Transcript
Introduction to Loose Neutrals
Hello, everyone, and welcome to today’s white paper webinar. Today, we’re talking about loose neutrals, and in particular, loose neutrals that are complicated by grounding issues.
A loose neutral is a pretty severe and possibly dangerous power quality issue. It’s really at its root isn’t a power quality issue, even though the complaint usually originates that way, but it’s really a safety issue, because it can cause equipment damage, it can start fires, it can even burn homes to the ground.
How a Loose Neutral Works
In a normal loose neutral situation, we have a break between the neutral at the single phase transformer and the connection at the meter base. With a loose neutral, you have a break between the center tap of the single phase transformer and the neutral connection at the meter base. It could be at the transformer, could be at the meter base, it could be along the way, it could be at the weatherhead, but there’s no connection or it’s intermittent.
What happens in this situation is that the difference current between the two 120 volt legs can’t flow back to the center tap of the transformer.
Single Phase Service Basics
In a single phase service, we have two 120 volt legs that are 180 degrees out of phase. We have one transformer winding, and it’s 240 volts across the full winding, but that center tap gives us two 120 volt legs. On a vector diagram, we have two 120 volt phasors that are 180 degrees apart, and the voltage across both of these is the full 240 volts on the transformer.
If we measure relative to the center tap, that neutral point, we have two legs that are equal and opposite in phase, two 120 volt legs. The neutral connection holds that point constant.
The Artificial Neutral and Voltage Divider
What happens when we have a break in the neutral is that we no longer have a solid connection to the center tap, so we have an artificial neutral. In the example we give in more detail in the white paper, we have a break in the neutral. We have two different loads on the 120 volt legs, nine ohms and eight ohms. This corresponds to roughly 1,600 watts and 1,200 watts of load on the two legs.
The difference in current between these two loads would flow through the neutral, but that can’t happen here because we have a break in that. So we have a voltage divider formed and an artificial neutral formed by the ratio of these two loads. If those two loads are equal, then this artificial neutral will still be at the midpoint of the transformer’s windings and there’ll be no voltage across here.
As the loads vary and get different on the two legs, this artificial neutral floats with respect to the real neutral. In this case, with nine ohms on one leg and eight ohms on the other, that puts seven volts across here. That means if we’re measuring with respect to the artificial neutral, one voltage leg goes down, one goes up. They both move by seven volts, but in equal and opposite directions.
Classic Signs of a Loose Neutral
That brings us to the classic signs of a loose neutral. One leg goes up in voltage, one goes down by the same amount, equal and opposite directions. The classic symptom from a customer is, “Well, my lights suddenly got brighter.” Any lighting that’s on the leg that gets bigger gets brighter. Any lighting on the other leg gets dimmer.
There’s a really good YouTube illustration of this. In the video, you see the link on the screen, they’re demonstrating this principle. We have D cell batteries representing the two 120 volt legs, four batteries each. We have incandescent bulbs, and there’s a different amount of loading on each leg. One leg has three bulbs, one leg has five bulbs, and this white wire in the center is the neutral.
When he connects the neutral, the bulbs are equal brightness because they have the same voltage across them. When he disconnects that white neutral, that forms the artificial neutral, and the voltage on one set of bulbs is higher than the other. The one with three bulbs gets brighter. The one with five bulbs gets dimmer when the neutral is open, because now we have unbalanced loading. One leg goes up, one leg goes down.
How Grounding Complicates the Situation
But life is more complicated than this because we have grounding that makes this more complicated. If there’s a solid connected ground, which in general there is on single phase residential loads or services, there’s an alternate path for this neutral current to flow back into the transformer secondary, and that’s through the ground.
That ground resistance is typically much, much higher than a real neutral conductor. A neutral conductor is gonna be in milliohms. The ground resistance is gonna be in tens of ohms, maybe even hundreds of ohms. And naturally, that ground resistance is gonna vary with the humidity, with rain, with soil conditions, with temperature. So there’s all kinds of variations here.
What this does is kind of blur the symptoms of the loose neutral, because as this ground resistance gets smaller and smaller, you have more and more current flow through it, and it looks more and more like a real neutral. The voltage shifts will be less as this ground resistance gets smaller. In the limiting case, if the ground resistance is tiny, meaning it’s a good conductor, it almost functions as a neutral. But in the opposite case, it gets higher and higher, it’s an open circuit, and you’re left with a standard open neutral.
Voltage on the Customer’s Ground
The other important thing to keep in mind is not only will it somewhat mask the symptoms, but it will also put a potential on the ground the customer is seeing. Now you have a resistance and current flow through the ground. Ohm’s law says you’re gonna have a voltage drop across it. Remember V equals IR. So if there’s current flowing through the ground and the ground has a resistance, there’s gonna be voltage across it, just like the open neutral.
