A Low-Cost Insurance Policy
While it is not the most traditional definition, all electrical protective equipment is basically just insurance. For example, when you buy a health insurance policy, you pay a premium for covering your health-care costs in the event you get sick. Likewise, when purchasing electrical protective equipment, you pay a premium to protect an investment—be it in a building, a process, a piece o...
While it is not the most traditional definition, all electrical protective equipment is basically just insurance. For example, when you buy a health insurance policy, you pay a premium for covering your health-care costs in the event you get sick. Likewise, when purchasing electrical protective equipment, you pay a premium to protect an investment—be it in a building, a process, a piece of equipment or employees.
Each element of an electrical protection system—breakers, fuses, uninterruptible power supply (UPS) systems, relays, reclosers, harmonic filters and transient-voltage surge suppressors (TVSS)—is like another rider on the basic policy, because each protective element protects the system from a different hazard. The breakers, fuses and relays insure the system against overcurrents ; the UPS insures the system against voltage dips and outages ; the harmonic filters mitigate the impact of harmonic distortion ; and the TVSS insures the system against overvoltages . Just like an insurance policy, the initial costs for the installation of this equipment will reduce the cost later if the corresponding condition occurs.
But of all this protective equipment, the TVSS offers the most economical form of electrical "insurance" available. To demonstrate the economies of a TVSS system, it is helpful to examine the relative costs and protections of the various protective components.
First cost comparison
First of all, the TVSS system offers protection of the distribution system at the lowest first cost. The single-line drawing in Figure 1 (at right) shows an example of the electrical distribution that could be utilized in a small, computer-intensive office building, showing the possible arrangement of the various protective elements.
While the example demonstrates the use of either a UPS or harmonic filters to protect sensitive loads, in an actual design it would be possible to utilize both of these elements according to the design requirements.
In an arrangement like that shown in the figure, an approximated first-cost comparison may look something like this:
Overcurrent protection elements —breakers in the switchboard and four branch circuit panels. Estimated cost: $32,000.
Undervoltage or loss-of-voltage protection elements —the two UPS' for the sensitive equipment. Estimated cost (to provide a 15-minute power backup): $82,000.
Harmonic filters (installed instead of the UPS systems) to protect the sensitive load feeders. Estimated cost (to reduce the voltage- and current-harmonic content below 5% total harmonic distortion): $18,000.
Overvoltage and surge-protection elements —TVSS units on both the incoming service and the sensitive load panels. Estimated cost (for such a two-level TVSS): $3,500.
This example is based on what is typically seen in distribution systems of this type, although scenarios certainly exist that necessitate more expensive TVSS systems—or other protective devices that are less expensive. Nevertheless, it is safe to say that there is a significant cost difference between TVSS and other equipment.
While TVSS comes at a low cost relative to other protective equipment, an economic comparison should not be the only factor in selecting one protective component over another. Analysis of the threats is likely the most important factor.
As a matter of fact, an evaluation of the potential damage caused by the different distribution-system disturbances can show that TVSS systems offer further economic advantages relative to other electrical protection equipment.
Overcurrent protection is a requirement of the National Electrical Code (NFPA 70), and as such, all electrical systems should have circuit breakers installed in the distribution system. However, the requirement—and, thus, the motivation—to install protection for harmonics, undervoltages and overvoltages is much less defined. An owner or user will often wait to experience one of these disturbances before deciding to install the corresponding protection system. Not protecting against these electrical threats can bring different consequences for each.
The impacts of harmonic distortion are typically progressive. Current harmonics use up a portion of the current-carrying capability of a component. For example, a 75-kVA transformer may only be able to carry 50 kVA of load without overheating due to current harmonics in that portion of the circuit. But the owner will not recognize the harmonics within his facility until a component overheats and fails, even though the load is well within its ratings. Voltage harmonics are more pervasive because they travel throughout the facility, but they are rarely encountered in commercial distribution systems. However, when they are present, they can cause malfunctions in overcurrent devices and variable-frequency drives. With either type of harmonic, however, when a problem does occur it is usually limited in extent.
When an undervoltage problem such as a power failure occurs, the served system ends up without power for a period of time. But this produces only operational problems, not equipment loss. While it is very rare for production equipment to suffer permanent damage strictly due to a loss of power, it is common for a power failure to cause a loss of production. Simply stated, when the utility power fails and there is no UPS, equipment will not work until power returns.
However, when there is an overvoltage —such as a lightning strike or switching surge—there may be extensive damage to much of the distribution system as well as the production equipment within a facility. Both lightning strikes and switching surges create the same conditions on a power distribution system, but there is a difference in magnitude: the elevation of the voltage level on the utility power lines is transformed to a higher incoming voltage than the distribution system is accustomed to seeing.
This higher voltage causes insulation breakdown in conductors, transformers, panels, breakers and electronic equipment. If the voltage is high enough, it can flash over within the equipment, burning anything in its path and causing massive failures in the distribution system. There have been instances where lightning has completely vaporized the conductors from within the insulating jacket of the wires, leaving only an empty, charred shell. Switching surges are not so dramatic, but can still create overvoltages within sensitive equipment, causing subsequent operational failure.
The economic advantage
Once the cost and protection parameters of the different protective equipment have been evaluated, it becomes evident that the basic cost-to-loss-potential ratio for a TVSS protection system is significantly greater than for the other systems considered.
