THE AIR CIRCUIT BREAKER 50YEARS OFTECHNOLOGY

W. Stuart Jackson, Vice President Instel, Inc., 1025 Thousand Oaks Blvd., Greenville, SC 29607

Abstract

The low voltage air circuit breaker, the work horse of in- dustrial electrical systems, is 50 years old. It has been changed and refined over its history. This paper reviews the development up to the air breaker as it is offered by today’s manufacturers.

Introduction

The low voltage air circuit breaker has beer) the backbone of industrial power systems since the 1940’s. It have been chal- lenged by competitive devices such as fuses, molded case breakers, and more recently insulated case breakers. The draw- out low voltage air breaker is still superior.

It is my purpose to provide an overview of the development of the air circuit breaker from the mid 1940’s io the offerings of today’s manufacturers. As with all products, there are trade- offs in performance and cost. It is quite remarkable that these early work horses are in wide usage providing years of ser- vice, much beyond the original design criterion.

In the 1940’s and ~ O ’ S , air breakers were heavy mechanical devices (still are). They were mostly removable, draw-out, in design. The protective devices have involved from electrome- chanical mechanisms to current day state-of-the-art digital protective systems with communication capabilities. Modern materials and safety features have enhanced their function and reliability. When properly selected, applied, and main- tained, low voltage drawout air circuit breakers remain an ex- cellent investment choice for industrial power systems.

Discussion

Breakers in the 1940’s were offered by several manufacturers. The most popular in the Southeast were Westinghouse, Gen- eral Electric, Allis Chalmers, and ITE. It is interesting to note that the name ITE stood for Inverse Tme Element, taken from the operational characteristic of overcurrent protection. Break- ers were, in most cases, manually dependent in operation; meaning the breaker contacts closed in proportion to the ve- locity of the operator’s movement of the closing handle. Teas- ing the contacts was possible. The breaker assembly consisted of a chassis, controls, an operating mechanism, a levering-in device, various interlocks, and three pole assemblies. Materi- als were steel, copper, and insulating materials (phenolic, as- bestos, molded composition, resin impregnated papers). Like most products of that era, they were over-engineered, a con- cept which is out of favor today. It is interesting that 50 years later, a good proportion of these breakers are still being main- tained and provide a reasonable level of protection.

The protective devices of this era were electromechanical. These devices were direct acting, utilizing thermal, electro- magnetic attraction or electromagnetic induction. By design, they responded to RMS current. Manufacturers claims in the early 1980’s would have you to believe they invented RMS (Root Mean Square) sensing with solid state trip devices. Elec- tromechanical devices and their many designs worked rea- sonably well to open the breaker under a short circuit or over- load. They allowed for time delay adjustment and coordina- tion with other protective devices. These electromechanical series trips, when properly applied and maintained, worked very well. When not maintained, their mechanical design and application as a separate device on each pole proved to trip more often than not; maybe not with electronic accuracy, but meeting the goal of protection.

The most popular early trip design was the oil dash pot. These devices had actually been used in the early 1900’s. The time delay element used adhesive discs restrained by an oil film. This oil film formed a cohesion between two surfaces. This film allowed for short duration overloads, with rupture of the oil film allowing the armature to move upward casing the trip bar to rotate, opening the circuit breaker. The instantaneous function was often accomplished with a spring restraint which would move to trip the breaker at a preset current with no intentional delay.

In the 50’s and 60’s there were refinements by all manufactur- ers. The insulation materials were improved with the addition of glass polyester, ceramic, melamine, and other inorganic insulation materials. Interlocks were improved and closing mechanisms provided closing speeds completely independent of the operator. Integrally mounted current limiting fuses were offered, allowing fault current rating up to 200,000 amperes with single phasing protection. There was a refinement of the early trip devices. The two most common designs were fluid displacementlpiston type and pneumatic. The fluid/displace- mentlpiston type was utilized by General Electric, ITE and oth- ers. These trip units were an improvement over the oil dash pot type, mainly because they used a sealed cylinder and pis- ton. The additional protective function, short time, was added. The overall operation of these trip units was similar to the oil dash pot type. The major difference was in the time delay ele- ment. The long time delay armature had a fixed air gap and a spring for current pickup adjustment. The armature was con- nected through linkage to a piston suspended in a sealed fluid displacement dashpot. The time delay was obtained by the displacement of fluid from one side of the piston to the other. After the armature had completed half of its total operating

0-7803-4090-6/97/$10.00 0 1997 IEEE 1

stroke, the piston entered an unrestrained position of the cyl- inder. This allowed the armature to break free causing it to strike and trip the circuit breaker. A highly responsive check valve allowed the armature to reset quickly, typically in less than one second. The short-time delay was accomplished through a fixed air gap and spring for current pick-up calibra- tion. The armature was connected to a mechanical timer, and this timer held back the armature until the end of the time delay period. The armature was able to move freely so it could strike the trip mechanism causing the breaker to trip. The in- stantaneous trip function worked similarly to the short time except the time delay mechanism was removed.

