dB/dt = 4800 nT/min
2017 June 15 Tech, Weather
One day you notice the lights flickering. Some get dimmer; some might even get brighter.
Then you notice there's something else not right about the power.
"Open neutral" is a dangerous condition that happens when the neutral line gets broken or disconnected. Not only can it destroy your electronics, but it can also be dangerous to you.
In this article we'll see how to detect an "open neutral" that's outside your building, using some basic methods inside the building. Warning: Testing mains voltage has significant hazards. If you don't have the skill set, please hire an electrician to do the measurements. (Disclaimer.)
In This ArticleSomething's Off About The Power
From Parallel To Series
One More Test
What Causes Utility-Side Open Neutral
Something's Off About The Power
One day you run something with an electric motor. Compressor, angle grinder, maybe just a refrigerator. That's when you notice something really weird: one of your LED light bulbs is blinking on and off.
But only when an electric motor is running.
And the motor won't run at full speed, either.
Some of your incandescent bulbs may be getting dimmer. Some might even be getting brighter. What's going on here?
Get yourself a three-prong outlet tester.
Test all the outlets in your building. You cannot detect a utility-side "open neutral" this way, but you can detect one that's on a particular outlet. You should do this test to rule out a problem on your side.
Both yellow lights "ON" and the red light "OFF" means the outlet is wired correctly. Again, a utility-side open neutral will not show up on this test.
Next we're going to do a test that shows if you've got a utility-side open neutral. Keep in mind the test results will depend on how things are wired. If your service is wired to use both L1 and L2, things could get more complicated than I've described here. That situation will have some things at 240 volts when they should be at 120. If you see that, it's a definite indicator of an open neutral.
Get one that can measure 0 to 250 volts AC, or better. If you're not near the service entrance, most any halfway-decent meter will do.
If you test any outlets within 30 feet (conductor length) of the main panel, you should use a CAT III multimeter. Here's one and here's another that would be suitable for that purpose. The reason to use CAT III here is that if there are any line transients, they can cause catastrophic failure of the multimeter (and you). Line transients can happen on a clear day, or any time. They actually happen quite often.
Keep one hand in your pocket when you're testing mains voltage. This greatly reduces your chances of completing a circuit with your free hand and getting electrocuted. Try to attach the multimeter leads without using both hands. One lead goes in each slot. The shorter slot is the "hot"; the longer slot is "neutral".
With circuit breakers "on" and no appliances plugged in, the voltage should read 120 to 125 volts AC. (The RMS voltage is actually 120).
Now, this is only a CAT II multimeter, so it's not entirely safe to use it this close to the panel. Next time it's gonna be a CAT III meter for sure. And by the way, avoid super-cheap multimeters for this, no matter what rating they claim. If you're going to test this kind of stuff, don't use a $5.98 multimeter.
Electric motors can be damaged by low voltage. Don't use your nice turntable as a test device for this; if you're having power issues, unplug it until they get fixed. Also, if you have some circuits that use L1 and others that use L2-- as most homes do-- an open neutral can put 240 volts where there should be only 120. Some things can get destroyed immediately by this condition.
Do any of these tests at your own risk.
OK, so let's say you found 125 volts when nothing else was plugged in. Then, when a test appliance is running, you might see something like this...
Wow, that's not the normal mains voltage. Definitely something wrong there.
In a series circuit, current is everwhere the same. That means voltages have to change.
To see these voltage drops, the device has to be running.
From Parallel To Series
Normally, 120-volt outlets are parallel-wired. This is why the light bulbs in your home don't get dimmer when you add more light bulbs.
When the neutral went "open line", it probably left three other things connected. L1, L2, and a bare "Ground" wire right outside the building.
The ground wire provides a return path for electricity if the neutral gets interrupted. (Make sure the ground wire is there!!)
With open neutral, the 120 volt outlets are acting as if they're wired in series now. This doesn't make intuitive sense; shouldn't they still be in parallel if the earth ground is intact? Well, a circuit diagram ought to show us why, but for now... the voltage drops are a pretty good indicator that there's a series circuit.
If you speak to someone at your utility company, be sure to explain what you tested, and what you found. Otherwise, the easiest thing for them to do is say "Nope, it must be in your building."
An open neutral outside your building has to be fixed by them. There is nothing else you can do at that point, except unplug all your stuff!
One More Test
If your circuitry uses only one leg of the two-line system, then all the current will go back through the ground wire. If it uses both, the current on the ground wire will be less, but it will still be at 120 volts or so. And current can still flow.
If you touch that wire, the current could flow through you. In fact, one thing that makes open neutrals so dangerous is that anything connected to ground could become energized. If you present a better path to ground than it currently has, you could get electrocuted.
A clamp meter makes it possible to test the ground wire for current.
I do not recommend doing this test unless you keep in mind that the ground wire is live. Be careful. If you absent-mindedly grab something grounded while you've got one hand on a "hot" ground wire, you could die. So, if someone were to do this test, they would want to keep one hand in a pocket at all times. Then there's only one free hand at any given moment.
When I tried the angle grinder, the clamp meter showed a couple of amperes going through the ground wire. In a normal situation there would be zero amps on it.
By the way, be sure there are no distractions and no one talking to you.
What Causes Utility-Side Open Neutral
An open neutral can happen for a variety of reasons. This is probably not a complete list:
- Heavy Rain
- Ice Storm
- Surge or electrical short somewhere
- Rust and corrosion
- General lack of maintenance
- Fasteners working loose by expansion & contraction
- Tree roots
- Damage by groundhogs, squirrels, etc.
There are lots of reasons why this problem could happen. It could occur without warning, out of the blue. If your power starts acting weird, it's time to start testing voltages.
The "motor test" can help you detect an open neutral. An electric motor may not run at full speed. You may also find light bulbs getting dimmer or even brighter.
If the voltage changes a lot on other outlets while something is running, there's an open neutral. If all the outlets have this type of issue no matter what circuit they're on, then the problem is outside the building.
And if so, get the power company there are soon as possible. You can't prevent a service-side open neutral, but you can minimize the damage. If you know what to look for, you might get the power company to fix it before there's a real problem.
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