Main causes and available abatement methods.
By Ed Butts, PE, CPI
So far, we have reviewed electrical circuit protection, power factor and correction methods, and power quality and harmonics in 2023. This month, we will examine the main causes of high and low voltage electrical power surges and the available abatement methods used to control these surges.
Electrical Power Surges and Surge Control
Power surges, also commonly referred to as transients or spikes, in an electrical system are another component of power quality although the root cause and outcome from these events are often quite different.
The definition for a surge varies, but in general, it is defined as an increase of at least 10% more than the base current and voltage that lasts for only a few microseconds.
The intensity of a power surge can be drastically different based on the source. Thus, power surges are often categorized as either major or minor power surges. And although the duration of a typical surge is short, the amount of electrical energy associated with one can be significant as the voltage can momentarily rise to as much as a few thousand volts on a 120/240-volt system during a minor surge.
Power surges can result from various origins and can be caused by sources within the same facility, such as when large electrical loads are rapidly activated or switched on and off (cycled) inside the facility.
The switching of power factor correction capacitors on the distribution system can also produce transients of up to two times the peak line voltage. These transients may cause variable frequency drives tripping due to overvoltage or total failure from component destruction or voltage breakdown.
Disturbances can also result externally from the application and transmission of surges onto primary power lines during routine operation of electrical machinery at an adjacent or nearby factory or large commercial facility. They can also be caused by the electric utility’s protective devices or during routine switching or activation of utility feeders.
The most damaging and common source of electrical surges in many regions, however, is lightning. Lightning—a common source of major power surges—is caused by the attraction of positive and negative static charges in the atmosphere. This results in a buildup and eventual discharge of this stored electrical energy to ground (Earth) in the form of a strike. Lightning strikes generally search for and find a path of least resistance to the highest nearby structure, such as a tree, tall building, or metallic TV, radio, or cell tower.
Lightning is extremely hot; a flash can heat the air around it to temperatures five times greater than the surface of the sun! This heat causes the surrounding air to rapidly expand and vibrate, resulting in the familiar rolling sound associated with thunder.
Depending on whether the strike is a positive or negative charge, current from a lightning strike can range from 20,000 to more than 300,000 amps at a voltage potential of a few million volts up to one billion volts. The average lightning flash will transmit 300 million volts and 30,000 amps of current.
In addition to the damage from direct strikes, lightning can induce high voltages onto nearby distribution and transmission lines that transmit power to feeders and into electrical services and equipment. Repeated minor power surges can result in a progressive loss and degrading of insulating integrity in motors and transformers. Major power surges—those caused by lightning or bolted faults—can result in immediate and catastrophic failure of electronic and electrical equipment including motors, switchboards, computers, and motor starters/ variable frequency drives.
Control of power surges is typically accomplished through use of a surge protection device (SPD) (Figure 1). As the name implies, the function of a surge protection device is to provide a degree of protection for electrical and electronic equipment from the damaging effects of electrical power surges.
The amount of surge protection provided depends upon the current and time rating of the device. An SPD protects an electrical circuit by limiting the voltage and current that can be applied to the protected circuit in the event of a surge.
This requires the SPD to immediately channel or divert the excess energy of the surge away from the protected circuit by directing it through the device. This function obviously requires the device to be fully and effectively grounded and bonded to a low impedance ground path. Proper grounding of an SPD is one of the most overlooked but critical functions.
One of the more common ratings for an SPD is the amount of electrical energy the device can absorb in a designated time without failing. This rating is sometimes called the joule rating because the joule (J) is a basic measurement unit for energy. However, this rating is also often referred to by other terms, such as a transient energy or single pulse energy dissipation rating.
In addition to effective grounding, surge protection devices must be carefully applied to ensure the device’s rating is adequate to withstand the greatest voltage and current it may encounter and that it can safely and rapidly pass these values to ground.
