Insulated digger derricks and aerial devices protect users and ground personnel who work around energized power lines. Following established work procedures – as is testing and maintaining the equipment – is critical so that the equipment continues to provide the insulation users expect. You cannot look at a unit to determine if it will provide the insulation expected; it must be tested.
Two types of dielectric tests must be performed on insulated aerial devices and digger derricks: the qualification and periodic tests. The qualification test is required to determine the voltage rating of the unit. The periodic test (Photo 1) is done at intervals to verify that the equipment continues to provide the expected insulation. Knowing who is responsible for these tests and when to perform them is essential to identify any defects or weaknesses in the insulation capabilities of the equipment. Once in use, the responsibility falls on the owners and users.
New insulated bucket trucks or digger derricks are first tested by the OEM (Photo 2), according to ANSI A92.2-5.3.2 or A10.31 5.3.2 requirements, respectively. This qualification test at the factory establishes the insulation rating for the equipment. The installer will then perform a dielectric test to confirm the insulation after the units are finished and operational. A qualification test is also required if the fiberglass boom is replaced. Then, the original qualification test is no longer valid.
Once insulated equipment is placed in service, maintenance tests (also called periodic tests or annual tests) are required to be performed for a variety of reasons. The maintenance test is required annually for most classifications of machines or more frequently, according to the user’s policy. Maintenance tests are also required after repairs or replacement of components in the insulated sections, such as hoses or leveling components. If a problem is suspected — or after incidents of unintended contact with energized power lines occur — a maintenance test will verify that the insulation is providing the expected insulation and is not damaged.
Dielectric testing must be done by a “qualified person” per ANSI A92.2 or A10.31 standards. A qualified person is: “A person who, by possession of an appropriate technical degree, certificate, professional standing, or skill, and who, by knowledge, training, and experience, has demonstrated the ability to deal with problems relating to the subject matter, the work, or project.”
If no periodic tests occurred within 12 months, the equipment can no longer be considered insulated. Category A units have different requirements if used for bare-hand work at least every three months.
Always follow test procedures established by ANSI A92.2, A10.31, and the manufacturer to establish consistent methods for measuring the leakage current at the specified voltage. The manufacturers will be able to provide the information, and it is included in the ANSI standards. The ANSI A92.2 Manual of Responsibility also provides the aerial test criteria.
Know your voltage ratings
Before conducting any type of dielectric testing, you should first know the machine’s voltage rating and category. Refer to the voltage ratings for the equipment. The ID plate (Photo 3) on your bucket truck or derrick will indicate if the unit is insulated and, if so, the voltage and category the insulation is designed and tested to withstand. The equipment manuals tell what areas of the machine will provide insulation.
Look on the ID plate for the “qualification voltage.” The number in this area is the voltage rating the unit was tested and qualified for per ANSI standards. The date of the qualification test is indicated on the ID plate as the “test date.”
There may be two voltages stamped on the ID plate near the lower controls. The qualification voltage is the most important. The design voltage may not be shown on the ID plate and is the maximum voltage for which this machine can be rated if it is properly equipped and tested. The machine cannot be used on lines at the design voltage unless the qualification voltage indicates the same voltage on the ID plate.
Qualification voltage is the maximum voltage for which the upper boom on aerial devices has been tested and rated. The same is true for digger derricks, which can be used as an aerial lift when equipped with a platform and upper controls to work on power lines. For digger derricks, the fiberglass boom must be fully extended, and the load line removed across the insulating section to provide this insulation. If used with a platform, the operation must be controlled by the person on the platform to work on energized lines or equipment. Digger derricks cannot be used with a platform attached unless the ID plate indicates a platform capacity, because it is not designed and tested for platform use.
Insulated equipment can be used to work near electrical system lines up to this phase-to-phase voltage if the proper personnel protective equipment (PPE) and procedures are used. The insulation provided by the booms is secondary to PPE. Only Category A units (when used following bare-hand work procedures) will provide primary protection. The ratings are given as the phase-to-phase voltage of the system. The booms are tested based on the phase-to-ground voltage, not the voltage between the phase lines. A 46kV-rated unit cannot be used on a 69kV system even though the phase-to-ground voltage of the 69kV system is less than 46kV.
Types of testing
Qualification testing (Photo 4) can only be done with an AC testing machine. This is performed by qualified service personnel. It is performed anytime the fiberglass boom section is modified or replaced. A record of dielectric tests should be maintained in written, dated, and signed form.
After the aerial device or digger derrick is installed on the chassis, one of the following dielectric tests is required to be performed by the installer. They can conduct another qualification test with AC equipment, or if the installer accepts the qualification test from the manufacturer, they can perform a periodic test using AC or DC test equipment. In the case of boom replacement, the OEM tests the boom before shipment, but this is not considered a qualification test because the unit must be fully assembled and operational with all components installed in the insulated section. The entity reassembling the unit must perform a qualification test before the unit can be returned to service.
Insulated units should be visually inspected daily. It is especially important in the spring when birds are nesting. If a unit sits for an hour, a bird may try to build a nest inside. If a dielectric test is performed without inspecting the interior, a fire may result.
A periodic test is necessary whenever there is a question of the dielectric properties, such as contaminated or deteriorated fiberglass. This type of test must also be done anytime repairs have been made to components that cross the insulating section. An example is when boom hoses or leveling rods are replaced. A periodic test can be performed with either AC or DC testing machines.
Boom positions should be recorded when performing dielectric tests, as the position can cause readings to vary, especially for AC tests. Testing the boom in the same position each time will give more consistent readings. By comparing the test results from year to year, any trend upward in leakage current may aid in determining if the fiberglass boom's insulating properties are deteriorating and if the fiberglass, hoses, or leveling rods need to be inspected or repaired.
Finally, as stated previously, an annual test must be performed. AC or DC testing can be used. Either a qualification test or a periodic test will suffice for meeting the Annual Test requirement.
Other items to test
In addition to the insulating upper boom sections of bucket trucks or derrick trucks, there are a few other considerations.
Insulating liners, if used, also require an annual dielectric test. The liner does not have a rating, only a requirement to be tested. The liner depends on the material thickness for its insulating properties. Damage to the liner that reduces the thickness, such as gouges and cuts, can reduce the insulating properties and can result in failing the test.
It’s a good idea to examine the hydraulic oil and perform a dielectric test on the oil at the same time the booms are tested, especially if the oil is discolored or milky looking. The dielectric strength of new oil should exceed 25kV, and used oil should remain above 15kV when tested per ASTM D877 (kV/0.1).
High-resistance control handles on aerial devices must also be tested using AC equipment for qualification. Maintenance tests can use AC or DC equipment. Testing must be performed annually or whenever maintenance is completed that may affect the dielectric integrity of the controls.
The aerial chassis insulating system or lower boom insert, if equipped, may provide some level of protection for people on the ground if contact is made below the upper boom insulating section. The insert does not have a rating — only a test requirement. The same test procedures apply as for the upper boom. The qualification test only uses AC test equipment, and the periodic test can use AC or DC equipment.
