Automatic protective devices (circuit breakers) are provided within aircraft systems to minimize distress to the electrical system and hazard to the aircraft in the event of wiring faults or serious malfunction of a system or connected equipment. Alerting devices provide the pilot with a visual and/or aural alarm to direct the pilot’s attention to a situation that may require an immediate intervention by the pilot.
Circuit Breakers and Alerting Devices:
Good operating practices suggest a popped circuit breaker can indicate that there is a potential problem being protected. The practice of attempting one reset should only be considered if the equipment rendered unusable is considered essential for the continued safety of the flight. Depending on the amperage of the circuit breaker and its location within the circuit being protected, resetting a popped circuit breaker may create a more adverse situation than simply leaving the circuit breaker out. Indiscriminately resetting popped circuit breakers should be avoided.
Crew members are cautioned against pulling circuit breakers on board an aircraft in order to silence an alerting or warning device that may in fact be providing a valid warning or alarm. Examples of such alarms include landing gear warning horn with certain flap/slat combinations, overspeed warnings, ground proximity warning system alerts and washroom smoke detectors. Deactivating the alerting or warning device by pulling circuit breakers compromises or may compromise the safety of flight. Exceptions would be acceptable for an obvious malfunction resulting in continuous erroneous warnings. In these cases, a defect entry in the aircraft journey log book must be made.
The FAA published AC 120-80 “In-Flight Fires” on 1/8/04 which contains the following guidance regarding the FAA’s position on reseting circuit breakers:
Resetting Circuit Breakers In Flight. The FAA reiterates its concern about resetting CBs during flight. Crewmembers may create a potentially hazardous situation if they reset a CB without knowing what caused it to trip. A tripped CB should not be reset in flight unless doing so is consistent with explicit procedures specified in the approved operating manual used by the flightcrew or unless, in the judgment of the captain, resetting the CB is necessary for the safe completion of the flight. A detailed entry in the aircraft’s maintenance log is a proven safety practice for tracking purposes, and may provide maintenance personnel with key information to enable prompt trouble-shooting and effective corrective action on the ground.
Air Carrier manuals and training programs should contain company policies and explicit procedures regarding resetting tripped CBs, both during flight and on the ground. The procedures shown in the manuals used by the air carrier’s crewmembers, maintenance personnel and airplane ground servicing personnel should be consistent with the airplane manufacturer’s guidance. Crewmembers should be reminded that a CB should not be used as a switch to perform procedural functions unless doing so is specified in approved company procedures or manufacturer’s operating procedures.
What are the potential hazards associated with tripped circuit breakers? The FAA has published guidance material that states that circuit breakers are slow acting devices and may not offer sufficient disconnect protection during events such as arc tracking or insulation flashover. Arc tracking is a phenomenon in which a conductive carbon path is formed across an insulating surface. The carbon path provides a short circuit path through which current can flow (e.g., electrical arcing.) The effects of electrical faults can include:
• Component overheating
• Toxic fumes
• Damage to wires, wire bundles, or parts
• Melting of holes in sheet metal parts by faulted, high current feeder cables
• Melting and burning of titanium bleed air ducts by a chaffed, high current feeder cable
• Electromagnetic interference (EMI) with equipment
• The simultaneous and unreasonable loss of both engine driven generators in a two engine airplane
Additional information may be found in AC 25-16, Electrical Fault and Fire Prevention and Protection.
May crewmembers use a CB as an on/off switch?
Since CBs are designed to open an electrical circuit automatically at a predetermined overload of current, they should not be used for day-to-day operational functions because they would not be performing their intended function, which is protection against overloads. Circuit breakers, even those suitable for frequent operation, should not be used as a switch to turn protected items on or off. Exceptions to this procedure should be published and included in an air carrier’s approved maintenance programs and flight operations manuals.
RegulatoryRequirements for Transport Category Aircraft Circuit Protective Devices - 25 CFR 25.1357
Similar requirements exist in parts 23, 27 and 29 for other categories of aircraft
The following (a – g) are the regulations for certifying transport category aircraft from part 25:
- (a) Automatic protective devices must be used to minimize distress to the electrical system and hazard to the airplane in the event of wiring faults or serious malfunction of the system or connected equipment.
