The typical home sees a 100V surge hourly and a 1000V surge monthly. However, most residential customers don't know how to protect their expensive entertainment centers, home networks, computers, and appliances from power quality anomalies. This provides significant opportunities for electrical contractors who can design a proper protection system and show customers why it's an excellent investment.
Because many devices are always plugged in and/or are always on, they are vulnerable. The potential loss of an inexpensive device or two isn't much motivation for investing in proper protection. But to repair or replace such items as televisions, VCR/DVD players, and high-end phone systems can get expensive, costing from hundreds to thousands of dollars each. Add up the replacement costs of programmable dishwashers, ranges, and refrigerators, and the eye-popping numbers can be quite motivating — so can asking what happens when that $9,000 programmable heat pump system becomes toast just before a holiday weekend.
Indoor gremlins. The typical residential customer doesn't know transients come from inside the home as well as outside. For example, the HVAC system that keeps the customer cool in summer and warm in the winter throws transients into the electrical system every time the AC cycles, the heat pump kicks on, or the electronic ignition lights the furnace. The electronic ignitions of gas ranges, dryers, and water heaters also generate transients. Don't forget that anything motor-operated throws pulses onto the system as well, such as vacuum cleaners, refrigerators, garbage disposals, and fans.
Although customers may argue “nothing ever happens” after these invisible events, the absence of an immediate failure doesn't mean the problems aren't real — and won't get expensive. Power events punch holes in semiconductor material, eventually bringing on premature failure and costly replacement. Maybe we can't explain why socks disappear in the dryer, but we can explain why electronics seem to fail shortly after the warranty expires. By protecting against equipment-generated transients, we can prevent mass replacement of costly possessions.
Outside influences. Back in the first season of Gilligan's Island, the typical home had a 30A service. Today, a 200A service is the new standard. As residential loads have increased, so have the complexity of the power grid, the amount of utility switching, and the number (and size) of fault surges. Typical transient-generating utility operations include capacitor bank switching, primary circuit disconnection to transformers, phase-to-neutral faults, and unbalanced loading of phases.
Lightning is the most spectacular and widely recognized source of transients (Photo 1). But it doesn't take a lightning strike to wipe out an entire household full of electronics. A single lightning-induced surge can do that — which might have come from a strike miles away.
Each year, lightning causes an estimated $400 million to $500 million in property damage in the United States (see Map). And that's just for known lightning events. If there's not a direct lightning strike, it's common to assume the failure wasn't due to lightning.
Flashover is another hazard. Check the typical home for conformance to the bonding requirements of NEC Art. 250, and you'll find violations. Even in your own home, are all of your incoming utilities bonded together? What about next door?
Arrest or suppress?. The two principal surge protective device (SPD) types for residential electrical systems are arresters and suppressors. While the NEC clearly distinguishes between arresters and suppressors, many installers and consumers do not.
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An arrester limits the surge voltage by discharging or bypassing surge current [280.2].
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A suppressor limits the surge voltage by diverting or limiting surge current [285.2].
Arresters handle higher energy levels than do suppressors — and typically at higher voltage levels. In fact, you cannot use a suppressor on circuits exceeding 600V [285.3]. Arresters have sufficient energy-handling capacity to withstand lightning-induced voltage surges, while suppressors do not. Arresters must be able to withstand surge currents of 10,000A.
In general, you use arresters on the supply side of the meter, and suppressors on the load side — but that's not a hard and fast rule. You may use arresters ahead of (and/or at) service equipment, on high-energy feeders, or even on high-energy loads. The determining factor is the energy level, not the location. You may use suppressors at panels, feeders, branch circuits, point of use, and service equipment (typically lower-energy locations). Use arresters “upstream” of suppressors, to limit the energy to levels that suppressors can handle.
Energy levels. Don't confuse the withstand voltage level with the degree of protection. It's critical that you ensure the clamping voltage is in line with the surge current. As the surge current increases, the clamping voltage increases. This raises the floor of when the SPD will start to turn on. So, for example, a 550V device does not start clamping until 1,000A (actual numbers vary).
The key is to match the SPDs to the energy level, based on their location on a one-line. Cascade the protection to reduce the energy level in discrete steps.
Adding SPDs. The effectiveness of add-on SPDs varies widely. Prices vary from $3 outlet devices to a $200 black box installed at the load center. An SPD that plugs into the load center (Photo 2) occupies at least one breaker space, which could be an issue in a crowded panel.
Two types of SPDs dominate the residential market. The simplest and oldest is the spark gap device (an arrester). Utility meters include spark gap protection that flashes over at 6,000V — that is, they limit the voltage on the line to 6,000V because the current jumps (to ground) across the air gap of the arrester when the line voltage exceeds 6,000V.
The varistor (variable resistor) is the other major SPD. At 120V, the varistor is non-conductive (Figure above). As voltage rises, the varistor begins to conduct and bleed off exponentially larger amounts of transient energy. It limits the pulse voltage to a level that the equipment being protected can presumably tolerate.
One problem with most varistor-based SPDs is the varistors are too small for the surges they must handle. Varistors having the same applied voltage rating will have energy ratings corresponding to the diameter of the varistor element. Typical varistors in residential SPDs have a 20mm diameter, with single current impulse ratings of 80 Joules (watt-seconds). The higher the Joule rating, the more energy a varistor can absorb and dissipate.
Another problem is that many SPDs use several small varistors wired in parallel to achieve high Joule ratings. Such designs are failure-prone because of small variations in operating characteristics between varistors. One varistor in a multiple-element suppressor will suffer a disproportionate share of the electrical abuse. It then degrades faster than the other varistors, until it fails. If that failure doesn't destroy the device outright, it leaves the device one varistor short (more overload on the remaining varistors). Then you get a cascading failure of varistors until the device stops working altogether.
Varistors also are rated according to their peak impulse current ratings, established via tests using an 8 × 20 microsecond peak current wave. Device capabilities vary widely. The varistor in a plug-in outlet suppressor is rated at 500Apeak (8 × 20 microseconds), while a permanently connected unit installed in the load center (where the energy level is the highest) is rated at 3,000Apeak (8 × 20 microseconds).
But the fuses that clear most of the varistor-based SPDs usually fail before the varistors self-destruct. This essentially derates the SPD. There are only three ways a customer can detect loss of surge protection:
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Inspect the fuse (unlikely).
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Notice if a device indicator light is off (possible).
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Notice that the new $7,500 plasma TV acted like a canary in a mine and is now dead.
Point of use. Because of their energy ratings, arresters and branch-circuit level SPDs cannot protect individual loads without some “downstream help.” Their job is to limit the surge to an energy level a point-of-use SPD (POU-SPD) can handle. The POU-SPD, then, is the final piece of the puzzle in a total protection solution.
The POU-SPD should protect across phase, neutral, and ground. It should have a suppressed voltage rating of 330V or 400V. Its job is to protect individual loads from those portions of transients that “pass under” the larger devices.
You can provide the best overall home protection through an intelligent combination of surge protective devices at the load center and high-quality outlet suppressors at specific points of use. Examine the total system, develop a coordinated transient protection plan, and compare that to the replacement costs of your customer's expensive devices. The wise customer will be happy to have you install the needed protection.
Vaughan-Birch is product manager for Residential Circuit Protection and Technology Products, Siemens Energy and Automation, Norcross, Ga.
