As these systems become more electronically sophisticated,
they demand a higher quality electrical environment
Reliable operation of security systems is obviously essential to protect company assets and ensure human safety. However, episodic equipment malfunctions can plague these systems and compromise system integrity. As pieces of data and communication equipment become more electronically sophisticated, they demand a higher quality electrical environment to ensure optimal performance and uninterrupted operation.
To resolve common power quality problems that can cause equipment malfunction and damage, it's crucial to understand the electrical sensitivity of various elements of a security system, best practices for a proper grounding system, and fundamentals of quality surge protection devices.
Visual surveillance, access control, storage of surveillance and access data, intruder detection, and monitoring and notification equipment are all components of security systems ((click here to see Fig. 1). Highly sensitive to poor power quality, these components are often on long cable runs located both inside and outdoors. Surges can follow these cables back into a facility, destroying expensive equipment or causing other hazardous situations. Following is a look at several potential problems.
From high-security keyways and proximity card readers to biometrics, most large organizations use some level of electronic access control. Biometric products, such as retinal scan and fingerprint or palm readers, can provide higher levels of secure access to network operations and R&D labs. Each subsystem presents a unique set of grounding challenges.
When the access control mechanism is located outdoors, it's typically mounted on a cement pad surrounded by asphalt, possibly integrated with an electronic gate controller. All system components should be safeguarded with surge protection devices (SPDs) and be properly grounded. Adequate grounding includes a buried site ground ring, or referencing back to the original neutral-ground bond in the building or the secondary neutral-ground bond of an associated step-down or isolation transformer.
Typically, a magnetic loop detector and underground burial loop are installed to ensure that the door or gate closes immediately after the vehicle has passed. Sensing the metal of a passing vehicle, the buried wire coil loop sends a signal to close the door or gate when the vehicle clears the opening. Any nearby lightning strike can induce a current surge onto this buried coil, which then can travel back to the gate controller. Hence, electronic access control gate equipment, including the motor controller, should be protected by an SPD and properly grounded.
Closed-circuit television (CCTV) systems that incorporate digital cameras, monitors, and storage have become commonplace in today's market (Photo above). Horizontal bars on monitor screens, a result of load unbalance in 60-Hz power distribution in the vicinity of a CCTV system, are a common problem for technicians. A crossover between the flow of AC power and the CCTV signal causes this anomaly. This happens when there is a common point of connection (system ground) between the CCTV system and the utility company. Complex systems that include multiple monitor locations, or camera cables connected through coaxial “patch panels” tend to have severe 60-cycle interference. In these cases, there are numerous variables to consider when deciding upon a solution. The solution should always include a single-point ground for the system. This grounding point would normally be at the equipment hub or monitoring location.
A CCTV network can have cameras located in numerous locations within the buildings and outdoors mounted on buildings and poles. In applications with cameras mounted to metal buildings, numerous instances of damage to cameras and digital video recorders (DVRs) have been documented. To mitigate this problem, you should connect an isolation transformer at each point in the system where additional grounds are attached, such as at cameras, lightning arresters, patch panels, and coaxial cable shields.
Cameras mounted outside on tall poles also need to be protected by SPDs with effective dedicated grounds and optional air terminals. Technicians installing outdoor cameras often error by attempting to isolate the camera through the use of an insulating pad on which they mount the camera. This method is ineffective because a lightning surge, looking for the quickest path to ground, will frequently arc over from the building to the camera housing. Another common mistake is to bond the ground wire to steel on the building.
If the steel beams are not properly bonded and tied to the ground ring, an isolated bond is not established. An SPD to protect the power and data lines should be installed and grounded locally. In this high-frequency lightning event, an SPD is needed to stop current from reaching both equipment within the building and the ground loop.
Improperly grounded cable shields frequently cause disruption of equipment operation. These shields are intended to block AC and high-frequency fields. Low-frequency power distributions that run in parallel can add noise or hum into low-amplitude signal circuits. High-frequency fields can alter data if there are transmitters in the area. Because of differing AC potentials in the soil, an equalizing AC current can exist upon a shield. This happens when the shield is grounded at more than one location, thereby introducing unwanted noise into the signal circuit.
A properly designed intrusion detection system identifies when and where intruders first enter a facility, pinpointing their current location. Heat and motion detectors are commonly used within buildings to detect intruders. Airports, harbors, nuclear power plants, pipelines, and other critical infrastructure sites require reliable outdoor surveillance capabilities for intruder detection. Today, many sites are introducing Millimeter Wave Perimeter Security Radar to aid in detection of targets in poor visibility environments. Obviously, these highly sensitive and costly technologies also require installation of grounding and SPDs.
Many organizations with multiple facilities have integrated security systems. Remote sites networked via phone or radio link need RF surge protection on the antenna feed. Surge protection on AC/DC power supplies for rack equipment is also needed. Once again, a single-point grounding system is essential for trouble-free operation.
To remove these power-related problems and keep a site electronically secure, there are certain best practices you should follow. The single most effective means of assuring a safe electrical environment is a high-integrity grounding system. Improper grounding can account for up to 40% of power-related problems that result in costly downtime.
