Nearly every application that uses sensitive electronic circuitry will require surge protection in stages to buffer damaging power surges from lightning or other random happenstance such as a severe power surge with the local electrical company. The same lines used to transmit signals form an ideal conduit for lightning strikes, and although lightning strikes may rarely occur, they can cause a catastrophic failure. Camera systems in video surveillance applications, radio equipment in wireless applications and power over Ethernet (PoE) equipment require some sort of surge protection at the systems and subsystems levels. The following information takes a deeper dive into the inner workings of surge protection equipment for wireless networks, IT infrastructure, industrial automation and video surveillance applications.
Multiple Stages of Protection
Varying levels of power surges happen frequently in a facility. Even the minor, frequent surges can corrupt data files. Larger, more infrequent surges will eventually damage equipment. An estimated 60 to 80 percent of power surges are smaller and are caused by anomalies that arise within the facility housing the electrical equipment (See “Protecting Commercial Facilities from Power Surges” at https://disastersafety.org/ibhs/commercial-power-surges.)
Surge currents from these events can be sent down the signal chain through power cords, telephone lines, data lines and coaxial lines. The most frequently cited high-power surges caused by lightning strikes or electromagnetic pulses (EMPs) can rapidly cause irreparable failures all the way down to internal equipment. Additionally, indirect lightning strikes can generate strong electromagnetic fields that induce power surges within a building. This is especially true for RF equipment that operates at much lower voltages than typical data lines and power lines. Coaxial cabling will be susceptible to strong electromagnetic (EM) fields and may carry excess voltage or added noise to sensitive equipment.
The cost of the smaller surges is difficult to gauge because they often go unnoticed. Nevertheless, they end up costing facilities millions of dollars in downtime. Causes of small surges can vary from external power issues with the local electrical grid to internal transient voltages caused by malfunctioning high-powered equipment, such as motors in the facility’s heating ventilation and cooling (HVAC) system.
Table 1 lists the various power surges and some of their common causes. Nearly all businesses are subject to various levels of power surges and should therefore take measures to protect their internal equipment from these risks.
Although it would be difficult to entirely protect a system from a massive electrical event such as a direct lightning strike, steps can be taken to ensure that the equipment in a facility is not entirely damaged. The same steps also provide protection from smaller transients. This level of protection requires several stages.
Often, the first line of defense is placed at the facility’s service entrance where power can enter a building. The second line of defense is placed at the distribution panel, buffering any excess surges that the first stage did not adequately absorb or redirecting surge current to prevent the excess power from being redistributed. However, these steps only protect a facility from external events. Remember, smaller transients from internal equipment in the facility generate the most surges. This means that sensitive, critical equipment will need protection.
Standards listed by the Underwriters Laboratory (UL) include the parameters and testing for lightning and surge protective equipment. Table 2 shows some of these standards. What follows will focus more on UL 1449-certified surge protective devices that are often used to protect sensitive internal equipment.
Deciding what to protect is often a customized, application-specific design process because any and all potential damage must be mitigated. Regardless, there are some protective devices that are often used. Here are some protective devices, relevant parameters and design considerations:
Gas discharge tube: The various surge protection devices (SPDs) can use a number of components, often in stages, to suppress or redirect any transient overvoltages. A gas discharge tube (GDT) is frequently used because it handles voltages from 70 volts to 6,000 volts. This is accomplished through the use of an inert gas that acts as an open circuit until triggered. Once triggered, the GDT shorts to ground, redirecting any high voltage pulses. To trigger a GDT, the device must be presented with a high voltage for a significant amount of time, a characteristic that potentially allows damaging surge currents to pass through them. Moreover, a GDT can handle only a few large transients in its operational lifetime.
Additionally, the gas within the GDT tends to remain ionized after the surge event concludes, because of the current supply from the power line. The sustained ionization only occurs when the GDT breakdown voltage is smaller than that of the power line. At this point, the tube enters what is known as a “discharge mode” in which it continues to decrease in voltage while increasing in current. This leads to an inevitable demise of the device, so typically various resistor or nonlinear devices are placed to interrupt the follow-on current. The follow-on current, combined with the relatively slow response time, accounts for why a GDT is often paired with some type of clamping device (e.g., a Zener diode, a metal-oxide varistor [MOV] or a transient voltage suppression [TVS] diode).
There are a few notable clamping devices, including the TVS diodes and MOVs. They are known as clamping devices because, when the voltage exceeds a certain limit, the device clamps the signal to a voltage level and often steers surge current into the ground rail. An MOV exhibits a high resistance at low voltages, but when voltages increases, its resistance drops in order to shunt excess current.
One major difference between the varistor and the TVS diode is the bidirectional nature of the varistor. (Varistor is short for variable resistor.) Some TVS diodes have bidirectional characteristics. The TVS diode has the shortest response time of all the transient overvoltage protection devices listed — on the order of picoseconds.
Another consideration when choosing between clamping devices is capacitance because capacitive loading degrades frequency response. The MOV has a higher capacitance than the TVS. An individual TVS diode will have a higher capacitance unless it is used in an array with other diodes. One significant consideration with TVS diodes is that although they can handle relatively high voltages, they cannot handle as high currents as an MOV. Table 3 offers a simple overview of these devices, some of their considerations, and an idea of their respective response times, voltage capability and capacitance.
Various Topologies of SPDs
When these major protective devices are combined with supplementary devices such as current-limiting resistors or suppression coils, the SPD can have cascaded protection that addresses some of the weak points of these individual devices. For instance, the GDT has a remarkably high overvoltage protection for its size compared with other protective devices, so it is often the first device to be placed in parallel with the line that needs surge protection. These devices can be used in tandem with clamping devices, such as a diode array or an MOV, because they have a higher response time.
