Quality Power for Security Systems

Reliable operation of security systems is essential to protect company assets and ensure human safety. Episodic equipment malfunctions plague security systems and compromise system integrity. These malfunctions often are related to the power quality environment. As data and communication equipment become more electronically sophisticated, they demand a higher quality electrical environment to e...

09/01/2007


Reliable operation of security systems is essential to protect company assets and ensure human safety. Episodic equipment malfunctions plague security systems and compromise system integrity. These malfunctions often are related to the power quality environment.

As 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 is 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.

Several sources cause changes in the characteristics of the power supply to the components of a security system, including:

  • Lightning activity

  • Utility grid switching

  • Utility power factor correction

  • Start-up or shutdown of equipment within a facility.

These occurrences last only a short period of time; however, when injected into power and data circuits, they cause equipment damage or destruction and create safety hazards.

There are best practices to remove these power-related problems to keep a site electronically secure. 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.

Equipment malfunction, damage and destruction can be avoided through the following:

  • Implementing a single-point grounding system

  • Following the National Electric Code when installing the safety ground and grounding electrode systems

  • Following the use of properly designed and appropriate integration of surge protection devices (SPDs).

SENSITIVE ELEMENTS

Visual surveillance, access control, storage of surveillance and access data, intruder detection, and monitoring and notification equipment are all components of security systems. All are highly sensitive to poor power quality. These components are often on long cable runs located both indoors and out. Surges can follow these cables back into a facility, and destroy expensive equipment or cause other hazardous situations.

Access control systems. From high-security keyways and proximity card readers to biometrics, the levels of access and security can be adapted to site needs. Most large organizations use some level of electronic access control, with either magstripe or proximity technology. Biometric products such as retinal scan, 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 is typically mounted on a cement pad and surrounded by asphalt. It may be integrated with an electronic gate controller. All system components should be protected with 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 stepdown or isolation transformer. A detailed discussion of proper ground ring construction is found later in this article.

Typically, a magnetic loop detector and underground burial loop is installed to ensure that the door or gate closes immediately after the vehicle has passed. The buried wire coil loop, sensing the metal of a passing vehicle, 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, and can travel back to the gate controller. Hence, electronic access control gate equipment, including the motor controller, should be protected by a SPD and properly grounded.

Video surveillance systems. CCTV systems that incorporate digital cameras, monitors and storage have become ubiquitous. Horizontal bars, a result of load unbalance in 60 cycle power distribution in the vicinity of a CCTV system, are a common problem for technicians. This anomaly is caused by a crossover between the flow of AC power and the CCTV signal. This happens when there is a common point of connection (system ground) between the CCTV system and 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 as illustrated in Figure 2. 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, connect an isolation transformer at each point in the system where additional grounds are attached (grounded cameras, lightning arresters, patch panels, required coaxial cable grounds, and any other possible source of grounding).

Cameras mounted outside on tall poles also need to be protected by SPDs with effective dedicated grounds and optional air terminals. Technicians, when installing outdoor cameras, often mistakenly attempt to isolate the camera by mounting it on an insulating pad. 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; however, 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 be 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.

Proper equipment operation is frequently disrupted by improperly grounded cable shields. 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 and causes unwanted noise to be introduced into the signal circuit.

Intruder detection systems. A properly designed intrusion detection system identifies when and where an intruder first enters a facility and pinpoints 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. It is obvious that these highly sensitive and costly technologies require installation of grounding and SPDs.

Remote site radio links. 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 need. Once again, a single point grounding system is essential for proper trouble free operation.

GROUNDING ESSENTIALS

NFPA 780, Standard for the Installation of Lightning Protection Systems, requires a separate physical lightning protection (air terminal) earth ground electrode be bonded to the main entrance grounding electrode system as shown in Figure 3 (p. 18). Its purpose is to direct the majority of a lightning discharge's current upon a building into the earth, away from its entrance grounding system, thereby significantly reducing the lightning current into the building electrical grounding system. Bonding of the two grounded systems as required maintains a reasonable degree of touch safety during the lightning event.

Single-point grounding. Referencing individual 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. The security system components are linked by control and communication cables with various voltages. If each component connects to the ground at different points, these 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 or a secondary neutral-ground bond of an associated stepdown 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.

