How to Test Electrical Outlets for Grounding

How to Tell if a 2-Slot Polarized Receptacle Is Grounded

If you have an older home, you may have outlet receptacles that have only two vertical slots for accepting plug prongs, not the 3-slot outlets found in newer homes. 

If you look closely at a 2-slot receptacle, you may notice that one slot is wider than the other. This is known as a polarized receptacle, and if it is wired correctly, the hot circuit wire will be connected to the narrower slot, with the neutral circuit wire connected to the longer slot. 

1. Test for Power

   To test for power on a polarized receptacle, place one probe of the neon circuit tester in the smaller slot and the other probe into the larger slot. If the tester lights, you have established that the receptacle is powered up and you can continue testing. The red and black probes are interchangeable for this part of the test.

2. Test for Ground

   Once you verify that you have power, remove the probe from the longer slot and touch it to the screw in the center of the cover plate. If the tester lights up or registers, the outlet is grounded and wired correctly, and your job is done. If not, continue to the next test.

   Tip

   If the cover plate screw is covered with paint, the tester may have trouble making contact. Make sure there is a good metal-on-metal connection between the tester probe and the screw. 

3. Test for Reversed Wiring (Reverse Polarity)

   Place one probe of the tester into the long (neutral) slot and the black lead on the center screw of the cover plate. If the tester lights, you have established that the receptacle is wired incorrectly, better known as reversed polarity. The hot and neutral wires are reversed and should be switched to make a correct connection. This will normally be a matter of switching the screw terminal connections on the receptacle.

4. Test for Absent Ground

   Now try placing one probe of the tester on the screw in the middle of the cover plate and place the other probe into each of the other slots (small and large slots) one at a time to see if the tester lights. If it doesn’t light for either slot, the receptacle isn’t grounded. This is a situation that may require the attention of a professional electrician to troubleshoot and correct the problem. 

How to Tell if a 3-Slot Receptacle Is Grounded

Three-slot receptacles usually indicate that you have a newer wiring system or one that has been upgraded to more modern standards. Here, the third, round receptacle slot is the ground connection. 

1. Test for Power

   To do a ground test on a 3-slot receptacle, place one probe of the tester in the small vertical (hot) slot and the other probe into the larger vertical (neutral) slot. If the test lights, the receptacle is active and functioning correctly.

2. Test for Ground

   Once you know a 3-slot outlet has power, test for a faulty ground by taking the probe out of the large (neutral) slot and touching it to the center screw on the cover plate. The tester should light if the ground connection is good and the receptacle is connected properly.

   Now move the probe from the screw and place it in the round lower slot on the receptacle. The tester should also light in this position. If the tester does not light, there are two possibilities: either the receptacle's wiring is reversed, or there is a missing ground.

   If it doesn’t light at all, continue to the next test.

3. Test for Reversed Wiring

   Place one probe into the long vertical (neutral) slot and the other probe on the center screw of the cover plate. If the tester lights, you have established that the receptacle is wired incorrectly—the hot wire is attached improperly to the neutral slot on the receptacle. While you might not notice the difference as lamps and other appliances will work correctly in this situation, it is potentially dangerous.

   Normally the solution is a fairly easy one: Reverse the screw terminal connections on the receptacle. 

4. Test for Absent Ground

   Now try placing one probe into the round hole and place the other probe into each of the two vertical slots (small and large slots) one at a time, to see if the tester lights. If it doesn’t light for either, the receptacle isn’t grounded.

   An ungrounded receptacle is a problem that should be addressed by an electrician. The problem may be that the circuit ground wire isn't attached to the receptacle, but it's also possible that the system itself is ungrounded and needs to be upgraded. 

If There Is No Power

If there is a situation where there is absolutely no power on the receptacle at all, you have another problem. Possible causes include:

• A simple loose connection on the receptacle is interrupting the flow of electricity. Check the receptacle screw terminal connections and make sure they are secure. 
• The wiring from the circuit breaker or fuse is damaged and not completing the circuit.
• A circuit breaker has tripped or a fuse has blown. In these cases, you can reset a breaker or replace a fuse if the circuit looks to have no visible defects. 
• A faulty device somewhere along the circuit is causing a short circuit or open circuit. Try unplugging everything connected to that circuit, reset the breaker or replace the fuse, then plug the devices back into the circuit one at a time to find the problem, if any. Often, a loose connection in one of the outlets in the circuit causes the interrupted flow of electricity to your outlet.