At the Institute of Electrical and Electronic Engineers (IEEE), members of working group 3.6.6 have been busy developing the Guide for the Application of Surge Protective Devices in Low Voltage AC Power Circuits. The guide applies specifically to surge protection devices (SPDs) that connect to the load sides of main overcurrent devices found at service entrances with 50 Hz or 60 Hz AC power circuits rated at 100VAC to 1000VAC (see the photo). The SPDs discussed are those manufactured for use with electrical power distribution equipment such as load centers, motor-control centers, panel boards, switchboards, switchgear, and end-use equipment installed in commercial and industrial facilities. The guide describes the effects of SPD operation in low-voltage power distribution systems as well as the coordination of multiple SPDs on the same circuit. The following is a brief overview of the information contained in the application guide.
One of the reasons for developing the guide is to clear up some misconceptions pertaining to SPDs. For example, installing one or more SPDs in a facility or within specific equipment doesn't provide total immunity from all power disturbances. In reality, properly connected SPDs provide protection from some transient voltage variations, but not from other events such as sags and swells.
Another misconception is that the installation methods and locations of SPDs don't influence their functionality. In fact, the functionality of any SPD directly relates to the specific location where it connects to a low-voltage power distribution system and the methods used to make the required electrical connections to AC power circuits.
These are just a few examples of the issues addressed in the guide. To help the reader easily identify the important elements of SPD specification and installation, the guide has been divided into 16 sections. I'll cover several of these sections in the remainder of the article so you'll know what to expect when the IEEE publishes the official document.
This section indicates, among other things, that the guide only discusses transient overvoltages or surge events that originate outside of facilities and impinge on power distribution systems through service entrance conductors. Overvoltages or surges that originate from equipment within a building are not within the scope of the guide, nor is protection against direct strikes to a facility.
The overview section also stresses the importance of preinstallation reviews and evaluations. Failure to consider the applications — or misapplications — of any SPD can negatively influence the expected performance of the device and result in undesirable effects on power distribution systems, end-use equipment, or both.
Power Distribution Systems
This particular section details a number of aspects of power distribution as they relate to SPD overcurrent protection specifications. For equipment intended to break current at other than fault levels, listing agencies and electrical codes require an interrupting rating at nominal circuit voltages sufficient for the interrupted current.
An interrupting rating is generally understood to mean fuses and breakers, but it has a broader interpretation. The ability of an overcurrent device to safely interrupt high levels of fault current does not necessarily mean that equipment connected downstream can withstand the let-through value of the available short-circuit current.
For example, let's assume that a component has been tested at 25 kA of rms symmetrical short-circuit current and has acquired a short-circuit current rating and listing based on a value of 25 kA. Let's also assume that the overcurrent device ahead of the component has a 65 kA interrupting rating and the available short-circuit current is 42 kA. In this example, there would be an improper match, unless the overcurrent device has the ability to limit the let-through available short-circuit current to a value of less than 25 kA. The guide suggests that merely providing an overcurrent device ahead of an SPD with a sufficient interrupting rating will not ensure adequate short-circuit protection for the SPD.
Another important aspect relating to SPD overcurrent protection involves maximum continuous operating voltage, or MCOV. Sustained overvoltages can dramatically affect SPD operation. The guide recommends that specifiers and users watch for potential fluctuations in service-entrance line voltages and choose SPDs with MCOVs above any expected higher-than-normal line voltages.
Grounded or ungrounded power distribution systems are determined by the intentional grounding conductor placed on the secondary windings of the distribution or power-class transformer that provides service-entrance power. The selection of a grounded or ungrounded power distribution system depends strictly on the purpose and intended use of the electrical system. There are advantages and disadvantages to each system, which the guide describes in detail.
For example, grounded, 3-phase systems provide a voltage stability that minimizes certain transient overvoltage conditions. Grounded wye systems have the advantage of operating both single-phase lighting loads and 3-phase motor loads without the installation costs of additional transformers. A “floating ground” condition is rare.
In addition, grounded wye systems easily locate phase-to-ground faults. During such faults, the voltage doesn't rise above the phase-to-ground potential. A potential disadvantage is the higher probability of damaging phase-to-ground arcing faults.
The traditional benefit of 3-phase, ungrounded systems is the ability to continue the operations of 3-phase motor loads if a phase-to-ground fault occurs during normal production or equipment operations.
However, ungrounded systems are not completely ungrounded and are often unstable. Although ungrounded, 3-phase power distribution systems have no intentional grounding conductor on the secondary winding of the supply transformer, a capacitive coupling exists between the system's conductors and the earth.
Ungrounded systems are susceptible to destructive transient overvoltages and continuous potential rises. The guide indicates that if engineers decide to use SPDs in ungrounded power distribution systems, they should make sure that all of the devices are rated and designed for the specific application.
