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Archive for December, 2010

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.       

Scrap Circuit Breakers

December 23rd, 2010 Comments off

 

ED3100 Cutler Hammer Circuit Breaker

ED3100 Cutler Hammer Circuit Breaker

This morning I walked past three drums of old, obsolete, used circuit breakers. All these breakers were going to the scrap heap. Actually they go to a recycling company that crushes them and recycles the metal parts. If someone was to ask what we do with the defective circuit breakers, this would be the answer.  These drums contained Square D, Westinghouse, Cutler Hammer, Siemens, GE General Electric circuit breakers and probably many other manufacturers. If a circuit breaker is mechanically or electrically defective, if it doesn’t pass our inspection and testing quality control, it gets tossed into the scrap heap, actually drums. 

If you were to look at the breakers in the drums, you would find many that look in perfect condition, some even mint. But, if you tore them apart, you might find the contacts blasted beyond any possible repair. This is found with an internal inspection, contact resistance tests, or by a voltage drop test during over current testing.  One might be surprised how many old and even new surplus circuit breakers are rejected based on the visual inspection. And, again, you would be amazed at how many new looking circuit breakers fail one or more of the quality control tests.

No one manufacturer or model, whether Square D, Cutler Hammer or GE General Electric circuit breakers, is exempt just because of the name on the label. And no breaker can be evaluated just based on its appearance. Sometimes a circuit breaker may fail a test and we really have to look hard to find the cause of the defect. Sometimes the defect may not be visible, for example a defective over current trip device.

 

So you can’t judge the condition of an old or new surplus circuit breaker by its appearance. MIDWEST is always testing new circuit breakers as part of our Acceptance Testing Services on new switchgear, a common practice for hospitals, data processing centers, and mission critical facilities. And we do find deficiencies in brand new circuit breakers.  So again, appearances can be deceptive. 

100% Rated Square D 2000 Amp Circuit Breaker

December 15th, 2010 Comments off

MIDWEST lost a battle with the maintenance dollar.  A manufacturing plant had a 2000 amp Square D circuit breaker that was loaded to over 1800 amps, sometimes hitting 1900.  We detected the heavily loaded breaker during an Infrared Scan of the facility.  This was an old Square D circuit breaker mounted in a tap box feeding a 2000 amp bus duct. The local contractor recommended they just replace the breaker with a 100% rated breaker. He said they could use Square D, Cutler Hammer, or General Electric, whoever was cheapest and would fit. Because of the bus configuration, it only made sense to use the same Square D circuit breaker. The customer bit on the idea that just replacing the old circuit breaker would solve their problem and save them a lot of money. We strongly disagreed. Replacing the circuit breaker would make zero difference, in this case. The old Square D molded case circuit breaker was an 80% rated breaker.  More recent replacement Square D circuit breakers, same frame, model number, current rating, were 100% rated, ie 2000 amps. The 100% rated replacement circuit breaker was specified 100% if it was in an enclosure with a much larger volume than the old breaker. In other words, to achieve the 100% rating, the breaker must be in a much larger enclosure so as to properly dissipate the heat generated from 2000 amps. This all makes sense. But to just replace one breaker, whether GE General Electric, Cutler Hammer, Siemens, or Square D, with a 100% rated breaker and not address the installation requirements to achieve the higher rating can be a waste of dollars.  A quick check of the old and new circuit breaker specifications, will tell what the installation requirements are. This is the technical added value that is often ignored, or just not known, in too many facilities, as experienced personnel disappear.

            This facility decided to replace the circuit breaker, but later transferred load off the bus duct.

Circuit Breakers in Switchboard Buried in Sand

December 6th, 2010 Comments off

Sometimes MIDWEST runs into switchgear and circuit breakers in such harsh environments that you would wonder how they don’t blow up, much less work properly.  An example is some switchgear and old circuit breakers found in foundry environments. The condition of electrical equipment in foundries is 100 times better than 25 years ago. But there is still one thing that has not changed for some foundries and that is sand in electrical switchgear. Some foundries still have their main panel boards and some switchgear in open foundry areas, rather than in clean positive pressure rooms.

 

We recently were called in to repair a 2000 amp circuit breaker used in an open foundry environment. It turned out the old circuit breaker was not a breaker at all, but rather was a 2000 amp bolted pressure switch. The electrical switchboard had over 6 inches of sand in the bottom and 3 or 4 inches on top. The main horizontal bus feeding the risers for the circuit breakers, was partially buried in foundry sand. The service technicians said they actually scooped the sand out before even trying to use vacuum equipment. Fortunately the sand didn’t carry anything with it that acted as a conductor. This isn’t always true. In this case, the sand was just more insulation.

 

Maintaining the bolt lock switch and the circuit breakers was a nasty job. The covers had to be removed from every breaker to clean the operating mechanism and to get the sand out of the contact and arc chute area. And all our efforts were only temporary since the environment was unchanged. More serious was the fact that foundry dust would be inside the over current trip devices of the circuit breakers. Therefore the operation of the trip devices was unreliable, even unsafe. It wouldn’t make any difference whether these old circuit breakers were Square D, Westinghouse, GE General Electric or Cutler Hammer. Foundry dust and sand doesn’t care who the manufacture is. Even a brand new circuit breaker would be a victim to the sand.

The illusion was the circuit breakers were okay because they didn’t trip. It was only when the owner tried and failed to operate the main switch did they realize that maybe the panel board and breakers needed some attention. This was not the first, nor will it be the last, switchgear, panel board, or circuit breakers that we find basically buried in sand.