SURGE PROTECTION:the enemy within

Jul 1, 1999 12:00 PM, Bill Whitlock

Most people have been convinced that ordinary power lines are teeming with equipment damaging spikes and surges and have, without hesitation, installed a myriad of protective devices they believe will prevent costly system problems. This protection business is a huge, fast-growing industry with lots of players. In researching this column, I found an article urging electric utility companies to get in on the action, "Cooper recommends that the utility lease the meter socket arrester to their customers for several reasons. First, the monthly revenue (often between $4 and $9, depending on location) is a long-term source of income" [ref 1]. All too often, science and reality take a back seat to sales and profits.

Do not misunderstand-such protection is good, but only if applied thoughtfully. In fact, the meter socket arrester is an excellent idea because it completely avoids the subject problem. The real problem is the haphazard use of common all-mode protectors at AC outlets or outlet strips. In many cases, this practice causes either system noise problems or hardware damage. The noise is heard as pops and clicks in audio systems, seen as specks or sparkles in video systems, or experienced as a crash or lock-up in computer systems. The hardware damage does not usually occur in the power supply, where you would expect it, but in the signal I/O interfaces that connect to the outside world.

What they are Normal 120 V (RMS) AC power alternates between peak voltages of +170 V and -170 V. Power-line spikes and surges are generally defined as short-term over-voltages with spikes characterized as having higher peak voltages but shorter durations than surges. The vast majority of protective devices use a component, such as a metal oxide varistor (MOV), to limit the peak voltages on power lines by drawing large currents when the voltage attempts to exceed the clamping or let-through level. Most simple suppressors use these devices to divert or shunt the resulting surge current, causing large pulses of current, hundreds or thousands of amperes, to flow in the circuit during the surge.

Underwriter's Laboratory (UL) has rated surge suppressors for safety per specification UL 1449 for some time, but in 1996, it collaborated with the U.S. government to produce the UL1449 Adjunct Classification performance specification. This spec classifies suppressors in several ways and helps to promote the use of standardized terminology. Mode 1 is defined as line (B or black wire) to neutral (W or white wire), and is also called normal-mode or differential-mode. Mode 2 adds line (B) and neutral (W) to safety ground (G), and is also called all-mode. Surge energy from a lightning strike to the power line, for example, will enter a facility in Mode 1. Note that surge voltage between neutral (W) and safety ground (G or green wire), also called common-mode, cannot exist at the service entry panel because code mandates that these conductors be bonded together as shown in Figure 1. Common-mode surge voltages are coupled from the line (B) to neutral (W) by branch circuit loads, tend to increase with distance from the bond at the service entrance, and are usually much lower in voltage than normal-mode surges [ref 2]. In spite of this, most commercial suppressors are Mode 2, which diverts surge energy from line or neutral to the safety ground. In real-world systems, these suppressors can be a liability-the dumping of surge currents into the safety ground can have dire consequences.

System level effects Nearly all equipment is grounded, via the third pin on its power cord, to the electrical system's safety ground. For reasons stated in previous columns and other writings, this ground is adequate, safe, and legal; do not defeat it! Therefore, depending on their physical locations and the building's wiring, any two pieces of equipment will have their grounds connected via some length of the building's safety ground wiring. If both devices are plugged into the same outlet, this length will be small, but, if the devices are powered from different branch circuits (breakers) or operate on isolated power (orange outlets with dedicated grounds), the ground wires may be quite long. Most transient over-voltages are high-frequency events, having most of their energy well above 100 kHz. At these frequencies, long wires, regardless of their gauge, have high impedance and will develop extremely high voltage drops when carrying the high current pulses created by MOV clamping. For this reason, the wires connecting the distant protected outlet are shown as inductors L1, L2 and L3 in Figure 1. Figure 1 shows the effects of a 6 kV spike arriving on the incoming utility power on a common computer-to-printer hookup. During the spike, a brief current of perhaps 2,000 amperes will flow in the paths indicated by the solid arrows. Under these high-current conditions, the clamp voltage of the MOVs may rise to about 600 V. Note that about a third of the spike voltage appears across the lengths of the neutral (W) and safety ground (G) wires connecting the protected outlet to the breaker panel. This outlet's ground and the ground of anything plugged into it, jumps to 1,800 V relative to the earth ground at the breaker panel. This voltage is likely to reduce interface circuitry in the computer, printer or both to silicon vapor. More frequent low-voltage spikes (down to the low-current MOV clamp of 300 V or so) will still cause high-current pulses to flow in the same loop. These smaller noise spikes between the grounds will cause errors or lockup. Remember that RS-232 and printer parallel ports are unbalanced and prone to ground noise. In my opinion, a great deal of unexplained computer behavior is caused by this kind of problem, and I am certain it causes many audio and video system noises.

Surge protection is something that must be designed and implemented methodically. The absolute best place to guard against incoming spikes and surges is at the service entry panel or a sub-panel that powers everything in an interconnected system. Unless your system operates on a branch circuit that is shared with spike-producing loads (air conditioners, refrigerators, light dimmers) this will most often be enough protection for even so-called sensitive loads. If surge suppression must be used at an outlet or outlet strip, do not use ordinary Mode 2 suppressors unless every piece of interconnected equipment is powered from the same protected outlet or strip. From a system noise (and hardware damage) point of view, the best suppressors operate in series mode. Although conventional Mode 1 suppressors may simply consist of an MOV placed across the line, they still cause high spike currents that circulate in wiring. Series type Mode 1 suppressors, however, use inductors to limit and a capacitor bank to absorb high-frequency energy, which is then slowly released into the neutral wire. Such suppressors from New Frontier Electronics (www.surgex.com) have met the highest possible A-1-1 performance and endurance ratings in UL1449 tests.

1. C. Plummer, Storm Trapper "HSE Residential Lightning Protection Program," The Line On-Line, Cooper Power Systems, April 1997.

2. F. Martzloff, "The Propagation and Attenuation of Surge Voltages and Surge Currents in Low-Voltage AC Circuits," IEEE Transactions on Power Apparatus and Systems, Vol. PAS-102, May 1983.