Key Takeaways
- The thermal circuit limiting fuse is a simple and reliable means of circuit protection that has been incorporated in electrical systems for a century.
- Qualification testing of fuses relies on the ability to isolate and identify the stressors that affect the component.
- The MIL-F-5372D guides qualification testing of fuses.
The thermal circuit limiting fuse has been a stable part of circuit protection for a century. The reason for this is that its high reliability for circuit protection needs has made it ideal for many applications, particularly those where resetting the circuit protection does not require an immediate override. In the case of personal protection, isolated equipment, and non-mission critical systems, this single-shot circuit protection device is reliable due to its simplicity. A single filament is connected between two terminals and isolated in an enclosure. The sizing of this single filament is based on known performance characteristics and can be sized quite precisely for an application.
Verification of such devices is straightforward and a good example of certifying such a device is included in MIL-F-5372D (Fuse, Current Limiter Type, Aircraft). This military specification for aircraft current limiting fuses has been around since the middle of the last century and guides the circuit protection qualification. Important factors to take from the assessment include the types of tests for fuse verification, how they are grouped, and how these might be updated to address the needs of modern aircraft design.
First and foremost, thermal fuses, when part of the power distribution network, are part of the aircraft EWIS. Though, fuses that are installed within devices, such as LRUs, fall outside of the EWIS definition as defined by 25.1701. However, even those devices that are not part of the EWIS still should have many of the same performance characteristics and tests performed for verification purposes.
Assessment Types
MIL-F-5372 outlines 12 different test groups for fuse verification. Within these groups, the following test types are included:
Shown here is a standard fuse. Within the cylinder is housed the single connecting filament which will melt when its thermal limit is reached, thus disconnecting the circuit.
Electrical
- Voltage drop
- Rated current assessment
- Overload damage
- Time current testing
- AC interrupt characteristics
- DC interrupt characteristics
Environmental
- Moisture resistance
- Thermal shock
- Vibration
- Salt spray
- Temperature rise
- Explosion proof
- Mechanical shock
- Sand and dust
- Ambient temperature influence
Some of the tests are straightforward and those familiar with the aerospace testing techniques are likely familiar with them. A couple of these tests are discussed more thoroughly below.
Fuse Life
The fuse life test is as follows:
“The limiter shall be inserted into the specified limiter holder and subjected to a 1000-hour life test at any convenient AC or DC voltage, consisting of 500 hours of continuous operation at each of the ambient temperature extremes of negative 54° c and 125° c. While at each of these temperature extremes, the limiter shall carry rated current at any convenient AC or DC voltage continuously for 250 hours and then be subjected to 2,500 on and off cycles with each cycle consisting of 3 minutes at rated current and 3 minutes at no current. The temperature of the limiter bodies shall be monitored continuously and the temperature rise determined and recorded. The maximum temperature shall not exceed 218° c. Time current measurements performed subsequent to this test shall be distributed evenly at 500 and 1,000% of the current rating and shall be conducted at 28th volts DC. The time current test method shall be in accordance with 4.8.13.”
Naturally, the expectation is that the current limiting fuse will perform without interruption within its normal operating temperature range without failure or opening the circuit. This is an important fact of fuse design as ensuring that the fuse does not trip at the rated current is critical for circuit design. If the fuse degrades and has a more rapid failure or opening of the circuit, this could have a significant impact on system reliability, and create unwanted regular maintenance activities for the system and question.
Overload Damage
Another test to highlight is the overload damage test:
“Using the equipment suitable for the time current test of 4.8.13, with provisions added to interrupt current ads predetermined times before limiter damage, apply the test current for the time shown on the applicable time-current curve for 70% of the melting current at 200, 500, and 1000% of the current rating. The test shall be conducted at 28 volts DC, or a higher voltage at the manufacturer’s option. With the limiter stabilized at the ambient temperature of 125° c, apply each of the test currents one at a time allowing a minimum of 10 minutes between applications. Stabilize the limiter at room temperature and perform the calibration test of 1000% of rating. Failure will be indicated by inability of the limiter to complete the above procedure or to melt at a time outside the calibration requirement of 4.8.13”.
Whereas the life test seeks to stress the fuse thermally, the overload damage test seeks to stress the fuse electrically. This test method is appropriate for any circuit that may have an inductor in line. For example, many electric motors have a very high inrush current, and many of those motor starter circuits include large fuses. The inrush current is over the fuse rating, however, as it is less than the time necessary for this circuit to open and the fuse the trip, the circuit remains operational without interruption. For those looking to verify fuse performance, those with specific applications, and/or desire higher reliability of their fuses, it may be advantageous to run a greater number of overcurrent cycles to verify fuse performance.
Test Groups
Fuses are designed such that once there is an overload condition, the fuse quickly opens and protects the circuit. Even with aging and environmental stressing, the expectation is that the trip curves only change slightly (if at all).
The qualification of aircraft fuses per MIL-F-5372 is more analogous to wire qualification than connector qualification. In the test groups defined within the standard, there are multiple tests in which the fuse is typically exposed to a single stressor, such as thermal shock, or vibration, and then assessed on its after-stress performance. Most wire qualification tests are done in a similar way avoiding the development of large matrices for various stressors to be done simultaneously and/or multiple sequential stressors on the same sample. The benefit of this approach is that it helps to identify specific weaknesses of a component without getting lost in multiple stages and being uncertain as to what stressor created the ultimate failure condition.
The one disadvantage of this approach is that, while it does help to isolate parameters, it makes it difficult to have confidence in the component’s performance in the extreme conditions of multiple scenarios. For example, if the fuse were to be used in a vibrational environment at the maximum rated level, at an elevated temperature, and subject to multiple overload conditions, the specific performance characteristics would be uncertain. This leaves the OEM engineers to select a product that is at or beyond the performance characteristics of the aircraft, and then perform testing to ensure the reliability under such conditions. This is only practical, as the alternative to this approach would limit part selection to those that have excessive testing and thus a very high price tag.
Conclusion
Aircraft electrical fuse testing follows the same overall test planning and assessment as other EWIS components, but with the specific testing that is appropriate for the device. The testing that a fuse should undergo before being fielded, includes electrical, mechanical, chemical, and thermal conditions that are representative of the aircraft in question. For those seeking to use fuses near the performance level of multiple parameters, caution should be taken to ensure that their liability is not overstated, and/or laboratory testing should be performed to verify component performance under severe service conditions.
Lectromec’s ISO 17025 accredited lab is ready and available to help our clients with their various EWIS component test needs.
Michael Traskos
President, Lectromec
michael.traskos@lectromec.comMichael has been involved in wire degradation and failure assessments for more than two decades. He has worked on dozens of projects assessing the reliability and qualification of EWIS components. Michael is an FAA DER with a delegated authority covering EWIS certification and the former chairman of the SAE AE-8A EWIS installation committee.