By Sam Massa

In recent years, the types of components plugged into fire apparatus electrical systems have changed significantly. Modern electronics have enhanced first responders’ ability to accomplish the tasks at hand quickly and effectively.

Some Fire Apparatus Manufacturers’ Association (FAMA) member companies build apparatus, while others make the generators that produce the power, and still others make the lighting and equipment that consume that power. As technology becomes more sophisticated, it is important that all those who manufacture the components work together to ensure coordination and that the firefighters who use the equipment recognize potential conflicts in equipment loads.

Equipment and tools have gotten lighter, more powerful, and more capable. With these advancements, equipment also has become more electronically sophisticated. In today’s technologically and electrically driven world, it is important to understand that when we plug individual components and equipment into a common power system, all of the individual components then work together to become just that: an entire system. Each part has an effect on the overall system. Subsequently, adding and subtracting components can have an effect on the other equipment sharing that common electrical bond.

Types of Power

Most fire apparatus have at least two types of electrical power on board. The 12- or 24-volt power is direct current (DC) and is supplied by the chassis’s alternator. This power runs much of the apparatus lighting and controls and is referred to as “low voltage.” If the apparatus includes a generator or inverter, it will provide 110- or 220-volt alternating current (AC)-just like a wall outlet. This power may be available in either single-phase or three-phase and is referred to as “line voltage.”

The reason it is called “alternating” is because the voltage alternates between positive and negative charges multiple times per second. This can be illustrated on a graph in a shape like a wave. The number of times per second the power alternates between negative and positive (and back) is considered its frequency. In the United States, the standard power frequency is 60 cycles per second, or 60 hertz.

Types of Loads

When working with a small electrical system, like that of a mobile generator, the load that is plugged into the system can impact the electrical waveform. There are two types of loads: a linear load and a nonlinear load. Linear loads are often the types of technologies used in legacy fire service equipment like 1,000-watt quartz halogen scene lights, single-speed AC box fans, or traditional hydraulic power units. Electrically speaking, these types of loads are very simple and consume power consistently and uniformly. Imagine a traditional 100-watt lightbulb attached to a switch. When you throw the switch, the lightbulb turns on and draws a constant amount of power until the switch is shut off. The lamp is essentially just a piece of coiled wire that gets hot and emits light. These types of loads do not typically cause problems with the electrical systems on fire apparatus.

When modern technology gets involved, things get more complex. Many of the computer circuits that control today’s technology require a more fine-tuned DC power source. In DC power systems, the voltage does not alternate; it remains constantly positive. To turn AC line voltage into DC voltage, a piece of circuitry called a switch mode power supply (SMPS) is often used. Unlike the example of the 100-watt light bulb above, the circuitry inside an SMPS module has a tendency to act more like someone flashing the light switch off and on multiple times per second while it converts the AC input into a DC output. This rapidly c