Circuit Breakers Blog – Expert Safety and Usage Information

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.

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.

TJJ436300 General Electric Circuit Breaker

MIDWEST buys surplus, old, used, obsolete, and even ugly circuit breakers.  We are paranoid about the history and condition of these previously owned circuit breakers.  We are particularly concerned someone will try to sell us breakers that look good, but have been damaged by water.  We are especially cautious because of dramatic flooding in recent years. This week we got a call asking us if we wanted to buy like new circuit breakers that were flooded with water.  Our polite answer was a screaming “No!”  They were honest and told us the breakers had been flooded.  We will not even consider buying moisture damaged circuit breakers. They could even be brand new circuit breakers and still, no. 

These breakers could be cleaned up cosmetically so they look new and they might even pass the basic electrical tests, but we know they are still junk. If you tore a water damaged breaker completely apart, including the trip device and the operating mechanism, you would find rust and caked on dirt, even mud.  Under the stationary contacts, on the trip device hold down bolts, under the trip element, inside the trip element, everywhere in and under the operating mechanism.

The breaker may pass the insulation resistance tests, the contact resistance tests, and maybe even the over current tests. And, if you don’t bother to take the cover off, you might never know that the breaker was damaged by water or extreme humidity.  We know that plants that have been flooded will go through a thorough switchgear on site reconditioning process. Switchgear, including circuit breakers, will be restored to operating condition in an effort to get the power back on as quickly as possible. But these are emergency situations and getting replacement equipment quickly may not be possible.  So one should not compare such an extreme circumstance with the normal world of a Switchgear Service Shop. 

We want to know the history of equipment we purchase and we can not rely on that information being always accurate. Everything must be completely checked out, reconditioned.  And because equipment passes standardized tests, does not mean it is reliable for reuse.  Reconditioning will determine if it will be reliable after passing the standardize tests and MIDWEST’s other reliability centered tests. But testing alone does not determine reliability.

MIDWEST had a customer that seemed to call us about every 5 to 6 months for a replacement circuit breaker. This went on for about three years, before we noticed the pattern. We recognized it because he started calling the same Engineer.   We asked why he needed so many reconditioned replacement circuit breakers over the past two or three years. It seems their facility was built in the 1970s when copper was so scarce and expensive that many projects were built using aluminum cables.  Unfortunately some installations did not use the correct lugs or failed to install the lugs properly. We were very surprised they didn’t have these problems, or worse, decades ago.  We asked them if they still had a couple of the old damaged circuit breakers laying around.  Could they send us one or more of the old circuit breakers that had been damaged, so we could inspect them and give some recommendations on how to prevent future problems.  Inspection of two of their used circuit breakers that had failed, showed extensive heat and arcing damage at one or more of the load side lugs.  We have seen this type damage many times and it invariably was caused by cables that had become loose inside the lug or lugs that had become loose at the breaker connection.  One of our senior engineers worked in electrical construction during the years aluminum cables were used.  He explained how this could be dangerous and he also was very surprised they didn’t have problems many years ago.  He said a lot of new circuit breakers and MLO, ie main lugs only, panel boards were damaged when the aluminum cable terminations failed in the years after installation. He had one strong recommendation.  Hire an old time electrician who remembers those days and knows from experience what has to be done to correct things.  He said this can be a challenge. But, if they don’t do something, they may lose more than an occasional old circuit breaker. He also said to be extremely careful when installing a replacement circuit breaker. Always turn all the power off, including the feeder into the old power panel or panelboard. Safety first.

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.   

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.