Electricity Today Magazine Article -- Past, Present and Future Circuit Breakers

The second thing that may have occurred to Thomas Edison after getting his first light bulb turned on was how will I now turn it off? This application is now a large and central consideration of everything we do in electrical systems.

The initial concept is quite easy to understand - current is flowing to an electrical load causing it to operate as designed. We now wish to turn this load off, hence stop the current flow. At lower voltages and currents, a manually operated switch is usually sufficient. Even at these lower levels though, most of us are aware of the potential for arcing when a switch is "teased" (slowly opened or closed). This arcing or ionization of the air between two separating potentials is the primary critical design criteria for a circuit breaker.

A circuit breaker by simple description is any device, which inserted in an electrical circuit will successfully interrupt the current flow on demand. In this definition all manner of switches, contactors and breakers are included.

Air Circuit Breaker

An early example of circuit breaker technology was recently highlighted in the Fall 1999 issue of N.E.T.A World Magazine. The vintage 1898 breaker implemented many of the theories still considered for air circuit breaker operation today. These considerations provided for, and described, at that time are:

- Arcing contacts: These contacts (carbons) which operate in parallel with the main current carrying contacts make first and break last. This allows the main contacts to remain relatively unaffected by the arcing phenomena (burning).

- Main contacts: Mention is made as to some flexibility and adaptability of surface to ensure tight mating contact. This is the key to low contact resistance and I2R losses. There is also mention of copper being the preferred material as opposed to brass and iron. As material technology advances, copper still remains the preferred cost effective choice for most contact applications.

- Close and Latch: As mentioned previously, this function has changed very little over time. The important concern mentioned in the text is relating to a latching mechanism that allows a relatively small force to open the breaker causing it to trip. An important factor stressed is a "sharp and quick break" which springs to "quicken the motion."

- Protection is provided by an electromagnetic coil with an adjustable core. This is very similar to the instantaneous targets of IAC and CO relays. Less iron in the core equates to a higher pickup level, therefore more current to operate. Release of the close latch is promoted as requiring a "very weak solenoid" action.

- To complete the action, a buffer is provided to avoid stress on the mechanism at the end of the opening cycle. I suspect this took the form of a rubber or felt pad.

This methodology of breaking a circuit with many theoretical and technological updates is still in service today. The wiping contact was a major step in increasing current handling capabilities. This wiping of contact surfaces as they mate and separate creates a cleaning action. This removes potential contaminants and surface imperfections. This is usually provided by a rocker action between the stationary and moving contacts. Before we look at the next major advance, we need to discuss the more common and somewhat standardized circuit breaker ratings. It is these ratings, and the need to expand them as power systems increase in size and capacity, that continues to drive new technology today.

Rated Voltage: Nominal phase-to-phase operational voltage.

Rated Current: Maximum continuous operating current (safe rule of thumb design utilizes 80 per cent of this).

Rated Short Circuit Breaking Current: Maximum current level the breaker can safely interrupt.

Rated Short Circuit Making Current: Maximum current level the breaker can safely close on and latch closed.

This air circuit breaker design has a few inherent problems, which limit its application in all cases, at every voltage level. The arc suppression is violent and must be vented to atmosphere. In metal clad applications this requires large cells and, in many cases, safe venting to the outside. This, of course, is not as critical in outdoor applications, but the design does not fit well with exposure to the elements. Secondly, the contacts must have sufficient travel (away from each other) to allow the arc to be extinguished. This distance of travel is directly proportional to the system voltage and current being interrupted.

Oil Circuit Breaker

The medium of choice in the early 1900s was insulating mineral oil. This choice was likely limited by technology at the time. The theoretical benefits of other mediums were explored, but oil was the economical winner. To my surprise, and likely a few others, it has also been widely utilized at lower voltages - in some cases applications at 2400VAC phase-to-phase are not uncommon. This selection was likely driven by choice rather than technology, as air breaker specifications were certainly up to the task at installation date. I am certain marketing also provided some driving force as it does today.

