b"FINDING THE CAUSE OF THE PROBLEM~ harmonic current makes this a distorted waveform. A lower-Depending on the voltage and measurement requirements,cost average-sensing meter will tend to read low, which you can use a variety of tools for power qualitywould lead you to assume that the circuits are more lightly troubleshooting, from digital multimeters to handheldloaded than they actually are.single and three phase power quality analyzers thatHARMONICS ~perform many calculations automatically.To check for the presence and level of harmonics, measure VOLTAGE LEVEL AND STABILITY current on the feeder neutral. This will typically be in the 80 Though this reflects in part the high pThe first step into 130 percent range of the feeder current, due to the fact checking to see if voltage levels and stability are thethat the third harmonic will add up in the neutral. Figure culprits is to measure voltage levels of the branch circuits,5a shows some readings that were made in an office, at phase-to-neutral, at the load side of the branch circuita lightly loaded panel. These waveforms were captured breakers. with a single phase power quality analyzer. Note that the neutral current (Figure 5b) is far in excess of what would Note: For safety's sake, when making voltagebe expected from unbalanced currents alone.measurements always keep a circuit breaker between youAlthough most of us are increasingly aware of the fact and the fault current ampacity of the feeders. that third harmonic currents (also called triplen or zero If voltage levels are low at the breaker, they'll be evensequence) generated by non-linear, singlephase loads add lower at the receptacle. This could be caused by low tapup in the neutral, we often wonder why.settings at the transformer. Other likely culprits include loose connections, long feeder runs, and overloadedGROUNDING~transformers, which create excessively high sourceNeutral ground bonds in subpanels are a violation of the impedance (impedance from the load to the source).NEC as well as of power quality performance wiring, but Source impedance and voltage drop are two sides of thethey are also quite common. Neutral ground bonds should same coin. be made at the transformer (although the NEC permits If intermittent voltage sags are suspected, start at theit to be made at the main panel). In any case, it should panel to isolate the cause of sags: Are the sags the resultnever be made downstream of the main panel, whether at of loads on the same branch circuit or are they causeda subpanel or a receptacle. When a neutral ground bond by loads elsewhere in the distribution system (includingis made at a subpanel or receptacle, the ground path utility-generated sags)? We can start to isolate the sourcebecomes a parallel return path for normal load current of the sag with a multi channel recording instrument, suchresulting in measurable current on the ground.as a Fluke power quality analyzer, that can trend voltage and current simultaneously. HOT SPOTS~UPSTREAM, DOWNSTREAM ~ Poor connections and the resulting heat losses are the What information are you looking for from the trendplots? single greatest source of system inefficiency (according to a 1995 study by the then Washington State Energy Office). If a voltage sag occurs simultaneously with a currentFrom the power quality point of view, loose terminations surge, the sag was caused by a load on the branchare a major contributor to excessive source impedance. circuit (Figure 2). In other words, the cause of the sag wasFortunately, they are easy to locate with a simple infrared downstream of the measurement point and therefore canthermometer.be thought of as a load-related disturbance. Infrared (IR) measurements with tools like the Fluke 60 If, on the other hand, the voltage sag coincides with aSeries are a safe and effective technique for non-contact very small change in current, the sag was likely causeddetection of panel hot spots. However, there are some key by something upstream of the measurement point andconcepts that are crucial to understand if we're making can be thought of as a sourcerelated disturbance. Typicalthese measurements.source-related disturbances are heavily loaded three-phase motors started across-the-line or sags originatingCIRCUIT BREAKERS~on the utility feed. If the sag is deep and approaches anA lot of people don't think of breakers as having finite outage, the cause is more likely to be the utility. The eventlifetimes. In reality, contacts and springs wear out. probably reflects a fault and breaker trip followed byMeasurements of circuit breaker voltage drop can help us automatic breaker reclosure. determine the condition of the breaker. Measure across the CURRENT BALANCE AND LOADING ~ line-to-load side of the branch breaker. If the voltage drop exceeds 100 mV, the breaker should be replaced. In the To check current balance and loading, measure each35 to 100 mV range, readings should be documented and feeder phase as well as current on each branch circuit.trended.When making these measurements, it is criticallyIn summary, the service panel is the crossroad of the important to use a true-rms clamp or true-rms digitalbuilding's electrical system and the place where an multimeter (DMM) with a clamp-on accessory. An averageexperienced electrical troubleshooter can start down the responding clamp-on meter will not provide an accurateright path to locate and fix any problems.measurement as the combination of fundamental and"