combined with their rugged construction, make them durable and reliable instruments.

The maintenance and cleaning of these instruments are easy. Maintenance consists of periodic cleaning, battery replacement, fuse replacement, and recalibration. Calibration on these meters should be performed every year. The exterior of the instrument can be cleaned with a soft, clean cloth to remove any oil, grease, or grime from the exterior of the instrument. Never use liquid solvents or detergents. If the instrument gets wet for any reason, dry the instrument using low-pressure "clean" air at less than 25 psi. Use care and caution while drying around the display protector and areas where water or air could enter the interior of the instrument.

CAUTION

All resistance measurements should be taken on de-energized circuits ONLY!

WARNING

When using compressed air for cleaning, wear chemical splash goggles. Do not direct the air toward eyes or skin.

MAINTENANCE OF DISTRIBUTION EQUIPMENT

The elements, accidents, and willful vandalism are the cause of most damage to power distribution equipment. To repair these damages, the lineman requires experience, a total commitment to safety, and the knowledge to accomplish repairs to the system as quickly and economically as possible.

The maintenance required on the poles, timbers, and crossarms in a power distribution system is minimal. Normally, this equipment lasts for a period of 20 years or more. However, the following problems may occur and create a need for maintenance action:

A pole can settle and require straightening.

Wood can shrink and cause all hardware to become loose and require tightening.

Over time, guys stretch and require re- tensioning.

Insulators get dirty and require cleaning, especially around sea water where there is salt in the air.

Connections become loose with age and must be re-torqued to prevent hot spots.

In time, conductors stretch and require re- sagging.

INTERFERENCE ELIMINATION

Another important area of maintenance is noise interference elimination in the power distribution system.

Power lines may be a source of interference with radio communications. Conductors, insulators, and hardware contribute their share by means of spark discharges, localized corona discharge, and cross modulation.

Spark Discharges

Spark discharges occur when localized excessive voltage stress exists. A conductor may become partially insulated by corrosion products or an insulator partially conductive because of cracks. A third source of stress occurs when a conductor is separated from another metallic part on a pole only by a small air gap.

Corona Discharge

"Corona" is defined as the luminous discharge due to ionization of the air in the vicinity of a conductor when the voltage gradient exceeds a certain critical value.

Cross Modulation

Cross modulation (often the result of a corroded connection that causes nonlinear rectification of currents) may occur when splices are made by twisting the conductors, rather than using a tighter mechanical splice. Additionally, when conductors of dissimilar metals are joined, corrosion occurs unless special connectors designed for the specific combination of metals are used.

Remedies for conductor, insulator, and hardware interference are relatively simple. Remember, the condition for hardware interference is set up whenever two pieces of hardware are not securely bonded to each other or are permanently separated by too short an air gap.