By Georgia Genovezos
Utilities are becoming increasingly reliant on communications as the industry becomes more competitive. But lightning strikes and high voltage surges can wreak havoc with wire-line circuits, particularly in electric power plants and substations. One bad storm, a lightning strike or a fault on the line, and thousands of dollars worth of equipment are gone. The only way to prevent the damage is by properly protecting the wire-line circuits.
The basic premise behind the use of wire-line isolation devices in high voltage environments is simple: When phone service is required at a site that may be subject to high voltage surges, special protection measures are required by various national standards to ensure personnel safety and prevent damage to equipment.
This brings about a number of questions: Who needs to be aware of these special protection measures? What type of equipment is required to achieve this protection? Where is this equipment installed? When should the equipment be installed? And, most importantly, why is it required?
Ground Potential Rise (GPR)
The “hows” and “whys” of ground potential rise (GPR) first must be understood before designing and implementing a safe and effective protection scheme. In a nutshell, when a fault or lightning strike occurs and a current reaches a substation ground grid, the result, according to Ohm’s Law, is a potential rise.
If equipment is all tied to the same ground grid and is not referenced to any external ground, it will not be damaged due to GPR. However, wire-line telecommunications, which are connected through equipment bonded to the substation’s ground grid, are also terminated to a central office (CO) by copper pairs. This CO is the remote earth, and the copper wire-line is a conductor tied between two ground planes. Therefore, a difference in potential between the two ground planes will cause a current to flow up from the ground at the substation, through the equipment and out onto the wire-line. This is dangerous to personnel and can damage the site equipment.
This situation can be compared to two glasses filled with water, one representing the ground plane at the substation, the other, the ground plane at the CO. Imagine one glass up on a shelf, and the other lower on the table. If there is no connection between the two, then no matter what happens to the water levels in the glasses (comparing variations in potential), no water will flow between the glasses (meaning no current will flow). However, if the two glasses are connected by a straw (i.e. connecting the two ground planes by a copper phone line), then sudden increases in the water level of the first glass, means water will flow down the straw (i.e. current on the wire-line) to the second glass. Anything tied to that straw would get wet. In the same way, anything tied to the wire-line will see the current. The only way to prevent this is to put a barrier in the straw. This is what isolation devices do.
While proper grounding is essential and standard communication protection methods, used properly, are critical at these sites, they are unfortunately ineffective in protecting equipment from GPR. For example, shunting devices normally are placed at each end of a cable communication facility and are designed to direct foreign voltage impulses into a grounding system. During a GPR, these devices merely offer an additional path to remote ground reference, and actually provide a path for current to flow in the reverse direction from which they were intended to operate. Thus, no matter how good standard protection devices are, equipment or cable facilities will become part of an electrical path between the GPR and remote ground. The only effective protection scheme against GPR is an isolation device.
The next step is defining what tools are available to solve GPR problems. A series of field-proven ANSI/IEEE and other national standards provide methods for protecting people and equipment from GPR. Although most of these standards address protection from GPR due to 60 Hz fault currents, lightning strike energy applications are basically the same when considering higher frequency impedance. Both currents generate a GPR and can harm personnel and damage or destroy communication facilities.
The standards define when a high voltage interface (HVI) device is necessary for wire-line protection. In general, an HVI should be installed when the calculated GPR is above 1,000 V peak asymmetrical, or the service performance objective (SPO) is for Class A, always requiring protection.
In summary, three issues must be considered before a protection scheme can be designed and implemented:
- Is the site a likely candidate for GPR? The answer is yes if a wire-line communication link enters a high voltage area or one that is prone to lightning.
- What is the calculated level of GPR at this site? If it is evaluated at greater than 1,000 V peak asymmetrical, then high voltage isolation is required by IEEE standard 487, rather than using shunting devices.
- What level of service performance objective is required at the site? If it is Class A, then again, HVI protection should be installed rather than using shunting devices.
