installed a replacement circuit breaker with a higher interrupting current level in this panelboard.  During their Arc Flash Hazard Analysis they discovered several old circuit breakers that did not have high enough interrupting current rating for their system. So they replaced these circuit breakers.  The replacement circuit breakers had high interrupting ratings and it was a straight forward replacement project. Their electricians were pretty sharp, so they were skeptical of our finding.  On third shift they powered down and checked connections and the electricians informed MIDWEST, in their own emphatic vernacular, that the bolts were tight and maybe MIDWEST was loose.  MIDWEST has run into this breaker problem before.  It doesn’t happen often, but we work with old and new circuit breakers every day, all day.  Here is the work practice one must follow when changing out old circuit breakers.  Always lay out the bolts removed from the old circuit breaker such that you know exactly which bolt came out of which hole. Use identical replacements for each bolt and pay attention to the bolt lengths. Replace the bolts with new bolts of the exact same length.  On their installation, the middle line side bolt was shorter than the other two bolts. The middle bolt was not a through bolt. The bolt hole bottomed out.  They used a bolt that was ¼ inch too long and even when it was properly torque tightened, it still was not a physically tight connection, because the bolt hit bottom before the attached bus was tight. We suspect their torque wrench was called “armstrong,” so we were sure they had tightened it enough. Again, on third shift, they replaced the bolt with the right length and the overheating circuit breaker problem disappeared.  MIDWEST recommends following a simple work practice when installing replacement circuit breakers. Keep track of exactly which bolt went where. Usually it does not make a difference, but one in a hundred does.  This customer was lucky we did an infrared scan shortly after the circuit breaker was replaced.