- (b) The protective and control devices in the generating system must be designed to de-energize and disconnect faulty power sources and power transmission equipment from their associated busses with sufficient rapidity to provide protection from hazardous over-voltage and other malfunctioning.
- (c) Each resettable circuit protective device must be designed so that, when an overload or circuit fault exists, it will open the circuit irrespective of the position of the operating control.
- (d) If the ability to reset a circuit breaker or replace a fuse is essential to safety in flight, that circuit breaker or fuse must be located and identified so that it can be readily reset or replaced in flight. Where fuses are used, there must be spare fuses for use in flight equal to at least 50% of the number of fuses of each rating required for complete circuit protection.
- (e) Each circuit for essential loads must have individual circuit protection. However, individual protection for each circuit in an essential load system (such as each position light circuit in a system) is not required.
- (f) For airplane systems for which the ability to remove or reset power during normal operations is necessary, the system must be designed so that circuit breakers are not the primary means to remove or reset system power unless specifically designed for use as a switch.
- (g) Automatic reset circuit breakers may be used as integral protectors for electrical equipment (such as thermal cut-outs) if there is circuit protection to protect the cable to the equipment.
Circuit Breaker Maintenance & Application:
A circuit breaker is a device designed to open and close an electric circuit and to open the circuit automatically at a predetermined overload current, without damage to itself. The primary purpose of circuit breakers in aircraft is to provide overcurrent protection for wire and cable and to minimize the danger of smoke and fire.
Correct circuit breaker selection should result in a protective device with the lowest standard rating that will not trip inadvertently. It must interrupt the fault or overload current by disconnecting the faulted line from the power distribution point before any wire or insulation damage occurs.
The nameplate current rating of circuit breakers is a nominal rating for identification and the actual useable rating for a particular application may be considerably different. Most circuit breakers must carry approximately 115% of their rated current indefinitely. This excess is to provide a tolerance for the effects of wear, vibration etc. The instantaneous trip current is usually in the order of ten times the current rating of the circuit breaker. The applicable Military Standard (MS) should be reviewed when determining the actual trip current for a circuit breaker. 26. When selecting a circuit breaker for a particular application all the variables should be considered. These variables include time-current characteristics of the circuit breaker, start-up surges of equipment, wire type, size and location (ambient temperature) and the maximum altitude at which the equipment is likely to operate. The current carrying capacity of a wire varies considerably depending on the application and should be determined using the graphs contained in SAE AS50881.
Both magnetic and thermal type circuit breakers are available however circuit protection in aircraft is primarily provided by thermal circuit breakers which are dependent on temperature rise in the sensing element for actuation. Operation is achieved by deflection of a bi-metal strip that will open the circuit at a pre-determined temperature. Temperature rise in the sensing element is caused principally by the load current however this is affected by ambient temperature which can raise or lower the actual current at which the circuit breaker will trip.
Trip-free circuit breakers are normally used for all aircraft applications. Manual resetting of this type of circuit breaker cannot be effected while an over current circuit fault remains. Non trip-free circuit breakers are used when the application requires over-riding of the tripping mechanism, in an emergency, when the fault still exists. Both types of circuit breaker can be manually operated to both ON and OFF positions with power applied, without damage to the electrical contacts however circuit breakers should not be used as switches unless specifically designed for this purpose.
The trip characteristics of a circuit breaker can change when the trip mechanism has been dormant for long periods. This phenomenon is due to the high internal spring forces which are inherent in most circuit breakers and which can cause a static type of wear to the trip mechanism when in the closed position. Test data suggest that this condition can be prevented by periodically operating the circuit breaker manually with no electrical load, however manual operation should be limited to two or three times a year to avoid excessive dynamic wear of the tripping mechanism.
Where practicable, ‘power in’ and ‘power out’ wires should be physically separated to avoid the possibility of a short circuit negating the effect of the circuit breaker.