The purpose of power system grounding is to stabilize the voltage rise between any phase conductor and earth or any phase conductor and neutral during normal operations, as well as to limit the voltage rise during abnormal conditions. Abnormal voltage conditions can originate from lightning or line surges, unintentional contact with higher-voltage lines, accidental grounding of system conductors, or arcing ground-fault conditions.
Power system grounding also provides a low-impedance path for the flow of current between system conductors and the earth, which initiates the operation of protective devices. An effective grounding system prevents excess voltage rises that can exceed equipment operating limits and insulation levels. An effective grounding system also ensures that staff members are not inadvertently exposed to the dangers of electric shock.
The IEEE application guide places paramount importance on a properly installed, low-impedance ground path for the satisfactory operation of an SPD. It highly recommends that specifiers and users evaluate the grounding requirements for SPDs. Here are a few example questions from the guide that can help in such an evaluation:
- How is the equipment to be protected — grounded or referenced to earth?
- Is there a common ground grid?
- Are all pieces of equipment effectively bonded via low-impedance conductive means?
- Are the distribution or power-class transformers, the building structure, and all equipment (to be protected) bonded and referenced to the same grounding electrode or ground grid?
- What is the resistance between the grounding terminal of the SPD; the equipment needing protection; and the reference grounding electrode, ground grid, or system?
- If the installation is not new, what is the respective age of the existing grounding system? Grounding systems often deteriorate over a period of time.
- Has the existing grounding system been regularly inspected, maintained, and tested?
It's important that the grounding system of any power distribution system receive as much attention in development, construction, and maintenance as its energized components and equipment. An SPD cannot be expected to provide adequate protection unless an effective grounding system exists.
Modes of Protection
Surges are coupled or transmitted in or through equipment by two modes. The first mode is normal mode, which is either line-to-neutral (L-N) or line-to-line (L-L). The second mode is common mode, which is either line-to-ground (L-G) or neutral-to-ground (N-G).
The guide suggests that any N-G mode of surge protection is unnecessary with an SPD installed in service-entrance equipment near the main bonding jumper. The same is true for panel boards, switchboards, and switchgear containing the first system-disconnecting means or overcurrent device of a separately derived power source. Any surge condition conducting through an N-G mode in service-entrance equipment would most likely be a direct lightning strike with high energy. Such direct strikes are extremely rare. However, in such cases, N-G surge protection would be the least of the equipment owner's worries.
The numerous, and sometimes conflicting, specifications presently used in SPD literature can become quite confusing. The guide distills many of the common items found in SPD specification sheets, including measured limiting voltages, suppressed voltage ratings, product response times, surge current ratings, and joule ratings. It also describes why some of these items shouldn't be used as part of an engineer's specification. For example, the guide specifically states that a “joule rating” should not be a factor in specifying SPDs.
Applying overcurrent protection within the electrical industry has become a fine art. However, the evolution of overcurrent protection devices and their coordination did not occur overnight. The concept of SPD coordination centers on the proper installation and connection of two or more SPDs at different locations within a power distribution system (i.e., one SPD is connected to the service-entrance equipment, and another is connected in feeder or branch-distribution panels or specific end-use equipment). Coordination is achieved when the SPD closest to the source of the impinging surge diverts the majority of the energy from the impinging surge, and a downstream SPD diverts the remaining or residual surge energy.
The successful coordination of SPDs requires a thorough understanding of the specific power distribution system and numerous variables that can affect the functionality of SPDs. Major variables affecting successful coordination of SPDs, as described in the guide, include the following:
- Waveform and duration of an impinging surge
- Distances between the SPDs and the power distribution system
- Distance between the origin of a surge and the sensitive end-use equipment requiring protection
- Voltage-clamping levels and response times of the components within a specific SPD
- An SPD's surge current capacity
- Age of an SPD
- Connections to and integrity of the grounding system of the power distribution system
- Modes of protection selected for each SPD in the power distribution system
- Configuration of a power distribution system
- SPDs integrally connected within end-use equipment
The Guide for the Application of Surge Protective Devices in Low Voltage AC Power Circuits is currently out for ballot. The approval vote and any ballot resolution requirements are pending. It should be available sometime in early 2003.
With the IEEE guide, specifiers and users will be able to apply SPDs with confidence, and in a manner that achieves safety, performance, and protection for the personnel, facility, and equipment involved.
The guide will ultimately become another addition to the IEEE's C62 family of documents, which deal with power system surges and surge protection. Other C62 papers describe the performance characteristics of SPDs, recommended standard test protocols for verifying performance, and guidance on the interaction between power system disturbances and SPDs.
This complimentary group of documents will provide all interested parties with a wealth of knowledge — knowledge that will help them successfully apply low-voltage SPDs in any situation.
PQ Doug Dorr is the director of business development for EPRI PEAC Corp. in Knoxville, Tenn. You can reach him at [email protected]. The task force that developed the draft version of the application guide is chaired by Frank Waterer, Square D Corp.