TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
(2) Time-delay units. Time-delay units act on
dures. Major repairs and testing should be con-
an electromagnetic induction operating principle,
ducted in a facility's testing laboratory, or by
contract personnel with access to any special testing
whereby torque is developed in a movable rotor
(disc or cup) which rotates between two faces of an
electromagnet. These units can only be used in ac
a. Electromechanical relays. Check contacts,
circuits. Time overcurrent and time under/over-
moving parts, connections, and the case and covers
voltage relays are generally of the disc design type,
of these relays.
while high-speed overcurrent, directional, differen-
(1) Contacts. Contacts must be kept clean. A
tial, and distance relays are more often of the cup
flexible burnishing tool should be used for cleaning
(cylinder) design type.
silver contacts. Silver contacts should not be
b. Solid-state relays. Solid-state relays are ex-
cleaned with knives, files, abrasive paper or cloth,
tremely fast in their operation, as they have no
as these items may leave scratches which can in-
moving parts. Other advantages are lower burden,
crease arcing and hasten deterioration of the con-
high seismic-withstand, and reduced panel space.
tacts. Abrasive paper or cloth may, in addition, leave
Many are programmable, allowing increased
minute particles or insulating abrasive material in
choices of time-current characteristics.
the contacts, and thus prevent closing. Contact wipe
c. Usage. There are no formal statistics available,
and resistance are important in all relays and
should be checked as part of the maintenance pro-
but one manufacturer estimates that 40 to 50 per-
cent of their relays sold in 1992 were solid-state
cedure. Contact resistance can be determined by
units. By the year 2000, this manufacturer esti-
mates that of their total relay sales 85 to 95 percent
on springs, the contact pressure should be checked
using a spring gage. High resistance of such con-
will be the solid-state type. Solid-state relays re-
quire no preventive maintenance, but they do re-
tacts may indicate insufficient spring pressure,
which will require replacement of the spring. The
quire a periodic maintenance check.
relay must be deenergized and disconnected when
11-4. Relay maintenance periods.
the contacts are tested.
(2) Moving parts. It is important that all mov-
Frequency of maintenance should be such as to re-
ing parts operate smoothly, so keep all bearings,
veal any possibilities of failure. Maintenance
shafts, linkages, and other moving parts free and
records will disclose trends which might lead to
clear of dirt or gum. Relays normally require oiling
only when replacing a jewel, shaft, or moving part.
a. Test considerations. Tests should simulate nor-
Too much lubrication of these parts can lead to
mal operating conditions. Avoid overtesting because
serious troubles and should be avoided. The relay
such tests can often cause more problems than they
disks should be cleaned with a thin brass or bronze
correct. Consider the variables that can cause prob-
magnet cleaner having a steel edge or insert. Relays
lems, such as relay complexity, environment, his-
should be quiet when operating. A noisy relay
tory, and facility relay-type experience. Other con-
should be checked for loose parts or excessive play,
siderations are relay age and relay stress (relays
and corrective measures should be taken.
operated at greater currents and/or control voltages
because of station expansions).
(3) Connections. Relay connections should be
thoroughly checked as part of the maintenance in-
b. Frequency. Inspections made every 2 to 3 years
spection. Check all screws and nuts for tightness.
is usually sufficient. Testing may be necessary after
Check the relays, and as much of the circuitry as
a relay operation. Visual inspections of the target
possible, for continuity, grounds, and shorts.
should be made any time other area visual inspec-
(4) Case and cover. To prevent dirt from enter-
tions are required. Relay settings should be checked
at least once a year and after any incorrect opera-
ing the case, ensure there is a tight seal between
tion or redesign of the system. These inspections,
the relay cover and its gasket. Any dust or dirt
supplemented by suitable tests, should be thorough
within the case should be brushed, blown, or vacu-
enough to detect any faulty relays, settings, or wir-
umed out. Care should be taken that dirt is not
ing errors before trouble is encountered.
blown deeper into the relay necessitating removal
and overhaul of the relay.
11-5. Relay general field inspection.
b. Solid-state relays. Many solid-state relays
have easy-to-use built-in operational test diagnos-
Relays should be completely disconnected from any
tics. Calibration tests are made in the conventional
live circuit when they are inspected or tested. Only
manner. Maintenance is generally not required, in
specially trained electricians should be permitted to
the usual sense of adjusting, cleaning, or lubricat-
repair and adjust relays. The manufacturer's in-
ing. Check external connections. It may be neces-
structions should be checked for the proper proce-