Answers to basic power-protection questions

The basic equipment for power protection includes surge suppressors, backup power supplies and power conditioners. Here`s a primer of these three types of devices.

Sep 1st, 1996

The basic equipment for power protection includes surge suppressors, backup power supplies and power conditioners. Here`s a primer of these three types of devices.

Ken Keil, The Wiremold Co.

Some of the frequently asked questions when considering surge protection and power conditioning are as follows:

- How much protection do I get from a particular model of surge protector?

- What is the difference between uninterruptible power supplies and standby power supply systems?

- Is a power conditioner the same as a transformer?

Getting the answers to these questions before purchasing power-conditioning and protection equipment can make the difference between effective decision-making and a costly mistake.

Surge-protection devices

Surge protectors may look alike from the outside, but they are not the same inside, and the performance characteristics of particular devices vary widely. Look for these three important specifications when choosing a surge-protection device:

- Underwriters Laboratories (UL) clamping category. Clamping refers to the voltage level of the surge remnant that remains after the bulk of the surge is diverted and dissipated by the surge-protection device. A lower number indicates more effective protection. UL (Northbrook, IL) groups surge protectors into categories that are based on a fairly wide range of actual clamping voltages. As a result, devices with significantly different performance may be found in the same category. The best UL category is 330V.

- Actual clamping voltage. The actual voltage that will pass through the surge-protection device to the equipment being protected is a measure of the let-through voltage, or actual clamping voltage. Once again, a lower number indicates better performance. Actual clamping voltage provides a more accurate assessment of a device than UL clamping category.

- Maximum surge current. It is important to keep in mind that the effects of surges are cumulative. A surge-protection device with a high maximum surge current will have a longer useful life than a device with a lower maximum surge current.

Some manufacturers classify their devices by joule rating or response time. These measurements are not recognized by UL, the Institute of Electrical and Electronics Engineers, the American National Standards Institute, the National Electrical Manufacturers Association, or the Independent Electrical Contractors as being relevant to AC surge-protection devices.

Backup power supplies

Although some use the terms interchangeably, uninterruptible power supply (UPS) and standby power supply (SPS) systems are not the same. Deciding which one to use depends on the sensitivity of the equipment the device is intended to protect.

Uninterruptible power supplies are intended to provide constant power to outage-sensitive equipment in case of a power failure. A true UPS continually provides power to the protected equipment. Therefore, no break in the supply of power to that equipment occurs, not even for a few milliseconds.

Standby power supply systems, on the other hand, provide power to the protected equipment in the event of a power failure, but they do so with a very short delay. This delay, typically referred to as transfer time, is on the order of a few milliseconds. Line-interactive standby power systems have built-in circuitry and capacitance designed to make the transfer time transparent to the connected equipment.

Some electronic systems, including most personal computers, have enough inherent capacitance in their internal power supply to hold over the power for a fraction of a second--long enough for the SPS to transfer from utility power to the backup battery in the event of a power failure. For this reason, the momentary power interruption caused by the transfer to the SPS battery may not be critical in stand-alone PC applications.

However, certain types of equipment, such as network servers and other mission-critical devices, demand a no-break transition from utility power to the backup supply. If this is the case, then a true UPS must be used instead of an SPS. Some manufacturers may promote standby systems as UPSs; however, if there is any transfer time involved, the unit should be classified as a standby system.

Another issue relating to the choice of a particular power-protection device is how to determine the correct size of the UPS or SPS for a given application. To avoid having too little protection or investing in unnecessary capacity, follow these steps:

1. List all equipment to be protected, including terminals, printers, and other peripherals.

2. Determine the volt-amperage (VA) required for all of the equipment. To calculate this, add together the amperes drawn by the equipment (this number is usually found on the nameplate or in the owner`s manual) and multiply the total by the output voltage. For example, if the total amperes drawn is 6 and the output voltage is 120V, the VA is 720 (6 x 120 = 720).

3. Add 15% to allow for future growth and round up to the nearest VA rating available in the product series you have selected.

Power conditioners

Isolation transformers, automatic voltage regulators and ferro-resonant power conditioners are devices that are often mentioned in the field of power conditioning. Although they are all varieties of transformers, they are not the same and do not offer the same benefits.

An isolation transformer generally isolates the load on the secondary side from the source on the primary side. Its main function is to suppress noise and ground interference.

An automatic voltage regulator provides voltage regulation through tap-changing transformer technology. These devices provide regulation by "tapping" the output voltage up or down based on changes to the input voltage. Automatic voltage regulators also usually contain devices for surge protection and noise filtering, but they do not provide any harmonic filtering.

Like an automatic voltage regulator, a ferro-resonant power conditioner provides voltage regulation, noise filtering, and surge protection. However, ferro-resonant power conditioners also offer excellent harmonic containment. The principle of ferro-resonance involves a saturated core, a magnetic shunt, and a loosely coupled transformer. The combination of these components produces a finely tuned circuit that resonates only at a specified frequency, usually 60 hertz. Therefore, changes on the input side are not reflected on the output side and vice versa.

The proper selection and application of these products in a coordinated and systematic approach to whole-building protection will help to ensure that all critical systems are properly protected. q

Ken Keil is product business manager for power- and data- quality systems for The Wiremold Co. in West Hartford, CT.

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