SECONDARY COCKPIT LIGHT SYSTEM WITH 17
The secondary cockpit lighting system provides lighting
for the overhead panel, instrument panel, and console.
The lighting system consists of eight floodlights, three
relays, two toggle switches, and a dimming control. The
relays and toggle switches are in the overhead panel.
Two of the floodlights are on the overhead panel and six
are on the instrument panel. An interlock turns on the
floodlights if ac power or No. 1 dc bus were to fail. An
ac power failure disables the normal instrument lights.
The secondary cockpit lighting system operates from
the 28 volt essential dc bus.
When INSTR FLOODLIGHTING circuit breaker
is closed, 28 vdc power from the essential dc
bus is connected to FLOOD DIM/BRT control.
When the control is turned from OFF, power is
connected through the control to pins 2 of INST
FLOOD and OVHD FLOOD light switches. When
either switch is set to ON, power is applied to the
respective floodlights. FLOOD DIM/BRT control
provides on-off and brightness control for all
secondary cockpit lights.
During normal operation, transfer relay K1 is
operated when 115 vac is available from No. 1
ac bus. Floodlight relay K2 is operated from 28
volt No. 1 dc bus through NO. 1 DC BUS CONT
circuit breaker and contacts of relay K1.
If 28 vdc is lost from No. 1 dc bus, relay K2
releases. If 115 vac is lost from No. 1 ac bus,
relay K1 releases, causing relay K2 to release.
When the pilots INST DIM/BRT control on the
PLT LTG panel is turned from OFF, control relay
K3 is operated by 28 vdc from the dc essential
bus through the COCKPIT LIGHTS DIM CONT
circuit breaker. Relay K3 arms a bypass circuit.
If the FLOOD light switches were at OFF in
an ac power failure, relay K3 bypasses the
open FLOOD light switches and turns on the
floodlights. Power to bypass the open switches is
routed from the FLOOD DIM/BRT control through
closed contacts A1-A2 of relay K3 and normal
contacts D2-D3, C2-C3, and B2-B3 of relay K2.