Switches and Contactor/Relay Switched Contacts Troubleshooting
Once a switch or set of contactor/relay switched contacts (F ig u re SP-8-4) have been identified as the probable cause of an electrical problem by using hopscotch troubleshooting, the contacts should be tested to confirm their position (open or closed). Switches and contactor/relay switched contacts can be tested with continuity measurements.
A continuity measurement can be used to determine whether a switch or contact is open or closed. If the switch is open, the VOM/DMM should indicate an infinite reading. If the switch is closed, the VOM/DMM should indicate a short (zero ohms).
Keep in mind that power must be off to perform the continuity check. As such, the check can only confirm the status of normally open and closed switch contacts. It may have no bearing on the status o f the contacts when the system is powered up. When working with contactor or relay contacts (Figure SP-8-5),
remember the following:
• Contacts that open when the contactor/relay is energized are called normally closed (N .C .) contacts.
• Contacts that close when the contactor/relay is energized are called normally open (N .O .) contacts.
• Note that the contacts of a contactor/relay may or may not be marked N.O. or N .C . on the schematic or wiring diagram. However, a schematic or wiring diagram always shows the contacts as they are positioned in the deenergized condition of the contactor/relay. Therefore, a contact that is shown closed would be considered a normally closed contact and a contact that is shown open would be considered normally open.
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