recorded for future reference. If it is out of tolerance, contact the power company to correct the
c) Semiannual checks
(1) Ground Resistance. Perform a ground-resistance measurement for each item of
equipment using a ground resistance tester (refer to par. 4.11). Record the readings and compare
with previous readings to discover deterioration in the grounding system. The lower the
resistance value, the better; a value of 5 to 10 ohms is desirable. If the resistance is greater than
25 ohms, immediate action must be taken to lower the resistance.
(2) Primary High-Voltage Buses and Ground Buses. Check the high-voltage bus
installation with particular attention to the condition of the insulators, supports, and electrical
connections. Keep the bus insulators wiped free of dust or any other deposits. Check the ground
bus carefully throughout its entire length. If the bus or any ground connection to the bus is
broken, make immediate repairs. De-energize the system before cleaning or repairing the bus.
(3) Relays. Inspect the protective relay, runway-selector relays, and auxiliary relay
panels when servicing the vault equipment. Check the operation of these devices, clean the
contacts, adjust release springs, and check contact arms and dashpots. Replace all unserviceable
(4) Oil Fuse Cutouts. Check operation and electrical connections of the oil fuse
cutouts. Check the contacts and check the oil level. Add oil when necessary. If fuses with
replaceable links have failed, replace them with fuse links especially manufactured for this
purpose rather than string fuse wire. If the oil fuse cutouts have a manual operating lever, check
the operation of the locking arrangement. Be sure the manual operating handle is locked in the
"OFF" position before servicing vault equipment that is being supplied through the oil fuse
(5) Oil Switches. Check the operation of the oil switches. Be sure that the
moveable handle on the oil switch, which has three positions, "MANUAL OFF," "MANUAL
ON," and "AUTOMATIC," is in the "AUTOMATIC" position at all times. This allows the
switch to be remotely controlled. Check the contacts and oil level and service when necessary.
(6) Power Transfer Switches. Check operation of power transfer switches. Check
contacts for dirt or corrosion.
(7) Control Panel. In some cases, an auxiliary control panel is installed in the
vault, or an outdoor-type control panel is installed on an outside wall of the vault. In such cases,
carefully check the operation of all parts of the panel. Clean all contacts and make sure all
electrical connections are in good condition. Clean the interior of the panel carefully.
(8) Photoelectric Time Switch. If a photoelectric time switch is installed, it should
be maintained according to the manufacturer's instructions. The light levels should be checked
with a photographic light meter to ensure that the control turns on and off at the proper ambient
light levels (see Appendix A, Table A-7).
(9) Astronomic Time Switch. If this switch is installed, it should be serviced
according to the manufacturer's instruction book. Inspect the operation, check clock for proper
time, clean motor commutator and main switch contacts, and check all electrical connections.
Since this is a precision instrument, repairs should be made by the manufacturer or an authorized