The proliferation of computers and other sensitive devices throughout our manufacturing and office environments has fostered the need to
design the electrical systems of buildings with an eye toward power quality issues.
The term power quality means different things to different people. One definition is the relative frequency and severity of deviations in the
incoming power supplied to electrical equipment from the customary, steady 60 Hz sinusoidal waveform voltage. These deviations may affect
the safe or reliable operation of equipment such as computers.
The sensitivity to such deviations varies from one piece of equipment to another. Poor power quality affects the reliable operation of computers and computer-based equipment, which are now so ubiquitous. Often more important than the physical effect on the equipment is the loss of productivity resulting from computer equipment failure, miscalculations and downtime.
Dealing With Power Quality Problems
The vast majority of power quality problems in a building originate within the same building. The Institute of Electrical and Electronics Engineers (IEEE), various governmental agencies and other organizations have issued design guidelines and recommended practices known to greatly reduce, if not eliminate, the incidence and severity of power quality related problems. In many cases, simply installing enhanced electrical
systems and better grounding systems will prevent (or cure) the problem.
Generally speaking, by following well-known formulas to determine the electrical loads for a given floor area, the designer of past decades
was reasonably assured of designing an adequate electrical installation that could be expected to serve the needs of the building and its occupants well into the future. There was seldom a need to be concerned about harmonics or transients. But time, progress and micro-computerization marched on.
Today most disruptions show up in random, difficult-to-reproduce ways, such as a PC that locks up, a PBX that loses calls or a motor that fails prematurely. According to the Electric Power Research Institute, as much as 80% of power quality problems relate to inadequate wiring or grounding.
The term grounded, refers to a system in which one of the elements is purposely connected to ground. Electrical systems need not be
grounded to function, and indeed not all electrical systems are grounded. But the voltages referred to when talking about electrical systems are usually voltages with respect to ground. Ground, therefore, represents the reference point, or zero potential point, to which all other voltages refer. And, for computerized equipment to communicate with other equipment, a zero reference voltage is essential for proper operation.
In most cases, the electrical service to most buildings installed over the past several decades is grounded. There are numerous exceptions. Whether or not a given electric service to a building is grounded - that is, purposely connected by a low impedance connection to the ground -
is determined by the rules of the Canadian Electrical Code (CEC) and the electric utility serving the facility.
Why Grounded Systems are Prefered
The primary purpose of grounding electrical systems is to protect personnel and property if a fault (short circuit) occurs. In simple terms,
if one of the three hot legs (phases) of an ungrounded electric service becomes grounded, intentionally or accidentally, nothing happens.
Ungrounded electrical systems were popular in industrial buildings of the first half of the 20th century precisely for the reason that motor-
driven loads, the most common at the time, would not stop simply because of a short.
But a consequence of this type of system is that it is possible for the frame of a piece of equipment to become energized at some voltage
above ground and present a shock hazard for personnel who may be touching the equipment and a grounded (zero volts) component of the structure simultaneously.
A second purpose of a grounding system is to provide a controlled, low impedance path for lightning-induced currents to flow to the earth harmlessly. The assumption in this document is of a grounded service installed in accordance with the Canadian Electrical Code.
Techniques that Help
There are a variety of techniques that can help prevent or alleviate the effects of poor power quality. Most simply involve better electrical
designs and installation of some additional wiring. These techniques are inexpensive to install, especially when a building is undergoing construction, and they may also be cost effective during retrofits.
The most serious consequence of poor power quality, frequently, is not the damage to physical hardware, but the lost data, reduced
productivity and costly downtime. Like most ailments, they are much easier and cheaper to prevent than to diagnose and cure.
Most techniques are part of the current IEEE recommended practice, and are contained in IEEE Standard 1100-1992 and/or Standard 142-1991.
Techniques for Handling Harmonics
1. Double-Size Neutrals or Separate Neutrals per Phase
2. Shielded Isolation Transformers
3. Harmonic Filters
4. K-Rated Transformers
5. Harmonic-rated Circuit Breakers and Panels
Techniques for General Wiring
Separation of Sensitive Electronic Loads from Other Equipment
Do not mix standard loads and sensitive loads on the same circuitry (or panelboards, if at all possible). A dedicated "computer" circuit in each office is a good idea, at least back to the branch circuit panel. A better idea is to power sensitive equipment from a separate electric
Limited Number of Outlets per Circuit
A maximum of three to six outlets (four is typical) per circuit is recommended instead of the twelve allowed by Code.
Metal conduit, property grounded, provides shielding of the conductors from RF energy. Always use a separate, full-size copper grounding conductor, irrespective of the conduit material, to provide a reliable, low impedance path to ground.
Although the CEC allows up to a 3% voltage drop in a branch circuit, recommended practice is to design for no more than a 1% voltage drop
at full load on branch circuits feeding sensitive equipment. Feeder voltage drop should not exceed 2%.
The chances of problematic connections which could cause voltage fluctuations in mild cases, and catastrophic failure in extreme cases,
are decreased with the use of copper conductors.
All metal enclosures, raceways, equipment grounding conductors and earth grounding electrodes should be solidly joined together into one continuous electrically connected system. All structural building steel should be bonded into a single electrically conductive mass and
connected to the required electric service ground at the service entrance, as well as to the equipment grounding conductor system and to
the metallic cold water system.
Isolated Grounds (IG)
Isolated grounding is a loosely defined technique that attempts to reduce the chances of "noise" entering the sensitive equipment through the equipment grounding conductor. There is no defined standard method.
A buried exterior ground ring is a technique to help achieve a low impedance from the building's grounding system to the earth itself and a convenient means to connect various conductors and other grounding elements leading from the building.
The resistance of the grounding electrode system should be checked upon installation and annually or semiannually, depending on the results.
Depth and Spacing of Grounding Electrodes
Generally speaking, deeper ground rods tend to be more effective than shallow rods, so a 20-foot rod is preferred to a 10-foot rod.
The general rule of thumb is that multiple rods should be spaced apart at least the length of one rod, and two times the rod length if possible.
That is, two 10-foot rods should be placed no closer than 10 feet apart, and 20 feet if space permits.
Lightning Protection Systems
In simple terms, if part of the lightning's "path of least resistance" to ground is through your wiring or equipment, that is where it will flow.
Lightning produces very high currents for a short time interval.
To provide the least resistive path, heavy gauge copper wire should be employed in the leaders and down-conductors.
Grounding of Lightning Systems
Design considerations covering lightning systems are found in the National Fire Protection Association's Code #780, Code for Protection
Against Lightning, and the Canadian Standards Association's CAN/CSA - B72 - M87, Installation Code for Lightning Protection Systems.
During construction or major renovation, when structures are exposed and workmen are on-site, the cost of extra materials or larger
conductors is minimal. The potential savings in lost production and downtime make these precautions a good investment.
In cases where power quality problems are encountered in an existing facility, a careful study will be necessary to determine the best course
of action. Solutions may be as simple as moving some loads among branch circuits, some minor rewiring or additional branch circuits. In some cases, installation of shielded isolation transformers or harmonic filters may be the best course of action. In difficult cases, professional engineering assistance is recommended.
Power quality problems frequently can be avoided entirely in new buildings by careful design of building systems, especially grounding and
Additional information on power quality, including a complete set of case histories and a videotape on harmonics, is available from the CCBDA
at www.coppercanada.ca, or toll free at 1-877-640-0946.
Click here to order the complete CDA/USA version
The information in this Primer is adapted from an Application Data Sheet, A Primer on Power Quality, published by the Copper Development Association (USA).
For a detailed report on Grounding and Bonding in Electrical Systems, click here.