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Posts Tagged ‘circuit breaker’

Circuit Breaker Broken Handle, Blasted Contacts

February 23rd, 2011 Comments off

 

KD3400 Cutler Hammer Circuit Breaker For Sale

KD3400 Cutler Hammer Circuit Breaker For Sale

We grabbed another circuit breaker out of one of the junk breaker barrels. This one was a Cutler Hammer KD3400.  Again, at first the breaker looked clean and happy. But it only took a second to realize the operating handle was broken off. It takes a pretty hard blow to break the handle off a Cutler Hammer circuit breaker. Handles can be easily replaced. But further inspection revealed serious carbon residue in the area of the line side terminals. You could wipe it off but if there is enough to be visible on the outside, there is something seriously wrong inside. Most of the screws were still missing from the cover, so a technician had removed the cover for inspection of the interior of the circuit breaker and put the cover back on with only a couple screws. A quick conversation with the technician reveal that the arcing contacts were destroyed on two poles and two main contacts were seriously damaged. There was heat damage to pole pieces and arcing damage to the interior insulating, dielectric, material. This Cutler Hammer KD3400 circuit breaker was trash. The breaker did not need to be tested. It was rejected based solely on the visible damage to the operating mechanism, main contacts, and arcing contacts. Square D, Cutler Hammer, GE General Electric or Siemens circuit breakers, it doesn’t make a difference. When this happens, they all fit into the same junk barrels.

 

 

Square D MAL361000 Circuit Breaker May Take 5 Minutes 40 Seconds to Trip

January 20th, 2011 Comments off

 

Square D MAL361000 Circuit Breaker For Sale

Square D MAL361000 Circuit Breaker For Sale

MIDWEST had a customer call because they had an old Square D MAL361000 and it took forever to trip when they had a serious overload problem. He bet it took 3 minutes. This brought up a common misconception concerning how circuit breakers provide protection. When we provide Hands-On Safety Training, this is one of the items we are sure to cover. First of all, whether Square D circuit breakers, Cutler Hammer circuit breakers, GE General Electric circuit breakers, or Siemens circuit breakers, they are not designed to protect people directly. They are designed to protect connected equipment, yet not nuisance trip due to a non harmful transient event. By protecting equipment, circuit breakers consequentially protect people. His old Square D circuit breaker may have taken over 3 minutes to trip and it may have performed the way it was designed. Breakers have a performance specification called a “Time Current Curve,” TCC. In basic terms, whether Westinghouse circuit breakers or ABB circuit breakers, they do not trip immediately at the trip setting. A 1000 amp circuit breaker does not trip right away at 1000 amps or even 1500 or 2000 amps. As a matter of fact, an old 1000 amp Square D MA circuit breaker may have a trip range of 45 seconds to 340 seconds when overloaded with 3000 amps, 300%.  In basic terms, it should not trip in less the 45 seconds and may take as long as 340 seconds to trip. This may seem crazy but, again, it is designed to protect the equipment connected to it while not nuisance tripping. The same breaker would have an instantaneous setting which would determine at what current value the circuit breaker would trip immediately. But, if that setting is over 300%, ie 3000 amps, the breaker would cook for a long time before tripping.  By that time you can smell the breaker overheating.

Circuit Breaker Large Over Current Time Delays

December 29th, 2010 4 comments

In MIDWEST’s training classes for qualified personnel, there is a segment where we explain the long time delay range within which a Square D 1000 amp circuit breaker should trip due to an overload. This information is received with anything from amazement to skepticism to outright disbelief, even though we show the Square D circuit breaker characteristic trip curve.  The overload time delay information is not restricted to Square D circuit breakers. It’s the same with Cutler Hammer, GE General Electric, Siemens, ITE, Westinghouse, Merlin Gerin, or Federal Pacific circuit breakers.