The pneumatic trip units were designed by Westinghouse. These units operated similarly to the fluid type, but used an air medium in concert with reset, check valves, and rubber dia- phragms.

Both style units were manufactured and offered through the 1970’s and even into the early 1980’s Heat and age deteriora- tion of rubber components were these units’ worst enemy. Because the rubber components were critical to providing time delay, nuisance (premature) tripping could occur. The popu- lar practice of retrofitting low voltage breakers (conversion from older style series trips to solid state types) was a result of trip unit component deterioration.

Interestingly, while these highly refined electromechanical trip units were being offered, these and other manufacturers were developing solid state trips. This began in the late 1960’s. Allis Chalmers Company, now the Siemens Company, introduced the first solid state trip device.

Manufacturers took advantage of technology made available by the development of semi conductors to design trip devices. These solid state trip devices providing a wider range of pickup setting and delay characteristics. General Electric’s Power Sensor electronic trip device offered a new feature, ground fault protection.

The solid state trip device was the brains of the circuit breaker. The earliest units were analog devices. They obtained both their operational power and current sensing from specially designed current transformers mounted around the breaker’s three current carrying paths. The actual tripping action was accomplished with a new invention called a flux shifter. This device is pretty much the same in today’s breakers. The latch is held in place by a permanent magnet while compressing a spring. When the trip unit sends a signal to an internal coil, the magnetic flux is reversed. This allows the spring’s stored en- ergy to be released and trip the breaker.

With the rapid advancement of solid state components and digital signal processing, the 1980’s and 1990’s saw rapid re- finement of solid state breaker protection. The modern air cir- cuit breaker protection. The modern air circuit breaker offers Long Time, Short Time, Instantaneous, Ground Fault, Mul- tiple Time Bands, Rating Plugs, I2t Selection, Zone Selection Interlocking, Power and Energy Monitoring, Harmonics, Wave Form Capture, Communications, Load Monitoring, Under/Over Voltage, Breaker Status, Thermal Memory, and more.

Each of these, in itself, is complex enough to discuss at length. Many, when not understood or programmed incorrectly, will leave your system with a less than desirable level of protec- tion and coordination.

Conclusion

The last 50 years have seen a steady refinement of the 1940’s air circuit breaker. Higher interrupting rating, integral current limiting fuses, dead front construction, quick close mecha- nisms, safety interlocks, and advanced insulation materials all make for an enhanced breaker. The protective devices are more accurate, repeatable, and functionally superior. Air cir- cuit breakers have been challenged by modern molded case and insulated case breakers.

These devices offer many of the protective features of the draw- out air circuit breaker; quite often with a smaller footprint and reduced investment. They have several shortcomings in an industrial environment. They lack a reliable design with no option for repair. Their application is most often fixed mounted and require a complete switchgear outage to troubleshoot or replace. Their high replacement cost and, for some designs, long delivery times quickly overshadow any up front savings. It is not unusual to see replacement prices in the $20,000.00 range for a 3000 amp breaker. Light duty mechanisms which are deemed non-repairable by the manufacturers can easily require complete breaker replacement vs. repair.

There exists a real problem with today’s state-of-the-art solid , state protective devices. This problem is common to low volt- 1

age air breakers and many other protective devices. The speed and sophistication of today’s solid state devices have made ’ available functions and choices limited only to a manufacturer’s design engineer’s imagination. These many functions should be reviewed, setting selection made, devices programmed and tested for operation. This complete procedure does not hap- pen very often, meaning that many systems are placed into service with limited or no program setup. These deficiencies can manifest themselves at commissioning or even years later with an event.

What is the future? One major manufacturer is in the pre-pro- duction states to offer Fault Interruption At Current Zero. This could greatly reduce contact damage. There, most likely, will be more sophisticated real-time load monitoring and setting adjustments; maybe a phone call to the plant engineer warn- ing of an impending trip with the call being placed by his new switchgear.

References

1. NFPA,70B. I

2. “Electrical Maintenance Hints”, Volume 3, Power

3. “Overcurrent Trip Devices for Low Voltage Circuit

4. “Electrical Equipment Testing and Maintenance, A S . Gill.

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Apparatus Maintenance.

Breakers”, Ron Cuthbert.

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