YOU NEED A WHOLE- HOUSE PROTECTION SYSTEM

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1 YOU NEED A WHOLE- HOUSE PROTECTION SYSTEM Dr. Richard L. Cohen Panamax San Rafael, CA, (INTRO/ABSTRACT) Hard-wired AC and telephone protectors at the building entrance can keep major disturbances out of the house. Complete protection of AC-powered electronic equipment requires additional plug-in protectors, at the equipment to be protected. These point-of-use protectors provide much lower (safer) limiting voltages than the building entrance protectors, provide for integration of AC and signal protection, and protect against overvoltages that develop within the building. This paper deals only with AC protection; a subsequent article will discuss protecting against transients on signal lines. (TEXT) For many decades, offices and houses have had telecom protectors installed on phone lines at the building entrance to keep out dangerous voltages from lightning and contact with AC power lines. During the last decade, many people have installed small AC "plug-in protectors, directly at the equipment, to limit AC voltage surges from utility problems and lightning. Only recently have people begun to install AC protectors (voltage limiters) at the power entrance/main breaker panel ("Service Equipment" in NEC terminology) in residences, and this protection is still very unusual. The main lightning risks for homes and their contents do not come from direct strikes. Although direct strikes can cause devastating damage, they are extremely rare, even in high-lightning areas. But the network of AC, phone, and CATV cables coming into the home acts as an antenna that can pick up lightning from miles away and bring damaging lightning currents to equipment. Surges on the AC power lines, often caused by lightning, can also create severe equipment damage. Because the vast majority of the damage is caused by disturbances brought into the home by wires from outside, some people have proposed a "whole house protector"- an AC protector, sometimes in conjunction with a supplementary telecom and coax cable protector, to be installed at the building AC entrance (Fig. 1). The idea is that all surges- from AC or lightning, can be stopped at the main power panel by an AC protector. Fig. 1 shows the AC primary protector grounded with the telephone protector and coax.

2 There are 3 major problems with this approach: First, houses of the kind we are talking about here do not look like those in Fig. 1. They look more like those in fig. 2: All of those wires running out of the rear of the house can also lead damaging voltages from nearby lightning into electronic equipment in the house. The only way to prevent damage is to equip all those connections with surge protectors. Surge protection at the power panel can't protect against surges that are brought into the equipment by wires that run outside the house. The second problem: Primary power panel protectors are designed to be "Coarse" protectors - they are capable of handling very large current surges, but have much higher surge limiting voltage - the "let-through voltage"- than the plugin, point-of-use protectors, used within the house. A typical installation is shown in fig. 3.

3 All panel protectors I know of designed for residential use are "Shunt" protectorsthey are installed across the supply wiring, and the load current does not pass through the protector. For large, fast rising pulses, typical of lightning, the inductance of the wiring to the protector (L1, L2, and GC in Fig. 3) develops very large voltages in the leads to the protector, and these voltages add to the intrinsic limiting voltage of the protector itself. The way UL tests protectors, and UL ratings for the "Suppressed Voltage Rating," create a misleading impression of the actual performance in the application. Table 1 shows the difference between performance of a high quality panel protector and a high quality plug in protector, under realistic conditions that could be experienced in service. The "Limiting Voltage" is the effective impulse voltage that appears on the bus bars of the breaker panel, and is transmitted to the loads in the house. Table 1(See also sidebar) Quality Panel Protector Quality Plug-in Protector UL TVSS Approx. Limiting Approx. Limiting Voltage, 10KA, Voltage, 10KA, 15 cm(6") Leads, 8uS Risetime 30cm(12") Leads, 3uS Risetime 500V 900V 2500V 330V 440V 440V

