Along with the tremendous attention paid to the state of aircraft wiring over the past three years, a small side effort has focused on the condition of circuit breakers. The news is mixed. They hold up under conditions and practices that are better avoided.
Circuit breakers, also known as circuit protection devices, generally seem to retain their basic function with years of service, unlike the cracking, chafing, and other problems that can afflict wire over time, and which has been documented in both large and small transport aircraft (see ASW, March 19, 2001 and Jan. 27). And, to be sure, circuit breakers mounted in racks offer less opportunity for the kind of installation mayhem that has been found with wiring, such as wires that run alongside fuel lines, next to heat sources and right alongside sharp metal edges (see ASW, Jan. 27).
Nonetheless, circuit breakers cannot be ignored. They should not be used cavalierly as “on-off” switches, and they should be kept free of dust, dirt, grease and other contaminants that can create a fire hazard. These are the main findings of a recent study conducted at the William J. Hughes Technical Center near Atlantic City, N.J. The center is operated by the Federal Aviation Administration (FAA). The aircraft age-related circuit breaker degradation study (Report. No. DOT/FAA/AR-01/118) was based on detailed examinations of circuit breaker panels removed from a 29-year-old B727 with 58,800 flights and more than 66,000 flight hours, and from a 28-year old DC-10 with about 25,860 flights and 73,600 flight hours. The breakers were examined as installed in the panels, to include by x-ray, and their electrical functioning was tested.
Don’t replace, periodically cycle
The good news is that their electrical functioning, in terms of the usual tests for voltage drop, minimum and maximum trip limit, and overload current trips (a test designed to prevent nuisance trips), were generally within the breakers’ design specifications. The study concluded:
“The circuit breakers in aging aircraft have not exceeded their design limits for protecting the wire from slight increased current levels caused by aging of the electrical distribution system. Based on the results … it is recommended the breakers not be replaced.”
Rather, the study found that periodically cycling the breaker can actually improve its 200 percent lower current overload characteristics and therefore recommended “the breakers not be replaced but periodically cycled on and off.” A Tech Center official explained that breakers can trap debris, so it is important to periodically “make sure they function manually.”
If used as a switch, install a switch
However, the study warned against frequently cycling breakers by using them as the functional equivalent of “on-off” switches. Breakers are designed to handle a certain number of on and off cycles, say 5,000 for a resistive load and 10,000 cycles with no load, but “a switch is typically designed for endurance four times that of [a] circuit breaker.”
A breaker used as a switch can reach the 10,000-cylce limit “pretty quickly on some aircraft,” the Tech Center official said.
The study found one breaker maddeningly continued to pop open during routine preparation for a low voltage test. Upon closer examination, “excessive wear of the latching mechanism” appeared to have been caused by “excessive on and off cycling of the breaker,” the study noted.
If a circuit requires routine on and off action, use a switch, the study recommended.
Clean as you go
The study found that roughly 50 percent of the breakers removed from panels were exposed to dust or small particles, which evidently accumulated over time after aircraft delivery. It was apparent that liquid contamination and copper particles found on the breakers were deposited during maintenance. Overhead panels seemed to provide the best traps for dust and small particles, as these panels had the most of both.
The study team reviewed maintenance manuals and found no clear specifications “that panels be routinely cleaned and protected when exposed to maintenance activity behind or above the panels.” Yet, as the study team noted, contaminated breakers “may develop voltage arc-over, current seepage, internal corrosion, and dry closure problems.”
For these reasons, the report said, maintenance manuals “should include clear instructions on how to protect the panels during maintenance actions.” Moreover, the area behind the circuit breaker panel should be cleaned and vacuumed “at least once a year.” As reported previously in this publication, the areas behind some panels of high-time jets have been found buried under an inch-thick layer of dust – a serious fire hazard in the event of electrical arcing (where the spark is the “match” and the lint serves as “tinder”).
Installation issues
The study found two major types of installation problems. Of the 316 breakers that were evaluated, “many of the lugs contained two wires and had two different size conductors.”
In addition, the study found, “In two cases, a terminal had three wires, and in one case a terminal had four wires … Depending on [the] amp rating, the categories would include signal and power circuits.”
The Tech Center official recounted that for the most part these cases of multiple wires connected to one breaker result from supplemental type certificate (STC) modifications. “It is not necessarily dangerous, but it is not recommended,” he said, explaining, “When you have all this thermal mass on one lug, [it] can throw the circuit breaker out of calibration.”
The Federal Aviation Regulations (FARs) clearly state that each circuit for essential loads will have an individual breaker (FAR 25.1357). The study cited two Society of Automotive Engineers (SAE) aerospace recommended practices, ARP 1199 and ARP 4404, saying both documents imply that each circuit should have its own breaker but neither standard provides any guidance “on when multiple conductors on a single breaker should be used.”
The study said that the SAE committee responsible for the 1199 and 4404 recommended practices should “develop clear guidelines on when multiple circuits on the same breaker should and should not be permitted.”
The study team found numerous instances where the breakers were darkened, indicating excessive heating, as well as the telltale evidence of electrical arcing, indicating loose terminals.
“The darkened areas may be due to chemical oxidation,” a sign of exposure to high temperature.
“Metal oxides are often nonconductive or semiconductive films that increase the resistance or voltage drop of the circuit, in turn increasing the potential for localized resistive heating,” the study said, presenting a “worst-case example” of its concern. Yet maintenance manuals “do not require inspection of circuit breaker panels for signs of overheating or arcing,” the study noted, adding that guidance needs to be provided “on what to do” when these indicators are present.
The study found numerous examples of stripped threads and other problems of wire-to-breaker connectivity, including broken lock washers, missing lock washers, washers against terminals and not lugs, and so forth.
“Aircraft maintenance manuals do not clearly specify that securing hardware should be replaced with identical parts, and there is no established process to permit substitutions until such time that the identical part can be added,” the study concluded.
In brief, the study found plenty of room to reform maintenance practices, and when breakers are used routinely as switches, they should be replaced with purpose-designed switches on a scheduled basis. The industry hopes that arc fault circuit interrupters (AFCI) will offer a leap in electrical system safety, as they will mitigate the hazard of “ticking faults” and will act faster in the face of arcing than is the case for today’s thermally-activated devices. However, AFCI technology is years away from deployment. The recent study of age-related breaker degradation shows the need to upgrade awareness and standards of breaker husbandry today. This complementary activity can improve electrical system safety quite independently of the installed state of breaker technology.
Circuit Breakers 101
There are several different designs of circuit breakers, but the most commonly used type … depends on a thermal-sensing element to release. Typically, the sensing element is a bimetal strip, which opens the electrical circuit at a predetermined calibration point based on a temperature rise caused by load current heating.
The electrical designer must choose a circuit breaker rating that matches the wire size the breaker is trying to protect. If the wire size is too small or too big for the breaker, the heat sink characteristics of the wire will shift the calibration curve up or down, respectively. If the wire is too big, the breaker will respond more slowly to a current overload, thus inadequately protecting the wire. If the wire is too small, the breaker will open earlier than designed, potentially causing electrical system malfunctions.
Although it is generally not recommended, when multiple wires are used on the same breaker … the breaker must be chosen to protect the smallest wire size … and the heat sink characteristics caused by the multiple connectors must be taken into account. This is … one of the principal reasons why wire … is always replaced with the same size wire … in order to prevent safety problems.
Source: Aircraft Age-Related Degradation Study on Single- and Three-Phase Circuit Breakers, November 2002, Report No. DOT/FAA/AR-01/118, p. 8