Transcript
Introduction to Loose Neutrals
Hello everyone, and welcome to today’s white paper webinar. Today we’re going to be talking about loose neutrals. We’re going to dive into the theory behind loose neutrals, and also give some guidelines for catching these issues in a power quality recording.
A loose neutral is really a dangerous power quality issue because it can damage equipment and start fires and cause actual harm because of the fire issue. What we have in a loose neutral is a break between the neutral conductor at the transformer secondary and the meter base. Normally this neutral conductor carries the difference of load between the two 120-volt legs.
Circuit Theory Behind Loose Neutrals
As we can see here in figure two, a circuit diagram representing a single phase service and the transformer has two windings that’s modeled as two 120-volt secondaries, and they’re 180 degrees apart, which means that across the full winding of the transformer you have 240 volts. So if you measure from leg to leg, that’s 240 volts. That’s the equivalent of this point here in the schematic.
In a single phase service we can have 120-volt loads that connect to the neutral on either 120-volt leg, and we can have 240-volt loads that connect across the full winding. Now for 120-volt loads, the current flows from the leg through the load and back to the transformer through the neutral. And this neutral conductor carries the difference in load currents between the two 120-volt legs.
Here we have loads modeled as R1, R2 and R3. And R1 and R2 are the two 120-volt loads. R3 is the 240-volt load. Now R3 really doesn’t play into a loose neutral issue because it’s not connected to the neutral itself. But the amount of current flowing to the neutral is the difference in current that is carried by those two loads from R1 and R2.
Now if R1 is equal to R2, that means that there is no difference in current, and there’s actually no current flow at all to the neutral. And in theory you don’t even need the neutral conductor because no current is flowing through it. But as the loading becomes different on the two legs, as customer switches lights off and on or other single phase loads, the neutral carries different amounts of current.
The Artificial Neutral and Voltage Divider
Now what happens in a situation where there’s a break in the current, we have the voltage on one leg going up, the other leg going down. When we have that break in the neutral current, we have basically an artificial neutral formed by this voltage divider. At this point here between the two legs is an artificial neutral. That voltage will float up or down compared to the real neutral based on the difference in the loading.
Here in this example, we have a different amount of load on the two legs, nine ohms and eight ohms, and that causes a seven volt difference between the real neutral and the artificial neutral. If we’re measuring at the secondary of the transformer, we’ll measure 120 volts, 120 volts on the two legs. If we measure past the break, now we’re measuring relative to this artificial neutral and one leg goes up in voltage, one leg goes down in voltage. They both move in opposite directions by this voltage across the break in the neutral.
Here in this example it’s a seven volt difference, so one leg goes down to 113 volts, one goes up to 127 volts. And they will always move in equal and opposite directions.
Vector Diagram Explanation
If you think of the vector diagram it makes it more clear what’s happening. Here we have a vector view of a single phase service, and we have 120 volts, 120 volts, and they’re 180 degrees apart. So if you measure across the full winding, we have 240 volts.
Now the real neutral is here in the center of the vector diagram, and that’s held by the winding of the transformer. The artificial neutral will float back and forth. It’s constrained to float in one dimension. It can float closer to one leg or closer to the other, and whichever leg it moves closer to, that voltage gets smaller, the other voltage gets bigger. But they move in equal and opposite directions.
Animated Demonstration
Now we actually have an animated view of this from a YouTube video. It is also a good explanation of open neutrals. As you can see here in this example, we have the two 120-volt legs modeled with DC batteries. And here we have a different amount of load in the two 120-volt legs. We have five lights and then we have three incandescent bulbs.
As he connects this neutral to the center tap, or the pretend center tap of this circuit, the brightness is the same on both sets of bulbs because we have equal voltage. And when he removes that neutral conductor, now we have an unequal amount of load on either leg. One leg goes up in voltage, the other goes down in voltage based on how many lights are different, and we have a change in brightness.
