Power Quality - Open Neutral

Here’s what it looks like when a tree falls across your powerline and the neutral opens up

I flipped the main breaker shortly after.


One leg rockets upward, the other downward…


Thank you @dmagerl for explaining what a user might see on their graphs when this scenario occurs. The next request to SENSE should be to program an immediate alert message / notification when this type of event occurs. All of my neighbors have had a problem with their neutral cables. I expect mine will be next. All of the homes in our neighborhood were built at the same time.


There have been some examples on the FB group recently of people who have open/degraded neutrals that are showing up in the PQ monitoring data. An alert explaining to them what’s going on would be very helpful! It’s a bit difficult to understand without an electrical background, and a message from Sense that shows the problem being detected would go a long way to getting them pointed toward resolution.


Open or loose neutrals are a huge danger to your appliances. IMO, it’s the greatest danger we face. Lightning strikes can be worse, but the variables required for that to cause greater damage are so rare that open neutrals are #1 on my list.

The result as shown above in the Power Quality plot is that one leg will have a voltage spike and the other will have a corresponding dip. As devices fail (go open circuit) on the elevated leg, their loads are removed from that circuit, reducing current flow [I] somewhat while greatly increasing impedance [R]. Ohm’s law tells us that the voltage [E] across the remaining devices step further upward with each device failure [E=I*R]. It’s a cascade of death.

After the open or loose neutral is corrected, many of the surviving devices will have reduced lifespans. It took almost two years to sort through all the damage I experienced in a concert production system where the neutral opened on the source.

Link: A simple tutorial to better understand open neutrals.

Link: A simple post that ties the tutorial to the real world.

Note: 240V appliances do not use the neutral, therefore won’t be affected by an open neutral. These appliances are fed with a 3-wire cable. Some 240V appliances include components that operate at 120V, thus requiring a neutral. These appliances are fed with 4-wire cables.


Thanks for posting @dmagerl. I’m glad you were able to flip off the breaker quickly.

As several of you have mentioned, Sense should alert you if we detect an open neutral. This is firmly on my radar for a future iteration, likely starting in Labs.

Appreciate the comments and resources, @lholland.


Some thoughts on Sense detection of open or loose neutrals:

  1. 240V equipment has no neutral reference and won’t be affected by an open neutral. The happy result is that the Sense monitor is safe and able to measure correctly in the midst of a neutral problem (edit: see Ben’s post below). This makes me jealous of most of the rest of the world.

  2. Kirchhoff’s voltage law tells us that the combined series voltages of the two legs, L1 and L2, must add up to the source voltage. With a good neutral, L1 and L2 will each be half the source voltage and of opposite polarity.

  3. An open neutral will result in a single catastrophic event where one leg goes over-voltage while the other leg goes under-voltage. The combined total of these two voltages will equal the source voltage. If no devices fail or are switched on or off on either leg, this over/under voltage relationship will remain constant. If you have a loose or intermittent neutral connection, the under/over voltage condition will vary (your lights will flicker when another device turns on or off). Lights should never get brighter - that’s a telltale of a neutral problem.

  4. For detection purposes, I would think it best to measure peaks in the two legs instead of RMS. In theory either should work, but RMS calculation slows the process down a bit.

  5. Thus, if Sense detects over and under voltages at the same moment on L1 and L2 that add up the the source voltage, you have a neutral problem event. The real-world source voltage will never be perfectly symmetrical, so some kind of fudge factor will be needed to prevent false positives.


Unfortunately I don’t believe this is true. I can’t find the thread, but I believe it was discussed that the Sense is a 120v device. The second connection is just used for voltage monitoring of the other Leg, not powering the Sense.

**Edit - Editing just to add, I don’t think this means there can’t be floating neutral reports from Sense. Just commenting that if it is bad enough, it may take the Sense with it.


Confirmed… I just measured the current of the black and red wires connected to Sense’s voltage input. The black wire showed typical capacitive input power supply current peaks. The red wire showed no measurable current. Definitely a 120V gizmo, hopefully with a very wide input voltage tolerance. Thanks!


I had this happen at my beach house. No telling how long it was that way as we hadn’t been down over the winter. Luckily, nothing was damaged. The power went out a couple times the next week which I could see from my remote monitoring. Ended up being because the PoCo put in a new pole about half the distance of the original and split my wire length in half.

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