According to IEEE/ANSI Standard C62.41-1991 (formerly IEEE Standard 587), Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits, the worst normal mode surge expected inside a building is a 6,000V, 3,000A, 90 joule (J) combination wave. A normal mode surge is one that occurs between the hot (line) and neutral wires. This standard also states that the worst common mode surge — referred to in the standard as neutral-ground mode — is only found far from the service entrance and is a very weak 3,000V, 100A, 0.645J ring wave. This worst-case common-mode surge is already about nine times smaller than the suppressed surge left over after typical suppression, also referred to as the let-through surge. See the Sidebar below for joule calculations.

How can this worst-case common mode surge, which is 140 times weaker than the worst-case normal mode surge (90J vs. 0.645J) and weaker than the let-through surge, be a threat? The answer is that either this surge isn't a threat or the typical suppression isn't adequate.

Surge diversion: which path is best?. The most severe power line surges are lightning induced. In such instances, the surge voltage is seeking an earth ground return path. The only truly reliable return path within a building is the neutral wire.

While most modern buildings in the United States have neutral and ground wires bonded together at the service entrance, many older buildings lack a ground wire. So when older buildings are re-wired, compromises may occur, such as bonding neutrals at metallic enclosures. Also, if the neutral wire connection is bad, there will be no power delivered to the connected load.

If you have the option of diverting surges and noise to the ground or neutral wires, both of which should return to the earth ground, you should know how much surge and noise voltage those wires can handle in sample situations so you can make an informed choice.

Ground wire noise tolerance. Component audio and video systems are interconnected by cables that, for safety reasons, are often connected in some way to the electrical ground wire circuit. Their signals can be degraded by even millivolts of noise. On the other hand, computer systems may be able to tolerate several volts of noise on the ground system and continue to operate without problems.

So what happens when a shunt surge suppressor diverts a 6,000V surge to the ground wire? Simple math shows the voltage will divide between the connected wires, and nearly half the surge voltage can appear on the ground wire momentarily. This puts audio, video, and computer circuits, which can be disrupted by as little as 1V, in the path of 2,000V. Although this is an extreme case, it demonstrates the severe susceptibility of these systems to electrical noise and highlights the importance of protecting them against even the lower levels of electrical noise that may be present on the system.

Neutral wire noise tolerance. How much surge and noise voltage can the neutral wire tolerate (common mode)? Since the neutral wire carries power and the neutral and hot wires may inadvertently become reversed, Underwriters Laboratories (UL) instituted the dielectric withstand, or Hi-Pot, test. For safety reasons, UL requires that both the neutral and hot wires on electrical equipment be able to withstand at least 2,000V with respect to ground (common mode). This mandated neutral wire tolerance (common mode tolerance) is significantly greater than the millivolts of tolerance on the ground wire that can degrade or disrupt audio and video signals.

A suppressor manufacturer must choose how to return the 6,000V surge back to earth ground. Should it use the building ground wire — if the building has one — which has a tolerance to surges and noise of only a few volts, or should it use the neutral wire, which has a tolerance to surges and noise of at least several thousand volts? If you're trying to protect audio, video, or computing equipment, it seems obvious that ground wire surge diversion is less desirable than diverting the surge to the neutral conductor.

Is common-mode protection necessary?. A major UPS manufacturer has published a 4-page technical note that shows switch-mode power supplies, as used in most modern equipment, to be inherently immune to common-mode surges and noise, due principally to the high-frequency transformers and filters used in these supplies. However, due to their relatively weak nature, common-mode surges may not deserve the attention. With split-bobbin transformer designs, it's very easy to achieve a 6,000V dielectric withstand capability, which more than easily meets the 2,000V common-mode immunity required by UL.

Since the ground and neutral wires are bonded together at the service entrance, common mode (N-G) surges don't enter a building and can only develop far from the service entrance. As previously stated, the worst-case common-mode surge at this distant location is only 0.645J, which is 1/140th the size of the 90J worst-case normal mode surge. As the UPS manufacturer's report shows and as mandated, modern equipment is inherently immune to common-mode surges and noise up to 2,000V, principally for safety reasons.

To claim “protection” against common-mode surge (neutral-to-ground surge), manufacturers divert all surges to the ground wire. However, as stated earlier, this procedure can lead to audio, video, and computer noise and reliability problems. This procedure also has no benefit, since common mode surges are a non-problem due to their extremely low energy and the fact that equipment immune to them.

What to look for in a surge suppressor. The U.S. government recognizes the importance of using the proper mode for surge suppressors. In its power line surge suppressor specification, CID A-A-55818, the government makes the following definitions:

  • Mode 1: Normal mode (line-to-neutral suppression).

  • Mode 2: All modes (line-to-neutral, line-to-ground, and neutral-to-ground).

If you're looking to protect audio, video, or interconnected computing equipment, then use a U.S. government Mode 1 product (line-to-neutral suppression only). Stand-alone products like microwave ovens can use a Mode 2 (all modes) product, provided that no audio, video, or computing equipment is connected to the same circuit.

If the application is important, such as the protection of expensive and/or critical electronic equipment, you should focus on the endurance rating of the surge suppressor. For example, a suppressor with a certified endurance rating of 1,000 worst-case surges should last at least 10 years.

UL offers a service they call “UL 1449 adjunct classification,” whereby the firm will certify a manufacturer's claimed performance and endurance rating for its product. While available to any equipment manufacturer, only the manufacturers of higher performance and endurance products have elected to use this service.

Beware the ground wire. The building ground wire is very sensitive to surges and noise if audio, video, or networked computers are in use on the electrical system. Therefore, it's undesirable to divert surges to the ground wire if you're looking to protect this equipment. On the other hand, the building neutral wire isn't sensitive to surge and noise voltage, which makes it a good choice for returning surges back to the earth ground.

If you're trying to protect audio, video, or networked equipment, use surge protection products that provide “Mode 1” (line-to-neutral only) protection devices. If you choose the alternative, excessive noise or damage to the respective equipment data ports may result.

Harford is president and chief engineer at Zero Surge, Inc., Frenchtown, N.J.

Sidebar: Joule Calculations

  • The matched impedance energy delivered to a load for a 6,000V, 3,000A, 20μs surge is: 6,000V×3,000A×0.25×20μs=90J.

  • The matched impedance energy delivered to a load for a 3,000V, 100A, 10μs ring wave is: 0.86×3,000V×100A×0.25×10μs=0.645J.

  • The matched impedance surge energy let through from a sample “good” shunt suppressor for a dangerous surge is: 400V×3,000A×0.25×20μs=6J.