The Apparatus Architect: Part 25 - Designing Rescue Squad Apparatus

One of the most important fixed components on any rescue vehicle is the line voltage generator system. The generator and 120-volt lighting system that is designed into the rescue truck will provide much-needed scene support on the fireground and other emergency incidents. The type of generator and kilowatt output rating will greatly influence the overall design of the rescue truck and, depending on the make and model of generator system, may impact other operational features on your new unit.

The National Fire Protection Association (NFPA) Standard 1901 offers excellent guidance covering the design and proper installation of line voltage electrical systems; see Chapter 23 and the appendix of the standard. There is no specific requirement for the size or number of generators that should be carried on truck company or special service units. With this in mind, we will offer practical advice on what fire departments should consider when outfitting rescue or squad apparatus.

One unique aspect of rescue squad and special service units is that there is relatively little guidance with respect to what units are expected or need to carry for equipment. In some parts of the country, such as the metropolitan Washington, D.C. area, several county fire departments have detailed standards on what equipment is to be carried on rescue squad and rescue engine apparatus. Beyond this, there are some locations that have are specific equipment requirements for urban search and rescue (USAR) units and hazmat apparatus. If your department is just getting started with designing a new rescue apparatus, take the time to obtain an inventory list from another department or jurisdiction to see what type and quantity of tools and equipment should be carried.

Portable gasoline-powered generators should be carried on every special service apparatus. Smaller 500- to 2,000-watt-output units equipped with lamp heads can be used to illuminate remote areas as well as provide power for smaller electrical equipment. Consideration should be given to carrying one or two of these types of light/generator plants together with a larger-output portable generator. Midsize portable generators can provide 2,500 to 6,000 watts of power and, depending on size, can be frame mounted or provided with rubber tires. These units can be particularly useful when you need to provide a remote power supply at a scene without tying up an entire apparatus for extended periods.

Diesel generators are commonly available in sizes from six to 15 kilowatts and provide a reliable source of power; however, they weigh in the area of 500 to 900 pounds and take up a considerable amount of room within a compartment. When they are mounted on the floor of a side-body compartment, make sure that there is adequate ventilation for the unit to run; if using a larger-output unit, you will have to balance out the body with other equipment on the opposite side of the unit. Diesel generators offer the advantage of being able to produce power when the vehicle is in motion as well as running when the apparatus is shut down. These features may be beneficial to the department, depending on operating conditions at the scene of an incident.

Power take-off generators work well with rescue vehicles in that they generally do not take up outside compartment space on the apparatus. These generators come in sizes from 15 to 40 kilowatts and can be powered from one of the PTO openings on an Allison automatic transmission. Larger output generators in the 50- to 150-kilowatt range are run by transfer case gearboxes similar to a fire pump. These high-capacity units are typically used on air/light units or special service units, which operate electrically driven breathing air compressors or other high-demand components.

The power take-off generators are located between the chassis frame rails and occasionally may protrude above the frame level into a compartment. Adequate protection needs to be afforded around the generator frame from road hazards and care must taken to avoid running these units through high water during periods of flooding. These types of generators require the vehicle engine to be running and cannot produce power while the vehicle is in motion. Care must be taken when powering motor-driven equipment, that these units be protected by a time-delay circuit or power switch to prevent the generator from starting under load.

Hydraulic generators, while generally more expensive than their diesel or power take-off counterparts, are becoming more common on all types of apparatus. Smaller six- to 15-kilowatt units work well on pumpers and aerial ladders with larger 20- to 30-kilowatt generators being used on rescue vehicles. Hydraulic generators offer the advantage of being able to produce their rated output within the full range of engine rpm as well as producing power while the apparatus is in motion. This enables units equipped with hydraulic generators to check street addresses or provide scene illumination while the apparatus is moving. The hydraulic generator has no engine, fuel supply or exhaust system and can operate over a wide range of engine speed, where the power take-off generator must run at a constant speed. For this reason, apparatus configured with both a fire pump and generator system lend themselves to using hydraulic generator equipment.

The use of both 120-volt fixed-body lighting and telescopic light towers can produce a vast amount of illumination at the scene of an incident. Consideration should be given to the total demand of vehicle lighting together with the demand for other motor-driven components that will be carried on the unit before choosing the appropriate size of generator. You should consider the startup loads for equipment, such as hydraulic rescue tool motors, large exhaust fans and other electrical equipment to make sure that you are not choosing too small a generator package. Some larger rescue apparatus are equipped with dual generators combining both a power take-off unit and a hydraulic unit, which offers the best of both worlds with respect to using valuable compartment space as well as providing power when the vehicle is in motion.

(On a personal note, while one of the authors was on a truck committee in the early 1990s working on the specifications on a rescue pumper, the size of the diesel generator was discussed. Unfortunately, we went too small with the generator and did not have enough power to run everything on the apparatus. However, with that said, it is unwise to specify the largest generator without the need, as it could substantially increase the cost of the apparatus.)

Apparatus manufacturers can be helpful in suggesting mounting locations and providing guidance in the proper sizing of generator components for special applications. If your vehicle is going to be equipped with computers or other sensitive electronic equipment, you need to ensure that a continuous source of clean power will be available for these components at all times. No matter what type and size of generator is chosen, make sure when designing the apparatus body that there is sufficient room to maintain and work on the unit when necessary. Many fire departments have made the mistake of "burying" the generator inside of the body, only to find out then when the unit needs attention that the mechanic cannot get into the area to work on the unit without cutting something apart.

The generator system on the rescue apparatus is the backbone of the entire rig. Spending an appropriate amount of time in selecting the right generator for your needs will pay off in the long run over the life cycle of the apparatus. In the next installment of The Apparatus Architect, we will discuss some of the other major body components that your department should consider when developing the specifications.

Tom Shand, a Firehouse= contributing editor, is a 33-year veteran of the fire service and works with Michael Wilbur at Emergency Vehicle Response, consulting on a variety of fire apparatus and fire department master-planning issues. He is employed by American LaFrance and is assigned to the Hamburg, NY, facility. Michael Wilbur, a Firehouse contributing editor, is a lieutenant in the New York City Fire Department, assigned to Ladder Company 27 in the Bronx, and has served on the FDNY Apparatus Purchasing Committee. He consults on a variety of apparatus-related issues around the country. For further information access his website at