3-15
SWITCH RATING
Switches are rated according to their electrical characteristics. The rating of a switch is determined
by such factors as contact size, contact material, and contact spacing. There are two basic parts to a switch
rating-the current and voltage rating. For example, a switch may be rated at 250 volts dc, 10 amperes.
Some switches have more than one rating. For example, a single switch may be rated at 250 volts dc, 10
amperes; 500 volts ac, 10 amperes; and 28 volts dc, 20 amperes. This rating indicates a current rating that
depends upon the voltage applied.
CURRENT RATING OF A SWITCH
The current rating of a switch refers to the maximum current the switch is designed to carry. This
rating is dependent on the voltage of the circuit in which the switch is used. This is shown in the example
given above. The current rating of a switch should never be exceeded. If the current rating of a switch is
exceeded, the contacts may "weld" together making it impossible to open the circuit.
VOLTAGE RATING OF A SWITCH
The voltage rating of a switch refers to the maximum voltage allowable in the circuit in which the
switch is used. The voltage rating may be given as an ac voltage, a dc voltage, or both. The voltage rating
of a switch should never be exceeded. If a voltage higher than the voltage rating of the switch is applied
to the switch, the voltage may be able to "jump" the open contacts of the switch. This would make it
impossible to control the circuit in which the switch was used.
Q15. What is the current rating of a switch?
Q16. What is the voltage rating of a switch?
MAINTENANCE AND REPLACEMENT OF SWITCHES
Switches are usually a very reliable electrical component. This means, they don’t fail very often.
Most switches are designed to operate 100,000 times or more without failure if the voltage and current
ratings are not exceeded. Even so, switches do fail. The following information will help you in
maintaining and changing switches.
CHECKING SWITCHES
There are two basic methods used to check a switch. You can use an ohmmeter or a voltmeter. Each
of these methods will be explained using a single-pole, double-throw, single-break, three-position,
snap-acting, toggle switch.
Figure 3-15 is used to explain the method of using an ohmmeter to check a switch. Figure 3-15(A)
shows the toggle positions and schematic diagrams for the three switch positions. Figure 3-15(B) shows
the ohmmeter connections used to check the switch while the toggle is in position 1. Figure 3-15(C) is a
table showing the switch position, ohmmeter connection, and correct ohmmeter reading for those
conditions.
