How do you test for an open Neutral in 120V circuits or outlets? In this video, I show you how how to test for an open or loose neutral in 120v outlets with a regular Multimeter, a LoZ meter, and a Voltage Pen. Yes, it is possible to test for such an open or loose Neutral condition by using just a standard multimeter and even a voltage pen, by using specific techniques. A LoZ meter is generally more informative when it comes to detecting electrical anomalies, and is also discussed in this video as it applies to perceived neutral issues with the mains service. Cllick HERE (or the picture below) for the Video or HERE to Subscribe to The Tech Circuit Video Channel.
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More about Open Neutral Testing in 120v Circuits
Please note that this blog is intended for Appliance Technicians familiar with safety procedures associated with live troubleshooting. Take care when performing live tests as lethal voltages are often present. Do not work on live circuits unless you are qualified to do so.
Diagnosing a Loose Neutral in an Outlet
The Problem: A Mysterious Outlet Failure
According to a voltage tester, this is a properly working outlet. However, when I plug a light into it, the light doesn’t work. Even more interesting, the lights go out on it. If I plug the light into another outlet, it works just fine. But when I plug it back into this one, the same issue happens.
So, what’s going on with this outlet?
The Cause: A Loose Upstream Connection
As it turns out, the outlet has a loose upstream connection. When when I plug the leads of themultimeter into the outlet, I’m read 119 volts which is what we would expect. However, when I plug the light in, the voltage drops down to just a few volts.
This means the outlet has a compromised voltage source, and the issue only appears under load. As soon as I remove the light bulb, the voltage returns to 119 volts.
Understanding a Loose or Floating Neutral
In the video, I show you how to test for a loose or floating neutral using:
- A traditional voltmeter
- A low-Z meter
- A non-contact voltage pen
Even though the voltage tester says the outlet is good, voltage testers primarily check for correctly operating or open lines. This is not an open line—it’s a loose neutral. This issue occurs due to an upstream loose connection that causes the outlet to fail when a load is applied.
Testing for a Bad Neutral Under Load
To check for a bad neutral, I’ll place a load in the socket. Notice how the voltage drops to around 4 volts when the light is turned on. When I turn the light off, it returns to 119 volts.
Why Does This Happen?
If we have a floating or loose neutral, that neutral wire can rise to the same voltage as the hot wire. If hot is stable at 120 volts and the difference between hot and neutral is only 4 volts, that means neutral is sitting around 116 volts.
Neutral should always be at zero volts, or very close to it—within a few tenths of a volt. To confirm this issue, I can place one lead on the ground and one on neutral. This test reads 115 volts, which means the neutral is way above ground level—it should not be there. That confirms we have a floating or loose neutral.
How a Loose Neutral Can Behave Differently
Neutral can sometimes behave differently in circuits where neutrals are shared. If there’s a load imbalance, neutral voltage can shift in the opposite direction, creating a voltage divider effect.
However, in this case, the issue is clearly an upstream loose connection, causing the outlet to fail under load. When I apply a load, the voltage drops drastically.
Using a Low-Z Meter to Detect the Issue
If you didn’t already know that the outlet had an upstream loose connection, how could you detect it?
This is where a low-Z meter becomes useful. I switch to low-Z mode, which inserts a low resistance (about 3.2K ohms) into the circuit. This resistance is typically much lower than the upstream loose connections that cause problems like this.
Now, let’s check the outlet in low-Z mode:
- The reading drops to 11.2 volts.
- In normal mode, it showed a good outlet.
This means that without a low-Z meter, this issue might go undetected.
Click the Image to Purchase the LoZ Meter in the Video
Determining If the Problem is Neutral or Hot
Now that we know the outlet has a problem, is the issue with the hot wire or the neutral wire?
To find out, I can measure between ground and hot:
- I read 119 volts, confirming that the hot wire is okay.
- By process of elimination, this tells us the problem is with the neutral wire.
A Warning About GFCI Outlets
If this was a GFCI outlet, you wouldn’t be able to perform this test the same way. The low resistance of the meter would trip the GFCI since it only takes 6 milliamps of current leakage to trigger it.
To test a GFCI outlet, simply apply a load and measure ground to neutral without using low-Z mode.
In this case, when I put the load back in and switch back to normal mode, neutral is still reading 118 volts. That confirms the compromised neutral.
Using a Voltage Pen to Detect a Loose Neutral
What if you only have a non-contact voltage pen? It can still help you identify this problem.
Normally, a voltage pen:
- Detects hot voltage (as expected).
- Does not detect neutral (since neutral should be at zero volts).
But when a neutral floats, it rises close to 120 volts under load. So, if I plug in the light and only have a small voltage drop, that means:
- Hot is around 120 volts with respect to ground.
- Neutral is around 118 volts with respect to ground.
Since neutral is being pulled up by a loose upstream connection, the voltage pen will now signal on the neutral side when the load is applied. This confirms the floating neutral.
Conclusion
A loose upstream neutral connection can cause an outlet to appear functional but fail under load. To diagnose this issue, you can use:
- A low-Z meter to reveal the voltage drop.
- A traditional voltmeter to check neutral voltage.
- A non-contact voltage pen to detect floating neutral conditions.
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