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 ring ground. The other legal, but much less prevalent are:

  • Concrete-encased electrode (metal bars encased in concrete, buried in the earth)

  • Grounded metal building frame

  • Plate electrodes (metal plates buried in the earth for a larger surface area)

  • Supplemental metal underground water pipe (this can be used as a secondary or third connection to the earth but is not legal as a individual means of grounding)

  • Underground local metal structures (gas piping, for example)

Rod and Pipe Electrodes. Approximately 90% of all grounding electrode system installations are rod and pipe electrodes. An 8- to 10-ft. stake is driven into the earth and connected to the neutral conductor in the main power distribution center. Unfortunately, many facility technicians drive additional ground rods to clear up problems and undermine this system. If NEC is followed, all earth ground references are directly bonded to the original neutral-ground bond at the building entrance.

This safety measure prevents harm to a person who touches a connected component. A person would be harmed if contacting one component connected to an independent earth ground rod and another component 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 circulating 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 creates 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.

Ring Ground. As illustrated in Figure 4 (p. 22), a ground ring is installed around the perimeter of a facility with, at minimum, bare #2 AWG wire buried no less than 30 in. under the soil. It is intended to be the path of least resistance for hazardous electrical currents, having no more than 5 ohms of resistance at any point upon the ring. Earth ground rods are driven into the ground and referenced to the ring at specific distances depending on 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.

SURGE PROTECTION

Mitigating harmful electrical surges in a security system requires the integration of RF, AC, DC and data line SPDs. 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 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 8/20ldings, from utility grid switching, and the power cycling of inductive loads.

Staying online at all times is critical to the integrity of a security system. Problems arising from power events can be mitigated by providing a proper grounding system and appropriate surge suppression. Incorporating a single point grounding system, and installing a premium non-degrading surge suppressor is essential to keeping these systems operating at peak performance for years to come.



Surge Suppression Technology

Most surge suppression devices are metal oxide varistor (MOV), silicon avalanche suppression diodes (SASDs) or a combination. Selection of one suppressor technology over another involves costing, maintenance and equipment sensitivity concerns.

MOVs are non-linear variable resistors with semiconductor properties. MOVs are highly popular because they are inexpensive and they dissipate reasonably high values of transient current. However, there are significant negative performance variables to consider.

Degrading technology

Higher voltage protection level

Subject to “thermal runaway”

MOVs conduct small amounts of electrical current as they are installed across an AC power source. The MOV's current conduction paths are directed through the zinc oxide particles. These particles are weakened and their resistive characteristics change after they have conducted current. This degradation cycle becomes more profound as the MOV conducts more frequently and as it conducts higher current value surges. As the cycle continues, the MOV's internal temperature elevates, and subsequently, it conducts even higher current values.

As this spiral continues under surge current conditions, thermal runway occurs. MOV-based suppressors have been documented as igniting fires resulting from thermal runaway disorders. If the SPD is located inside the AC panel, breakers will melt and bus bars will arc. To prevent these catastrophic failure modes, the initial voltage protection level (VPL) of properly designed MOV-based suppressor products is set to higher values. This undermines the SPD's surge protection.

SASDs are solid state semiconductors. The advantages associated with the use of SASDs in suppression devices, over other suppression technologies, are numerous. Unlike MOVs, they are not plagued with voltage protection limitations or thermal runaway problems. Silicon responds faster to transient overvoltages than MOVs, and maintains a tighter, stable voltage protection level. It suppresses much closer to the peak voltage value of the AC sine wave. Silicon products do not degrade with use or over time. A quality silicon surge suppressor incorporates multiple diodes to handle the transient currents under normal transient conditions.

Some SPD manufacturers incorporate SASDs and MOVs to create hybrid designs. These configurations employ the superior response time and stable voltage protection levels that SASDs offer. At the same time, they engage a secondary or backup MOV stage to dissipate higher current surges.

Beware designs that offer primary MOV components with a token axial leaded SASD technology. This type of design allows the manufacturer to make bold claims of superior silicon protection, while offering little true advantage.

Premature suppressor failure often occurs at the SASD stage because not enough diodes are incorporated to dissipate the proper levels of surge energy. While the MOVs continue to function, they are still plagued with the same deficiencies of a pure MOV-based product. Properly designed surge suppressors utilizing 100% SASDs as their sole suppression technology preclude the need for hybrid designs.



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