Aerospace, Military and Defence
Closing the gap on military circuit breakers
By Chris Leek, product specialist at Charcroft C
omponents are discontinued for many reasons, but a manufacturer of circuit breakers has recently discontinued supplying components for
use in military applications. This decision leaves designers of military equipment and vehicles facing the need to identify an alternative breaker for each application. Circuit breakers are used widely in the military, in generators, radio equipment, missile launch systems, and signal jamming equipment, as well as in armoured personnel carriers and tanks. Tanks in particular integrate a high number of electronics and have hydraulics which are powered by large motors, as it takes a lot of power to turn the turret. So, the circuit breakers used in tanks need to handle large currents.
The replacement challenge An existing aircraft or MIL circuit breaker can be ordered under a manufacturer part number, or under the Military Specification (MS) number, but the solution is not always as simple as finding an identical breaker from a different manufacturer. Certainly, some breakers are unique to the manufacturer which has withdrawn from supplying for military use. The first step in finding a replacement breaker is to see if there is one with the same thermal, thermal magnetic, or hydraulic magnetic technology, and the same physical size. For breakers with current rating from 1 amp to 25 amp, there is often a head-to-head alternative and many hydraulic magnetic breakers can be cross-referred to breakers from the Sensata range.
Initially, the aim is to identify and deliver a drop-in replacement to avoid the need to modify control panels for different mounting methods. Also, matching the size of the breaker and the terminal type can affect the connections and harnesses that connect to the breaker.
So, while the replacement breaker may have the same technology and a similar
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magnetic breakers can be replaced by using a magnetic breaker or, in some cases, by customising a multipole breaker. Unlike thermal or thermal magnetic breakers, hydraulic magnetic breakers are not affected by ambient temperature and offer different trip characteristics to suit specific circuits and equipment. These trip characteristics can allow a hydraulic magnetic breaker to be used as a replacement for an existing thermal or thermal magnetic breaker.
Although non-temperature-compensated breakers are still available, most applications now use temperature-compensated breakers. Designers also prefer a breaker which can be switched on and off, rather than using a non-switchable breaker.
High-vibration breakers needed on armoured vehicles
physical size, voltage and current rating, some breakers have no direct alternative. These breakers include military (MIL) breakers with current up to 500 amp, where it may be necessary to engineer a special breaker as a replacement for the original.
Engineering a solution
Tweaking an existing breaker is more cost- effective, and quicker, than developing a new replacement breaker from scratch. The process of engineering a replacement starts by working with the customer to understand how the breaker is used in the application. Then a breaker with similar trip characteristics, but perhaps with a different size or technology, is identified from the wide range of existing breakers which have different mechanical and electrical options. While the important elements of a breaker are the voltage, current, trip times, and the short-circuit breaking capacity, it is also vital that the replacement breaker can fit the available space. For example, one circuit breaker which was fixed with a neck thread was replaced by a breaker which had the same physical size and current rating, but was fixed to the front panel with two screws.
Standard 2TC circuit breaker can be customised
This adds a design change to the front panel, but still enables the customer to stay within the given space restrictions. A high-vibration version of a standard breaker was
engineered using a 2TC breaker, which was customised by replacing the standard
M12 neck thread, with a 7/16” neck thread. This delivered a high- vibration version of the 2TC, which was already fit for purpose, and in-house vibration testing was used to prove the design theory.
Engineering a replacement is an option for some of the breakers which are no longer available for military use.
Breaker technologies and approvals
Looking at alternative breaker technologies can also deliver a solution. Some thermal-
Another factor is that MIL-STD and VG approvals are often required, and some Airpax breakers use MIL-STD-202 testing and approval for shock and vibration. But for many years now, the MIL industry has been looking at, and using, commercial-off-the-shelf (COTS) breakers which are capable of doing the job. The withdrawal of the manufacturer from supplying breakers for military use may push more OEMs down this route. To complicate the design further, the same manufacturer has also withdrawn from quoting on commercial breakers which would be used in military applications.
As the supply chain for military circuit breakers changes, to find a suitable replacement it is essential to have a thorough knowledge of circuit breaker technologies, and to understand the demands of military applications.
https://www.charcroft.com/ Components in Electronics March 2023 21
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