What do these statements have in common?
• Lightning doesn’t strike the same place twice.
• Electrical utilities correct a fault by sending a power surge to clear a short in a service line, transformer or other load grid short circuit.
• TVSS systems will reduce energy consumption by a minimum of 10 to 15 percent.
All three are purported to be fact by some people, even though they run contrary to reality and the laws of nature. The world is full of folklore about many different subjects, even about transient voltage surge suppression (TVSS) devices. Ads for TVSS devices claim that they will solve most power quality problems, even though sags are typically the most common PQ phenomena (as much as 60 to 70 percent) experienced in an industrial facility or residence. This doesn’t seem to affect the millions of dollars of annual sales of such mitigation devices. So, perhaps a little knowledge about such may help to make for “a more educated consumer.”
Surge protector products are primarily designed to eliminate transients (also referred to in the past as “surges”) and sold under the names of TVSS or SPD (surge protection devices). Transients are sub-cycle duration changes in the voltage and current waveforms, often measured in microseconds (millionths of a second). They are often very sneaky PQ disturbances, normally not visible to the human eye until they leave a path of destruction in their wake. They can be repetitive transients that gnaw away until causing a breakdown of insulation or components within equipment; or, they can be as dramatic as a directly coupled lightning strike, measured in tens of thousands of Amperes. By the way, lightning strikes because the potential difference in energy between two points is higher than the breakdown voltage between them. When this happens, the step leaders from the clouds and the earth make a connection that allows the massive amount of energy to traverse between the two points with the brilliant flash that people associate with lightning. This flow can happen several times in one strike, and can repeat itself later at the same place when the potential difference increases again to the breakdown or flashover point.
There is a major difference between “surge arrestors” and TVSSs. Surge Arrestors (SA) and TVSSs are treated differently by Underwriters Laboratories, as covered in UL 1449, and the National Electric Code (NEC). Surge Arrestors are generally hardwired protectors at the service entrance. The 2002 NEC allows SA-rated devices to be installed before the service disconnect [Articles 280 and 230.82(3)], with TVSS devices applicable only after the disconnect [Article 285]. In general, the SA devices are designed to withstand much larger current transients (up to 10kA). In lightning-prone areas, such as Florida, the service entrance is a very good place to install such protection. The distance between the line and neutral conductors, and the earth grounding electrodes, is probably the shortest here. Since Ohm’s and Kirchoff’s Laws still apply in the transient world, having the shortest distance would also probably yield the lowest impedance in the grounding conductor. It is here where the “unwanted” energy is going to be dumped; less impedance equals better protection
Since the “surge protector strips” are among the most prolific PQ mitigation devices out there, especially in a home or other residence, we’ll focus on those. If you open up one of the surge protector products (not recommending that you do, especially if it is electrically live), there are probably one or more of the following components: MOV (metal oxide varistors), SASD (silicon avalanche diodes), spark gaps or gas tubes, fuses, and/or filters made of inductors and capacitors. Their ability to protect your equipment is a function of what exactly is inside and how it is installed.
MOVs are devices that lower their resistance when higher voltage transients occur versus nominal line voltage (whereas a resistor is a constant impedance component). The result is that they will limit or clamp the voltage level across two points by providing a lower impedance path. They can be connected Line-to-Neutral for “normal mode” transients as well as line-to-ground and neutral-to-ground for “common mode” transients. When they absorb the transient, there is a resulting heating effect. The amount of energy (voltage ¥ current ¥ time) that they can absorb is their joule rating. The more joules, the more “heat” that they can take.
According to the Control Synergy Web site (www.controlsynergy.com.au), “While a 20mm MOV can withstand 1,000 650A 8/20 usec current pulses, it self-sacrifices as it suppresses a single 6,500A 8/20 pulse. Its surge current capacity decreases significantly as it is subjected to the more common longer duration 10/1000 usec transient current pulses.”
In general, MOVs can take more punishment than SAPD, but their clamping voltage or voltage protection level, VPL, is not a fixed value but a function of the magnitude of the transients. SAPDs are usually very fast-acting devices, measured in nanoseconds (billionths of a second). The VPL doesn’t vary with the magnitude of the voltage transients, as with the MOV. However, since it has less current-carrying capacity, it takes more of them in parallel to absorb the same joules as an MOV. They also tend to cost more than an MOV, making the overall protection equivalent a more expensive proposition.
An MOV can be subjected to continuous transients, either because of a constant source of such or because of an overvoltage condition resulting in a peak voltage above the clamping voltage. The peak voltage is typically 1.414 times the nominal rms value, as in 170Vpk for 120Vrms. This heat buildup can cause the MOV to degrade, with the “on” impedance getting smaller. Since the VPL is also a function of the current and the “on” impedance, this value can change. Eventually, this can lead to failure, and it often fails catastrophically in a near short circuit. This draws excessive amounts of current, until the leads melt off or some type of circuit protection devices opens. Rather than relying on the distribution circuit protection (breaker or fuse), it is a good idea to purchase a surge protection product with such overcurrent protection built into the product. Note that there is no visible marking difference in the UL marking for a product that passed the second edition tests versus those covered by the first edition. An example of an MOV failure can be seen in Figure 3.
Since these devices are normally placed downstream of the electric revenue meter, it is difficult to see how they can actually save energy costs. The electrons that are diverted by the clamping devices when limiting the voltage downstream to the equipment they are protecting will still flow through the wattmeter. Energy is still being consumed, though maybe not by the equipment itself. In addition, such high-frequency disturbances are probably above the bandwidth of most wattmeters anyway, so they wouldn’t even be recorded as such. Residential electric bills are in the tens or hundreds of kilowatt-hours per month; it takes thousands of 10 usec-wide, 500V transients to result in even one kilowatt-hour.
Protecting just the electrical distribution circuits in an industrial facility or residence isn’t often adequate. Telecommunications lines, including phones and cable TV lines connected to modems and other electronic equipment, can provide a path for transients to cause damage to equipment, as well as couple into the power circuits and spread to other circuits in the facility. Similar type surge protection devices are available for them, as well as combination devices that offer protection for both.
Since the common mode protection diverts energy into the grounding conductor, it is important that this path be one of low impedance to the grounding electrodes. Having several surge protection strips daisy-chained together, or plugged into partition walls or circuits without adequate ground paths, isn’t going to do too much for you. Remember, if you are diverting a large amount of current, the impedance will result in a proportional voltage being developed across it. If some of the circuits experience this voltage rise and other parts do not, the equi-potential ground system is no longer at the same levels for equipment interconnected between such, as with a local area network or other communication interfaces. Hence, misoperation or damage may result elsewhere in the building, even if the TVSS does its job at the piece of equipment.
Correctly installing the properly rated surge protection device will go a long way to protecting your computers, HVAC controls, microwaves, answering machines and other equipment from the evil effects of transients. Of course, they wouldn’t do much for a voltage sag, as they don’t generate energy, only absorbing the potentially damaging transient instead of your equipment. And lastly, no, the electric utilities don’t generate surges to clear the lines of faults. Surges or transients can occur during fault conditions, but they are also the result of the laws of physics, not an attempt by the utility company to “clear the fault.” EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680. KINDER is principal engineer at Dranetz-BMI performing new product research and development on present products.