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Coordination Guidelines for Susceptible Electronic Loads — Part 2

As noted in Part 1 of this article, starting on page 22 of the September 2006 issue, there are many factors to consider before deciding how to provide suitable power, grounding, communications, and environmental control for a facility with significant amounts of susceptible electronic equipment. In the last installment, we discussed continuity of processing operations, facility location considerations,

As noted in Part 1 of this article, starting on page 22 of the September 2006 issue, there are many factors to consider before deciding how to provide suitable power, grounding, communications, and environmental control for a facility with significant amounts of susceptible electronic equipment. In the last installment, we discussed continuity of processing operations, facility location considerations, commercial power issues, and coordination and planning with the CPU vendor. Now let's get into the actual design parameters.

Designing/upgrading the power system Having gathered as much of the required background information on the facility and equipment as possible, you can begin the actual planning and engineering of the new or upgraded power system that will serve susceptible electronic equipment in your facility or your client's. Just follow the steps listed below.

Step 1: Compare power requirements with available power. Here's where you should compare the requirements of any susceptible electronic equipment with the existing or available power source. Remember, you can use a shielded transformer at the point of use for both voltage transformation and common-mode voltage isolation. You do the latter by referring the secondary voltage to a central grounding point.

Typically, the feeder voltage to the IT room or area is 480V, 3-phase, while the utilization voltages at most of the solid-state devices is 208V, single-phase or 120V, single-phase.

Step 2: Determine the supply voltage to the PDU. If the equipment in the IT room or area is to be fed by a power distribution unit (PDU) containing an isolation transformer and output circuit breakers, you must verify what input voltage is best and most efficient for the application. The voltage can be 208V, 3-phase, 480V, 3-phase, or 600V, 3-phase. The higher voltages are more efficient, have a lower percentage line voltage drop, and generally cost less to install for a given kVA rating.

In this type of installation, the secondary output circuit breakers and conductors feeding individual load units are not called branch circuits because they are system interconnections rather than part of the building wiring. As such, the power peripherals and their interconnecting cables are subject to UL 60950, “Safety of Information Technology Equipment,” examination and listing rather than inspection under the NEC or other applicable electrical codes for building wiring.

Step 3: Itemize loads and prepare wiring connection schedule. First, place each susceptible electronic load that draws more than 5A on a separate power peripheral circuit, with its own circuit breaker and interconnecting power cable.

Second, arrange all single-phase loads so that they will be evenly distributed over the three phases and neutral. In other words, balance these loads as best as possible.

Third, decide which loads must be fed from the PDU. Remember, not all electronic loads need to be fed from the same power source to avoid ground potential differences. By using separate shielded isolation transformers, you can feed specified loads from the UPS in the PDU. Other less critical loads, such as printers, for example, can operate on “commercial” power.

Fourth, use separate shielded isolation transformers to feed easily disturbed loads, such as memory, for example.

Fifth, check for loads having a DC component of current. These may include devices with half-wave or unsymmetrical rectification or SCR control.

Finally, check for loads having a very high harmonic content in their load currents. These can create operating problems due to saturation or overheating the first upstream transformer or motor-generator set. Besides high temperature rise, you may see excessive operating costs, poor efficiency, and distorted output voltage and input current waveforms. Such effects can render UPS systems inoperative and can cause very high peak magnetization current pulses at the inputs to some distribution transformers. You should refer such issues to the device manufacturer to verify what can be done to eliminate or reduce the problem. Otherwise, you may need to install special line conditioning equipment such as tuned filters.

Step 4: Examine site layout proposals for environmental compatibility. Here, you should be asking the following questions:

  • Has the plant manager or engineer placed and sized processing cooling locations according to heat-producing areas on the floor?

  • Has the IT manager addressed present and future underfloor space needs?

  • What flexibility has the IT manager planned for expansion of the power, grounding, and communications protection systems?

  • Has the IT manager planned for a central operator station for environmental control or access?

  • Has the IT manager planned the routing, type, and physical location of data communications for both present and future compatibility with the site?

Step 5: Coordinate vendor-provided equipment power conditioning with overall power quality goals. Individual vendor-provided power conditioning techniques may interact with each other to create unfavorable conditions, resulting in disrupted data processing. Make sure you examine each vendor device for its internal impedance so that you can coordinate all equipment having high and low dynamic impedance. By doing so, you'll soften and possibly reduce the distortions between individual solid-state power supplies.

