Emergency Power Units


Emergency power unit (EPU) systems, which allow operators to bypass the normal system in a malfunction, are common in the fire service. But many firefighters don’t know what they are or their importance. Most fire companies drill on firefighter personal survival skills but often overlook fireground equipment emergency operation skills. Everyone on the fireground must commit to understanding our equipment and how it works. Trusting that everything will work when we need it is not realistic in emergency operations. We should drill on manual pump overrides to supply water to interior operating handlines in controlled environments so we can guarantee that supply on the fireground. Routinely, we should train on how to use batteries or generators as alternatives to maintain the power supply to the aerial so we can readily do so when our firefighters are in harm’s way on the fireground.

As firefighters, we are obligated to understand everything there is to know about our job and the equipment we use. If we expect that the brother to our right and the sister to our left will have the answer, we are headed for trouble. How often have you asked, “What does that switch or button do?” and received no answer? Studying operation manuals is key, but how many times have you climbed under a rig to see how things really work? If every fire station had a Curious George, he would be our go-to guy. Who in the fire station makes us think, and why do we need this person?

Let’s take a basic look at how fire companies can review, exercise, and operate our equipment so we can ensure our EPU systems function when we need them. Today’s apparatus manufacturers design equipment to help us out of just about any fireground problem we can face.


We must first understand interlock systems and their purpose. National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus, defines an interlock as “a device or arrangement by means of which the functioning of one part is controlled by the functioning of another.” It requires manufacturers to install several interlock systems to keep us safe, forcing us to operate in a specific sequence to safeguard the operation. If we fail to follow the proper sequence, the interlock system will engage. You can often activate manual controls to override the interlock, but often they are purposely not within easy reach and out of our line of sight to avoid accidental or casual engagement. So when we need to operate them, it must be an actual emergency.

For example, consider your vehicle’s parking brake and the transmission’s gear selector (photo 1). We have all witnessed a truck that will not go into pump mode at the emergency scene but yet find no problem when we check it after the alarm. Typically, this indicates that the operator erred by trying to engage the pump out of the proper sequence. Most commonly in this situation, the operator left the apparatus in drive and applied the parking brake. Since 2009, all new fire apparatus are required to automatically shift into neutral when the parking brake is applied to avoid this. But if this doesn’t correct the problem, what is our backup plan?

Photos by author.
Photos by author.

Station drills. See if your motor pump operators understand how to override an electronic pressure governor when the engine will not throttle. Check the apparatus in the station before starting your shifts. With some apparatus today, when the parking brake has been applied and the vehicle is placed in pump, you cannot operate the throttle from the cab. Check to see if this is how your vehicle operates. If not, this is an easy and quick training exercise for all firefighters. Maintain a steady throttle using your foot, and notify members to evacuate the building.

Some builders provide a mechanical pump shift override, which includes a 12-volt electronic circuit that bypasses the interlocks and attains converter lockup in fourth gear. A placard will detail the proper sequence necessary to use the circuit, which is independent of all other systems on the apparatus (photo 2). This system is known as the redundant electronic transmission lockup circuit, and every apparatus operator should drill on it, including operating the manual pump shift controller that needs to be operated (it is normally located on the pump panel), then overriding the transmission electronics to enter the pumping mode. These are great training exercises, and your operators should be as familiar with them as they are with putting on their gear.

photo 2


The EPUs supplied on most aerials today are either a 12- or a 120-volt alternating current hydraulic pump system (photo 3). These pumps are not designed to set the aerial up but to enable removing it from harm’s way if the main hydraulic pump or apparatus engine power should fail. Warning: Before operating these systems, always read and understand the manufacturer’s operational procedures. Most EPU drive motors can operate for only a few minutes before they need to cool down. Some generator-operated EPUs can provide continuous duty, eliminating periodic cool-down time. Still, we need to understand that these are for emergency use only.

photo 3

Station drill. Set up your aerial to elevate the lower section about two feet above the cradle, and shut down the engine power. Then, using the EPU motors, stow the aerial back in its cradle for road travel. This drill will allow multiple members to do this exercise again and again without overheating the EPU motors. This is a quick, simple, and safe way for firefighters to become comfortable with activating this system.

Blocking diverter valve bypass. This control allows us to stow the outriggers/stabilizers so the rig can be removed from a scene (photo 4). Warning: Use extreme caution when opening these valves because the safety interlocks are not active. You can test this system by lifting the aerial just a few inches out of a stowed position and then trying to move an outrigger. With the valve in its normally closed position, there should be no movement in the outrigger hydraulics. When the valve is manually activated, the system interlock is disabled and the override will provide hydraulic power to stow the outriggers. Use this system only after you visually verify that the ladder is bedded but the interlock failed to recognize the safe condition. Never try to move the outriggers with the aerial ladder elevated.

photo 4

Always work safely when using these systems. I hope we never have to use them, but if a situation should arise, we need to think and quickly implement these devices.

DANIEL PELUSO is a 28-year veteran of the volunteer fire service on Long Island, New York. He has more than 30 years of experience as an automotive technician and has serviced fire apparatus for the past 10 years. He has Emergency Vehicle Technician (EVT Certification Commission), Automotive Service Excellence (National Institute for ASE), and Factory Mutual training certifications. Peluso is the vice president of the Long Island Emergency Vehicle Technician group and has taught fleet maintenance for the past several years.

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