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’ 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 light bulb attached to a switch. When you throw the switch, the light bulb 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 in to 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 changing high-frequency load is known to distort the power source’s AC waveform and can wreak havoc on the other things plugged into the system.
Power Waveform Distortion Can Create Problems
Today’s AC LED scene lights are examples of loads that often incorporate SMPSs. They have circuitry inside the fixture that converts, or “rectifies,” the AC power to DC power, then reduces the voltage to a usable range. The LEDs are then “driven” by their individual computers at a very specific voltage to produce light output. As the voltage fluctuates on the AC input side, the DC output to the LEDs must remain constant. The circuitry required to accomplish this can have a negative effect on the timing and relationship between the voltage and the current on the small electrical system or can cause spikes or surges in voltage that are outside the operational range of other pieces of electronic equipment plugged into the system.
Equipment that causes significant distortion to the electrical waveform can cause problems that may cause the generator to shut itself down and throw an error code or cause other equipment in the system to act incorrectly. This is because some pieces of equipment (battery chargers, fans, cardiac monitors, and some generators) have circuitry that is highly sensitive to requirements for a clean sine wave power source for proper operation.
The tricky matter is that when this same array of equipment is plugged into a station on the U.S. power grid, the problems that occur when plugged into the truck’s generator often go away. If a scene light, fan, or some other piece of equipment on a truck consistently causes problems with the generator but works 100 percent at the station, it is possible that the device is causing a distortion in the waveform on the small electrical system.
The reason that equipment may work in the station, but not work when using a mobile generator platform, is because the mobile generator is significantly smaller than the power grid. When you think about the effect of a 1,000-W load on an 8-kW generator, it is important to note that the load accounts for greater than 10 percent of the total available capacity. There are not enough fans and scene lights in the world to account for 10 percent of the available power in the nation’s power grid.
Think about it this way: If the generator is a plastic baby pool and the load is a large rock, when the rock is thrown in to the baby pool’s water, it will cause a big splash and ripples will travel from side to side throughout the entire pool. Take that same rock and throw it into one of the Great Lakes (analogous to the power grid), and the effect of the rock on the lake as a whole is unnoticeable in comparison.
It is important to note that not all modern electronic devices have SMPS modules that cause waveform distortion. To make things more complicated, even in the same model of devices from the same manufacturer, variances in the semiconductor circuitry from one run to the next can cause minor differences in the effect on the waveform. The component manufacturer can include circuitry to correct a device that is known to be potentially disruptive.
To avoid problems with your new apparatus, the key is communication. When writing specifications calling for mobile generators, lighting equipment, or fans, reach out to the equipment’s engineering teams and ask if they have any recommendations for best practices when pairing their equipment with the other brands and models of equipment you intend to use. A small investment of time on the front side can save a tremendous amount of time after the system is built.
If you suspect a power-factor-related issue on your mobile generator power system, plug a true linear load such as a portable 500-Watt quartz halogen scene light, into the truck before the piece of problematic equipment. Keep the load energized while operating the other equipment and, very often, the linear load can help smooth the waveform and resolve the system problem. This might not be electrical rocket science, but it’s a quick tip that can save a lot of headaches when you are depending on the apparatus to work as intended at your next incident.
The custom nature of fire apparatus and the infinite ways they are used in the fire service make this discussion particularly important. Many times a specific fire department is using its equipment in a way that may have never been seen before. FAMA member companies recognize this situation and want to be a part of the solution. Work with your apparatus and equipment manufacturers to avoid problems before they occur and don’t hesitate to reach out if your equipment fails to live up to your expectations. Together, we can keep the fire service as efficient as possible for the ultimate benefit of those we protect.
FAMA is committed to the manufacture and sale of safe, efficient emergency response vehicles and equipment. FAMA urges fire departments to evaluate the full range of safety features offered by its member companies.