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Retrofitting can reveal a host of power-quality problems

The common practice, when renovating any structure, is to move in, get the renovation work done fast, and get out. This way, the owner gains a revenue-producing facility in the shortest possible time.With all this haste, though, it's important to survey the electrical power system in any structure undergoing renovation to make sure that it doesn't have elements, components, or wiring connections that

The common practice, when renovating any structure, is to move in, get the renovation work done fast, and get out. This way, the owner gains a revenue-producing facility in the shortest possible time.

With all this haste, though, it's important to survey the electrical power system in any structure undergoing renovation to make sure that it doesn't have elements, components, or wiring connections that can compromise the power system's performance. In the long run, any tenant occupying a renovated facility wants the connected equipment in the facility to perform satisfactorily.

An electrical power-distribution system review is best done at the outset of any renovation, when it's easiest and less costly. Any initial additional cost is small and the payoff can be huge. This applies especially to computer networks, communications systems or other electronic equipment since they will provide optimum performance.

The next concern is to pay close attention to workmanship during renovation and to make sure that no short cuts are taken and that contract specifications are followed. Why all this attention? Because today, most facilities are serving a computer-intensive environment involving lots of solid-state electronic equipment.

Let's look at a typical single-story commercial/industrial building. It's common practice to have the lighting loads served through lighting branch circuit distribution panels that are independent of other equipment or receptacle panels. But wiring modifications over time may have mixed up the feeder and branch circuit layout of many electrical loads. It's easy to think that even though there may be no National Electrical Code violations involved, adverse interactions of equipment could occur. For example, the building may have high-pressure-sodium or metal-halide lighting circuits and large motor circuits connected to the same distribution panel. Startup of large motors could cause a reduction in the circuit voltage at the lamp ballast that is below the minimum needed to sustain the lamp arc. So when the motors are energized, the HID lamps go dark.

Let's now go to particular case histories to see how renovation work can uncover power quality problems.

Case history No.1

An existing 20-year-old, single-story commercial building was renovated on a tight construction schedule to accommodate a data processing/telecommunications center. No close attention was paid to the actual condition of the existing power system during the retrofit. The concept: Let's do everything to help the tenant move in as soon as possible. After using the building for less than a year, the tenant noticed problems ranging from unpredictable and intermittent loss of data to outright equipment failure.

One problem was the ongoing failure of a capacitor used in the power supply unit of the numerous computer monitors. These failures were occurring so regularly that, on average, an individual monitor failed about twice a year. Additionally, they found that about 4,000 transmission errors occurred on the terminal server equipment in an average month. They also had re-occurring failures of printed circuit boards within some telephone equipment.

Responding to the company's request for help, a consultant specializing in power-quality concerns did a survey of the building. The consultant noted that, from the 1,200A, 480V/277 V, wye service, two feeder circuits extended to four step-down power transformers. The transformers consisted of two 75 kVA, one 45 kVA, and one 9kVA unit, and the secondaries of these transformers served associated load panels, which are commonly called lighting and appliance panelboards. The fluorescent lighting loads were served at a voltage level of 480/277V. A 120/208V three-phase, four-wire distribution system served the branch circuit receptacle loads.

Using a variety of portable testing equipment, the consultant could not detect any kind of power anomalies at the main service of the building. He surmised that all of the problems were to be found in the building's electrical distribution system.

Harmonics galore. One major deficiency uncovered in the building was excessive neutral-to-ground voltages in the branch circuits serving computer loads, partially caused by harmonic currents and specification for most modern telephone switching equipment.

Installing a master ground plate in the electrical room. A master ground plate was installed in the electrical room to serve as a common grounding point for the electrical system ground, the telephone equipment, the phone call processor, and the interior water pipe. Similar to what is called the "CO plate" in a telephone central office, this ground plate is a copper bus bar, 4 in. wide and about 20 in. long, secured to the wall with standoffs.