 

In our training example we use an old Square D 1000 amp MA type circuit breaker.  If we tested this circuit breaker at 3000 amps, that’s 300%, the minimum to maximum trip range is about 45 seconds to 340 seconds. It might trip in 45 seconds or it might not trip for 340 seconds.  This is an old thermo-magnetic circuit breaker, which typically works by heating a bi-metal in the over current trip device. Many newer breakers use electronic over current devices which have more repeatable overload time delay test results.

 

The illusion is that these Square D, Cutler Hammer, Westinghouse circuit breakers are designed to directly protect people. They are not. The breakers protect the equipment connected to them and they protect the electrical system. They are designed for the characteristics of the equipment connected, such that connected equipment will not be damaged by an overload or fault. This is a basic limited explanation. So, when you think of molded case circuit breakers, power circuit breakers or air circuit breakers, it’s important to know these breakers don’t just trip right at the breaker trip device rating.       

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.

Why didn’t the Circuit Breaker trip in a Surge Suppressor Strip?

August 12th, 2010 2 comments

MIDWEST received a call for help lately when a client had a dozen surge suppressor strips (with circuit breakers) virtually start on fire.  The interesting thing about this client was that it was a large metropolitan area’s city hall.  The surge suppressor strips were powering the computers that controlled the jail.  The surge suppressor strips had been in place for 10 years without incident.  All of a sudden, on two different floors, a dozen surge suppressor strips overheated.  The strange part is that the circuit breakers inside them did not trip.  Forensic examination  by MIDWEST revealed that the metal oxide varistors inside had overheated, causing the varistors and printed circuit board to burn up;  these were essentially carbonized.  This carbonized material supported arcing, and generated great quantities of heat.  But why didn’t the internal circuit breaker or any building circuit breaker trip when the arcs occurred?

 

First, a circuit breaker is an electromechanical device that interrupts a circuit when a large current flows through it.  But it has to be a large current, like three to ten times rated current, depending on the particular circuit breaker’s curves.  This is the breaker’s trip current; in reality, there are engineering curves and graphs that define a circuit breaker’s exact trip.  For a better description, see “How Circuit Breakers Work” and Wikipedia’s entry on circuit breakers.

 

Why didn’t the circuit breakers trip?  The answer is that if the resistance of the carbon tracking is high enough to limit the current flow to less than the trip current, then the breaker won’t trip.  And the heat just keeps building up inside the strip, resulting in fire.  Thus circuit breakers are not a cure-all; in general, a circuit breaker will trip when presented with a high current short circuit.  But if the fault impedance is current limiting, the circuit breaker gives no protection.  This current limited situation is the cause of most electrical fires and is highly dangerous.  These can be very elusive. 

 

But, one great solution to the problem is offered by MIDWEST’S Infrared Thermography Service.  But that is the subject of another blog.

 

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.

 

Knock Out for that Circuit Breaker Panel

April 30th, 2010 Comments off

During an Acceptance Testing procedure on new circuit breakers and electrical switchgear used in a manufacturing plant, we ran into a squabble over a 2 inch hole in the side of a power panel.  This was strange because the cost of a 2” knockout closure is nothing.  It was just a small detail on our checklist. But the owner’s representative was determined to make his point that it was unnecessary because the new and the old circuit breaker panel boards were in a locked room and only electricians and other authorized personnel were allowed to enter.

 

This really wasn’t going to cost anything and usually MIDWEST just tells the electrical contractor about these little things and they take care of them willingly. In this case, the contractor was already off the job and we were just following up on more important details, like several breakers that did not meet the interrupting rating required.

 

When we spoke of code and construction in a “workmanlike manner,” it didn’t seem to make a difference.  When we spoke of rodents or accidental entry of something conductive, we were met with disbelief.   

 

Then MIDWEST politely explained examples of rodents causing electrical failures. And we recounted a tragic accident where two ceiling workers were seriously injured when the fixture wire they were working with, was accidentally extended into an opening in similar electrical equipment, causing a horrible electrical fault on the line side of the protective circuit breaker. At this point there was silence and a simple instruction to just take care of it.  Sometimes the human factor is more difficult to deal with than circuit breakers and switchgear.