Immersing the breaker contacts in mineral oil greatly reduced the travel required to have the arc extinguished. Anyone who has done an ASTMD877 oil di-electric test (with and without oil in the cup) easily understands the di-electric differences between oil and air. Of course, this is obvious. The benefits of reduced mechanical operating mechanisms, sealed contacts, cooling properties and oil analysis capability among others, have promoted this technology to the multitude of in service applications we see today.

Due to the high di-electric properties of oil, contacts will extinguish the arc in a short distance of travel when separated under load or fault. This provides little change in practical theory from the 1898 vintage unit previously discussed (but at much higher voltages). The energy of the arc is absorbed and cooled in the relatively dense medium of the oil and vented to the atmosphere through an oil block transfer. The residual carbons and gases are usually suspended in the oil and diffuse over time in the overall volume of oil in the tank.

The key to the successful operation of an oil breaker is the quality and operating condition of the contacts, and most importantly, the condition of the oil. In many cases the successful interruption of a high magnitude fault will require filtering (cleaning) of the oil or, in extreme cases, wholesale replacement. Most of us have been exposed to the result of a misapplied or poorly maintained oil breaker in failure mode. The primary contacts are burnt to a condition of very high contact resistance, and the oil will appear as dirty black or have significant carbon "wisps" suspended in the clear oil.

The advent of minimum-oil circuit breakers circumvented some of the problems inherent in the bulk oil breaker and provided a compact package for metal clad applications to 38KVAC. It did not eliminate some of the root problems associated with using a medium which is inherently contaminated every time it is exposed to the arcing phenomena.

Air Blast Breaker

The next development was, and still is, primarily used at the extremely high voltages of 380KV to 800KV. The benefits of air blast technology over oil-immersed equipment are numerous and have been constantly updated since the first in- service units appeared in the 1940s. The elimination of oil and its inherent fire hazard and handling issues were the first advantages. The use of dry compressed air to extinguish an arc has promoted cleaner operation, faster operating times and shorter maintenance periods. Operation is promoted to 800KV with 40KA interrupting normal and 80KA available in special applications. Operating times of two cycles are not unreasonable for this technology.

The arc is quenched in a sealed chamber. When the contacts are separated, a blast of high-pressure air is blown into the chamber to both cool and separate the arc. Restrike is minimized by contact separation. In higher voltage applications, the arc is separated in two stages or series chambers.

SF6 Breaker

The next or possibly parallel step in technology is the SF6 breaker. This breaker utilizes the inherent arc suppression properties of Sulfur Hexafluoride gas. It first appeared for wide in-service use in the late 1960s and early 1970s but had been experimented with since the end of the Second World War.

This medium requires constant monitoring of the gas density, which is accomplished by pressure measurement with temperature compensation. This monitoring normally activates contacts to alarm and activate controls as follows:

- Satisfactory Pressure - Normal Operation
- Low Pressure - Open Under Load
- Loss of Pressure - Block Trip

With all the given benefits, it is hard to understand any reluctance for the widespread use of this technology. To further strengthen this statement SF6 is biologically inert and is treated with no extra concerns than any other gas not normally found in our atmosphere. This is under normal considerations.

The release of SF6 gas and its byproducts to the atmosphere is to be avoided. The equipment typically has a pressure release device bursting disc, to vent to the atmosphere should maximum operating pressure be exceeded. This operates much as an overpressure relief on oil-filled equipment. In the event of catastrophic failure or burnthrough, the gas remains in a plasma state and arc extinction is not realized. In this case, the plasma and byproducts are expelled in a stream when the containment is compromised. When this discharge combines with atmosphere, it becomes extremely corrosive and can deplete oxygen content. Immediate exit from the area and re-entry by qualified and properly equipped personnel is the highly promoted action.

