How to Test an AC Relay With an Analog Multimeter
AC (alternating current) relays are used as remote switching devices. Relays typically employ two separate, but mechanically connected, parts for the operation. A solenoid coil magnetically activates a set or pair of electrical contacts. A single set of contacts is also referred to as a pole. Various AC relays may have many poles or sets of contacts. An analog test meter uses a display with a needle indicator on the face. Digital meters have now replaced analog meters, but analog devices are still highly accurate. Does this Spark an idea?
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Instructions
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Remove all electrical power to the AC relay device. This may entail pulling a control voltage fuse or switching off a circuit breaker that powers the relay control circuit. In some cases, there may be two separate power feeds for the relay. The control power that feeds the solenoid coil may be separate from the power feeding the relay contacts. Consult any electrical schematics to ensure all power is disabled.
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Use the screwdriver and remove one of the two wires that are connected to the solenoid coil. Switch the analog meter to the "ohm" position. Insert the red lead of the meter into the "ohm" connector. Plug the black lead into the "com" or "common" connector on the face of the meter.
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Touch the two ends of the meter leads together. The meter display needle should deflect all the way to the right-hand side. Use the side adjustment knob to "zero" the meter. In other words, when the two leads are touching, the meter display should rest on exactly "0." The adjustment knob will move the meter's needle back and forth to secure the "0" indication.
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Touch the two leads, one each, to each of the solenoid's screw terminals. The meter display should indicate a resistance reading between 10 and 100 ohms. Various solenoid coils will have different resistance readings. Consult the relay's manufacturer specifications for exact resistance readings. If the meter fails to deflect at all, the coil connection is broken and the relay needs replacing.
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Locate, on the center top of the relay, the manual activation button to push the relay contacts together. Use the slotted end of the screwdriver to push the button inward. If the relay has more than one set of contacts or poles, test each set of poles separately by holding the contacts in place with the screwdriver.
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Touch the red lead of the meter to the topside of the screw terminal contact on the relay. Touch the black lead to the screw terminal on the direct opposite side of the relay. While the manual button is being pushed inward, the meter should read "0" ohms. If the meter reads any more than zero ohms, the contacts may be bad. Pitting or corrosion can cause a resistance reading to the set of contacts. If you are having problems with the set of contacts, this can be the cause.
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Test the other poles on the relay while pushing the manual button with the screwdriver. Consult the manufacturer's specifications of the relay for specific screw terminal identification and labeling.
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Tips & Warnings
Follow all local electrical regulations whenever installing or servicing any electrical equipment.
Relays placed in wet environments may have internal corrosion disabling the relay's mechanical actions.