Figure 1 shows a sample protective circuit for an RS-485 control line. Equipment failure caused by the GDT’s slow response time can be mitigated with the use of the fast-responding diode clamp array. In the case of a sustained pulse, the current limiting resistors limit the power dissipation of the diode array, thereby allowing the GDT to flash over, that is, to make an electric circuit by sparking across a gap. Other iterations can involve suppression coils to limit the current through the clamping device, allowing time for the GDT to respond.
Equipment used for video surveillance that is subject to lightning strikes can be compromised without proper protection. This is especially true for high-security facilities, such as nuclear power plants, military bases, and oil and gas refineries. Across industries, video surveillance uses various technologies from analog video to high-definition (HD) over coax, and even power over Ethernet HD cameras. More often than not, these installations use a 75-ohm coaxial interface to allow the video surveillance system to be incrementally improved without the need for a complete overhaul.
Commonly used pan, tilt and zoom (PTZ) cameras can use three main lines: an analog video line, a RS-485 control line and a power line. Each of these lines requires different levels of buffering. Various device topologies are used to protect the different lines of a PTZ camera. For analog video lines, the BNC connector shield can be directly tied to ground or can be isolated for installations prone to ground loops.
The Ethernet backbone has infiltrated many applications including industrial automation with PoE inspection cameras and IT infrastructure. The IEEE 802.3 standard states that power can be delivered to a powered device (PD) in one of three ways:
The nature of powered devices can vary from security cameras and touchscreen devices to access points and call stations. For this reason, there is a broad range of applications for PoE. A midspan PSE would most likely be used to add PoE capability to a network that previously had none. An endpoint PSE would be for newer networks that already have PoE capabilities.
Figure 2 shows a sample connection between an Ethernet hub and DC injector to a PoE device. Many companies use injectors because it is often more costly to change an existing non-PoE switch.
A PoE PSE can also take the form of one of two power topologies:
Most endpoint PSEs are Mode A; midspan PSEs are compatible with Mode B. Some SPDs have both Mode A and B compatibility. Although there is no explicit IEEE requirement for protection, the PSE controllers often come with pins to connect to equipment for surge protection because it is increasingly necessary. A PD can be damaged by power surges from an electrostatic discharge (ESD) carried by the Ethernet cable or the device’s input power connection. In the IEEE 802.3af standard, a field-effect transistor (FET) switch is recommended for letting the power go through to the PD. The FET is particularly susceptible to transient electrical surges. Moreover, facilities that use long unshielded twisted-pair (UTP) cable runs to outdoor PDs create a large antenna that attracts large transients. For this reason, more and more installations use fiber-optic cabling for its inherent lightning resistance.
Figure 3 shows a sample Mode B injector SPD circuit. Despite the benefits of using PoE SPDs, these devices add capacitance that can cause an unavoidable slight signal degradation. The use of the low-capacitance three-pole differential gas tubes, diode array and silicon diode for alternating current (SIDAC) allows for a low-line-to line (L-L) and line-to-ground (L-G) capacitance that, in turn, minimizes distortion of any high-speed signals.
It is ultimately more costly and risky not to implement any kind of protective device for PoE systems as Ethernet speeds become faster and components become much more sensitive to surge currents. As speeds increase, integration also tends to increase where the Ethernet interface is now integrated into the main printed circuit board (PCB) assembly. If any caused by a surge current occurs, the entire motherboard or even the entire piece of equipment will require replacement. This result not only has the obvious cost of equipment repair or replacement, but also the less apparent cost of downtime caused by the troubleshooting and repairing.
Coaxial Surge Protection
Telecommunications systems and various wireless applications can be particularly sensitive to transients because of their inherent low-voltage operations. Fortunately, most transients have a low frequency. The average lightning strike has a frequency between DC to 1 MHz. Therefore, a signal filter often is adequate for shunting lightning surge current to ground and maintaining the desired transverse electromagnetic (TEM) mode of propagation along the coax. This can be accomplished in a number of ways. One example is the use of quarter-wave stubs because they effectively block low-frequency surges to ground (see Photo 1).
The coaxial quarter-wave stub SPD appears as a tee that is perpendicular to the signal path. During normal operation, a quarter-wave stub will take a portion of the signal through the stub portion and it will be scattered down the length of the stub. The signal will be reflected off the short-circuit in the stub and travel back down in phase with the desired signal, because the length of the stub is one-quarter of the center-frequency wavelength. A low frequency, however, has a much longer wavelength than the normal operating frequencies and will therefore scatter into the stub and short-circuit to ground, thereby diverting a transient to ground. The benefit of this is that it operates with a low VSWR so that it minimally impedes signal flow during normal operation. GDTs can also be used to shunt extremely high voltages to ground. The downside is that replacing this technology when necessary can be costly.
SPDs are essential to protect sensitive electronic equipment from small to large transients. As technology advances, this equipment requires extra precautions against damage because of the increased use of smaller trace widths, extensive use of low-voltage circuitry and increased integration. Although all of these factors allow for remarkably fast data speeds within small packages, this increases the cost of any damage occurring with these components. Because this trend can be seen across countless applications, it is critical that lightning protection be a heavy consideration in any cable installation. Long runs of copper cabling tend to act as a fantastic antenna that attracts transients, so they must be adequately connected to ground via a grounding lug that provides a tie point for earth ground. There is a fairly vast repertory of protective devices from which to choose. All of these devices come with their own set of considerations when implementing them in an SPD. A designer most likely will have to assess all these factors when implementing SPDs in specific application.
Dan Rebeck is product line manager at L-com.
This article ran in the June 2019 issue of AGL Magazine.