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Power Quality Measurements at Receptacle Branch Circuits

17 Sep 2021 | Power quality

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Many power quality problems show up at the branch circuit level. There's a simple reason for this: that's where most of the sensitive loads (and sensitive employees) are located. It's also the "end of the line" of the electrical system, and the place where shortcomings can't be hidden. Let's assume you've been called in to solve the problem. You've already talked to the people involved, have a rough idea of the symptoms (equipment lock-ups, intermittent resets or crashes, etc.) and as much sense of the timing and history of the problems as you can get. So it's time to gather hard evidence: it's time to take measurements.

Our primary focus with troubleshooting at the receptacle level is to determine if the Line- Neutral (L-N) voltage available is of sufficient stability and amplitude to supply the needs of the load(s).

Measurement

1. Waveform

The waveform gives us quick snapshot information. An ideal waveform would be a sine wave. In this case, (see Figure 1) the voltage waveform is flat-topped, which is typical of a building with many non-linear loads such as computers and other office equipment (see Figure 2). Our other measurements will tell us whether this flat-topping is excessive.

2. Peak voltage

The peak value is critical to electronic loads because the electronic power supply charges its internal capacitors to the peak value of the line voltage. If the peak is too low, it affects the ability of the caps to charge fully and the ability of the power supply to ride through momentary dips in the line voltage. For an RMS voltage of 115 V, the peak value would be 1.414 x 115 V = 162.6 V, if the waveform were a sine wave. However, as we just saw from the flat-topped waveform, what we have is far from a sine wave and will have a lower peak value.

3. RMS voltage

Nominal line voltage is measured in RMS (root-mean-square) which corresponds to the effective heating value. Equipment is rated in RMS, not peak, because their main limitation has to do with heat dissipation.

RMS voltage can be too high or too low, but it is usually the low voltage that causes problems. Low RMS voltage combined with flat-topping (low peak) is a deadly combination for sensitive loads.

Voltage drop is a function of both the loading of the circuit and the source impedance, which in effect means the length and diameter (gauge) of the wire run. The NEC (210-19.a, FPN No. 4) recommends a limit of a 3 % voltage drop from the branch circuit breaker to the farthest outlet, and a total voltage drop of less than 5 % including the feeder and branch circuit.

4. Recording (short-term)

The limitation of the above measurement is that it is static. Many loads require more current, usually referred to as inrush current, when they are first turned on. This momentary high current may cause a momentary low voltage (sag) because of the additional IR drop through the conductors. Such sags are often caused by loads drawing inrush currents on the same branch circuit, or on the same panelboard.

You can measure a worst-case sag of 100 ms or more (about 6 cycles at 60 Hz) by using the MIN MAX function of a Fluke 87 DMM while energizing the load. What if you want to know if there are recurring sags? Use the Sags & Swells trending feature on a Fluke Power Quality Analyzer to continuously capture sags. A four-minute to a one-hour recording time (i.e., anywhere from a single cup of coffee to a lunch break) may be enough to tell you if there are recurring sags and swells.

5. Recording (long-term)

For longer term recording, the Fluke VR101S Voltage Event Recorders will record sags, swells, outages, transients and frequency deviations while plugged into the outlet. The device can be left on-site, unattended, for days and weeks, all the time catching intermittent events (4000 event buffer). Now you can see why it's so important to ask the user to keep a troubleshooting log: correlation of equipment malfunction with voltage events is hard evidence of a PQ problem.

6. Neutral-to-ground voltage

Let's say that you make a simple L-N measurement at the outlet and get a low reading. You can't tell if the reading is low because the feeder voltage is low (at the subpanel), or if the branch circuit is overloaded. You could try to measure the voltage at the panel, but it's not always easy to tell which panel feeds the outlet you're measuring and it's also sometimes inconvenient to access a panel.

N-G voltage is often an easier way of measuring the loading on a circuit. As the current travels through the circuit, there is a certain amount of voltage drop in the hot conductor and in the neutral conductor. The drop on the hot and neutral conductors will be the same if they are the same gauge and length. The total voltage drop on both conductors is subtracted from the source voltage and is that much less voltage available to the load. The greater the load, the greater the current, the greater the N-G voltage.

Think of N-G voltage as the mirror of L-N voltage: if L-N voltage is low, that will show up as a higher N-G voltage (see Figure 4).

N-G voltage exists because of the IR drop of the current travelling through the neutral back to the N-G bond. If the system is correctly wired, there should be no N-G bond except at the source transformer (at what the NEC calls the source of the Separately Derived System, or SDS, which is usually a transformer). Under this situation, the ground conductor should have virtually no current and therefore no IR drop on it. In effect, the ground wire is available as a long test lead back to the N-G bond.

Shared neutrals

Some buildings are wired so that two or three phases share a single neutral. The original idea was to duplicate on the branch circuit level the four wire (three phases and a neutral) wiring of panelboards. Theoretically, only the unbalanced current will return on the neutral. This allows one neutral to do the work for three phases. This wiring shortcut quickly became a dead-end with the growth of single-phase nonlinear loads. The problem is that zero sequence current from nonlinear loads, primarily third harmonic, will add up arithmetically and return on the neutral. In addition to being a potential safety problem because of overheating of an undersized neutral, the extra neutral current creates a higher N-G voltage. Remember that this N-G voltage subtracts from the L-N voltage available to the load. If you're starting to feel that shared neutrals are one of the worst ideas that ever got translated to copper, you're not alone.

Solutions

Performance Wiring vs. Code Minimum

Any experienced PQ troubleshooter will tell you that the first place to look for most problems is in the building wiring system (including its grounding system). Quality power depends on quality wiring; the term the industry uses is performance wiring (See Table 2). The basic intent of performance wiring is to maintain or restore L-N voltage to the load. There is a distinction between "performance wiring" and "code minimum" wiring. The NEC sets the absolute minimum requirements for a wiring job and is primarily concerned with fire prevention and personnel safety. The NEC should, of course, never be violated, but it is also important to understand that the Code's objective is not to establish standards to achieve power quality. However, many facilities are finding that it pays to take the extra step and install or even retrofit a performance wiring job. As one veteran said, "If every building were performance wired, I'd be out of business. . . But there's no fear of that happening."

Power conditioning

There are also situations where receptacle-installed power conditioning devices are a good solution, either as a complement to the wiring changes or as an economically viable alternative to some wiring changes.

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