a heat source, such as an exhaust manifold, before it enters  the carburetor. [ Figure 4-23 ] Refer to the RFM (see Chapter  5, Rotorcraft Flight Manual) for the specific procedure as to  when and how to apply carburetor heat. In a fuel injection system, fuel and air are metered at the fuel  control unit but are not mixed. The fuel is injected directly  into the intake port of the cylinder where it is mixed with  Fuel Injection Figure 4-24.  An electrical system schematic like this sample is included in most POHs. Notice that the various bus bar accessories are  protected by circuit breakers. However, ensure that all electrical equipment is turned off before starting the engine. This protects sensitive  components, particularly the radios, from damage that may be caused by random voltages generated during the starting process.  4-15 A V I O N I C S B U S B A R A V I O N I C S B U S Panel Position Beacon L I G H T S Starter Battery + 60 - 60 -30 +30 0 A M P Ammeter Avionics relay (Optional Avionics) Trim Instr Lndg Lt Radio Xpdr Clutch B U S B A R Alternator switch Avionics master switch Starter relay Battery relay Left magnetos ADV RET L Right magnetos ADV R Battery switch Clutch actuator (internal  limit switches shown in  full disengage position) Alternator control unit Mag switch Off L R Both Starting vibrator Alternator + F1 F2 Starter switch M/R gearbox press switch Release Hold Engage Clutch switch