Exam CFPS Topic 6 Question 73 Discussion

The upcoming (1996) edition of NFPA 1901, Pumper Fire Apparatus, if approved by the organization, will recommend a secondary braking device on apparatus weighing more than
31,000 pounds and require it on units exceeding 36,000 pounds.
Secondary braking devices in the forms of transmission retarders, driveline retarders, and engine brakes, when activated by the driver, apply a retarding force to the vehicle`s drive wheels without the use of friction. These auxiliary braking devices increase safety as well as economy when used properly.
The foundation or friction brakes on an apparatus were designed to fulfill two main requirements--to maintain the vehicle in a stationary position for indefinite periods of time when the parking brake is applied and to bring the apparatus to a standstill in the shortest possible distance without loss of control such as is experienced in a panic stop condition.
The braking duty cycle of fire apparatus on a response is such that the vehicle is accelerated and braked in rapid succession. The friction involved in braking causes heat to rapidly build up in the brake block and drums. This excessive heat can cause a major loss of efficiency in the braking system by affecting the friction surface of the brakes and expanding the drums. Some estimate that brakes that heat to over 520 degrees lose 60 percent of their efficiency. This, of course, increases stopping distance dramatically. On a long downgrade, this results in brake fade. Disc brakes are somewhat less prone to fading, as they dissipate heat better than drum brakes.
Some secondary braking devices provide up to 85 percent of the vehicle`s deceleration prior to the service brake engagement. By keeping the friction brakes cool, they are ready to respond to panic stop conditions, and stopping distances are greatly reduced.
An additional benefit is the reduction of downtime for brake maintenance. Many high-use emergency vehicles require frequent brake lining changes due to premature wear. Using auxiliary braking devices increases lining life by decreasing brake temperatures. The savings on brake maintenance can provide a substantial return on an investment in an engine brake or retarder.