Posts Tagged ‘overheating’

Removing Cover from an Energized Breaker

November 19th, 2010 No comments


MIDWEST had a customer call and ask if it was okay to remove the cover from an old 400 amp circuit breaker, live. Our Infrared Scan indicated the load side connection was overheating. They wanted to repair it, but didn’t want to turn the power off to the whole panel board. They needed to remove the cover of the circuit breaker to make the repair and thought they could just remove the four screws holding the cover on and carefully remove the cover. We explained politely that they were crazy to try such a thing. This was an old molded case circuit breaker and the arc chutes for this breaker were not fastened in place as they are in some breakers. In addition, the arc dividers were metal and they were held together with an insulated band. On some of these breakers we have to tape the band to hold the arc dividers together or they just fall apart. So the danger would be that you remove the cover with the line side still hot and one of the arc chutes falls out and the metal arc dividers fall apart. It would be almost certain that one of the metal arc dividers would short a stationary contact to a moveable contact and cause a horrific arcing blast, arcing fault.  Depending on the instantaneous, ie fault, setting of the main breaker, the fault might last for seconds and result in tremendous damage to the equipment and expose anyone nearby to serious injury or death from an arc blast. Because the fault is on the line side of the breaker, it wouldn’t take much to create a panel board bus fault. This is a good way to get someone seriously injured or killed and a good way to destroy a whole panel board. To remove the cover off any circuit breaker with the line side hot is a very bad idea. But to remove the cover of some of the older circuit breakers, with the line side hot, is just crazy because of the construction of the breaker. Inside molded case circuit breakers there are other devices that may fall out when you remove the cover. Besides all this, there may be something defective inside the breaker, just waiting there, for the first unfortunate person to take the cover off, and then it falls apart or breaks completely. You could get very unlucky. We call these things incipient failures and they can be some of the most nasty and dangerous defects in electrical equipment, because you are not expecting them. This is true whether it’s a Square D, Cutler Hammer, Westinghouse, GE General Electric, or Siemens circuit breaker or any other breaker manufacturer. Turn the main power off!

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.

Circuit Breakers Are Not Always Trouble Free

June 10th, 2009 No comments

Circuit breakers are unlike fuses as they are designed to open and close all three phases at once.  This is a definite plus as the power can be restored after the system problem has been corrected.  But even circuit breakers have their warts.  They can be slower acting than fuses and sometimes one or more of the poles can fail in the ‘open’ or ‘closed’ position.  If they have not been maintained for years, they could act even slower than usual or worse they may not react to a faulted condition at all.  This could result in the same kind of overheating condition you could experience with two out of three fuses providing some power to a machine.  You know where one fuse blows or ‘goes’ prematurely and the other two hold and then you slowly cook the machine’s motor to death.  Why? Because the power from the remaining two phases, if it will spin the motor at all, will not be able to turn the fan blade on the motor shaft fast enough to provide adequate cooling. 


Best remedy to keep circuit breakers fit is to routinely open and close them to keep the mechanisms moving a few times, maybe ten or so times, so all of the moving parts break through any accumulated buildup of grease and grime.  Remember just because a machine is not moving does not mean the power is turned off. 

May 21st, 2009 No comments

A question asked by one of our customers:


You guys work on circuit breakers all the time.  I have a used circuit breaker.  I don’t know why it was laying around in the shop, but I checked it with a VOM and operated it.  Checks okay and sounds okay.  Is it okay, or is there more to it?  My breaker is a 3 pole 100 amp plastic type circuit breaker.




Yes, there is more to it.  MIDWEST’s tests are (1) Voltage Related, (2) Current Related, and (3) Mechanical Related.  The most serious deficiencies may be current, ie heat, related.  MIDWEST overcurrent tests used and new circuit breakers.  In addition to testing the overcurrent functions and insulation quality, we also test the condition of the contacts, including contact resistance.  Poor breaker contacts can cause overheating and lead to serious damage.