Enhancing Surge Protection Performance and Equipment Safety
Protecting electrical equipment isn't a new concept — lightning arresters were recognized in the NEC as far back as 1897. However, protection for electronic equipment within facilities is relatively new. Most companies that manufacture transient voltage surge suppressor (TVSS) products didn't start doing so until the late '70s or early '80s. The UL product safety standard for TVSS products wasn't even released until August 1985. That was followed by the NEMA LS-1, an industry-developed specification guide, in 1992.
With the addition to the 2002 NEC of Art. 285, Transient Voltage Surge Suppressors, the electrical industry has marked the latest milestone in its mission to protect electronic equipment. This document outlines the proper installation of TVSS equipment as an integral component of the electrical system. As industry experience and understanding has grown in the area of surge protection, the focus has shifted to understanding how to effectively use the surge protection component within the arsenal of power quality solutions. A key component of that is understanding how to effectively specify and install surge protection devices.
The primary function of any surge protection device is to identify, in a fraction of a cycle, an overvoltage on the electrical system and divert it to protect other equipment in the system. Although end-users expect a surge protection device to perform this function many times throughout its life, manufacturers in the electrical industry recognize that these devices may have a limited life, in comparison to the facility's entire electrical system. Based on these elementary principles, consider the following TVSS installation factors that will ensure a better performing and more reliable electrical distribution system.
Common knowledge dictates that connecting the surge device to the electrical system with the shortest conductor length possible provides the most effective protection. A TVSS installed as an internal part of a panel or switch provides the optimal design for the following reasons:
The impedance in the conduction path is minimized.
Protection levels don't become a variable based on the proximity of the TVSS outside the equipment.
Conductor length variability is no longer an installation issue.
The listing of the equipment with which the TVSS is integrated is maintained.
The 2002 NEC provides guidance to electrical system designers/specifiers or installers/contractors of TVSS products. In addition, UL 1449, Standard for Transient Voltage Surge Suppressors, requires TVSS devices to be installed only on the load side of the service disconnect and overcurrent device. The IEEE Emerald Book also supports this line of demarcation for the evaluated ratings of a TVSS for installation at the service entrance equipment.
Surge protection devices located at the service entrance equipment can divert surges that enter the system on the service conductors, but surges can also enter through branch circuits or feeders external to the building. Equipment operating within the facility can also generate surges. Equipment that cycles or switches on and off within the facility affects the quality of power being delivered to the rest of the facility. Consequently, you'll need to make decisions about adding protection at other critical panels that may feed computers or electronic loads.
Connection requirements for a TVSS at the service entrance are clearly defined in the 2002 NEC, but they continue to be misunderstood. The requirements of 230.71 allow only six service disconnecting means for each service. When you install a TVSS at the service as a component of a six-disconnect application, the 2002 NEC counts the disconnecting means for a TVSS at the service as one of those six disconnects. In other words, the TVSS disconnecting means isn't permitted as a seventh disconnect. During the development of the 2002 NEC, the NFPA Technical Committees (CMP-4 and the TCC) discussed a proposal to allow for a seventh disconnect specifically for a TVSS. However, the provision for a seventh disconnect specifically for the TVSS never made it into the 2002 NEC.
A proposal is on the table for the 2005 NEC that would no longer count the TVSS disconnect as one of the six service disconnects as long as it's installed within the service equipment. The proposal is based on the idea that when the service disconnecting means are turned off, no electrical power exists external to the service equipment enclosure.
Short circuit current rating.
The short circuit current rating (SCCR) might appear to be a non-issue for specifiers and contractors now that the 2002 NEC requires the TVSS be marked with this rating and UL 1449 requires UL-witnessed testing and certification of the SCCR. However, many devices aren't properly marked. It's your responsibility to ensure that the product you intend to use is marked correctly. This issue should be resolved by late 2003 when the UL product standard will begin enforcing the marking requirement.