Varistors are voltage-sensitive devices designed to protect circuits against transient voltage surges. Multilayer varistors are surface-mounted devices that utilize a ceramic multilayer construction and are used to protect circuit boards in small electronics from transients generated from electrostatic discharge, inductive loads, switching, and lightning surge remnants.
Metal oxide varistors (MOVs) (Figure 2a) are epoxyencapsulated zinc oxide discs with radial or axial leads. MOVs are mid-range devices used to protect small machinery, power sources, and components. Industrial MOVs are larger, rigid terminal-mounted devices made with zinc oxide discs enclosed in epoxy polymer cases that provide complete electrical isolation and are used in high-energy industrial applications.
An MOV is a device commonly used to provide a path for the release of excessive energy by routing the transient around the protected device (Figure 2b). There are two characteristics of MOVs that make them desirable for surge protection.
First, the resistance of an MOV decreases with an increase in voltage, making them a favorable device to pass high-voltage surges. In addition, MOVs are fast-acting and can respond to a surge in just a few nanoseconds. This often results in effectively suppressing a surge before it has a chance to damage sensitive electronic equipment.
The ratings of an MOV are typically the clamping voltage and peak or impulse current ratings. The clamping voltage is a measure of the voltage-limiting capability of an MOV. Voltage at a lower level than the designed clamping voltage is passed onto the load. When a surge occurs, the MOV attempts to limit the excess voltage to the level of the clamping voltage by redirecting the excess energy around the protected device to ground.
The peak current rating specifies the maximum current that can be dissipated from a single surge without causing failure of the SPD. Over time and repeated surge events, however, even the best MOVs degrade in performance and may need eventual replacement. Many MOVs are equipped with integral sensors and indicator lights to call attention to satisfactory operation or alert to impending or actual failure.
A surge protection device can refer to a device designed to provide a degree of protection against electrical surges, regardless of its placement with respect to the service entrance main overcurrent protection device.
Changes to the 2020 edition of the National Electrical Code (NEC) have further defined the application and required locations of the various types of SPDs. In most cases, surge arresters apply to just the line side of the utility watthour meter while transient voltage surge suppressor (TVSS) devices apply to just the load side. And while some devices are listed for use in either location, surge protection devices are typically designed for use in one location or the other.
The general hierarchy for surge protection devices is for lightning protection to be provided from service surge protection using a TVSS for the service and feeders, surge arresters for feeder and branch circuits, and point-of-use surge protection devices at and for equipment protection.
The primary standard for surge protection devices, Underwriters Laboratories (UL) Standard 1449, 4th Edition, Section 1, Effective August 20, 2014, specifies UL types covering enclosed and open-type surge protection devices designed for repeated limiting of transient voltage surges on 50 Hz or 60 Hz power circuits not exceeding 1000 volts and photovoltaic applications up to 1500 VDC provided by plug-in devices (power strips), UPS systems, line conditioners, etc.
However, NEC Article 242 specifically identifies the following types of SPDs:
Type 1. An SPD that is permanently mounted between the service transformer’s secondary and the service’s main overcurrent protection device. This type of SPD is also referred to as a utility surge arrester.
Type 2. An SPD permanently mounted on the load side of the service’s main overcurrent protection device. This type of SPD is referred to as a transient voltage surge suppressor or TVSS.
Type 3. A point-of-use SPD. This type is also referred to as a transient voltage surge suppressor.
Type 4. An SPD component or assembly, such as MOVs, and assemblies with MOVs and related components.
Type 1, 2, 3 Component Assemblies. An SPD assembly with internal or external short-circuit protection.
Type 5. Discrete component surge suppressors, like MOVs, that may be mounted to an enclosure, back panel, connected by its leads, or provided within an enclosure with mounting means and wiring terminations.
Figure 3a is an example of a Type 1 SPD for service entrance protection while Figure 3b illustrates a din rail-mounted point-of-use (POU) Type 3 SPD commonly used for control circuit and component protection.
