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Measure the current on each feeder phase and branch circuit (Figure 9-5) It is important to make these measurements using a true-RMS ammeter or digital multimeter (DMM) Because the combination of fundamental and harmonic currents results in a distorted waveform, a lower-cost average-sensing meter will tend to read low, leading you to assume that circuits are more lightly loaded than they actually are Loading on the three phases should be as balanced as possible Unbalanced current will return on the neutral conductor, which may already be carrying a high load due to harmonics caused by nonlinear loads In an ideal, balanced, three-phase electrical distribution system, there is little or no load on the neutral Neither the panel feeder nor branch circuits should be loaded to the maximum allowable limit (80 percent of the overcurrent device rating, for continuous loads) There should be some spare capacity to allow for harmonic currents
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Harmonics
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Harmonics are frequencies that are multiples of the fundamental frequency (120 Hz, 180 Hz, 240 Hz, and so on) High-frequency harmonic currents caused by nonlinear loads such as computers, adjustable speed motor drives, programmable controllers, and fluorescent fixtures with electronic ballasts can cause significant heating in power distribution systems, particularly in grounded (neutral) conductors Harmonics affect the operation or 197
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9-5 Branch-circuit panelboard (Courtesy of Schneider Electric Company) 198
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equipment such as motors, transformers, and overheating of neutral conductors Triplen Harmonics Triplen harmonics are the following multiples of the fundamental frequency: 3, 6, 9, 12, and so on They are very harmful for power quality because triplen harmonic currents can add up in the neutrals of the three-phase power systems, as shown in Figure 9-6 Nonlinear loads include such common electrical equipment as switched-mode power supplies used in computers and their peripherals, and fluorescent or HID fixture ballasts Overloaded neutrals are a potential fire hazard because, unlike phase conductors, they are not protected by an overcurrent device Third harmonics can overload system neutral conductors even when loads have been balanced among the three phases For this reason, National Electrical Code 31015(B) (4)(c) requires that On a four-wire, three-phase, wye
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100 A Nonlinear 100 A Nonlinear
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100 A Nonlinear
200 A0
Amps
100 A3 Amps
Up to 200% of the phase current for harmonic neutral current
9-6 Effect of harmonics due to nonlinear loads 199
circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor In effect, this requires that neutral conductors of such three-phase, four-wire systems be at least the same size as the phase conductors In practice, neutrals of systems serving a high proportion of nonlinear loads (such as office areas with multiple computers and fluorescent lighting) are sometimes even larger, up to double the size of the associated phase conductors (Figure 9-6) Multiwire Branch Circuits Common neutrals shared by either two or three singlephase branch circuits are subject to the same overloading as neutrals of three-phase panel feeders, due to asymmetrical loading and third harmonics Harmonic currents in feeder or branch circuit grounded (neutral) conductors can be measured using a DMM, or by using a probe-type meter to measure the potential from neutral to ground (Figure 9-7) Grounding The neutral and grounding electrode conductor should be bonded together only once, at the service entrance or distribution point of a separately derived system Other neutral-ground connections elsewhere in the system, such as subpanels or receptacle outlets, are a violation of the National Electrical Code Unfortunately, improper downstream connections between neutral and grounding conductors are also 200
Ground bus
PLUKE 87 III
RANGE
HOLD
mA A
A mA A COM V
Neutral bus
9-7 Measuring neutral current
very common, and they are frequently a source of power quality problems When the neutral and grounding electrode conductors are bonded at a subpanel or other location, the ground path becomes a parallel return path for normal load current, and there will be measurable current on the ground To determine whether improper connections exist, measure the current on the grounded (neutral) conductor and then on the grounding electrode conductor and look at the ratio between them For example, if the neutral current is 70 A and the ground current is 2 A, the small ground current probably represents 201
normal leakage But if the neutral measures 40 A and the green ground measures 20 A, this probably indicates that there are improper neutral-ground connections The smaller the ratio of neutral-to-ground current, the more likely it is that neutral-ground binds exist All neutral-to-ground connections not permitted by the National Electrical Code should be removed This will improve both safety and power quality
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