Cirrus DESIGN SR20 Information Manual page 301

Cirrus Design Section 9
SR20 Supplements
Should both alternators fail, the battery will supply system current and
a discharge rate will be indicated on the ammeter. Under these
conditions, depending on electrical system load, the LOW VOLTS
warning light will illuminate when system voltage drops below
approximately 24.5 volts.
Power Distribution
The power distribution system for the SR20 consists of the primary
electrical power bus in the MCU, which distributes electrical power
from the alternators, battery, and external power receptacle to the
airplane systems through the circuit breaker panel and internal circuit
breakers or fuses. The circuit breaker panel main busses (Main Bus 1
and Main Bus 2) and a non-essential bus receives power through 25-
amp circuit breakers on the primary power bus in the MCU. The
Essential Bus in the circuit breaker panel is dual sourced receiving
power from the ESSENTIAL 1 and ESSENTIAL 2 circuit breakers on
the respective Main Bus. Nonessential avionics are powered from
Main Bus 1 through the associated AVIONICS circuit breaker.
Essential avionics is powered from the Essential Bus through the
associated AVIONICS circuit breaker. During normal operation the
essential and non-essential busses operate in parallel, but during
power system failures, the non-essential bus can be disconnected to
provide load shedding of non-essential equipment loads. This load
shedding system is designed to increase emergency operating power
capacity and to decrease pilot workload during emergency situations
by providing the capability to remove all non-essential loads in a single
action.
BAT & ALT Master Switches
Rocker type electrical system MASTER switches for the battery (BAT)
and both alternators (ALT2 and ALT) are installed on the bolster switch
panel. The right switch, labeled BAT, controls the battery contactor.
When the BAT switch is set 'on,' battery power is available to the
airplane electrical circuits. The ALT2 master switch controls the aft,
gear-driven alternator. The ALT master switch controls the forward,
belt-driven alternator.
Normally, all master switches will be ON. However, the BAT switch can
be turned on separately to check equipment while on the ground. To
check or use avionics equipment or radios while on the ground, the
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Original: 09-28-99