Basics of Surge and Transient Protection – Part 8

In this multipart series we will cover surge and transient protection for all types of signal, control and power lines. In Part 7 we looked at the more sophisticated and solid state types of crowbar devices including silicon controlled rectifiers (SCR), and thyristors. In this part we will address questions that have been sent regarding previous parts of this series

Brent Lehmkuhl wrote: I have seen some cases where a manufacturer will install a TVSS on the line side of the main service switch. Article 285 explicitly disallows this (285.21-A (1) and (2)) unless the device is also listed as a surge arrester. I have not seen a TVSS also listed as a surge arrester and am not convinced that in the manufacturers of the switchgear are using these correctly. The argument that I have heard is that since it is part of the manufacturers equipment and that the unit is listed (110.3(B)) that it is not subject to NEC. Is this correct? What gives a TVSS a surge protector listing? What is the main reason behind surge arresters only allowed to be installed on the line side and TVSS' only installed on the load side?

Underwriter’s Laboratories UL1449 is used to investigate the performance of the Transient Voltage Surge Protectors (XUHT). These devices are for installation on the load side of the Mains service disconnecting means.

The basic standard used to investigate products in the Surge Arrester Category (OWHX) is ANSI/IEEE C62.11, "Standard for Metal-Oxide Surge Arresters for AC Power Circuits." All other types of surge arresters are investigated to IEEE C62.1-1989, "Standard for Gapped Silicon-Carbide Surge Arresters for AC Power Circuits."

There are a number of devices listed as both TVSS devices and surge arresters. You can verify if a device is certified as a surge arrester or a TVSS or both by visiting www.ul.com and going to the UL Online Certifications Directory. Devices that comply with UL 1449 version 2 and ANSI/IEEE C62.11 are eligible for either UL registration or listing as both a TVSS and a surge arrester. That is categories XUHT and OWHX.

The use of a UL recognized surge arrester within a panel board or service entrance panel prior to the main disconnect would be eligible for a UL listing. The reason that devices are listed as surge arresters or TVSS devices is generally centered on their ability to clamp a surge and also their ability to interrupt current without needing an external overcurrent protective device. Thus a surge arrester has internal overcurrent protection capable of interrupting the available fault current present on the secondary of the facility transformer while a TVSS requires, as a minimum, the protection of the overcurrent protection device located at the main service disconnecting means (service entrance).

Matt Egloff wrote: The most common thing is the surge suppressor power strip or UPS, installed in the office or home with often questionable facility wiring. The big hitter is the home with the old 2-prong outlets or 3-prong with the ground pin tied to nothing. There are a number of bad ways to deal with this issue. I'd like to see more on this. With personal computers and servers, everyone has a UPS or at least a surge protector. How well do they do in a typical home or office, not the factory's lab where they have wiring up to code? The biggest problem is what happens to you computer when the lights go out, or the guy next door is welding or running a big half dead motor off a VFD?

The office and home of the future may have state of the art TVSS, but 99% of us work in buildings made before World War II, or in my case before World War I, and live in homes almost as old.

Dealing with surges on a two wire (no safety ground) system does complicate things a bit but it certainly does not create an insurmountable problem. The design of any surge protectors for use in such a facility would be line to line. If we assume a 115volt service the design would even be less costly than a standard protector. Unfortunately, I do not know of any protectors that are specifically designed for such circuits. If we employ a standard category A protector at the point of use there is protection between a floating point (the ground terminal) that is typically referenced to the hot and neutral conductors through two other MOVs. Certainly, this reduces the effectiveness of the protection especially if protection for LAN or TV cable is included in the TVSS. Reduction is not the same as elimination of the value of the point of use protector. At a minimum homes and businesses with two wire AC distribution should be equipped with IEEE Category C protection at the service entrance.

Next segment we will look at some more of the feedback that has been received concerning this series. I will try to answer any questions received by the end of this month either on this website or sent to ed.roberts@efrobertsassoc.com .

Ed Roberts

ed.roberts@efrobertsassoc.com

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