Transcript

Introduction

Good afternoon, everyone, and welcome to today’s white paper webinar. Today, we’re talking about loose neutrals, and in particular, how to spot a loose neutral with the RMS capture graph. You may be familiar with how to do this with the strip charts and our traditional loose neutral strip chart graph, but we’re gonna show you another way of doing this that sometimes is easier to use with the RMS capture graph. And I have Wes here as the author of the paper to go into details.

What Is a Loose Neutral?

To start out, a loose neutral or open neutrals in a residential 120/240-volt split-phase service lets the neutral float. That makes one leg swell above 120 volts while the other leg sags below, stressing equipment and creating safety hazards. The paper shows what causes it, what it looks like in data, and how to prove it fast using PMI Provision and PQ Canvass, especially the RMS capture view.

Normal Residential Service Operation

In a residential service from a single-phase transformer with a center-tap secondary, the center tap is grounded as neutral. Each hot to neutral is 120 volts, and hot to hot is 240 volts, as shown in figure one and figure two. In normal operation, loads share the solid neutral reference.

For example, in figure three, there’s a six-amp computer on one leg and a four-amp light on the other, which means two amps flow through the neutral, the difference between legs, and both loads see 120 volts.

What Happens When the Neutral Opens

If the neutral connection goes resistive or open, return current loses its direct path. The two 120-volt loads effectively go in series across 240 volts. The center tap no longer pins the midpoint, and artificial neutral floats based on load impedances. This results in one leg voltage rise while the other one falls.

In the paper’s example, one leg can run 144 volts and the other 96 volts under an open neutral condition, the same current through as when the open condition wasn’t present, but the voltages are split unevenly. In severe or intermittent opens, leg voltages can swing towards 240 volts or zero volts momentarily, as shown in figure four.

Typical Symptoms and Complaints

The typical symptoms and complaints of loose neutrals, people will complain about flickering lights, frequent UPS beeps, failed surge trips, or odd appliance behavior. You can inspect for burnt or corroded connections to the weather head or meter base. You may even see ground potential rise as current rises to return via the grounding electrode system. Tight and neutral to ground bonds can mask or soften symptoms, but don’t eliminate the hazard.

Identifying Loose Neutrals in Data

Strip Charts

When looking at your data on strip charts, modest cases can be easy to miss. The hallmark pattern is simultaneous swell on one leg and sag on the other, while the sum stays at 240 volts. PMI’s loose neutral graph mirrors the two lines to neutral voltage to see this DSOL pattern jump out, exactly what you want for a quicker view.

Loose Neutral Report

To find it fast, PMI has a loose neutral report, which you can find on PQ Canvass and on Provision. Quickly scan over multi-day recordings using three checks:

1. The difference between the leg voltages.
2. How closely their sum tracks to 240 volts.
3. How long it persists.

Great for triage, but note it’s less sensitive than waveform RMS methods because it relies on one-second RMS samples.

Waveform Capture

Waveform capture lets you confirm instantaneous behavior. In figure six, the voltages look okay at a glance, but the leg behavior isn’t symmetric. That’s your hint. An event is hiding there.

RMS Capture View

In RMS capture, you can switch the same record to RMS voltage, and the event becomes obvious. Figure seven shows channel one and channel two sitting around 121 volts. During the event, channel one spikes to 130 volts, while channel two sags to 112 volts. Yet channel one and channel two are still approximately 242 volts before and after. The constant sum with the opposite leg movement is a textbook loose neutral signature.

Steps to Deal with a Loose Neutral

1. Run the loose neutral report over the whole dataset to spot likely windows.
2. Open the loose neutral graph to confirm the mirror swell, sag with 240-volt sum.
3. Jump into waveform, then flip to RMS capture to show the leg split numerically and nearly constant sum.
4. Check neutral connections at the service head meter base, and panel logs, and repair, re-torque as needed.
5. Take a follow-up recording to confirm stable balance line-to-neutral voltages have returned to normal.

