Type Rating Knowledge — Cessna 172S NAV III

Electrical system

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Three sources, one 28-volt system

The C172S NAV III carries a 28-volt direct current electrical system. In normal operation a belt-driven 60-ampere alternator powers everything and holds the system at about 28 volts. A 24-volt main battery, mounted inside the engine cowling on the left firewall, starts the engine and backs the alternator up; both are controlled through the MASTER switch. A third source — a small standby battery between the firewall and the instrument panel — exists for one job only: keeping the essential bus alive if the alternator and the main battery have both failed. The system’s relays, the alternator control unit and the external power connector all live in a power distribution module on the left forward side of the firewall.

C172S electrical system (NAV III, GFC 700)EXT PWRMAIN BATT 24VALT 60ASTBY BATTPWR DISTRIBUTIONELECTRICAL BUS 1ELECTRICAL BUS 2AVN SW 1CROSSFEED BUSAVN SW 2AVIONICS BUS 1ESSENTIAL BUSAVIONICS BUS 2
  • EXT PWR — External power receptacle — ground power for engine starts and servicing, connected on the main battery side.
  • MAIN BATT 24V — 24 V main battery, mounted on the left forward firewall.
  • ALT 60A — 60 A belt-driven alternator — the primary power source in normal operation; holds the system at 28 V.
  • STBY BATT — 24 V standby battery — powers the essential bus for at least 30 minutes when main bus voltage drops below 20 V.
  • PWR DISTRIBUTION — Power distribution module — alternator and battery power meet here and feed the two primary buses.
  • ELECTRICAL BUS 1 — Electrical bus 1 — one of the two primary distribution buses.
  • ELECTRICAL BUS 2 — Electrical bus 2 — the other primary distribution bus.
  • AVN SW 1 — Avionics master switch 1 — gates power from electrical bus 1 onto avionics bus 1.
  • CROSSFEED BUS — Crossfeed bus — ties the two primary buses together.
  • AVN SW 2 — Avionics master switch 2 — gates power from electrical bus 2 onto avionics bus 2.
  • AVIONICS BUS 1 — Avionics bus 1 — powers its share of the avionics behind avionics master switch 1.
  • ESSENTIAL BUS — Essential bus — keeps flight-critical equipment powered; fed from both primary buses and, in an emergency, the standby battery.
  • AVIONICS BUS 2 — Avionics bus 2 — powers its share of the avionics behind avionics master switch 2.
Redrawn from POH 172SPHBUS §7, Electrical System — structure restated, not traced. C172S NAV III with G1000 and GFC 700 (serials 172S10468 and on).