Members of the surge protection industry are also attempting to address a concern they have with the UL product standard. Many industry professionals have written articles that discuss the failure of TVSS products at low levels of fault current, which make up the majority of fault conditions due to arcing and high-impedance fault conditions. TVSS manufacturers' instructions also indicate that this issue exists. For example, some TVSS products include 300kAIC (current limiting) rated fuses on surge protective devices for installation in systems with a much lower overall rating. Yet another instruction will require the TVSS to be installed on an electrical system with a minimum available fault current at the point of connection to the electrical system. These instructions indicate that the industry recognizes the issue but has yet to arrive at consensus on how best to address it. So TVSS design and appropriate overcurrent protection coordination (internal and external) present an equipment safety concern.
You shouldn't assume that a minimum fault current, such as 1,500A, will flow during a fault condition to clear the failing metal oxide varistor (MOV) inside the TVSS, because a high-impedance condition may reduce the current. Nor should you assume that a current limiting fuse will perform a current limiting function at low-level fault conditions. Discuss this SCCR issue with your TVSS provider. If you can't get a good explanation of how to resolve this issue, then the manufacturer hasn't addressed it as part of the TVSS product design.
Surge current ratings.
The industry continues to debate the surge current ratings on surge devices. Identifying the appropriate rating is challenging, especially when you consider the results of an industry report on lightning strikes conducted by the National Lightning Detection Network that presents data on 33,863 cloud-to-earth lightning strikes in the Tampa Bay area. More than 80% had a measured current magnitude of less than 30kA, and less than 2% were over 60kA. Based on this data, it's questionable whether there's any benefit to installing a TVSS with a rating higher than 60kA or 80kA.
While the higher rating on the device might seem superfluous, it will enhance the endurance performance of the surge device. If the electrical system is located in an area where lightning strikes are prevalent or line disturbances are a regular event, then increasing the rating may have some merit and extend the life of the device. At some point, however, higher ratings provide diminishing returns. The addition of TVSS components increases the suppression capability beyond expected surge levels, at which point it's possible to reduce the stress placed on each component and ultimately extend the life of the TVSS in an environment where it encounters repeated stress.
To ensure a properly protected and reliable electrical system with enhanced power quality, you need a basic understanding of TVSS design and application as a component of the overall electrical system. To achieve a reliable electrical system with enhanced power quality, make sure that the installation is NEC-compliant, paying special attention to the SCCR and the number of disconnects at the service. You also need to specify the appropriate surge ratings, lead length, internally mounted TVSS, and placement in the system.
Manche is a TVSS engineering manager for Square D/Schneider Electric in Lexington, Ky.
Sidebar 1: Follow These Tips for an NEC-Compliant TVSS Installation
Review the grounding system and its connections to verify an effective ground path for surge current [250.4(A)]. Remember, you can't install a TVSS on an ungrounded electrical system [285.3(2)].
Make sure a recognized certification body lists the TVSS device. You may find surge arresters installed that are listed as secondary surge arresters and permitted on the line side of the service disconnect [280.4(A)].
Make sure the specification and product markings address the SCCR on permanently connected TVSS products (285.6). Verify that your TVSS supplier is addressing intermediate short circuit current conditions between 5A and the SCCR on the device.
Install the TVSS device on the load side of the service overcurrent device [285.21(A)] and review the TVSS markings for the required overcurrent protection.
A TVSS installed at the service equipment must be connected to one of the six disconnects. Seven disconnects is a violation of the requirements of 230.71.
Review your panelboard specifications or a marking indicating the panel is listed for use with an internal or integral TVSS. Installing a TVSS within equipment not marked for use with that equipment is a violation of the requirements of 110.3(B).
Sidebar 2: The Importance of Surge Protectors
As they relate to the overall power quality effort in an electrical distribution system, surge arresters and TVSSs provide fundamental functions, such as:
Limiting the failure of electrical and electronic equipment caused by transients in the electrical system that exceed the intended operational properties of the equipment.
Extending the life of equipment that can be stressed over time due to surges imposed on the power system.
Removing surge events imposed on an area of the electrical system before they can affect the power quality of the system in other areas of the facility.