The Wire-line Isolator Concept
The basic objectives for the protection of wire-line facilities are to ensure personnel safety, protect the telecommunications plant and terminal equipment, maintain reliability of service, and accomplish these in the most economic way.
A good wire-line isolator will isolate and protect telephone facilities and personnel from the hazardous voltages associated with GPRs. Inserted into the wire-line link at the terminal end, the wire-line isolator unit breaks the copper continuity of the telephone line as it enters the high voltage site, thus eliminating the conductive bridge, which links the high voltage site and the CO ground planes. Basically, it acts as a dam between the local site services (also called station side) and the CO side, allowing all of the communication signals to pass through transparently, but preventing any fault currents from passing on the phone lines. It accomplishes this either through an isolating transformer or a fiber optic link. This basic concept is illustrated in Figure 1.
Design and Installation of High Voltage Wire-line Protection
Once the requirement for use of the high voltage isolation device is determined, and the device purchased, then proper installation procedures must be followed. If the installation is not done properly, the equipment will not perform as specified. The device is installed on the wire-line telecommunication link that feeds into a high voltage area, such as an electric power plant, substation, PCS or cellular site, or E9-1-1 site, where telco wire-line service is required.
The installation of a high voltage interface (HVI) is a detailed procedure. It is also a simple procedure if the necessary steps are done in the right sequence and safety regulations are observed. The different elements which constitute a typical HVI installation are: the telephone cable junction (splice point) outside the zone of influence, the dedicated cable in PVC conduit, the lightning arrester, a non-metallic splice case, the high voltage isolation device proper, and the secondary protection to which the local site services are connected. Figure 2 shows a typical high voltage protection installation using wire-line isolation devices.
To ensure personnel safety, all the local communications connections at the substation site should be made first and tested before connection to the dedicated CO cable. The reason being that since a GPR may occur at any time when the wire-line cable is connected to both the communications at the substation and remote ground planes, the connection to the dedicated cable should be made last. Additionally, it is recommended that a single crew, the members of which can see each other at all times, should perform the installation. Work should always be done while standing on a 20 kV insulating rubber mat, and wearing insulating rubber gloves and boots. Work should be accomplished on a clear day when lightning strikes are less likely to occur.
In figure 2, the wire-line from the central office comes into the HVI through the 4-inch PVC conduit. This is then routed through a lightning arrester and spliced to the shelf. This shelf is a central part of the isolation device. It is where the CO and the station cables are kept separate by an isolation gap. The station cable is connected to the upper right hand side of the shelf, and feeds through a secondary protector before going on to the local services. The CO cable comes in from the lower left-hand side. These two sides are kept separate from each other at all times, to maintain the integrity of the isolation. Power is supplied to the shelf when needed.
This describes the bones of the system. The meat of the system is how to get the telecommunication signals across the isolation gap. Isolation circuit cards accomplish this. They are the heart of the system. These cards plug into the shelf and are the telecommunication bridge between the CO and station sides of the assembly. The cards are designed to suit just about any communication requirements.
To design a system, the type and number of lines that require protection need to be determined, such as POTS, four-wire AC data, T1 carrier, etc. Then the size and quantity of shelves to house the cards is determined. An installed system can be as small as about 13-by-11-by-6 inches for isolating a single circuit. The entire assembly shown in figure 2 would take up about 35 by 40 inches of wall space and protect up to 8 circuits. At this stage, the user should keep in mind any future expansion needs when selecting the shelves. It is sometimes useful to reserve one or two empty slots in a shelf for this purpose. Powering requirements need to be determined and the appropriate power supply selected. Finally, other equipment needs should be considered, such as lightning arrestors, mutual drainage reactors, etc.
The end result will be a comprehensive high voltage wire-line communication asse-mbly that will protect site communication services from damage caused by GPR.
Georgia Genovezos is the media and communications manager for Positron’s Power Division. Positron engineers and manufactures telecommunications equipment for high reliability, critical service applications, including communication protection for high voltage environments and public safety emergency response systems (E9-1-1). Genovezos can be reached at firstname.lastname@example.org, or through the Web site at www.positronpower.com.