b) Next, a high voltage is applied across the electrodes, gradually increasing up to 22
c) If the oil can withstand a voltage of 22 kV, it is in good condition. Sparking across
the electrodes indicates that the oil should be changed or filtered.
d) Do not use or add any type of oil that has not been approved by the manufacturer of
the transformer. If the oil level changes appreciably from the normal range for the operating
temperature, the cause should be identified and necessary repairs made.
e) As little as 10 parts per million of water in the oil will reduce the dielectric strength
below a satisfactory value. Breathing of a transformer through a defective seal may bring in
enough moisture to cause a problem. Exposure to air or excessive temperatures may cause
formation of sludge. If enough sludge and water accumulate in the oil, the entire oil supply may
require filtering to gain acceptable dielectric strength. If too much water gets in the transformer,
it may require drying out.
Ground Resistance Tester
a) The ground resistance tester, Figure 13, is used to measure the effectiveness of
grounding systems. It does this by measuring the resistance between the grounding system and
the earth ground. Follow manufacturer's instructions closely to obtain an accurate ground
resistance reading, thus avoiding a false, lower than actual resistance-to-ground measurement
that can result from incorrect use. The grounding system in question may be used for beacon
towers, lighting vaults, engine generators, and for other lighted navigational aids, or it may be a
counterpoise system for underground cables.
b) The maximum acceptable ground resistance is 25 ohms. It is preferable that the
resistance be 10 ohms or less.
c) In many locations, the water table is gradually falling. In these cases, the ground
electrode systems that were effective when initially installed are no longer effective. This
emphasizes the importance of a continuous program to periodically check the grounding system.
It is not sufficient to check the grounding system only at the time of installation.
Safety. A grounding system is a very important integral safety feature in airport
lighting systems. To be effective, the grounding system must have a very low resistance-to-
ground. The higher the inherent resistance of the grounding system, the greater the voltage that
can build up on a grounded chassis or frame. When this built-up voltage discharges through a
person, injury or death may result. For this reason, the effectiveness of the grounding system
must be checked regularly.