Recognizing Loose Neutrals in a Power Quality Recording
Now, the way to recognize this in a power quality recording is to use a loose neutral graph, as we show here in the presentation and also in the white paper. You want to look for equal and opposite movements. Here in this graph we have channel one minimum voltage graphed along with channel two maximum voltage. These are one cycle minimum axes, and on the second graph we have channel one maximum and channel two minimum, so that’s kind of the opposite.
What we’re looking for are equal and opposite movements. In this situation, it’s intermittent. There are large portions of the recording where everything is good. You got a solid neutral. You have, for example, a voltage sag on both legs. But here in this section, we have one voltage going up, the other voltage going down by equal opposite amounts. And you can see this visually by looking for kind of a mirror image effect. If the top half of the graph is a mirror image of the bottom half, that’s that characteristic.
Here’s another example where, again, it’s intermittent. There are long stretches where the neutral is solid, and then there are stretches where it breaks into the intermittent loose neutral. But during that time, you can think about drawing a line down the middle of the graph, and is the top half a mirror image of the bottom half in either plot? If that’s the case, that’s the symptom of a loose neutral because we have blocks of movements on the two 120-volt legs.
Customer Symptoms of a Loose Neutral
Your customer may report this in a lot of different ways. They may report lights changing brightness, especially getting brighter. They may call it light flicker. They may have a UPS that beeps at them or operates even though there’s no outage. They may have a blown light bulb or equipment damage. They may have surge suppressors that blow.
Another might be their inverter. If they’re generating power with solar panels on the roof, the inverter, even though it’s not injecting power into the neutral, may actually sense the loose neutral and take itself offline. So the fact that the inverter is shutting off could be a symptom of a loose neutral. They could have arc fault breaker or GFCI breakers or receptacles trip.
Another one is tingle or shock when they touch something that should be grounded. This happens because typically ground and neutral are tied together at the service entrance, and that ground can form an alternate path for a neutral current to get back to the transformer. And when you have current flowing through the ground, you’re going to have a voltage drop across it, and that can put a voltage on what is the local ground for the customer. So the customer may be able to feel voltage on grounded exposed metal in their house. And so if they report getting a tingle or a shock, of course, that’s a serious situation in any case. But that could be a symptom of a loose neutral.
Alternate Paths and Complications
And there are other complications, too, where there are alternate paths back to the transformer. For example, current flowing through the cable TV braid. If the customer has cable TV, that braid is often grounded at their service entrance. And if we have another customer on the same secondary that has a good neutral that also has cable TV, that can provide an alternate path back through the braid of the coax.
Summary and Recommendations
Going back to the white paper, a loose neutral happens because of a break in that neutral connector. And because of the circuit theory here, we can see that the voltage forms on the two 120-volt legs is based on the resistor divider formula. If the two leg loadings are exactly equal, there’ll be no difference in the real neutral and the artificial neutral, and you won’t notice any symptoms at all. But as the customer switches lights off and on, you’ll see that.
Here in figure four, we actually have a graph of how this varies with the resistive load. It’s a nonlinear effect. And how much power is delivered with that, that’s the nonlinear aspect of the loose neutral.
But the bottom line is that you want to look for the customer symptoms of a loose neutral. As we saw on the slide and also listed here in the last section of the paper, when the customer complains about flickering lights, it may be a flicker problem, but it may be a loose neutral problem. This can be masked by LED lights that are well-designed that don’t change their brightness very frequently.
But the effect, really, it’s easy enough to check the loose neutral graph in every single phase recording. So regardless of why you’re making that PQ recording, if it’s a single phase service, go ahead and look at that loose neutral graph and look for that symptom of a mirror image. If you can draw a line down the middle of the plot and the top half is a mirror image of the bottom half, that is characteristic of a loose neutral. So go ahead and check that in any case if you have that PQ data available.
Contact Information
If anyone has any questions about the white paper or about loose neutrals, give us a call anytime at 1-800-296-4120, or send an email to support@powermonitors.com. Everyone have a great afternoon.