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Abstract
Loose/open neutrals can present a dangerous and even life-threatening situation in a residential service. The classic voltage symptoms suggesting this condition are one single-phase leg rising in voltage, the other falling, while the total remains constant. Observing these signs requires monitoring both voltage legs, ideally at the service entrance. However, a 120V receptacle PQ monitor such as the Eagle 120 can also be useful in detecting indications of a loose or open neutral. A method for analyzing 120V receptacle data for these symptoms is shown here.
Loose Neutral Mechanism
As detailed in the white paper Spotting Loose Neutrals with Waveform Capture, this situation is caused by a high resistance or complete open in the neutral wire between the distribution transformer and the service entrance. With balanced loads on both 120V legs, in theory the return current through the neutral from each leg cancels out, leaving no actual neutral current flow in the neutral wire.
With any unbalance, the difference in current returns to the transformer through the neutral. The transformer provides a low-impedance center point that keeps the voltages (roughly) balanced regardless of load conditions. With a break in the neutral connection, the transformer center point is not available to the loads. Instead, the “neutral” point is now formed by the loads themselves - they form a voltage divider circuit, with the artificial “neutral” voltage determined by the ratio of the loading on one leg to the loading on the other. Meanwhile, the real neutral point provided by the transformer is essentially disconnected, separated by a high resistance from the rest of the service. As the load balance changes, the artificial “neutral” shifts in voltage potential relative to the true neutral, and also to ground.
Since the full 240V transformer output is unaffected, the classic loose neutral symptoms are:
- One leg rises in voltage
- The other falls in voltage
- The sum of the two leg voltages remains constant
Of course, to see this pattern in a recording requires both leg voltages to be measured. But there is another way to see the effect of a neutral shift.
The Eagle 120
The Eagle 120 (shown in Figure 1) is a full PQ analyzer designed for use with standard 120V receptacles. It has the same advanced capabilities as the 3 phase Eagle, including 256 sample/cycle sampling, waveform capture, harmonics, IEEE 1453 flicker, etc. More importantly for loose neutral detection, it measures and records two channels of voltage - hot to neutral, and neutral to ground. In the Eagle 120, power is drawn from the hot-neutral connection; this is measured as voltage channel 1. The ground voltage (as referenced to the neutral) is recorded as channel 2 (see Figure 2). For loose neutral detection as described below, the receptacle must be wired correctly and grounded.
Figure 1. Eagle 120 receptacle PQ analyzer
Figure 2. Standard 120V receptacle with hot (red), neutral (blue), and ground (green) connections
Receptacle Loose Neutral Detection
With the Eagle 120 connected to a branch circuit within a residence, one of the two voltage legs is monitored - the one feeding the circuit. If a loose neutral occurs which results in a neutral shift, that voltage leg will either rise or fall. While suggestive of a problem (especially if the voltage rises), this movement could also be a result of other normal operation. If the neutral at the service drop is open , the neutral seen by the Eagle 120 is now floating, defined by the loads themselves. Since the neutral is the voltage reference for the Eagle 120, it’s not possible to detect this change directly. The ground connection (assuming it is not also open) gives the extra information needed. If the neutral point moves (as happens the instant the voltage balance shifts due to changes in load balance), this will appear to the Eagle 120 as a change in neutral-ground voltage, i.e. a shift in the voltage on channel 2. The fact that the ground potential doesn’t shift allows the detection of the neutral shift, even with the neutral acting as the Eagle 120 measurement reference.
An example is shown in Figure 3. Here the RMS voltage increased from 119.7V to 127.0V on channel 1. While a voltage swell in a residential location can be suggestive of an open neutral by itself, even more evidence is shown in the bottom plot. Here the neutral-ground voltage is shown. The same increase (7.4V) is seen at the same time. A simultaneous increase by the same amount in both channels is very suggestive of an open neutral, due to the mechanism involved. Fundamentally, the change is a movement in the neutral reference point as it shifts from the transformer-supplied value to one determined by the loads themselves. Since the Eagle 120’s voltage measurement reference is the neutral, a shift in its potential shows up as a shift in the two voltages its monitoring, and by the same amount.
Figure 3.RMS voltage increase on hot leg due to open neutral (top plot), with matching neutral-ground voltage increase (bottom plot)
Figure 3 shows a branch circuit where the voltage increased during the open neutral. The Eagle 120 could easily see the opposite case, since half the service experiences a voltage swell, and the other half a voltage sag. The voltage sag case is shown in Figure 4. Here the channel 1 voltage fell from 118.8V to 114.4V, while the channel 2 voltage (neutral-ground) shifted by the same amount (0.2V to 7.6V). In both cases, the channel 2 voltage rises due to the lack of polarity in the RMS measurement - there is no negative RMS reading. Both potentials (channel 1 and channel 2) shift in concert again due to the movement of the neutral point during the loose neutral condition.
Figure 4. RMS voltage decrease on other hot leg due to open neutral (top plot), with matching neutral-ground voltage increase (bottom plot)
It’s not unusual for normal loading to cause RMS voltage sags, and if this current flows through the neutral in a branch circuit, some neutral-ground voltage change will also occur (for more on this topic, see the white paper Measuring Neutral Voltage Drop with the Eagle 120. Legitimate neutral voltage drop can make it more difficult to distinguish a loose neutral condition from normal loading. Two characteristics can help separate them though:
- The neutral-ground voltage change from a loose neutral is more likely to equal the voltage change on the hot-neutral channel. In some cases, normal loading will cause the neutral-ground shift to be half the value of the hot-neutral shift. This effect is due to current-induced voltage drop appearing on both the supply (hot) and return (neutral) wires. Since the neutral is the measurement reference, the hot-neutral voltage sag will include both those drops, while the neutral-ground increase will only reflect the change in neutral.
- Loose neutral conditions are often intermittent, causing very quick fluctuations. Zooming into the excursion from Figure 4 and enabling waveform capture markers (Figure 5) shows many waveforms triggered and much fluctuation even on a second-by-second basis. This erratic voltage movement is more suggestive of a loose or intermittent neutral than normal load changes.
Since the artificial neutral created by the loads varies, often the branch circuit feeding an Eagle 120 will see voltage swells as well as sags at different times in the same recording due to a loose neutral. The presence of both these conditions in one recording is even more suggestive of a neutral problem.
Conclusion
Although loose neutral is more typically detected by measuring both voltage legs in a single-phase residential service, there is still useful information for gathering evidence with 120V receptacle recorder such as the Eagle 120. Only one voltage leg is available, but the addition of the ground channel allows for detection of neutral potential movement. It’s important to note that the signs given above are only suggestive of a loose neutral condition, not definitive. Depending on the specific load balance at the time of the incident, it’s possible that few or no voltage movements are seen during a specific recording. Consequently, if other indications are present (e.g. customer describing erratic increase in incandescent light brightness, equipment overvoltage, etc.) a loose neutral should be suspected in any case. But if these voltage signs are detected in a recording, a more detailed physical investigation is warranted.
Chris Mullins
VP of Engineering and Operations
cmullins@powermonitors.com
https://www.powermonitors.com
(800) 296-4120