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Archive for August, 2010

3 Second Sophisticated Circuit Breaker Test

August 23rd, 2010 No comments

Here is another MIDWEST real world experience. This time dealing with circuit breakers, specifically three old power air circuit breakers.  Because of the extremely difficult location of the breakers, getting equipment to test the old circuit breakers was nearly impossible. And the cost would have been astronomical. There seemed to be serious problems with the circuit breakers. These were bolted in place generator breakers and the switchboard they were in had to provide power at all times.  A short outage could be catastrophic. But the breakers were tripping and the generator operators needed to know if the breakers were the problem or if they had load problems somewhere in their power system. The circuit breakers had old oil dashpot type over current devices and the operators wanted to know if retrofitting the circuit breakers with new electronic over current devices would make their problem go away. So, during a rather tense meeting, MIDWEST recommended the following three tests on two poles of each breaker.  The first test would take about 3 seconds. The second test would take about 8 seconds. And the third test could take as long as 90 seconds. We thought these tests would be nearly 100% as effective as actual high current testing in determining the condition of the dashpot over current devices. These were not calibration tests. They were function tests. The first sophisticated test procedure was to look at the oil dashpots. If we saw signs of oil weepage out of the circuit breaker oil filled dashpots, the breaker over current protective devices, ie the oil dashpots, were deficient and the breaker should be retrofitted. Simple. The second test involved manually pushing the instantaneous lever of the old oil dashpots.  If the action and resistance was normal and the dashpot reset properly, no nuisance problem.  The third and last test was to simulate an over current condition by a slow steady push on the delay component of the dashpots on the outside phases. For a veteran in the maintenance, repair and testing of these old air circuit breakers, these checks are very simple and very reliable in detecting defective oil dashpots. The actual test time per circuit breaker took us less than 45 seconds. We were not calibrating anything, but we did determine that all three circuit breakers had defective over current protective devices and needed to be retrofitted with new electronic over current protection. This was done and their problem was solved. How the retrofitting was done is a good subject for another blog. By the way, these circuit breakers were located in Antarctica.

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.