Control switch Fuselage bus Loadmeter Control panel Control circuit breaker Nacelle bus Shunt Power relay Power relay Timer or  cycling unit To other propellers To other pair of blades Deicing power lines Internal or external heating elements Deicing power circuit breaker or limiter Figure 7-28.  Typical electrical deicing system.  Figure 7-29.  Deicing brush block and slip ring assembly.  Brushes Brush block Slip rings which terminate in slip rings and brushes. Flexible connectors  are used to transfer power from the hub to the blade elements. A deice system consists of one or more on-off switches.  The pilot controls the operation of the deice system by  turning on one or more switches. All deice systems have a  master switch and may have another toggle switch for each  propeller. Some systems may also have a selector switch  to adjust for light or heavy icing conditions or automatic  switching for icing conditions. The timer or cycling unit determines the sequence of which  blades (or portion thereof) are currently being deiced, and for  what length of time. The cycling unit applies power to each  deice boot, or boot segment, in a sequence or all on order. A brush block, which is normally mounted on the engine  just behind the propeller, is used to transfer electricity to  the slip ring. A slip ring and brush block assembly is shown  in  Figure 7-29 . The slip ring rotates with the propeller and  provides a current path to the blade deice boots. A slip ring  wire harness is used on some hub installations to electrically  connect the slip ring to the terminal strip connection screw.  A deice wire harness is used to electrically connect the deice  boot to the slip ring assembly. A deice boot contains internal heating elements or dual  elements.  [Figure 7-30]  The boot is securely attached to the  leading edge of each blade with adhesive. Icing control is accomplished by converting electrical energy  to heat energy in the heating element. Balanced ice removal  from all blades must be obtained as nearly as possible if  excessive vibration is to be avoided. To obtain balanced ice  removal, variation of heating current in the blade elements  is controlled so that similar heating effects are obtained in  opposite blades. Electric deicing systems are usually designed for intermittent  application of power to the heating elements to remove ice  after formation but before excessive accumulation. Proper  control of heating intervals aids in preventing runback,  7-19