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Theory and Application of Wire-Line Isolation Devices in High Voltage Environments
By: HVI-TEC Luescher, formerly know as Telbit AG
The basic premise behind the use of wire-line isolation devices in high voltage environments is simple: when phone or data service is required at a site which 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.
The purpose of this presentation is to review who needs to be aware of these special protection measures, what type of equipment is required to achieve this protection, where this equipment is installed, when should the equipment be installed and, most importantly, why it is required.
The main reason why it is installed is to prevent injury to personnel, damage to equipment and loss of the communication link. The associated monetary cost to both the telco providing service and the organization using such a service can be very high if the service is lost.
The sections herein present an overview of the theory behind the need for protection, the Isolation concept, the installation of a wire-line protection assembly and an introduction to the different configurations of an isolation interface.
2 GROUND POTENTIAL RISE
A high voltage environment can be a dangerous place. All kinds of voltage surges can occur. Of particular interest to communication / protection engineers, is the surge known as a ground potential rise (GPR).
As the old saying goes, "Know thine enemy". The "hows" and "whys" of GPR must first be understood, before designing and implementing a safe and effective protection scheme. Starting first with the technical description of IEEE standard 487-2007:
"When a power system ground fault occurs, all or some of the fault current returns via the earth through the ground grid and produces a potential difference between the ground grid and remote earth. This potential difference is defined as power station ground potential rise (GPR)."
In a nutshell, when a fault occurs, and a current reaches a 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, then 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 a copper wire. This CO is the remote earth, and the copper wire-line is a conductor tied between two ground planes. Therefore, a current will tend to flow, which is dangerous to personnel and can damage the site and its equipment.
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