4 The UL limiting voltage (TVSS) rating is 330V for the plug-in, and 500V for the panel protectors. So the impression is that the panel protector is "almost as good" in stopping damaging surges- after all, 500V is not so much worse than 330V! However, as the surge current increases, and for faster-rising pulses, the difference becomes MUCH greater! Still worse, the panel protector is subjected to the full, undivided force of the incoming surge. Protectors plugged into the building wiring see much smaller surges, because of the impedance (inductance and resistance) of the building wiring, and because the surge currents divide as they leave the panel and go into all the different building wires. Generally accepted "credible threat" surge values from nearby lightning strikes are A for plug-in protectors at interior locations, but 10,000A for surges at the panel. So the surge voltage reaching that delicate electronic equipment is now up from 380V for the plug-in protector (at 3,000A), to 2500V for the panel protector (at 10,000A)! This 2500V level is definitely enough to cause damage to some PC and consumer electronics equipment. The third problem: Fig. 3 also shows the wiring ("service drop") that connects the utility power to the building. A center-tapped 240V transformer provides two 120V lines (phases) carrying power to the building, and a neutral, which is grounded at both the pole and the house. One of the commonest forms of electrical system failure is breakage or damage to the neutral conductor, at some point "x" between the pole and the meter panel. In that event, 240V comes into the building between the 2 phase wires, but the (broken) neutral connection is no longer controlling the division into two 120V circuits. Either phase can have up to 240V, with the exact value depending on the loads connected to the two phases, and the effective resistance values at the building and pole grounds. This AC overvoltage will continue until the wiring is repaired. As long as the voltage between either phase and neutral is less than ~200V(RMS), virtually all panel protectors will not react; the overvoltage passes through without being limited. If the voltage is greater than ~200V, the varistors will begin to conduct to try to limit the voltage. Under this continuing power input, the varistors and/or the protective fuses will open within minutes, allowing the excess voltage to pass into the equipment in the house. Because panel protectors are shunt protectors, there is no easy way to remedy this vulnerability. Plug-in protectors to the rescue! So the overall conclusion is that there are 3 major factors that prevent an AC power-panel protector from being a "whole-house" protector. Rather, we need to install a WHOLE HOUSE PROTECTION SYSTEM, that includes a quality panel protector, AND plug-in protectors at the equipment worth protecting.

5 Fig. 4 shows what is called a "cascade" or "staged" or "zoned" protection system, in which the panel protector takes the brunt of the primary lightning or power surges, and reduces the surge to a level easily dealt with by smaller plug-in protectors at the equipment. The plug-in protectors deal easily with the vast majority of surges coming in from the back of the building, solving the first problem discussed above. When major lightning strikes or utility surges come in on the utility connection, the panel protector reduces them to ~2500V level shown in Table 1. The surge is further weakened by the impedance of the house distribution wiring. So by the time it gets to the plug-in protector, it has been reduced to a level which is easily controlled, and the plug-in protector really will reduce the surge to ~ volts, which should not be damaging for equipment. This eliminates the second problem. The thorniest problem, the third, is that of the "open neutral", where, we stated above, the panel protector will not help much. Here also, the plug-in protector can greatly reduce the hazard. Typical good plug-in protectors use a fused disconnecting circuitry (Fig. 4). Refs. 1 and 2 discuss the use of disconnecting circuits in plug-in protectors. If there is a sustained overvoltage, the varistors get hot, and a thermally sensitive fuse opens, disconnecting the incoming power. The varistors in the plug-in protectors typically respond to lower overvoltage than those in panel protectors. Typically, panel protectors use 150VAC(or greater) varistors; plug-in protectors use 130VAC varistors. The plug in protector begins to clamp at ~170V(RMS) on the AC line, and if the voltage stays at that level, the

6 varistors will rapidly heat and open the fuse, disconnecting the protector and the load, and saving the load from damage. So we see that the use of good plug-in protectors at the equipment is an excellent complement to the panel protector- the strengths of one compensate for the weaknesses in the other. Signal line protection: All of the material above has only discussed protection against AC surges and overvoltage. When equipment has signal/data connections in addition to AC connections (fax machine, TV set with cable, PC with modem connection), damage can come not only from the AC(power) side, but from the signal wires as well. In fact the commonest cause of damage to equipment is large voltage differences (from lightning or AC power surge) between the signal lines and the AC wiring. These voltage differences need to be suppressed, if the equipment is to survive. Many manufacturers offer commercial protectors that combine AC and signal line protectors. The arrangement is called "surge reference equalization", "portable ground window", "bubble of protection", multi-port surge protection, or a number of other names. The correct application of these protectors, and the integration with primary signal protectors and AC protection systems will be discussed in a later article. References 1. R. L. Cohen, in Power Quality Assurance, July/August 1998, p F. Martzloff, in Power Quality Assurance, July/August 1998, p SIDEBAR: Good panel protectors use either 1 large varistor per phase, or a bank of smaller varistors, and are typically rated for ~50,000A or more (8x20us impulse) per phase. Good plug-in protectors will survive 10,000A or more, and will disconnect ("fuse open") from the AC input if the surge exceeds what the protector can withstand. Both protectors are controlled by UL Standard The older UL test for "Lightning Arresters", category OXHD, is very much less stringent than 1449.