Step 6: Incorporate unit cabling restrictions with room layout plans. Make sure you locate each unit so that it will not interact physically with any surrounding sensitive units, such as locating printers near open disk drives, allowing print dust to infiltrate the drives.

Matching system power requirements to power conditioning alternatives Part of your design process should include finding answers to the following questions.

How much capacity (kVA, kW) is needed? While the sum total of power in a specific list is the total “connected” load, the actual measured power usage may be less because each unit may not have all the options installed and may not draw the maximum connected load continuously or simultaneously.

First, make sure you include allowances for future system growth. Power requirements for memory and controllers, multiplexers, and exchanges needed to address and share large memory may grow faster than the rest of the system.

Second, divide the total power (W) by the floor space devoted to data processing use. Typical large systems use 50W/ft2 to 60W/ft2. So, a 50-ft × 100-ft room might involve 250kVA of power capacity. If your proposed installation's power capacity is substantially more or significantly less than this, you should verify the reasons why.

Can the power source handle short-term demands? In other words, verify if the power source's internal impedances and momentary overload characteristics are adequate to handle short-term demands without installing more capacity than needed for the steady-state load. Basically, the internal impedances should be low enough so that transient currents will not create excessive load-induced line voltage disturbances.

Have you verified the operating efficiency under partial load conditions? The cost of additional electric energy for a drop of 15% in efficiency (say from 88% down to 73%) can be in excess of $10,000 per year, for every 100kW of load.

You should compare the estimated total load with the expected steady-state actual running load of the system, and then use the result in determining the “partial load” ratio as a percentage of the nameplate kVA/kW of the power conditioning device. Rarely do installed systems run higher than 50% to 60% of the nameplate capacity of the power source.

To ensure optimum efficiency, make sure you request, in writing, specific equipment manufacturers' guarantees at 50% load operation, with the guarantee containing wording noting compensation to the end-user for operations not meeting this standard.

What are necessary power conditioning characteristics? Without supplemental energy storage devices, many units will ride through a 5 msec to 20 msec interruption of power without malfunction, provided that noise impulses associated with the interruptions do not reach and corrupt digital signals by other paths.

Ferroresonant and synthesizer transformers may slightly enhance ride-through capability in some cases, provided the loads are not sensitive to phase shifts during their correction of line voltage variation, and that the loads can be limited to 75% of the device rating.

Motor-generators can extend ride-through to as long as 20 seconds. Make sure you check the manufacturer of the equipment to be protected to determine whether this equipment needs synchronous 60-Hz power or whether it can tolerate induction motor drives with lower (varying) frequency of output.

UPS systems can typically extend ride-through from 5 minutes to 30 minutes or longer. UPS installations with emergency standby diesel engine-generators can extend ride-through to almost indefinite lengths of time.

Examine the power conditioner or independent power source for its limits on load current, kVA, and kW output. This will vary with the power factor of the load. Some devices, such as static UPSs, have very little overload capability and depend upon a stiff, low-impedance bypass power source (usually the serving public utility) to supply large starting loads. In addition, verify that the power source and any bypass will supply all inrush needs and still provide the needed power quality.

If you intend on using voltage regulators, make sure you verify that their response times are fast enough to follow line voltage changes, yet not interact with regulators in sensitive loads.

Finally, determine if a step-by-step approach to power conditioning will benefit your installation. For optimum lowest dollar outlay, first consider a power conditioner with provisions for in-the-field conversion back to battery back-up support at a later date, without loss of initial investment.

What are the predicted line voltage sags, swells, and impulse transients? You can expect ordinary switching of loads to create momentary impulse voltages as high as the peak value of the sine wave.

What are the significant sources of load-induced transients? You should consider the following ideas to minimize the effects of these transients:

  • Specify and order “soft start” system operations.

  • Put units with high inrush on separate circuits.

  • Put sensitive units and other units that create disturbances on separate shielded isolation transformers.

Grounding for consistent noise suppression One of the most important items you should consider in any design is the required signal and safety grounding. During the design process, determine the following.

Are grounding requirements of solid-state equipment manufacturers consistent with the IEEE Emerald Book? If not, you should discuss any differences and evaluate any underlying rationale. In fact, the manufacturer may have good reasons for its differing requirements.

