How to Test Electrical Outlets for Grounding
Using a Neon Circuit Tester to Keep Your Home's Electricity Safe
If your home's electricity isn't functioning properly, you may notice flickering lights, appliances functioning improperly, or even feel a slight shock when you plug a cord into an outlet. There are a variety of tests you can perform on your outlet receptacles and several different tools you can use to do it. For example, you can test for ground using a multimeter to look for a reading of zero volts or ohms, which confirms that the ground is not carrying a current as desired. This broad-purpose tool is used by electricians to perform a variety of tests. But for most homeowners, a very simple little tool called a neon circuit tester can check the ground on an outlet and perform several other diagnostic tests. Best of all, it costs just a few dollars.
Among the most important tests is determining if your outlets are properly grounded. The home grounding system is an important safety feature, designed to help channel electricity "to ground" in the case of a short circuit. A properly grounded electrical system is much less likely to cause fires or shocks if a short circuit should occur. Checking the ground means testing the receptacle to ensure that the ground wire is not conducting electricity.
Testing receptacles are also helpful if you do your own electrical repairs. It will help you determine if the power is off before you work on wiring, and will also verify that you've done repair work correctly.
Depending on the age of your house, your receptacles will be one of two types: 2-slot polarized outlets or 3-slot grounded receptacles. This article will show you how to use the neon circuit tester to test your receptacles for power and reversed wiring, plus how to test ground wires.
What You'll Need
Equipment / Tools
- Neon circuit tester
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.
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.
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.
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.
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.
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.
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.
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.
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.
What is high resistance for a ground?
Less than 5 ohms is an acceptable reading for an electrical ground, though a reading of zero is desired.
Is ground equal to zero volts?
Ground readings in electrical projects are equal to zero volts.
Does a ground wire have current?
Ground wire should not carry any current when wired properly. This wire is intended to redirect currents into the earth in the case of short circuits or power surges.