Remote location power surge protection
By David Torres
Remote sites include locations such as telecommunications shelters, cellular towers, water/wastewater treatment plants, railroad bungalows, and HVAC systems. These installations can be challenging to protect from lightning, line surges originating from utility switching, damage to power utility lines, and power surges originating from area industrial equipment. Devices in remote locations are typically closer to the service entrance and lack other power paths for energy to flow. This makes them more susceptible to surge damage than other applications.
When planning a remote installation, the design engineer or installer must consider proper grounding and bonding, the correct surge protection device (SPD) for the job, and proper installation. When installed correctly, it is possible to protect a remote site from lightning or other surge events.
Facility grounding, bonding
Like all electrical and electronic systems, SPDs require proper grounding and bonding to operate successfully. The basic grounding and bonding requirements are guided by the National Fire Protection Association (NFPA) in the National Electric Code (NEC).
AC power systems are available in many different voltages, currents, configurations, and grounding schemes. There are three types of grounding configurations for AC power systems: solidly grounded, high-resistance grounded, and ungrounded. Solidly grounded AC power systems are the most common today. In this configuration, the AC power system is grounded at the service entrance using one or more grounding electrodes.
The advantage of solidly grounding the AC power system is the limitation in amplitude of sustained overvoltage conditions that can occur during fault conditions. The disadvantage is the amplitude of current occurring during fault conditions can be very high (65,000 amperes or more).
Remote applications typically require less power than large industrial or business applications-typically 120/240 V AC, single-phase, three-wire + ground system rated at 200 amperes or less. Thanks to the lower current requirements, these applications can achieve neutral to ground bond within the service entrance panelboard.
Although remote systems are usually small, they can still be complex. In addition to AC power systems feeding the facility, there might also be communication, fire alarm and security, or other specialized systems.
The NEC requires all systems have a common ground. This includes the AC power system, all communication-type systems, and the mechanical structure of the building. If a lightning protection system (e.g., Franklin Rods) is installed, this must also be bonded to the common grounding electrode.
The eventual goal is to bond all systems to one point (single-point grounding). For example, if a pumping station has a wireless radio, the antenna will have its own ground rod. This rod must be bonded to the AC power ground. If not, even with surge protection installed, the radio and other equipment can be damaged.
Selecting the best SPD technology
SPDs use two different technologies: voltage-limiting and voltage-switching.
The National Electrical Manufacturers Association (NEMA) defines a voltage-limiting SPD as one "that has a high impedance when no surge is present but will reduce it continuously with increased surge current and voltage." The most common voltage-limiting technologies are the metal-oxide varistor (MOV) and silicon avalanche diode (SAD).
A voltage-switching SPD is "an SPD that has high impedance when no surge is present, but can have a sudden change in impedance to a low value in response to a voltage surge." Common examples include spark gaps and gas discharge tubes.
The two technologies can be combined to form one hybrid SPD. The hybrid can provide more robust protection and minimize the disadvantages of each technology.
One hybrid SPD design is paralleling MOVs and spark gaps. When these two technologies are combined, the MOV turns on first during a surge event and conducts the surge current to neutral or ground. The surge voltage transitions the MOV from the "off" state to the "on" state. After the MOV has transitioned to "on," the electronic circuitry triggers the paralleled spark gap, which then transitions to the "on" state. The voltage then collapses toward zero volts, deactivating the MOV. These units are capable of diverting very large surge currents (up to 50 kA) with very long pulse widths (up to 350 µs).
A second benefit of the hybrid design is longevity. These SPDs can handle large amounts of energy with little degradation. The combination of large surge currents and longevity make it the most suitable SPD for remote applications.
The third benefit is if the MOVs become damaged, the spark gap still provides surge protection until the MOVs are replaced.
It is important to choose an SPD that meets the operating and performance parameters for the particular AC power system. Parameters to review include voltage protection capability, surge current rating, short circuit current ratings, enclosure ratings, and listing requirements. The SPD must also meet the longevity, serviceability, and uptime requirements: notification at 50% capacity, hot swappability, indication lights, NEMA 4x housing, and surge counter.
