3-18
The same configuration in regard to momentary or locked positions.
A voltage rating equal to or higher than the original switch.
A current rating equal to or higher than the original switch.
A physical size compatible with the mounting.
In addition, the type of actuator (toggle, pushbutton, rocker, etc.) should be the same as the original
switch. (This is desirable but not necessary. For example, a toggle switch could be used to replace a
rocker switch if it were acceptable in all other ways.)
The number of poles and throws of a switch can be determined from markings on the switch itself.
The switch case will be marked with a schematic diagram of the switch or letters such as SPST for
single-pole, single-throw. The voltage and current ratings will also be marked on the switch. The number
of breaks can be determined from the schematic marked on the switch or by counting the terminals after
you have determined the number of poles and throws. The type of actuator, number of positions, the
momentary and locked positions of the switch can all be determined by looking at the switch and
switching it to all the positions.
PREVENTIVE MAINTENANCE OF SWITCHES
As already mentioned, switches do not fail very often. However, there is a need for preventive
maintenance of switches. Periodically switches should be checked for corrosion at the terminals, smooth
and correct operation, and physical damage. Any problems found should be corrected immediately. Most
switches can be inspected visually for corrosion or damage. The operation of the switch may be checked
by moving the actuator. When the actuator is moved, you can feel whether the switch operation is smooth
or seems to have a great deal of friction. To check the actual switching, you can observe the operation of
the equipment or check the switch with a meter.
Q17. What two types of meters can be used to check a switch?
Q18. If a switch must be checked with power applied, what type of meter is used?
Q19. A double-pole, double-throw, single-break, three-position, toggle switch is faulty. This switch has
a momentary position 1 and is locked out opposition 3. The voltage and current ratings for the
switch are 115 volt dc, 5 amperes. No direct replacement is available. From switches A through I,
in table 3-1, indicate if the switch is acceptable or not acceptable as a substitute. Of the
acceptable switches, rank them in order of choice. If the switch is unacceptable, give the reason.
Q20. What should you check when performing preventive maintenance on a switch?