The Bottom Line

A loose neutral swaps a solid reference for a floating midpoint. In data, look for one leg up, one leg down, where the sum is still 240 volts. This is best seen in the RMS capture. Catching it early prevents equipment damage, reduces liability, and improves safety.

Why RMS Capture Is Easier to Read

The bottom line here is, like Wes said, the hallmark of an open or loose neutral is that equal and opposite shift in movement of the two 120-volt legs. As you see here in figure five, this can often bring this symptom out. But sometimes it is easier to see in the waveform capture.

In figure four, this is a really obvious one, but it’s not always that obvious. Like we see in figure six, the two retraces look like just sine waves. It’s really hard to see in this view that one channel is rising in voltage because the peaks of the voltage are almost the same visually when you’re graphing from negative 200 volts to positive 200 volts. So a few volt shift is only a pixel or two tall in that kind of view. It’s hard to see that this voltage is a little bit lower, and this voltage is a little bit higher from an RMS standpoint.

But if you look at the RMS capture view of the same waveform, it’s a lot more obvious. So here in figure six, PQ Canvass is computing a sliding RMS graph from the waveform capture. What we see in figure seven is the same data as figure six, but an RMS view of that data.

PQ Canvass has a one-cycle-long window. It computes the RMS value in that cycle and then shifts that window. It slides it one sample at a time to give you this continuous RMS graph over that same millisecond-level time span. And now it’s much easier to see that we’re going from 121 volts to 130 volts on channel one, and then dropping by that same amount down to 111 volts on channel two.

So here, the RMS movement is real obvious. Think about just drawing a line between these two graphs, and the top half is a mirror image of the bottom half. That’s that equal and opposite shift, even on a millisecond-level basis. And the shape of this is exactly the mirror image of the shape of that. That’s a lot more obvious than it is in the sine wave version of the waveform capture.

When RMS Capture Is the Best Tool

On something that’s very intermittent or very short, it may not even be enough of an RMS change to be that noticeable in the strip chart. So sometimes this RMS capture graph is the best place to spot this.

Current Behavior as a Clue

Another way to tell if it’s an open neutral as opposed to something else is that the current doesn’t really change much with it. Here, the small change in current is the load reacting to the voltage change, not causing that. So, if you do have a very asymmetric load, like an unbalanced load on a service, you will see some opposite movements in the legs. But here, there’s almost no current change. It’s all voltage change.

PQ Canvass Live Demo

Here I can show you in PQ Canvass what this looks like. Here we have an example of a loose neutral recording, and we can look at the waveforms. Some of these waveforms have already been identified by the AI or Merlin that they are a loose neutral. We have a step where channel 2 decreases, channel 1 increases. That is an open neutral, same clue that we’re looking for here.

But we can also go right to the waveform. And again, looking at sine waves, it’s not so obvious. Looking at the RMS capture graph, this is another instance of it. We see this equal and opposite movement right here where one goes up and one goes down by the exact same amount. And this one persists for longer than the waveform duration. So, after 130 milliseconds, it’s still shifted like that. This is a lot longer in duration than the one in the white paper.

Whenever you see this equal and opposite movement, you may not see it in the sine wave, but if you look at the RMS version, it jumps right out at you.

General Tips for Analyzing Waveforms

In general, if you’re looking at waveforms, if you see all sine waves all the way through, that’s a sign that it might be an RMS event of some sort, a sag or a swell. Click on the RMS button and take another view of the same data, and you’ll see things that aren’t as obvious looking at sine waves or may not be as obvious in a strip chart.

Or if you’re using Merlin, the AI tool, it will automatically flag this for you so that you can have the AI detect this. In fact, in this recording, AI determined it was a loose neutral based on those kind of patterns. But that’s what Merlin was looking for, that equal and opposite movement, and that’s what you should be looking for also if you’re analyzing the data manually.

Contact Information

If you have questions, give us a call anytime, 1-800-296-4120, or send an email to support@powermonitors.com. Thanks for attending everyone.