Mil-spec is not enough for circuit breaker specification

By Ken Phillips

Engineers who design military vehicles and equipment face the challenge of selecting circuit breakers that protect multimillion-dollar machines while ensuring reliable performance in mission-critical situations.

Unfortunately, many engineers assume that a circuit breaker will provide ideal protection if it meets military specifications, yet the presence or absence of a mil spec is not the most important consideration for selection. In fact, designers should exceed or even ignore mil spec in many circuit-protection applications.

More than fifty active military specifications and hundreds of private industry specifications apply to circuit breakers. Yet understanding the fundamentals of circuit breakers and their operating characteristics is a big help to designers in choosing the circuit breaker best suited for their applications.

Circuit breaker type

Choosing the correct circuit breaker technology is crucial for any application. There are four kinds of circuit breaker technology to choose from:

  • thermal;
  • thermal-magnetic;
  • magnetic; and
  • high-performance.

Thermal circuit breakers are composed of a thermal actuator and mechanical latch. The actuator is made of a bimetal strip or disc that carries the current and responds to heat. Because the two metals have different rates of expansion, heat causes the bimetal to flex until it triggers a spring-loaded latching mechanism. The design can distinguish between safe in-rush and temporary current surges and harmful sustained overloads. A typical thermal circuit breaker will trip within one hour at 140 percent of its rating

Thermal circuit breakers are available with temperature-compensating elements that eliminate sensitivity to ambient temperature and track environmental conditions. Thermal circuit breakers perform consistently under harsh conditions such as shock, vibration, and temperature extremes, which suits them well for military and other demanding applications.

Magnetic circuit breakers operate on a solenoid design and trip almost instantly under short-circuit conditions. Magnetic circuit breakers are most susceptible to nuisance tripping in applications such as motor startup that experience normal current surges. To combat the magnetic characteristics, designers often combine the solenoid coil with a hydraulic delay to enhance the breaker's tolerance to minor current surges. However, when selecting a breaker, designers should remember that even hydraulic-magnetic circuit breakers could trip 'prematurely' during high current surges common to volatile environments such as military vehicles and equipment.

The construction of the magnetic coil and the characteristics of the latching mechanism make magnetic circuit breakers highly susceptible to shock and vibration. Most importantly, magnetic coils generate magnetic fields that can interfere with sensitive electronic equipment. In an aircraft cockpit, where many electronic instruments are vital to the flight, magnetic circuit breakers add an unnecessary risk.

Thermal-magnetic circuit breakers possess characteristics of magnetic as well as thermal circuit breakers. High overcurrents cause the solenoid to trigger the release mechanism rapidly, while the thermal bimetal responds to prolonged low-value overloads and temporary inrush conditions. Thermal-magnetic breakers have a two-step profile trip curve.

High-performance circuit breakers are specifically designed to protect aerospace systems, off-road vehicles, and other critical applications where extreme vibration, excessive mechanical shock, and exposure to harsh environmental conditions cannot hinder performance.

Vibration

Because of their latching mechanism, thermal circuit breakers tolerate vibration well. For this reason, engineers specify thermal breakers extensively for military vehicles and equipment. In addition to circuit breaker type, designers need to determine the amount of vibration tolerance for each application. MS 3320 and MS 14154 require vibration tolerance of as much as 10 gs at 57 to 500 Hz, and mechanical shock tolerance of as much as 50 gs. Even so, in many applications a circuit breaker meeting these mil-spec minimums will not be adequate.

One military contractor, for example, had problems with thermal circuit breakers made to military standards know as MS-3320 and MS-14154. The breakers were installed in a helicopter control panel. The circuit breakers performed during flight, but extreme vibration from firing the helicopter's cannon would nuisance-trip the devices. The manufacturer resorted to mounting circuit breakers on a shock-absorbing panel to solve the problem.

An alternative solution would be to specify MS 3320 and MS 14154 circuit breakers that exceed the vibration specifications defined by the mil spec. Alternative circuit breakers are available that withstand vibration of 15 gs at 57 to 2000 Hz, as well as 75 gs of mechanical shock and random vibration. This spec exceeds the mil specification enables cannon firing without circuit protection interruption.

Temperature

Military vehicles and equipment must operate flawlessly regardless of ambient temperatures. To ensure performance it is critical for engineers to specify the appropriate circuit breaker. Designers should consider that ambient temperature could vary greatly within the same piece of equipment, depending on the location of the circuit breaker. A pressurized aircraft cabin has a stable temperature, for example, while a circuit breaker located close to the skin of the aircraft must be able to withstand extreme temperature fluctuations from varying altitudes. Ambient temperatures on some military aircraft can range from -55 degrees to 125 degrees Celsius within a single flight.

In applications where electronics are located very close together, such as in a cockpit control panel, increases in ambient temperature are common. Thermal breakers are sensitive to changes in ambient temperature. In a cold environment, the circuit breaker has a relatively long delay. Conversely, when exposed to high temperatures, thermal circuit breakers will trip at a relatively low current level. For this reason, military specifications call for temperature-compensated thermal circuit breakers, which have a second bimetal strip connected in the opposite direction to the sensing bimetal. This eliminates concerns about rising or decreasing ambient temperature.

There are many situations in which a non-compensated thermal circuit breaker will better match shifting performance requirements. At 90 C, for example, motor windings need greater protection from currents that could cause overheating than the same windings would need at -20 C. In a cold environment, a longer delay enables motors to break 'frozen' grease on startup, without reacting to the startup power surge.

Current rating and size

A prime consideration in designing military vehicles and equipment is weight and size. In some cases, a circuit breaker with equal or higher-than-mil-spec performance will actually weigh less and have a smaller footprint than a mil spec product of the same rating.

Most aircraft circuit breakers are designed to U.S. military specification MS 3320. This spec was issued more than 20 years ago, and since then advances in aircraft and systems technology have lead to significantly greater demands on electrical generation and distribution. Higher-rated circuit breakers are required to meet these needs. Some fighter aircraft, for example, include high-performance circuit breakers rated for 35 amps that have the same footprint of circuit breakers meeting the mil spec rating of 20 amps. (MS3320).

Military standards change relatively slowly compared to the pace of technology. Circuit breakers are becoming lighter, denser, and they increasingly meet high-performance requirements.

One example of a technology that allows circuit breakers to exceed mil specifications is dual-chamber construction. Most circuit breakers have contacts and sensing elements installed in one chamber. When arcing in the contacts occurs, heat is generated which can affect the sensing element and cause premature tripping. Dual-chamber construction isolates the two functions and eliminates the possibility of unwanted heat transfer from the contacts to the sensing elements. Dual-chamber construction enables circuit breakers to exceed mil spec tolerances in shock, vibration, altitude and endurance while reducing the weight of mil spec product.


Ken Phillips is senior applications engineer at E-T-A Circuit Breakers of Mount Prospect, Ill., a manufacturer of circuit breakers for military and aerospace applications. For more information, contact him by phone at 847-827-7600, ext. 124, by e-mail at kphillips@ etacbe.com, by post at 1551 Bishop Court, Mount Prospect, Ill. 60056-6060, or on the World Wide Web at http://www.etacbe.com/n_america/.


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October 2014
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