At the same time, each facility has a different tolerance for risk, and the differing electrical threats can be more or less significant based both on the equipment that the electrical-distribution system is serving and the likelihood of each event occurring. It is the charge of the electrical engineer to quantify these threats and suggest the electrical protective equipment that best serves the specific facility and owner.
Health insurance is not necessary for maintaining a healthy body, but is indispensable when an unfortunate malady strikes. Similarly, electrical protective equipment is not necessary for a functional electrical system, but forms an important protection policy against electrical threats that attack the livelihood—and bottom line—of businesses.
TVSS is only one part of that protection plan, albeit an important part that guards against truly life-threatening occurrences and offers some of the best electrical "insurance" money can buy.
TVSS Installation Requirements
TVSS systems, like any other, require proper installation. Here are a number of important rules to follow when implementing a TVSS system:
Keep conductor lengths as short as possible, i.e., less than 5 circuit ft. Short conductors have low impedances, so more of the overvoltage has an opportunity to reach the TVSS. High-voltage surges have a tendency to follow the path of least resistance, and shorter conductors have less resistance.
Avoid bending the conductors sharply. High voltage surges also have a tendency to travel in a straight line. If the conductor changes direction quickly, the surge will try to go straight and may arc to a near component at the sharp bend.
Install the largest conductors that the TVSS lugs will accept. The lower the resistance of the larger conductor, the more easily the surge will travel toward the TVSS.
Don't splice phase or ground conductors in the 5-ft. circuit length. Splices in the conductors add resistance to the conductor, working against the goal to keep the conductor resistance as low as possible.
Verify that the TVSS ground point measures 25 ohms or less. The ground point of the TVSS is where the TVSS will try to send the surge: so if the ground point is higher than 25 ohms, it could actually be at a higher impedance than the main service ground point. This makes it much more difficult to direct the surge through the TVSS for dissipation.
Confirm that all TVSS conductor terminations are secure and fully torqued. Secure and fully torqued connections have a lower impedance than poor, loose terminations.
Selecting the Right TVSS System
The design of a transient-voltage surge suppression system is not just selecting a box that will meet the withstand criteria of the installation. For the best protection, one should install a multistage TVSS system, not just a single TVSS on the service entrance.
In addition, each element in the system should correspond to its assigned task, starting with the main service-entrance device and on down to the second- or third-level of surge suppression:
For the main service-entrance device, the unit selected should have an "ultimate withstand" rating based on the sensitivity of the loads and the maximum event anticipated. While this unit should have excellent withstand characteristics, the clamping voltage will not be low enough for sensitive loads.
The second-level TVSS, at the utilization equipment, does not need a withstand rating as high as that of the service entrance unit, but should have a lower clamping voltage response.
The third-level TVSS, for sensitive loads, should have the lowest clamping voltage to limit the let-through voltage to its minimum value, maximizing the protection.
Other than high withstand ratings and low clamping voltages, several other characteristics are useful for optimum TVSS selection.
One beneficial characteristic is the ability to accommodate multiple surges, as they typically come in groups. Repetitive surge tests (see "TVSS Testing Standards," page 33) should indicate the ability of a particular TVSS to withstand multiple surges of given characteristics.
Additionally, having redundant protective elements is useful so that the failure of one—or even several—protective elements does not prevent the unit from continuing to protect the sensitive loads.
Also, some TVSS manufacturers provide individual fusing of each of the metal-oxide varistors (MOVs), which allow higher protective levels to be achieved. Manufacturers combine various technologies with the MOVs—such as silicon avalanche diodes, selenium rectifiers or series inductors—to provide better voltage clamping and greater withstand capabilities.
Engineers must carefully evaluate equipment to select TVSS systems that have proven track records of withstanding the high current and voltage surges caused by lightning and power switching. In particular, performance in all of the UL-test sequences of any considered TVSS must be carefully scrutinized to assure that the selected TVSS will truly protect the sensitive loads from these surges.
TVSS Codes and Standards
UL 1449 - Standard for Transient Voltage Surge Suppressors
UL 1383 - Standard for Electromagnetic Interference Filters
NEC Article 280 - Surge Arresters
Military Std. STD-220A
NFPA 780 - Installation of Lightning Protection Systems
ANSI/IEEE C62.41 1991 and ANSI/IEEE C62.45 1992 - TVSS Testing Standards (see below)
TVSS Testing Standards
Part of ANSI/IEEE C62.41 1991 and ANSI/IEEE C62.45 1992 is testing that assures every TVSS meets minimum standards— i.e., that the TVSS is not just a box filled with potting compound that has three leads in and three leads out.
There are four tests that a unit must pass for a TVSS unit to meet the above standards. The first three tests demonstrate the capability of the TVSS to absorb repetitive surges, and include one category A pulse , one category B pulse and at least 1,000 category C3 pulses . The voltage, current, frequency and timing standards for the various test waves are:
Category A. 0.5
Category B. 8 x 20
Category C3 Bi-wave. 8 x 20
In addition to the three repetitive surge tests, the unit is tested for its "ultimate withstand capability." This is the maximum single-pulse surge current that a TVSS unit is capable of protecting against, which is determined by testing the largest current pulse that a unit can take while still maintaining the maximum clamping voltage specified for the unit.
These four tests can provide the engineer with the most useful comparative data to ascertain exactly which TVSS unit will meet the installation design requirements for a project.