Is a shielded isolation transformer required or recommended? This affects the point where logic ground conductors and power source neutral grounding points will come together at a common point.

How will all conductors be brought to the point of delivery to your system? Verify that all power, communications, and grounding conductors will come through one very close-coupled “entrance.” Scattering the entrances and exits for the wiring increases the risk of noise voltages and transient impulses circulating through the system.

Where will the system's central grounding point be located? If a modular power center, such as a PDU, is used, the central grounding point may be located within this equipment.

Is the IT room raised floor support structure suitable for use as part of the SRG? In other words, does the raised flooring's structure come with interconnected bolted horizontal struts that are suitable for use in a signal reference grid. This could save much money compared with construction using copper conductors or straps, and could also enhance performance.

Is there separation between ground conductors for different equipment? Grounding conductors for susceptible electronic equipment should be separated from those for nonsensitive equipment, except at some upstream common connection, which typically is at the building service entrance or other common separately derived power source.

Will the communications and power grounding systems be bonded together? This should be done at an appropriate upstream point. It's needed for safety and to minimize noise voltage differences without providing conducting paths through the grounding conductors of the susceptible systems.

After answering these questions, you should verify all of the following items:

  • All grounding conductors and conducting pipes penetrating the sensitive IT area are bonded together by a short, robust connection outside the area.

  • All susceptible units and their accessories are listed or approved by UL or other acceptable agencies that are recognized by the municipality in which the units are to be installed.

  • The premises wiring meets the local and NEC requirements.

  • The installation complies with the listing by the manufacturer.

Redundancy requirements Now that you've designed the basic power system for the susceptible electronic equipment, you need to explore the necessity for redundancy as part of further protection by following these steps.

Step 1: Perform a simple failure analysis. Here you would assume that each piece of equipment, its power source, wiring, and specific wiring device could fail or must be de-energized for maintenance or service. First, determine if the electronic systems will continue to function and recover in such instances. Then, determine if redundant bypass paths in the power sources around conditioning equipment and major pieces of electrical apparatus will provide the ability to continue in the event of failure, service, or replacement. Keep in mind probable failure rates and frequency of necessary maintenance.

Step 2: Determine how future growth relates to redundancy. In other words, should you include a margin for future growth in the form of smaller distribution units rather than initially selecting a single unit that is adequate for current and all future growth?

Step 3: Determine if floor space and HVAC capacity are sufficient. These are important considerations in that they affect the ability to install new systems and get them operational before dismantling the existing ones. You may not need power conditioning for this purpose because of reserve capacity, but you may face the problem of transferring the power source, with a minimum of service interruption, to the new system.

Lightning protection Protection from transient voltages caused by lightning can be a major factor in areas having a high incidence of thunderstorms. Following are important items you should address in providing this protection.

First, ensure the new or existing facility structure has a lightning protection system installed by a Master Label contractor in accordance with NFPA 780, “Standard for Installation of Lightning Protection Systems.” As a point of reference, buildings in which structural steel is bonded together by welding (as opposed to reinforcing steel in concrete, which can be electrically discontinuous or merely touching) offer better lightning protection of circuits.

Second, make sure that conductive parts from roof-mounted equipment (lightning rods, etc.) do not create a path to ground via susceptible circuits. Air conditioning cooling pipes from the roof to air handlers should not become a direct path for lightning to reach critical circuits or central grounding points. Also, verify that all metallic conduit and/or pipe runs enter and exit at a single access point and that all parts are bonded together and to building steel.

Third, make sure that lightning protection down conductors are separated from power and communication circuits by at least 6 feet to protect against induced noise. Also, these conductors should be taken to ground via a path separate from the equipment grounding system.

Fourth, protect all incoming power and communications equipment conductors with surge protection devices having shunt overvoltage paths to ground and series impedances to limit surge currents. The place for the secondary lightning protection is at the building service entrance. You should also place supplementary protection at the input and output of load devices such as rectifier/chargers for UPS installations, motor-generators, and voltage regulators, as well as the load devices themselves and their communication ports.

The location in which susceptible electronic equipment is installed requires a special electrical infrastructure that will reduce and even eliminate power quality problems and ensure reliable equipment operation. Basically, your electrical infrastructure design should include a dependable source of power, a grounding system that provides a stable platform for consistent equipment operation, and protection against transients.

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