Suppressed voltage rating
The suppressed voltage rating (SVR) indicates the SPD's ability to attenuate a defined transient overvoltage event. Suppressed voltage ratings are used to coordinate the protection capabilities of the SPD to the protection needs of the AC power system.
To determine the SVR, the SPD is subjected to a specific test regimen defined in UL 1449. The outcome of the test regimen is a rating of the SPD to a combination wave transient consisting of an open circuit voltage and a short circuit current.
Testing consists of subjecting the SPD to the combination wave transient mentioned previously. Testing is conducted on three samples. The voltage measured at the connection point of the generator, and the SPD are then averaged. The SVR is then chosen from a list of values.
The SPD should have an SVR that protects the equipment without interfering with the nominal voltage of the AC power system.
Obtaining the lowest SVR value (330 volts) is typically where SPD and system interactions occur. SPDs intended for operation in a 120 V AC system that have obtained an SVR of 330 volts commonly have a maximum operating voltage of 130 V AC. In this case, the maximum operating voltage of the SPD is very close to the nominal operating voltage of the power system, approximately 8%. The voltage in many AC power systems fluctuates 10% or more.
An SPD with greater tolerance would be a better choice. The example above requires the SPD to have an SVR greater than 330 volts. An SPD with an SVR of 500 volts would provide sufficient immunity to fluctuations in the AC power system and protection from transient overvoltages. While this example uses the 120 V AC systems and an SPD SVR of 330 volts, there are other combinations of AC power system voltage and SPD suppressed rating voltages that can cause SPD failure.
SPD uptime, serviceability
A high-quality SPD should perform for many years without failure. If the SVR is too low to meet the voltage tolerances or short-circuit current capabilities of the AC power system, the SPD will not last as long.
However, eventually, the day will come when the SPD stops working. Choosing an SPD with an indication system will provide immediate warning to the maintenance staff. Ideally, the indication should be provided on the enclosure and inside the SPD.
Point of application
Remote facilities are usually smaller than traditional facilities, so conductor lengths from the service entrance to the point-of-use devices are relatively short. Therefore, it is important to provide transient protection at the service entrance, any branch panel and at the point-of-use device. This is called the cascade or zone approach.
Applying properly connected SPDs at various points along the AC power system ensures the transient overvoltage is reduced to magnitudes that will not interfere with the operation of electronic systems. In some remote locations, the distance between service entrance and point-of-use is very short, and cascade protection cannot be implemented. A properly connected hybrid SPD can reduce the voltage of a very large surge to an acceptable level.
Lead length effects
The SPD limits transient overvoltages by diverting current through the SPD itself. Conductors that connect the SPD to the AC power system have a number of parameters. These include inductance, resistance, capacitance, and conductance.
Inductance is the key factor that influences the performance of the SPD. While the size of the conductor (for example, #14 AWG, #8 AWG, #4 AWG, etc.) minimally changes the inductance of the conductor, the length of the conductor has a large effect. As the conductor length increases, the let-through voltage associated with the inductance of the conductor increases. As the transient current through the conductor increases, the let-through voltage also increases.
The shorter the conductor length, the better the protection the SPD can provide. The effect of the voltage associated with the conductor is less notable at low amplitudes of transient currents. As the transient current increases, the effect of lead length increases dramatically. A properly connected SPD is closely coupled to the electrical panelboard.
Short circuit current
UL 1449 defines the test regimen for determining the short circuit current rating (SCCR) for an SPD. The NEC requires the SCCR of the SPD be coordinated with the point of installation on the AC power system. Therefore, any SPD with a rating greater than the calculated short circuit current is acceptable.
Protecting remote systems from lightning strikes can present special challenges. However, by choosing the proper surge protection and following NEC and local codes, it is possible to keep the system running.
ABOUT THE AUTHOR
David Torres is Lead Product Marketing Specialist for Phoenix Contact's Power & Signal Quality products, TRABTECH. He has more than 19 years of experience in the electronics industry.