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349 

Federal Aviation Administration, DOT 

§ 25.1353 

E

LECTRICAL

S

YSTEMS AND

E

QUIPMENT

 

§ 25.1351

General. 

(a) 

Electrical system capacity. The re-

quired generating capacity, and num-
ber and kinds of power sources must— 

(1) Be determined by an electrical 

load analysis; and 

(2) Meet the requirements of § 25.1309. 
(b) 

Generating system. The generating 

system includes electrical power 
sources, main power busses, trans-
mission cables, and associated control, 
regulation, and protective devices. It 
must be designed so that— 

(1) Power sources function properly 

when independent and when connected 
in combination; 

(2) No failure or malfunction of any 

power source can create a hazard or 
impair the ability of remaining sources 
to supply essential loads; 

(3) The system voltage and frequency 

(as applicable) at the terminals of all 
essential load equipment can be main-
tained within the limits for which the 
equipment is designed, during any 
probable operating condition; and 

(4) System transients due to switch-

ing, fault clearing, or other causes do 
not make essential loads inoperative, 
and do not cause a smoke or fire haz-
ard. 

(5) There are means accessible, in 

flight, to appropriate crewmembers for 
the individual and collective dis-
connection of the electrical power 
sources from the system. 

(6) There are means to indicate to ap-

propriate crewmembers the generating 
system quantities essential for the safe 
operation of the system, such as the 
voltage and current supplied by each 
generator. 

(c) 

External power. If provisions are 

made for connecting external power to 
the airplane, and that external power 
can be electrically connected to equip-
ment other than that used for engine 
starting, means must be provided to 
ensure that no external power supply 
having a reverse polarity, or a reverse 
phase sequence, can supply power to 
the airplane’s electrical system. 

(d) 

Operation without normal electrical 

power.  It must be shown by analysis, 
tests, or both, that the airplane can be 
operated safely in VFR conditions, for 
a period of not less than five minutes, 

with the normal electrical power (elec-
trical power sources excluding the bat-
tery) inoperative, with critical type 
fuel (from the standpoint of flameout 
and restart capability), and with the 
airplane initially at the maximum cer-
tificated altitude. Parts of the elec-
trical system may remain on if— 

(1) A single malfunction, including a 

wire bundle or junction box fire, can-
not result in loss of both the part 
turned off and the part turned on; and 

(2) The parts turned on are elec-

trically and mechanically isolated 
from the parts turned off. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–41, 42 FR 36970, July 18, 
1977; Amdt. 25–72, 55 FR 29785, July 20, 1990] 

§ 25.1353

Electrical equipment and in-

stallations. 

(a) Electrical equipment and controls 

must be installed so that operation of 
any one unit or system of units will 
not adversely affect the simultaneous 
operation of any other electrical unit 
or system essential to safe operation. 
Any electrical interference likely to be 
present in the airplane must not result 
in hazardous effects on the airplane or 
its systems. 

(b) Storage batteries must be de-

signed and installed as follows: 

(1) Safe cell temperatures and pres-

sures must be maintained during any 
probable charging or discharging con-
dition. No uncontrolled increase in cell 
temperature may result when the bat-
tery is recharged (after previous com-
plete discharge)— 

(i) At maximum regulated voltage or 

power; 

(ii) During a flight of maximum dura-

tion; and 

(iii) Under the most adverse cooling 

condition likely to occur in service. 

(2) Compliance with paragraph (b)(1) 

of this section must be shown by test 
unless experience with similar bat-
teries and installations has shown that 
maintaining safe cell temperatures and 
pressures presents no problem. 

(3) No explosive or toxic gases emit-

ted by any battery in normal oper-
ation, or as the result of any probable 
malfunction in the charging system or 
battery installation, may accumulate 
in hazardous quantities within the air-
plane. 

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350 

14 CFR Ch. I (1–1–24 Edition) 

§ 25.1355 

(4) No corrosive fluids or gases that 

may escape from the battery may dam-
age surrounding airplane structures or 
adjacent essential equipment. 

(5) Each nickel cadmium battery in-

stallation must have provisions to pre-
vent any hazardous effect on structure 
or essential systems that may be 
caused by the maximum amount of 
heat the battery can generate during a 
short circuit of the battery or of indi-
vidual cells. 

(6) Nickel cadmium battery installa-

tions must have— 

(i) A system to control the charging 

rate of the battery automatically so as 
to prevent battery overheating; 

(ii) A battery temperature sensing 

and over-temperature warning system 
with a means for disconnecting the 
battery from its charging source in the 
event of an over-temperature condi-
tion; or 

(iii) A battery failure sensing and 

warning system with a means for dis-
connecting the battery from its charg-
ing source in the event of battery fail-
ure. 

(c) Electrical bonding must provide 

an adequate electrical return path 
under both normal and fault condi-
tions, on airplanes having grounded 
electrical systems. 

[Amdt. 25–123, 72 FR 63405, Nov. 8, 2007] 

§ 25.1355

Distribution system. 

(a) The distribution system includes 

the distribution busses, their associ-
ated feeders, and each control and pro-
tective device. 

(b) [Reserved] 
(c) If two independent sources of elec-

trical power for particular equipment 
or systems are required by this chap-
ter, in the event of the failure of one 
power source for such equipment or 
system, another power source (includ-
ing its separate feeder) must be auto-
matically provided or be manually se-
lectable to maintain equipment or sys-
tem operation. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–23, 35 FR 5679, Apr. 8, 
1970; Amdt. 25–38, 41 FR 55468, Dec. 20, 1976] 

§ 25.1357

Circuit protective devices. 

(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 de-

vices in the generating system must be 
designed to de-energize and disconnect 
faulty power sources and power trans-
mission equipment from their associ-
ated 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 com-
plete circuit protection. 

(e) Each circuit for essential loads 

must have individual circuit protec-
tion. However, individual protection 
for each circuit in an essential load 
system (such as each position light cir-
cuit 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. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–123, 72 FR 63405, Nov. 8, 
2007] 

§ 25.1360

Precautions against injury. 

(a) Shock. The electrical system 

must be designed to minimize risk of 
electric shock to crew, passengers, and 
servicing personnel and to mainte-
nance personnel using normal pre-
cautions. 

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