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Posts Tagged ‘infrared scanning’

Overheating 1200 Amp Square D Circuit Breaker – Bus Bolts Bottomed Out

March 31st, 2011 No comments

Square D 1200 Amp Circuit Breaker - Cat. No. MAL361200 For Sale by MIDWEST

MIDWEST found a seriously overheating 1200 amp Square D circuit breaker during the annual Infrared Scan of a Foundry.  The upper left corner of the newly installed circuit breaker lit up like a light bulb when viewed with Infrared. The foundry third shift electricians tightened all the bus connections to the back of the circuit breaker. They said a couple bolts turned a quarter turn and they exercised the breaker. Rescanning showed no change. They didn’t want us working on the circuit breaker in place, so MIDWEST provided a temporary rental circuit breaker so they could remove the defective Square D circuit breaker for us to repair, if possible. Before testing the circuit breaker in our shop, we inspected it. The technician immediately noticed there were different bolts holding the line and load side breaker bus details. In our famous words, “We’ve seen this before.” Got to love experience. As soon as he removed the bolts he determined the bolts, holding the bus detail in the overheating area of the breaker, were bottoming out before the bus detail was tight. This was a quick fix because no overheating damage had occurred. This was an old Square D molded case circuit breaker. But we have seen the same problem on Westinghouse 2000 amp circuit breakers and less frequently on GE circuit breakers. We have never seen it when the circuit breakers have been installed by the manufacturer. It happens when a breaker has been replaced for some reason. 

Circuit Breaker Infrared Scanning Disagreement – Lugs

October 27th, 2010 No comments

Infrared Scan of a Circuit BreakerWe read about a disagreement between two bloggers over whether or not infrared scanning, or thermography, was needed if you torque tightened the wire connections to power circuit breaker terminals during routine maintenance.  What occurred to MIDWEST were all the possible deficiencies we find in old, new, and replacement circuit breakers, using infrared scanning, that have nothing to do with whether or not the load terminals were tight.  One of the nasty deficiencies is when the cable lug in an old circuit breaker is very tight, but the lug is overheating because the screw, holding the lug to the breaker output tab, is loose. We’ve seen brand new circuit breakers and replacement circuit breakers fry the load side tab of the breaker so bad that the breaker had to be replaced. This is true for new or old Square D, GE General Electric, Westinghouse, Siemens, Cutler Hammer, ABB, any manufacturer. It has nothing to do with a specific circuit breaker manufacturer.

 

Sometimes the lug is welded to the tab from the arcing between the lug and tab. There is a very sophisticated test one can perform during a maintenance outage to check for this defect. First, check for voltage at the load and line side of the de-energized circuit breakers. Don’t care that the main breaker is off and all the feeder breakers are open.  Check voltage anyway.  You are checking for something that shouldn’t be, not for something you know should be.  We, rather I, have personal experience with getting my hand blasted because a breaker was back fed. Very bizarre set up, unbelievable, just waiting to injure someone.

 

After checking for voltage, carefully and gently try to move the conductor coming out of each phase of each circuit breaker.  You are trying to see if the cable is loose in the lug and you are trying to see if the lug is loose, moves or turns, in the circuit breaker. You are not trying to force it to move. Just use enough force to see if it is loose in the circuit breaker. If the lug itself is loose, the cable or cables will need to be removed from the lug; The mounting screw for the lug properly tightened; The cables properly reinstalled; And the cables tightened in the lug. Again, don’t be too forceful. On small breakers, you can always make the lug move. Repeating, you just want to use enough force to see if the lugs for that old obsolete circuit breaker are loose.

 

If the conductive interface, between the lug and the circuit breaker, is damaged from severe overheating or arcing, the defective circuit breaker may need to be replaced. Sometimes the damaged area can be repaired.  MIDWEST does not recommend replacing power circuit breakers while the switchboard is energized. Be safe. Turn things off. Check for voltage everywhere.

When is a Circuit Breaker not a Circuit Breaker?

August 16th, 2010 1 comment

Question:  When is a circuit breaker not a circuit breaker?

 

Answer:   When it doesn’t trip and break the circuit.

 

This is not just a silly riddle.  It unfortunately is a fact of life.  Circuit breakers, either like the small ones in your home, or large industrial ones as supplied by MIDWEST, usually only trip when presented with an extra heavy load, or a short circuit.  This is great news.

 

But there is bad news also.  In the case of a poor connection somewhere in the circuit path, or a not-so-short short circuit, a circuit breaker will not trip.  To be specific, the electrical engineers at MIDWEST would say that the first case is a high impedance series circuit, and the second is a low impedance parallel circuit.  Either way, very significant heat can be generated in places where heat can cause a fire.  And because the current flowing can be below the trip value, the circuit breaker will not trip to break the circuit. 

 

Examples of a high impedance series circuit might be a loose screw on a lug, or poor wire nut connection, or a bad solder joint.  These essentially become an additional series resistive component in the circuit.   

 

Examples of a low impedance parallel circuit are carbonized arc paths on a printed circuit board, a bare wire brushing up against something it shouldn’t, or the failure of a normal load.  These essentially become an additional parallel resistive component in the circuit.   

 

All of these conditions can easily result in an electrical fire, or even a catastrophic arc flash.  In an industrial setting, one of the products that MIDWEST offers is arc flash and fire resistant Arc Flash Personal Protective Equipment.  MIDWEST also offers Infrared Scanning and Ultrasonic Scanning Services, which is a great way to locate the troublesome series or parallel faults causing dangerous high temperatures.

 

It is probably safe to say that most electrical fires can be attributed to a circuit fault with just the right impedance resulting in circuit current that does not trip the circuit breaker or blow a fuse.  Often these faults occur in out of way places such as walls or electrical sockets.   

 

So, when is a circuit breaker not a circuit breaker?   

 

Answer:  When the impedance of the circuit is such that the current is less than trip current, and the circuit breaker does not trip to break the circuit.