SURGE PROTECTION:the enemy within
Jul 1, 1999 12:00 PM, Bill Whitlock
Most people have been convinced that ordinary power lines are teeming withequipment damaging spikes and surges and have, without hesitation,installed a myriad of protective devices they believe will prevent costlysystem problems. This protection business is a huge, fast-growing industrywith lots of players. In researching this column, I found an article urgingelectric utility companies to get in on the action, "Cooper recommends thatthe utility lease the meter socket arrester to their customers for severalreasons. First, the monthly revenue (often between $4 and $9, depending onlocation) is a long-term source of income" [ref 1]. All too often, scienceand reality take a back seat to sales and profits.
Do not misunderstand-such protection is good, but only if appliedthoughtfully. In fact, the meter socket arrester is an excellent ideabecause it completely avoids the subject problem. The real problem is thehaphazard use of common all-mode protectors at AC outlets or outlet strips.In many cases, this practice causes either system noise problems orhardware 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 orlock-up in computer systems. The hardware damage does not usually occur inthe power supply, where you would expect it, but in the signal I/Ointerfaces that connect to the outside world.
What they areNormal 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-termover-voltages with spikes characterized as having higher peak voltages butshorter durations than surges. The vast majority of protective devices usea component, such as a metal oxide varistor (MOV), to limit the peakvoltages on power lines by drawing large currents when the voltage attemptsto exceed the clamping or let-through level. Most simple suppressors usethese devices to divert or shunt the resulting surge current, causing largepulses of current, hundreds or thousands of amperes, to flow in the circuitduring the surge.
Underwriter's Laboratory (UL) has rated surge suppressors for safety perspecification UL 1449 for some time, but in 1996, it collaborated with theU.S. government to produce the UL1449 Adjunct Classification performancespecification. This spec classifies suppressors in several ways and helpsto 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 callednormal-mode or differential-mode. Mode 2 adds line (B) and neutral (W) tosafety ground (G), and is also called all-mode. Surge energy from alightning strike to the power line, for example, will enter a facility inMode 1. Note that surge voltage between neutral (W) and safety ground (G orgreen wire), also called common-mode, cannot exist at the service entrypanel because code mandates that these conductors be bonded together asshown 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 fromthe bond at the service entrance, and are usually much lower in voltagethan normal-mode surges [ref 2]. In spite of this, most commercialsuppressors are Mode 2, which diverts surge energy from line or neutral tothe safety ground. In real-world systems, these suppressors can be aliability-the dumping of surge currents into the safety ground can havedire consequences.
System level effectsNearly all equipment is grounded, via the third pin on its power cord, tothe electrical system's safety ground. For reasons stated in previouscolumns and other writings, this ground is adequate, safe, and legal; donot defeat it! Therefore, depending on their physical locations and thebuilding's wiring, any two pieces of equipment will have their groundsconnected via some length of the building's safety ground wiring. If bothdevices are plugged into the same outlet, this length will be small, but,if the devices are powered from different branch circuits (breakers) oroperate on isolated power (orange outlets with dedicated grounds), theground wires may be quite long. Most transient over-voltages arehigh-frequency events, having most of their energy well above 100 kHz. Atthese frequencies, long wires, regardless of their gauge, have highimpedance and will develop extremely high voltage drops when carrying thehigh current pulses created by MOV clamping. For this reason, the wiresconnecting the distant protected outlet are shown as inductors L1, L2 andL3 in Figure 1. Figure 1 shows the effects of a 6 kV spike arriving on theincoming utility power on a common computer-to-printer hookup. During thespike, a brief current of perhaps 2,000 amperes will flow in the pathsindicated by the solid arrows. Under these high-current conditions, theclamp voltage of the MOVs may rise to about 600 V. Note that about a thirdof the spike voltage appears across the lengths of the neutral (W) andsafety ground (G) wires connecting the protected outlet to the breakerpanel. 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. Thisvoltage is likely to reduce interface circuitry in the computer, printer orboth to silicon vapor. More frequent low-voltage spikes (down to thelow-current MOV clamp of 300 V or so) will still cause high-current pulsesto flow in the same loop. These smaller noise spikes between the groundswill cause errors or lockup. Remember that RS-232 and printer parallelports are unbalanced and prone to ground noise. In my opinion, a great dealof unexplained computer behavior is caused by this kind of problem, and Iam certain it causes many audio and video system noises.
Surge protection is something that must be designed and implementedmethodically. The absolute best place to guard against incoming spikes andsurges is at the service entry panel or a sub-panel that powers everythingin an interconnected system. Unless your system operates on a branchcircuit that is shared with spike-producing loads (air conditioners,refrigerators, light dimmers) this will most often be enough protection foreven so-called sensitive loads. If surge suppression must be used at anoutlet or outlet strip, do not use ordinary Mode 2 suppressors unless everypiece of interconnected equipment is powered from the same protected outletor strip. From a system noise (and hardware damage) point of view, the bestsuppressors operate in series mode. Although conventional Mode 1suppressors may simply consist of an MOV placed across the line, they stillcause high spike currents that circulate in wiring. Series type Mode 1suppressors, however, use inductors to limit and a capacitor bank to absorbhigh-frequency energy, which is then slowly released into the neutral wire.Such suppressors from New Frontier Electronics (www.surgex.com) have metthe highest possible A-1-1 performance and endurance ratings in UL1449tests.
1. C. Plummer, Storm Trapper "HSE Residential Lightning ProtectionProgram," The Line On-Line, Cooper Power Systems, April 1997.
2. F. Martzloff, "The Propagation and Attenuation of Surge Voltages andSurge Currents in Low-Voltage AC Circuits," IEEE Transactions on PowerApparatus and Systems, Vol. PAS-102, May 1983.