The NEC allows a single-point grounding system to be connected to the earth in seven different ways. The two most common (described in detail below) are rod and pipe electrodes and a ring ground. The other legal, but much less prevalent, methods include:
Concrete-encased electrode [Sec. 250.52(A)(3)] (metal bars encased in concrete, buried in the earth);
Grounded metal building frame [Sec. 250.52(A)(2)];
Plate electrodes [Sec. 250.52(A)(7)] (metal plates buried in the earth for a larger surface area); and
Other local metal underground systems or structures not bonded to metal water pipe [Sec. 250.52(A)(8)].
Approximately 90% of all grounding electrode system installations are rod and pipe electrodes. An 8- to 10-foot rod or pipe is driven into the earth and connected via a bare copper cable to the neutral bus in the main power distribution center. Unfortunately, many facility technicians drive additional ground rods throughout the facility, in an effort to clear up problems, but actually end up undermining this system. If the NEC is followed, all earth ground references are directly bonded to the original neutral-ground bond at the building entrance.
This safety measure prevents a person touching a connected component from danger. A person would be harmed if contacting one component connected to an independent earth ground rod and another connected to the main building ground system, especially during lightning events or fault conditions. Remember, the main reason for connecting an electrical distribution system to the earth is for touch safety.
Driving extra rods (multiple grounding) can also cause equipment downtime. Multiple grounding can create ground loop currents that circulate throughout the equipment cabinets between the different grounds. The proper method is to reference each cabinet back to the main building ground point or the nearest neutral-ground bond at the secondary of an associated transformer.
Wiring additional connections to the earth with varying amps of current running through equipment cabinets is a dangerous and common mistake made by technicians. This condition causes voltage spikes in the cabinets by creating fluctuating currents. Voltage surges are inherent to this condition. To solve this problem, you can legally add another ground rod at a specific minimum distance and connect it to the original building entrance ground rod. It is illegal, however, to ground equipment cabinets out to separate earth grounding systems.
As shown in Fig. 2, a ground ring is installed around the perimeter of a facility with, at minimum, No. 2 AWG bare wire buried no less than 30 inches under the soil. This ring, having no more than 25 ohms of resistance at any point upon it, is intended to be the path of least resistance for hazardous electrical currents. Earth ground rods are driven into the ground and referenced to the ring at specific distances, depending upon soil composition, to meet resistance requirements. System components should have the shortest distance possible between their installation point and connection to the ring. Larger wire sizes are needed, depending on length of wire stretch.
Several problems are inherent to ring grounds, while others are associated with improper installation. Installers connect different equipment cabinets to different points around that ring. Large lightning voltages can exist upon the ground ring because of the inductance of wire, regardless of wire size. This inductance leads to large voltage differences between the cabinets connected at different points on the ring. Instead of this installation, all equipment cabinets should be connected to a single point in the facility.
Up to this point, we've focused on the importance of installing a proper grounding system to mitigate the affects of harmful electrical surges in a security system. However, you must also properly integrate RF, AC, DC, and data line SPDs into your design. The sole function of a quality surge suppressor is to protect sensitive electronic equipment from transient overvoltages. It must limit transient overvoltages to values that do not surpass the AC sine wave peaks by more than 30% as it initially absorbs intense amounts of transient energy. The suppressor must immediately respond to transients to prevent impulses from reaching their uppermost voltage values. In addition, its performance characteristics should not degrade with use or over time as it is called upon to suppress extremely high levels of transient energy. Suppression devices should be installed at every copper building entry point, including the AC service and low-voltage control and sensing circuits.
The IEEE categorizes transient surges by waveform. The most frequently referenced are C Low and C High category combination waves used to simulate lightning. These waves are characterized by short-duration high-frequency 8/20µsec current and 1.2/50µsec voltage waveforms. SPDs are tested to their ability to mitigate and withstand these currents and longer duration 10/1000µsec voltage and current test impulses. These simulate transient activity originating from sources other than lightning, such as surges generated within buildings, from utility grid switching, and the power cycling of inductive loads.
In summary, you can avoid equipment malfunction, damage, and destruction of your security system by doing three things: Implementing a single-point grounding system, following the NEC when installing the safety ground and grounding electrode systems, and integrating premium, non-degrading SPDs.
Adams is an application engineer with Transtector in Hayden, Idaho.
Employ the following best practices to ensure good power quality.
NFPA 780 (Lightning Protection Code) requires a separate physical lightning protection (air terminal) earth ground electrode to be bonded to the main entrance grounding electrode system. Its purpose is to direct the majority of a lightning discharge's current upon a building into the earth away from its service entrance grounding system, thereby significantly reducing the amount of lightning current into the building electrical grounding system. Bonding of the two grounded systems as required maintains a reasonable degree of touch safety during a lightning event.
Referencing individual pieces of equipment to earth (grounding) at different points creates safety and damage hazards. The earth is a very poor conductor; therefore, steady-state and momentary voltage differences exist in the ground. Control and communication cables with various voltages link security system components. If each component connects to the ground at different points, the resulting voltage differences can cause equipment downtime and pose a safety threat. A single-point grounding system, where all references to the ground come to one main ground in the facility before referencing the earth, is essential to all facilities.
In most AC installations, individual grounds are referenced back to the building's original neutral-ground bond or to a secondary neutral-ground bond of an associated step-down or isolation transformer. In DC power applications, single-point grounding is accomplished by using a master ground bar, such as a large piece of copper.