NEC Article 285, Surge-Protective Devices (SPDs), for voltages of 1 kV (1000 VAC) or less, states where in an electrical system SPDs can be installed or connected. As previously stated, grounding is critical to the proper performance of an SPD and simply connecting a surge arrester to a ground rod driven into shallow ground is generally insufficient as electricity is always trying to return to its source by selecting the lowest resistance path available, and a single ground rod will likely not provide this required path.
Therefore, one of the four types of ground sources specified in NEC Article 280 must be used. Deep well steel casings are generally one of the best and most effective ground paths due to their inherent depth and intimate contact with a conductive path (water).
In addition, surge arresters must be connected to each ungrounded (line or load) conductor and connected with a minimum NEC-dictated wire size (AWG) using the shortest length possible to provide the lowest overall resistance path to ground. The minimum wire size and maximum length are generally specified by the arrester’s manufacturer.
Although they are often used on all motor types, a lightning arrester, a Type 3 secondary surge arrester, is primarily used in the water well industry to protect submersible well pump motors. Well pump motors are particularly vulnerable to major voltage surges, and especially from direct lightning strikes. This is due to their inherent location within or at the bottom of a water well, providing the perfect and lowest resistance feasible grounding path.
Lightning arresters, manufactured by various firms such as Joslyn (ABB), Eaton, and Siemens, are designed to route high-voltage surges to ground at the wellhead, bypassing the conductors leading to the motor and hopefully protecting the motor from these surges.
Similar to the function of a conventional spark plug, they are built to allow high-voltage surges to jump a gas-filled electrode gap between the power line side and ground; thus, they are often referred to as gap or spark arresters. They are available and used in both single-phase (Figure 4a) and three-phase (Figure 4b) styles.
The well casing, if metallic and sufficiently deep, is generally preferred and used as the grounding path. Figure 4c illustrates wiring diagrams for single- and three-phase lightning arresters and the typical performance of each type, although the specifics of clamping voltage and peak current ratings may vary between manufacturers.
It is vital that anyone needing to abate power surges understand the limitations associated with each type of SPD and select and apply the device to control the specific type and voltage value of anticipated power surge.
Lightning and surge arresters should be periodically inspected and tested, particularly following a lightning storm, as even though there may not be any apparent or obvious damage, a single high-voltage surge may render the device inoperable, requiring replacement.
Additionally, referencing and observing the applicable NEC Articles that apply to the type and voltage level of the power surge is necessary to ensure adequate protection requirements are met.
SPD Application and Location
For optimum effectiveness and protection, surge protection devices should be installed in the appropriate circuit location. There are three primary levels to install the various types of SPDs (Figure 5):
Type
1) Main (Primary) Level Protection: The protection at the origin of the electrical installation (i.e., primary protection) shunts most of the incident energy (common mode overvoltage delivered by the power system) to the equipotential bonding system and then to Earth.
2) Distribution (Secondary) Level Protection: Circuit protection (i.e., secondary protection) supplements protection at the main by coordinating and limiting residual current mode overvoltage arising from the configuration of the installation.
3) Application or Proximity (Ancillary) Point of Use Level Protection: Proximity protection at the device performs final peak limiting of overvoltage, which is the most dangerous for the powered equipment.
For best performance and effectiveness, a few general wiring recommendations should be observed:
- If possible, limit the maximum wire length to 18 inches. Use a larger wire size as the length increases.
- Use #10 AWG minimum wire size (#8 or #6 is better) for the connection; stranded wire is best.
- Use a secure (bolted or cad weld) connection between the SPD and ground.
- Minimize all sharp wire bends or kinks.
- Do not connect the SPD to the main circuit breaker or power bus.
- Place a properly sized circuit breaker or fused disconnect switch between the SPD and protected equipment.
- Observe all applicable NEC sizing, grounding, and connection requirements.
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This concludes this month’s edition of Engineering Your Business. Next month we will begin a three-part discussion on electrical wire and cable basics.
Until then, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.