Grounding Utility Fleet Trucks
There is probably not a fleet mechanic or fleet manager who has not heard something about grounds for trucks. But for all the talk about grounding trucks, including rules and commentary, there is very little consideration for how grounding connections are made to a vehicle. Unlike most every other procedure in the utility industry, there are no OSHA guidelines, consensus standards or best practices for connecting the truck to the truck ground. There are rules requiring grounding of trucks, but there are no best practices, procedures or methodologies for connecting grounds or ground attachments to trucks to allow grounding. So, that’s what we’re going to discuss in this installation of “Focus on Fleet Safety.”
For fleet managers to effectively facilitate the grounding of trucks, we need to understand the purpose of grounding and why it fails to do the job expected. Actually, the job expected often is the bigger issue because it may not be what you think. Grounding a truck does not directly protect workers from electrical shock, nor does it eliminate a shock hazard. Grounding trucks has one purpose: to cause immediate operation of a protective device. The protective device is the circuit breaker through which voltage and current are delivered to the electrical system. A truck is not an electrical conductor – it is a mechanical device, meaning that to ensure that current flow across the truck is sufficient to cause operation of the circuit protective device, an electrical connection must be employed to bypass the vehicle’s nonelectrical isolation from ground. Nonelectrical isolation refers to mechanical interfaces of bolted parts, the rubber tires and the outriggers on earth. Current has to pass these mechanical barriers to initiate circuit-breaker operation. In an electrical contact with a truck, the circuit breaker feeding the system in contact with the truck may not trip. As often happens, the circuit will continue to feed current into the vehicle, resulting in fire as well as a continuing electrical shock hazard to any person near the truck. So, the purpose of grounding is to create a good path for electrical current to flow. That good path causes a higher current to flow, resulting in fast tripping of the breaker feeding the electrical circuit. Fast tripping minimizes the damage to the electrical system and the truck.
Understanding Why Grounding Does Not Protect Workers
Grounding trips the circuit. It does not stop current and voltage from flowing on a vehicle that is in contact with an electrical source. A well-grounded truck will trip a circuit sometimes within a fraction of a second. However, even at a fraction of a second, the electrical contact is more than sufficient to cause serious injury or death to persons in contact with the truck or even standing near the truck. We refer to electrical charges as “potentials.” All conductors have different electrical flow qualities referred to as “resistance to current flow.” Conductors at different resistances have different potentials. Where there is a difference in potential, there is current flow. A person standing on the ground and in contact with a truck when it becomes energized is at a different potential than the truck, exposing them to current flow and injury. The only way to protect that person is to ensure they are at the same potential as the truck. If they are at the same potential, current cannot flow, and the person cannot be injured.
We use bonding to ensure a worker is at the same potential as the truck. To accomplish bonding, we use equipotential mats. An equipotential mat is an electrically conductive portable mat electrically connected to the truck. Anyone standing on an equipotential mat connected to the truck will be protected.
Making Grounding and Bonding Effective
Now that we understand the electrical requirements, let’s talk about how we as equipment managers and mechanics make grounding and bonding effective. Obviously, a first resource would be the users of the equipment. Crew foremen or safety personnel should be able to tell you where they need the ground connectors on trucks. If they haven’t told you before, this is a good time to establish a common specification. This also is the case for mobile compact backyard machines as well as aerial lifts, digger derricks, cranes, tensioners, tuggers and low-drills, and anything else that has an aerial extension that will be near overhead conductors. Don’t leave out underground equipment that may have tuggers or cable reels that get backed close to pad-mounted equipment or vaults.
For many companies, the first item is specifications for the truck build. Truck outfitters do not have a standard for the installation of truck ground connectors. If you order a truck equipped with grounding stirrups or studs, be specific about how they are installed. If you install them in-house, establish a specification. Consideration of the pathway is what is missing from most specs. The pathway is the most likely scenario. If the vehicle is an aerial device, the path will be from the metal components of the boom, through the turntable and chassis, to wheels and outriggers into the earth. Wheels and outriggers are the high-resistance part of the path. Outriggers usually are set on a pad to improve stability, which also increases resistance. Even outriggers set on bare earth, especially dry earth or sand, have high resistance and limit current flow. The most effective connection will eliminate that higher-resistance path across the wheels and outriggers. We find most ground connectors on the exterior body of the trucks, usually the bumpers. If the bumpers are effectively bolted to the chassis, that may be a sufficient electrical path to bypass the resistance of wheels and outriggers. Adding a bonding jumper between the bumper and chassis is even better. The bonding jumper creates a low-resistance electrical path from the chassis to the ground connection, bypassing the high-resistance path across the wheels. The result is improved grounding causing a faster tripping of the circuit breaker feeding the electrical system in contact with the truck.
Some companies install grounds in a bin on the truck. The ground cable hangs on a hook. The ground end is connected to the interior of the bin using a lug that is permanently connected. A best practice would be to connect a bonding strap on the backside of the bin connector to the truck chassis. Using a grounding reel assembly is another popular method. The grounding reel keeps the ground cable rolled up on a spring-loaded reel. The frame of the reel is the pathway to the truck. The reel assembly typically is delivered with a bonding jumper to be used to make an electrical connection from the reel’s mounting location to the vehicle chassis.
The last issue is the electrical connections themselves. If connectors are to be made to a checker plate, the raised ridges need to be ground down to provide a reasonably flat surface for the electrical lug to bolt against. Most lugs will require 3/8-inch or half-inch bolts. Bolts should be stainless and installed with flat and spring washers. The lug and truck surfaces where the connector meets should be sanded free of paint and oxidation before assembly. Painting the connection after assembly will seal it against oxidation.
We also encourage you to train your mechanics and service workers about the importance of truck grounding connections for safety. Lineworkers expect certain outcomes for the safety procedures they employ. The quality of the installations done in the shop helps to ensure those expected outcomes.
About the Author: After 25 years as a transmission-distribution lineman and foreman, Jim Vaughn, CUSP, has devoted the last 20 years to safety and training. A noted author, trainer and lecturer, he is a senior consultant for the Institute for Safety in Powerline Construction. He can be reached at jim@ispconline.com.
