11 Myths of Surge Protection

by Wendell H. Laidley

Myth #1. ANY SURGE PROTECTION IS BETTER THAN NO SURGE PROTECTION.

This is perhaps the most reasonable, yet the most misleading of all. With no surge protection at all, incoming surges will hit only the computer's power supply (which is considerably more surge tolerant than the data line circuitry), and will not affect the system ground level at all. Since shunt surge suppressors divert power line surges into data lines, using the wrong type of surge suppression can actually cause failures throughout the network. Thus the network is likely to be better off if individual nodes have no surge supp- ressor than if they have ones that shunt power-line surges into data-lines.

Myth #2. A UPS WILL PROVIDE DEPENDABLE SURGE PROTECTION.

Because a UPS costs far more than a surge protector, it is often assumed to provide better surge protection. However, virtually all UPS units designed for microcomputers simply combine an inexpensive MOV surge suppressor with a battery backup power source. The MOV surge protection is designed primarily to protect the UPS's own circuitry, and diverts incoming surges to ground, just like a common surge protector. Unfortunately, once the surge hits the UPS ground, it will then couple directly into the computer's data lines. Since many microcomputer UPSes are used in the context of local area networks, this problem must be addressed or the UPS will endanger the network data line. Some UPS makers show how surges which encounter the UPS battery are effectively eliminated. This is true for those surges which actually reach the battery; but most are diverted away from the UPS circuitry to ground before they ever reach the battery. Thus the belief that the battery in a UPS is an effective surge sink is not entirely relevant or dependable. Just like the basic surge suppressor, the UPS protects the computer power supply; but in doing so, it endangers the data lines. Another problem with UPSes is the existence of an alternative surge path around the battery and inverter. So called standby UPSes normally provide direct utility power to the computer, with only the MOVs at the UPS power inlet offering any surge protection. On-line UPSes generally have a bypass circuit to enable utility power to flow directly to the load in the event of UPS failure. Both these circuits provide paths for incoming surges. In the case of the standby UPS, the surge must pass through the transfer switch. These switches are often solid-state components with modest tolerance for high energy surges, so they may not prevent a surge from passing through.

Myth #3. TRANSFORMERS ARE THE BEST SURGE PROTECTORS.

Transformers are designed to transmit power, not to suppress it. The two major advantages of transformers are their surge-absorbing mass (tech- nically, their leakage inductance) and their availability as a complete subassembly, which eliminates the need to design a custom surge-processing circuit. And while the transformer's leakage inductance offers some surge protection, it provides much less than would inductors designed spec- ifically for surge suppression. In fact, a transformer is far from being an ideal surge suppressor, and it presents significant disadvantages, including ringing, regulation, increased source impedance, and efficiency loss. Transformers also have substantial parasitic capacitance to ground, and this can couple surges to ground. Finally, transformers used for surge protection often incorporate MOVs, since the transformer itself may be unable to handle the higher voltages in surges. The often-cited benefit that isolation transformers protect against common-mode surges is some- what of a red herring when applied to computers, as will be discussed under myth 5.

Myth #4. VOLTAGE-REGULATING TRANSFORMERS ARE HELPFUL WITH COMPUTERS.

Most modern desktop computers use switch-mode power supplies rather than older style, linear designs. A switch-mode power supply draws from the AC power line only as much energy as it requires to maintain its output power. In this sense it responds spontaneously to voltage fluctuations. If the line voltage drops, the power supply draws current for a longer period, until it replenishes the energy it put out since the previous cycle of the power wave. Because of this natural ability to accommodate varying source voltages, a switch-mode power supply gain no benefit from a voltage regulating transformer. However, switch-mode power supplies are more sensitive to source impedance than source voltage, and the increased impedance inserted into the line by the transformer may actually hinder the power supply by restricting the current available. A tap-switching, voltage-regulating transformer may also introduce noise if the tap switch hunts back and forth between adjacent output taps. Computer switch-mode power supplies often have a wider tolerance for input voltage than do regulating transformers themselves. Thus the primary benefit of a voltage regulating transformer is its leakage inductance, which is much less than that of an isolation transformer, but the regulator introduces offsetting disadvantages. Moreover, the transformer's promary function, voltage regulation, offers no material benefits.

Myth #5. COMMON-MODE SURGES CAUSE COMPUTER PROBLEMS.

