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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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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