THE AERIAL OPERATOR HAS THE MOST AFFECT ON THE SAFE OPERATION OF AN aerial device. He has an impact on receiving proper training, proper operation of the equipment, use of equipment as intended, and maintenance of the equipment. All are essential components for safe operation.
Safety rules have been developed as a result of repeated accidents, and these rules are written in blood. Following the safety rules every day is your only option. Let's look at just two items that can be hazards for an arborist: falls and electrical contact. Learn and follow proper work practices, because they have been developed to prevent accidents, including falls and electrical incidents.
Falls
When working from a bucket truck, taking precautions to prevent a fall is imperative. The operator is required to wear a fall protection system with the lanyard attached to the manufacturer supplied "D" ring.
Although not all from bucket trucks, four people fall every working day. Overreaching and not standing on the basket floor with both feet often results in falling out of a basket. However, being ejected from the basket is the majority of falls from buckets. There are several ways this can happen.
* The bucket truck is struck by another vehicle.
* An object, such as a tree branch, can strike the basket or boom.
* The basket or boom can snag on something and suddenly release, causing the boom to recoil.
* A lift rope can break, or the load slip while using a material handling aerial device.
* A rope used to tie off a branch to a tree can slip and allow the branch to fall or swing into the boom or basket.
* A mechanical part of the leveling system can fail from overload or improper maintenance causing the basket to tilt.
* A cut can go wrong and a branch fall the wrong way.
A fall to the ground can be prevented if the operator wears a fall protection system. The equipment owner and the operator must determine what type of OSHA-approved fall protection system they will need to use. Then it is up to the operator to always wear the harness, inspect it daily before use, and adjust it to fit, according the fall protection manufacture's instructions. An improper fitting harness can increase the chance of injury. Lastly, do not forget to attach the lanyard to the attachment point provided by the aerial manufacturer. Proper maintenance and skillful operation and techniques can prevent most accidents.
Electrical Contacts
You cannot tell what an object's electrical potential (voltage) is. Whenever you work near power lines, there is the possibility of an electrical contact. Proper precautions and following proper work rules will provide some protection from this unseen danger. Only a line clearance arborist can work around power lines. You must have the proper training to recognize the hazards in line clearance and know the techniques and equipment required to work in the vicinity of power lines. Unless you are a trained lineman or are a qualified line-clearance arborist authorized to work near energized lines, you and the equipment you are operating must maintain a minimum 10-foot clearance from all power lines. This 10-foot clearance increases with voltages of 50KV or greater. If you are an arborist and see a downed or damaged line, do not try to repair it. Contact the electrical system owner/operator of the power line. When your work may be too close to a power line, contact the power line electrical system owner/operator. They are much better equipped to handle problems safely.
To define some of the terms used: current is the flow of electricity, measured in amps; voltage is the force--the difference of potential--causing the current to flow, measured in volts; and resistance is the property trying to prevent current flow, measured in ohms.
Your body operates on electricity, the nerves use an electrical signal to see, think, make muscles move, indicate temperature, and let you know when you have an injury (pain). Low levels of an outside source of electrical current vary from undetected to where it will disrupt the body's electrical system and cause pain. At higher levels it can cause the muscles to contract intensely and interfere with the heart's function. At high levels it can cause severe burn injuries.
It does not take much current to affect the human body. Very small currents can be fatal. As the current flow increases, the tissue damage to the body increases. The damage will be determined by the power to which a body is exposed. This power is a function of the length of time of contact and the amount of current flowing through the body. Keep in mind that an electrical burn caused by a high-voltage contact causes current to flow through the blood and bone marrow. It literately cooks the body from the inside out. Further, a high-voltage electrical contact keeps on destroying tissue even after the voltage has been removed. High voltage is defined as equal to or greater than 600 volts. A distribution power line primary phase to ground potential can be anywhere from 2,400 volts to 19,900 volts.
Electrocution occurs when a person's body becomes a path for electric current. You place your body in a position to become an electrical path by touching, or getting too close to, two objects at a difference of potential (voltage). The voltage difference causes the current flow between the parts of the body in contact. You do not have to touch an object to be a path. The current can jump through air if the distance, voltage level and weather conditions are suitable. Current can also flow over what are normally considered non-conductive parts caused by surface contamination. These include dirt, moisture and oils. Here's what's important; any time any two parts of your body are placed between a difference of potential, current will flow. The amount of current flowing through your body is determined by the amount of resistance. The higher the resistance, given the same voltage, the lower the current flow. Conversely, given the same voltage, the lower the resistance, the higher the current flow. Lineman's rubber gloves and insulated booms provide an extremely high resistance. Leather gloves and moist skin provide a low resistance.