Just as modern switch-mode power supplies compensate spontaneously for voltage variations, they also naturally attenuate common-mode noise (i.e., voltage differences between the neutral and ground lines). Desktop computers have five orders of magnitude of common-mode noise attenuation built-in, from the EMI/RFI filter and the high frequency isolation transformer in the power supply. Low-voltage, low-frequency ground potential differences will not cause disruption or damage, because the primary cause of disruption is coupling, which depends on frequency and amplitude. Computers are inherently immune to common-mode disturbances below a certain threshold, but problems occur when high- enregy incoming normal=mode surges (i.e., differences between the hot and neutral lines) are converted to common-mode surges by the action of shunt surge suppressors. Surges originating from outside your building are always normal mode, since neutral and safety ground lines are tied together to an earth ground at the service entrance. Keeping shunt surge suppressors off circuits powering computers will eliminate the conversion from normal mode to common mode.

Myth #6. COMPUTER MODEM DAMAGE IS CAUSED BY SURGES ON THE PHONE LINE.

The phone line is a high-impedance circuit which cannot support high energy surges, so they die away rapidly after the inducing source (e.g., lightning) disappears. In contrast, the low-impedance power- line provides an ideal propagation network for high-energy surges. Also, the telephone service entrance is protected to under 300 volts, while power-line surges can reach 6000 volts before they will arc over in 110-volt fixtures. Most computer modem damage is caused when high energy power-line surges are diverted to the reference ground and coupled into the digital side of the modem. This elevated voltage then seeks the phone line ground reference on the analog side of the modem and arcs through the modem. As a corollary to this, phone- line protectors which provide shunts to the power-line ground (commonly found as cube taps which provide two phone line jacks that plug into a 110-volt receptacle) may introduce more disturbance to the phone line than they relieve, creating more problems than they solve.

Myth #7. SIGNAL GROUND IS ISOLATED FROM CHASSIS GROUND.

Some manufacturers attempt to isolate signal ground from frame ground, but all such isolation configurations have coupling coefficients and dynamic ranges which are likely to be exceeded by high-energy surges. The effectiveness of most such isolation circuits is generally limited to short-duration, low-energy noise.

Myth #8. THE ONLY RISK FROM THE POWER LINE IS HARDWARE DAMAGE.

Computers are vulnerable to data alterations as bit streams pass through microprocessors. Stray power surges can alter data or programs, causing data errors or lock-ups which cannot be traced. The consequential cost of such soft damage can be very high, especially if errors are not found and data files are contaminated.

Myth #9. SURGE PROTECTORS ARE PERMANENT DEVICES.

Most point-of-use surge protectors use metal oxide varistors as their primary protection component. Despite all its strengths, this inexp- ensive (15 cent) component wears out a little with each surge above a very modest threshold... a threshold that is exceeded mant times a day in most environments. Unfortunately, the race among surge protector manufacturers to provide the "best" protection (i.e., the lowest let- through voltage) has led them to use lower voltage MOVs which age faster and fail sooner. The normal failure mode for an MOV is overheating, and they have been known to cause fires. Thus MOVs wear out and should be replaced periodically. Unfortunately, equipment to test MOVs is very expensive (on the order of $20,000). Indicator lights purporting to show that protection is operational ,are not always reliable; in fact, those are sometimes wired across the power line and thus only indicates that the power line is live.

Myth #10. NOTHING CAN STOP LIGHTNING.

While this simple statement is true in absolute terms, it is misleading for all but the rarest of cases. Two important factors limit damage from lightning. First, a direct lightning strike is extremely rare, although in that event equipment may be destroyed and people killed. But lightning normally manifests itself in the power line as induced currents caused by the lightning's magnetic field. Thus we normally need only deal with the induced surge, not the lightning strike itself, and the energy of that induced surge will be limited by the capacity of the conductor to carry the surge energy. The second factor is that surge voltages are limited to 6000 volts because any higher voltage will cause 110-volt circuit fixtures to arc. Thus surge protectors need only deal with voltages up to 6000 volts, and currents determined by the circuit imp- edance. There are surge protectors available which suppress surges up to 6000 volts and unlimited current to under 250 volts without degradation, and without disturbing the critical reference ground.

Myth #11. YOU GET WHAT YOU PAY FOR.

The assumption that higher priced surge protectors provide greater effect- iveness and reliability is often not valid. Almost all surge suppressors priced under $200 rely on the same fundamental MOV components. Much of the supplementary circuitry is actually peripheral to the surge protection function, such as lights and switches, or it provides a minimal level of noise filtering that will be ineffectual in the face of an actual surge. Many users would be just as well served with a $3 hardware store MOV protector that they discard and replace periodically, as they would with an expensive protector using the same MOVs, which will also wear out.

Written by Wendell H. Laidley, President, Zero Surge Inc.