That means the ground over at the pole or the transformer is gonna be a different voltage than the ground at the customer, and a customer may be able to feel that. If they’re grounded and they’re touching something that’s grounded differently, they may be able to feel that voltage. It can be several volts.
Symptoms of Loose Neutral with Grounding
That brings us to another symptom of loose neutral: customers reporting that they’re feeling tingles or getting shocked when they touch something that’s normally grounded. Here are the symptoms of loose neutral:
- Lights changing in brightness, especially getting larger in brightness
- Ground fault breaker trips
- UPSs that operate frequently even though there’s no outage
- Reports of feeling a tingle, getting shocked, or feeling something when touching something that should be grounded
Alternate Current Paths Through Other Connections
There’s another situation where the grounding can make things more complicated, and that’s when there’s alternate paths through other types of connections. In this example, we have two homes on the same secondary. The house on the right has a break in the neutral, so normally current can’t flow back to the transformer secondary due to imbalances in this house.
But there’s a complication. Both houses on the transformer secondary have cable TV, and the cable TV ground is connected to earth ground typically near the service entrance. Also, neutral and ground are bonded at the service entrance. So that provides a sneaky path back to the transformer secondary.
Neutral current can flow through the shield of the coax cable through to the other house, through its cable TV shield back to where its neutral is connected to ground, and then through its neutral conductor, which is still good, back to the transformer center tap. In this situation, one house has an open neutral, one house doesn’t. The house that still has the solid neutral provides that return current back through the cable TV shield and then back through the regular neutral.
Cable TV Coax as a Current Path
The cable TV shield is much higher resistance than the neutral connector itself, but it’s still in the ohm or sub-one-ohm range. So you’re still not gonna see a lot of voltage movement relative to an open circuit, because that braid, which is not designed to carry a lot of current, can still carry a fair amount. If we look at a typical RG-59 coax braid, that’s about one point ohms per 1,000 feet, somewhere between number 10 and 12 gauge wire. So that is not gonna show a lot of voltage movement, maybe a volt or two, not the many tens of volts you might see in an open neutral.
But keep in mind the current carrying capacity of number 10 or 12 wire. If you’re wiring something up to number 10 or 12 wire, that’s typically 20 amps or 30 amps for NEC and conduit. But a customer may have much more neutral current than that, and you end up with something like the picture here, where the braid is carrying so much current that it starts melting the insulation on that coax.
Field Checks for Alternate Current Paths
Sometimes the utility will get a report from a cable TV serviceman or technician that’s measuring the current. You can clamp an ammeter around the entire coax, and if that current is an amp or two, that’s a red flag from the cable TV technician, and he will often contact the utility. If you are the utility and you’re out in the field, you can clamp on an ammeter around that coax just as a spot check. Another possibility is measuring the voltage at an outdoor water faucet to the ground rod to see if there’s any voltage there.
Summary and Key Takeaways
The bottom line is to keep in mind the fact that a loose neutral is not always as simple as the classic situation where there’s a break in the neutral and that artificial neutral can float wherever it wants. In many situations, there is that alternate path either through a cable TV coax, or if both houses have natural gas or some other metallic piping that is connecting them together that’s also grounded. There are sometimes alternate ways of getting current back to the transformer.
In this situation, we don’t have current flowing through the earth ground. We have current flowing through an actual conductor that’s grounded, giving us an alternate way back to the transformer. But even with just one house on the secondary, you still have that ground resistance that can alter things.
Not only does that somewhat mask the symptoms of a loose neutral, it’s not a perfect mask because the ground resistance is still tens of ohms or higher, much higher typically than the neutral conductor. So you still have a fair amount of voltage shift. But what that does is put voltage on that local ground for the customer. If the voltage across there is 10 or 20 volts, one leg moved up by 10 volts, one leg moved down by 10 volts, that’s gonna cause the classic flickering lights situation. But it’s also gonna put 10 volts on their ground, and they may feel that if they can touch something that doesn’t have the voltage across it.
So keep in mind the possibility of an open neutral if you have complaints from customers or reports from customers that they’re getting a tingle or can feel electricity on something that’s metallic or are actually getting shocked.
Contact Information and Further Learning
That wraps up this whitepaper webinar. If you have questions about the whitepaper or questions about loose neutrals in general, give us a call anytime at 1-800-296-4120, or send an email to support@powermonitors.com. If you want to sign up for our full one-hour class on open neutral, we have an entire series of free online classes, including one that’s dedicated purely to open neutral, where we go into more detail on how to recognize the signs in power quality data. Everyone, thanks for attending.