There is a false impression that current follows the path of least resistance. Current follows every path it can find. Lower resistance paths have more current flowing through them, but all paths will have a current flow--this is Ohm's Law. Even though there may be a lower resistance path for current flow, it does not take much current to be fatal. Consider a condition called a ground fault. This can have a momentary current surge in distribution power lines up to 15,000 amps. Remember, current will seek ground through every path available.
Current flow is not always from a hot wire in one hand to the ground under your feet. It can be:
* From a tree branch touching an energized phase or part, through your saw, to you and down to the ground through the tree you are climbing.
* From an energized phase wire, through you, to a neutral or ground wire.
* From an energized phase wire, through you, to a different phase wire.
* From a branch you are trying to remove, through the rope to the person handling the rope.
* From a tree branch you are holding, through you, to the controls you are operating when part of the aerial boom tip touches an energized phase or part.
* From an energized phase wire in contact with, or too close to, a non-insulated portion of a boom, through you, while standing on the ground touching the truck or attached chipper.
* From one foot, through you, to the other foot if standing near an energized vehicle or tree that has become energized (note:this is known as step potential).
* From the ground you are standing on, through you, to a tree you are removing, if an energized phase is contacting a branch of the tree.
* From an energized phase hidden in the branches through the branches and down the tree to you when you drill it to inject chemicals.
This partial list gives examples of contact possibilities. There are many possible combinations where a person's body can become a path for current, whenever a difference of potential exists. Remember, whenever any two parts of your body are at a difference of potential, current will flow. You must be aware of the clearance distance required to power lines. See ANSI Z133.1 for proper clearance distances.
Your bucket truck has three components that will provide some protection if they are properly maintained by being kept clean, dry and periodically tested per ANSI A92.2:
1. A basket liner will protect only that portion completely inside the liner. Anything conductive that extends out of the liner will conduct electricity into the liner and make it ineffective.
2. The insulating section of the upper boom will prevent current flow from the boom tip through the boom to the elbow only.
3. The lower boom insert will provide an insulating section between the elbow and the truck chassis.
The boom tip does not provide insulation because it contains metal components to provide structural support. Manufacturers place a band of arrows on the upper boom to show the end of the insulated section. Past the band of arrows on the boom, any part of the boom tip that contacts a energized line or part will become energized at that potential. Also, any part of the boom tip that contacts a grounded component will ground the entire boom tip. This includes the controls and all other components that are past the band of arrows. Covers and guards may provide limited protection but you cannot depend on them. They are not tested or maintained to provide electrical protection. Contact of fiberglass covers and guards with energized parts may arc along the surface or through the fasteners to metal under the cover and energize the entire boom tip. You must maintain proper clearance from all sources of electricity.
Just because you are in an aerial device you are not protected from all contact with an energized object. If you touch or are part of a path between two objects at different potential you can be electrocuted. The aerial device will only prevent one energized source having a path to ground through the boom. You are not like a bird on a wire.
Tool Hoses
Another hazard on aerials involves the tools you use. Many aerials are equipped with hydraulic tools such as power saws and loppers. The tools are connected to the bucket truck's hydraulic system through the tool hoses, which must be non-conductive. They are orange color and have NON-CONDUCTIVE printed on them. Inspect the hoses in the area just beyond the fittings for any wrinkling. Wrinkling is an indication of hose failure that's about to occur. Black tool hoses may be reinforced with wire braid and they are conductive. If a conductive tool hose bridges across electric lines it can cause an arc. A sustained are will melt a hole in the hose and ignite the mist that escapes. Since the hoses and hydraulic oil are flammable, the escaping oil will form a blowtorch and could spray the operator with ignited hydraulic oil. There have been far too many failed basket rescue attempts with a fire at the boom tip and an operator on fire because of poor or no training on basket rescue techniques, or failure to practice this necessary procedure periodically. The operator and the ground crew must know how to perform a basket rescue, especially in the case of a fire or an electrical contact. The operator's life is at stake here. If you don't know basket rescue techniques, find out.
An arborist's job is hazardous enough. That's why we follow the safety work rules and use the equipment properly in an earnest effort to prevent accidents.