This installation meets the requirements of TIA/EIA 607: Commercial Building Grounding and Bonding Requirements for Telecommunications, satisfies electrical safety needs and insures proper telecom and computer equipment operation and protection.

Installing four new K-13 rated (or K-factor) step-down transformers. These transformers, which can accept harmonic currents, were installed to replace the original transformers. The electrical contractor noted that all of the original transformers had the neutral bonded to ground at both the transformer and first disconnect. The dual connection caused a considerable amount of 3rd harmonic (180 Hz) current to flow through the equipment grounding conductors and conduit raceways. This was a problem that existed when the building was erected, but the problem did not manifest itself until the non-linear loads were installed.

A K-factor transformer is a dry-type power transformer-with an oversized neutral-that serves varying degrees of nonlinear load without exceeding rated temperature-rise limits. Specifically, the K-factor ratings are 4, 9, 13, 20, 30, 40 and 50. The desired K-factor is calculated from the harmonic current content.

Connecting two new load distribution panels to the secondaries of the transformers. These panels have a 200% rated copper neutral bus. The beefed-up neutral bus in these enclosures can handle the harmonic currents and high crest factor present on the computer branch circuits.

Replacing all 20-A receptacle branch circuits serving the computers. This was done to accommodate a separate neutral conductor for each phase conductor and also a separate grounding conductor for each branch circuit run. All of the No. 12 conductors were upsized to No.10 THHN conductors to handle the harmonic currents and high crest factor loads. This minimized voltage drop and reduced neutral-ground voltages to less than 0.5V RMS. The IEEE 1100 Emerald Book recommends no more than 1% branch circuit voltage drop for sensitive electronic loads, and this criteria was met. The EMT raceways were increased to a 3/4-in. size to satisfy the larger cross-section area of conductors in the metal raceway.

Replacing aluminum feeders with copper. A 150-ft long 200A feeder using aluminum conductors was removed and new copper feeder conductors were installed. The copper conductors have a greater current-carrying capacity that aluminum conductors for equal cross section, and the new conductors did not exceed the code requirement on the cross section area fill of the PVC conduit in the slab. This change-out was done because the engineer was concerned about the harmonic current loading on the existing neutral conductors.

Case History No. 2 Again let's look at one floor of a renovated office building, in which a fast track construction procedure was used to get a computer intensive business up and operating fast. When the space was occupied, the tenant noticed repeated failures at various communications ports (network cards, modems, printer connectors). The tenant and the building owner thought that by putting in a piece of power conditioning equipment the problem would be solved, but that was not the case.

To uncover the problem, an electrician pulled the covers off of all the receptacle outlets and withdrew the outlet boxes so he could get a closer look at the wiring.

All of the conductors were correctly wired to the receptacles. However by using a flashlight to make a closer inspection, the electrician noted that, on some receptacles, the grounding was achieved by using a short pigtail from the ground terminal of the receptacle to a screw on the back of the device box.

This looked like a satisfactory procedure except that some of the outlet boxes are plastic. Thus, all of the receptacles attached to these plastic boxes lacked a proper ground connection. Replacing the plastic units with metal boxes solved the problem.

In summary You can extract some key ideas from these two case histories. First and foremost, don't think that you can buy a power conditioner or other device that, of itself, can overcome infrastructure problems. There is no plug-in solution for a missing neutral or lack of equipment grounding.

If you have power quality problems at a site where you are involved as an installer, always start at the power source. Do you have proper grounding and bonding? A sign that someone misunderstands grounding is a ground rod driven into a concrete floor and not connected to the main service ground. Yes, such a ground makes an excellent supplement to an existing ground system, but it does not provide an isolated ground. In fact, it promotes the circulation of ground currents.

Today, you can't get the assistance of the local electrical utility the way you could before deregulation. So, you will most likely need an electrical engineer familiar with power systems to assist you. Be sure to document what you find before making any changes or recommendations to the building owner or client.

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