Dielectric Rating and Changing Levels

Knowing and understanding the dielectric rating of a bucket truck is imperative when it comes to using the equipment properly. The dielectric rating refers to the unit’s ability to insulate against electrical currents when the truck is being used near live power lines or other electrical hazards. At times, owners may want to change the dielectric rating if the bucket truck will no longer be used and maintained as originally rated.

Categories A, B, and C

There are three classifications of dielectric ratings for aerial devices. These are based on the level of insulation required for the voltage that they are designed to work close to and the type of work performed. Category A are for the use as a “bare hand unit”, followed by Category B, and then Category C. Only Category A units can be used for bare hand work, where the boom is the primary insulation. Category B and C are used when the boom insulation is secondary to use of PPE and hot line tools. Category A aerial devices can be de-rated to a Category B or C. Category B aerial devices can be de-rated to a Category C. Depending on which conversion is taking place components may need to be removed and a dielectric test will need to be performed if the unit is to be maintained as insulated.

When Re-rating is Required

A unit does not have to be re-rated to be used as a lower category machine. As an example, a 69 kV Category B machine can work any voltage below the 69 kV Phase to Phase rating. Category B machines can be used for Category C work without any modifications provided all covers at the boom tip are in place. Re-rating to Category C (46KV), would only be necessary if the owner wanted to eliminate the Category B components and specific testing requirements. All units used for rubber glove work must have fiberglass platforms and covers over all exposed metal at the boom tip.

If the machine is rated as a Category A it can be used for Category B and C work by simply removing the metal liner and disconnecting the bonding cables at the platform. If the machine is equipped with a metal platform, it will need to be changed to fiberglass or it can only be used using hot line tools . If a Category A machine is not used for bare hand work every 3 months, it will require an annual dielectric test. A thorough inspection will be required before being used for bare hand work again.

Regardless of machine type or category rating if an annual dielectric test is not performed on the equipment, it must be considered as not insulating. The exemption is Category A units, when used for bare hand work at least every 3 months, only require a dielectric test every 3 years.

For Category A to B, the parts that are removed can be stored for future use if the unit is ever retested and rerated back to a Category A.

Tech Tips

Terex Tech Tip 154 covers Dielectric Reclassification and Rerating Procedures. Terex Utilities regularly distributes Tech Tips to provide service personnel with information to make their jobs easier.