Because transient analysis is complex and not widely understood by computer users, a number of myths have grown up around the subject of surge protection over the years.

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.