1.18.5 Circuit Protection Devices
A regulatory requirement exists that electrical wires and cables be protected from an over-current condition. Typically, a circuit protection device (CPD) is used to provide this defence. CPDs are designed to protect the wire or cable; that is, they are not designed to protect the associated electrical components, such as line replaceable units (LRU), which may require their own internal CPDs.
The majority of CPDs used in aerospace applications are the resettable thermal CB type developed as a replacement for fuses. These conventional CBs typically contain a circuit consisting of a bimetallic element and two electrical contacts, one of which is spring-loaded. When an over-current condition occurs, the circuit heats as a function of current flow and time. (STI1-104) When the heat exceeds a preset amount, the bimetallic element bends causing the spring-loaded contact to trip and open the circuit. The design is known as a "trip-free" CB in that it cannot be reset in the presence of an over-current condition. After a predetermined interval for cooling, the CB is capable of being manually reset.
This type of CB has proven to be effective in accomplishing its primary role, which is to protect wire and cable from damage owing to an over-current condition. Specifically, this type of CB successfully protects the circuit when the temperature and time duration characteristics of the over-current condition are within the CB's design limits.
However, some types of wire and cable failures involve arc faults. Arc faults can create circumstances that do not fall within the design limits of the over-current/time protection curve of conventional CBs. One such phenomenon is an intermittent metal-to-metal event (conductor-to-conductor or conductor-to-frame) known as a "ticking fault." Such events can generate extremely high temperatures at the location of the insulation failure; however, the current draw may not be sufficient to heat the bimetal element to the temperature necessary to cause the CB to trip. In some cases, a breakdown of wire insulation can lead to other types of arc fault failures, such as arc tracking. The arc-tracking phenomenon involves carbonization of the wire insulation material that can result in intermittent arc faults between conductors, the aircraft frame, or other grounded conducting material.
Although the hazards created by ticking faults and electrical arc tracking are widely known, existing technology is such that there are no CPDs available for use in aircraft that can accurately and reliably detect faults associated with wire insulation breakdown. The USN, the FAA, and aircraft manufacturers are sponsoring initiatives to address this shortfall in CPD technology. The goal is to develop an arc fault circuit breaker device appropriate for aircraft use.
Inconsistencies exist within the aviation industry regarding CB reset philosophies, which have resulted in the evolution of inappropriate CB reset practices. For example, there is a widely held view among flight crew and maintenance personnel that one reset of any tripped CB is acceptable. Consequently, often the first step in troubleshooting a tripped CB was a reset attempt. There is also a view that the reset of a low ampere CB is less dangerous than the reset of a higher ampere CB. However, while the consequences of resetting a low ampere CB may be less pronounced, under the correct conditions an arcing event involving a low ampere circuit could readily ignite a fire. Since it is impossible to know whether these conditions exist in any given situation, a tripped CB should not be reset before any associated fault is located and eliminated.
The adverse consequences of a CB reset may not be universally well understood within the aviation industry. An inappropriate reset can exacerbate the consequences of the initial fault and lead to an arc or arc-tracking event; however, there is no clear regulatory direction to the industry on the issue of CB resets. In AC 25.16, the FAA recommends that all AFMs should contain guidance that states the following:
The crew should make only one attempt to restore an automatically-disconnected power source or reset or replace an automatically-disconnected CPD that affects flight operations or safety.
Precisely what action is expected from this statement is open to interpretation. In addition, there is no regulatory requirement that the AFMs are to inform flight crews of the adverse consequences of CB resets, or to state categorically that no resets are allowed except for a single reset of those systems deemed by the pilot-in-command to be flight essential.
Likewise, the FAA guidance material for maintenance personnel does not address the issue of resetting of tripped CBs. The FAA AC 43.13-1B does refer to the SAE Aerospace Recommended Practice (ARP) 1199, which deals with over-current protective devices. This ARP does not specifically discuss the immediate consequences of resetting CBs; however, it does advise that CBs should not be allowed to develop a "history of tripping."
In 1999, the major aircraft manufacturers summarized their existing CB reset policies by issuing statements to all operators that give clear and unambiguous direction concerning CB resets. However, operators are under no obligation to act on manufacturer recommendations; they are only required to act on requirements imposed by regulators.
Subsequently, on 21 August 2000, the FAA issued a Joint Flight Standards Information Bulletin entitled Resetting Tripped Circuit Breakers in an effort to standardize the industry's approach to this issue. The goal of the bulletin was to ensure that air carriers had training programs and manuals in place for flight crews, maintenance personnel, and aircraft ground servicing personnel, and that these programs and manuals contained company policies and procedures for resetting tripped CBs that reflected the FAA's position on this issue. This bulletin was only applicable to air carriers in scheduled operation, with aircraft having a passenger-seat configuration of 10 or more seats, or a payload capacity of more than 7 500 lb. This bulletin's expiration date was 31 October 2001.
The use of CBs as switches, either by design, or as a consequence of the system's in-service performance, is not recommended. The FAA guidance on this issue is contained in AC 43.13-1B and states "Circuit breakers...are not recommended for use as switches. Use of the circuit breaker as a switch will decrease the life of the circuit breaker."
SAE ARP 1199 expands on the guidance:
As there are no regulatory restrictions preventing the use of CBs as switches, it appears that this guidance is provided as a means to enhance system reliability as opposed to establishing the minimum requirements for system safety.
As certified, and installed on the occurrence aircraft, the original IFEN system design did not incorporate an ON/OFF master switch. The ON/OFF capability was achieved by the installation of the 28 V DC Interactive Flight Technologies (IFT)/video entertainment system 28 V CB. Although it was determined that this configuration was not related to the initiation of the fire, it did have the potential to be problematic, as suggested in the guidance material in the SAE ARP 1199. As modern aircraft use more software-based equipment, it is not uncommon for systems to be designed in this manner. CBs are being used more frequently as switches, as they are viewed as a convenient method of "rebooting" the system when the software gets "hung up."
Although no particular unsafe conditions were validated during the investigation regarding the use of CBs as switches, questions remain concerning the practice, including the potential for frequent "switching" to induce a change in the physical properties of the CB so as to alter its reaction time in the face of an over-current condition. The routine use of a CB as a switch also has the potential to influence an individual's perception about the actual use and function of a CB.
The CB is known in the aviation community to be a simple, long lasting, and reliable component that is designed to provide protection for the aircraft's wires and cables throughout the life of the aircraft. Due in part to its dependability, CB maintenance is usually confined to the replacement of a failed CB.
Typical CB failure modes include welding, erosion of electrical contacts, and contamination.
In addition, the mechanical characteristics of the CB will change when a CB trip mechanism has been inactive for long periods. Such changes could lead to inappropriate performance during an over-current condition, resulting in inadequate protection of the circuit. Routine inspections, including the unpowered cycling of the CB mechanism, can be useful in ensuring the reliability of a CB. Such cycling serves to enhance CB performance by "exercising" the CB trip mechanism and cleaning contaminants from the contact surfaces. Both the FAA and SAE recommend this practice to enhance CB reliability.
 Society of Automotive Engineers Aerospace Recommended Practice (ARP) 1199 defines an over-current as any current exceeding the rated current of the protective device. This includes both overload and short-circuit currents.
 FAA's AC 43.13-1B and SAE ARP 1199 Rev B.