As can be established from the preceding, the benefits of SF6 breakers are not without inherent drawbacks. It is extremely important to insure the properties of the SF6 gas are maintained at all times. The ability to interrupt 63KA fault levels up to 800KV ensure an expanding installed base and ongoing development. Metal clad applications to 36KV are possible, but indoor installations are sometimes avoided due to the SF6 release issues. Gas detectors are available to detect SF6 concentrations as low as two per cent volume in atmosphere.

Vacuum Breaker

The next technology to arrive was a milestone in breaker design. This one change has almost overnight replaced the oil and air breaker in most non-substation applications. It has also driven a huge retrofit market to which we have all been exposed.

The vacuum breaker takes advantage of the simple fact that an electrical arc cannot exist in a vacuum - that is, no ionization can take place to allow the arc to be sustained. This phenomenon was known for some time, but the practical application in breaking large electric currents was not possible until the early 1970s. The breakthrough came with the development of high quality ceramics and exotic materials, which allow for zero crossing current interruption.

The initial design of vacuum breakers had limitations in ampacity and interrupting capacity. The first major application was in medium voltage motor control. In many cases, surge packs and arrestors were recommended and suggested due to voltage rise considerations. This voltage rise is especially prevalent when switching inductive and capacitive loads, due to the phase position relationship between current and voltage (i.e. current zero/voltage maximum).

The development of vacuum breaker technology has vastly improved since inception. Due to the relatively small contact separation requirements, operating mechanisms are also quite tiny. In addition, the contacts themselves are hermetically sealed and virtually maintenance free with 10,000 full load operations considered normal (once per day for 28 years). This has provided an almost 100 per cent application for metal clad applications to 35KVAC. The size of new medium voltage equipment using this technology is truly amazing. The modern design of vacuum breakers now allows for specifications superior to air or oil immersed contacts in almost every area of consideration. The only item to which an outstanding concern may exist is vacuum integrity. Without the integrity of the vacuum, the contacts will not be able to interrupt even-rated current. To my knowledge, other than by routine maintenance tests (out of service), no successful on-line monitoring system has been developed.

The development of this technology will likely progress to higher voltage applications due to its simplistic operation and suggested longevity. At present operation, it is limited to 38KV, except in special applications. There is also a concern with X-ray emissions and as previously noted, vacuum integrity.

Future development will include more use of solid-state devices. We already see extensive use at 5000VAC with special applications to 25,000V for drive and soft-start applications. The technology exists to switch higher voltages and currents, but there is industry reluctance to accept a semiconductor as a means of isolation. New applications include DC utility tie points, low loss DC transmission lines, static var compensators, high-speed transfer switches and others. The future appearance of the drawout, high voltage, solid-state breaker is inevitable. Fault interruption capacity remains a stumbling block.

The breaker, with all its development and advances remains, in final analysis, an automated switch. In this, the criteria for our field tests to evaluate condition are fairly simple. We need to measure and guarantee good conductivity when the breaker is closed. We need to ensure adequate line-to-load isolation when the breaker is open. At all times, the phase voltages must be isolated from each other and the ground. As a matter of course, the breaker must operate mechanically - that is, open and close with no concern for reliability. The other testing required is more of a performance nature related to the specific breaker type and application. The technician with a ductor and suitable insulation tester can evaluate the basic condition of any breaker.

The correct and safe operation of the circuit breaker is critical to electrical transmission and distribution system operation. It is of utmost importance that the scheduled routine and special testing and maintenance of equipment be adhered to. Many times misoperation during system upset is the first evidence of poor condition. This can and sometimes does result in catastrophic damage and loss of electrical service.

Since inception, the breaker has become the key component of the controlled power distribution system. The majority of accidents involving electrical personnel occur when this device is inadvertently left closed, accidentally closes or is misunderstood in its electrical location. How many times have we heard "I thought this was off" or "all breakers are open." The one breaker overlooked, not properly isolated or erroneously closed is all it takes for the catastrophic events to occur which we all strive to prevent. ET

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