Testing a Light Switch
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How can I use a voltage tester to check a light switch? The Most Common Methods for Testing a Light Switch, Testing a Switch with a Continuity Tester, Testing a Switch with a Voltage Tester. © By: Dave Rongey |
Testing a Light Switch
Electrical Question: How can I use a voltage tester to check a light switch?
- I have an older home. The bathroom light switch to turn the light on is a little different.
- The two wires there when I measure across them gives me 120 volts.
- On the light I checked before beginning to work on it with my tick tracer and shows there is power in one of the legs.
- So I check across the two wires and nothing happens no power but when connecting the two switch wires and then measuring there are my 120 volts, this was the way this switch has worked for many years.
- I was just curious how this could be?
Any theory will help.
Thank you.
This electrical wiring question came from: Angel, a Handyman from Joliet, Illinois.
Dave’s Reply:
Thanks for your electrical wiring question Angel.
How to Test a Light Switch using Electrical Testers
Application: Testing a Light Switch.
Skill Level: Intermediate to Advanced – Best performed by a qualified electrician.
Tools Required: Basic Electricians Pouch Hand Tools and Voltage Tester or Continuity Tester.
Estimated Time: Depends on personal level experience and ability to work with tools.
Precaution: Identify the light circuit, turn it OFF and Tag it with a Note before working with the wiring.
Notice: Testing with the circuit power on can be dangerous and should only be done by qualified or experienced electricians.
The Most Common Methods for Testing a Light Switch
- Testing a Switch with a Continuity Tester
A continuity tester can be used to test the light switch while the circuit is OFF or the switch has been removed. This type of tester has two lead wires that uses the battery voltage of the tester to test a circuit, which in this example is the circuit components of a switch. Toggle the switch into the on and off position with the test leads attached to the terminals of the switch. If the switch is good then there will be a reading or indication, otherwise no reading or indication means that the switch is bad. - Testing a Switch with a Voltage Tester
A voltage tester has two lead wires, one for positive and one for negative, however since a switch only has a positive power source connected to it the one test lead will need to be attached to either a ground wire or a neutral wire, while the other test lead is attached to the screw terminals of the switch, one at a time while turning the switch on and off. The tester will produce a voltage reading if the switch is good, however sometimes you can get false voltage reading, which I will explain below.
Why a Tester May Read Voltage while the Light Switch is Off
- Testing a Light Switch
This is a question that comes up often, and here is the answer. A standard incandescent light bulb has a filament wire that makes the connection between the two wires of the circuit. Therefore when the light switch is in the off position, the connection between the neutral wire is made through the filament and enables the tester to connect back to the neutral wire of the light fixture. - The Tester as a Light Bulb
So in essence, your tester is becoming a light when the two tester leads are placed onto the two switch terminals. - How to Get an Accurate Test Reading
To prove this, simply remove any light bulbs from the light fixture and the 12o volt reading will stop because the connection to the neutral wire has been disconnected. - Light Circuit Wiring
Similar situations occur when one wire has become disconnected within a circuit, and a load is present, which can create a back-feed of electricity which causes lights to glow dim, because the available power is feeding back through other devices in the circuit wiring, and these devices are creating resistance on the circuit.
More about Light Switches
- Electrical Testers for Light Switches
- Using Electrical Testers
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Guide to Electrical Testers
- When working on home electrical wiring using voltage testers can play an important part in electrical safety. Electrical testers enable you to identify electrical circuits and help prevent the possibility of accidental electrical shock.
- Troubleshooting Electrical Wiring
- For more information about Light Switch Wiring
Light Switch Wiring
Wiring a Light Switch – Diagram 1
Fully explained pictures and wiring diagrams about wiring light switches describing the most common switches starting with photo diagram 1. - Home Electrical wire
Electrical Wire for the Home
Complete listing of electrical wire types and parts used for home projects with electrical code information serves as selection guidelines.
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