High Voltage Isolation and Ground Potential Rise


High Voltage Isolation and Ground Potential Rise

This article outlines the principles of High Voltage Isolation (HVI) and Ground Potential Rise (GPR). Most often questions of concern arise to anyone associated with the use of any communication signal (analog or digital) via a metallic or fiber optic cable. Providers of telecommunications, electric service, wireless service or CATV, or operating in an isolated manufacturing or mining location, may be working in a hostile electric environment. An unprotected or improperly protected system is unsafe for the public and personnel, and exposes equipment to service interruptions or permanent damage.

Importance

People who are new to this profession or those who have many years of experience, whether an engineer, architect, tower erector, equipment installer, manager, tradesperson or safety supervisor, need to know or learn the following:

* what the level of exposure may be
* the protection equipment necessary for the work location
* the necessary safety precautions
* the accepted codes, standards and practices for addressing

The Basic High-Voltage Problem

When hydro power faults occur in electric power systems or when lightning strikes, a Ground Potential Rise (GPR) occurs, which can damage equipment and injure personnel working on the equipment unless proper isolation or protection is provided. The GPR produces a dangerous potential difference between the power station or PCS tower and a remote ground connection located at a telephone company central office, remote terminal, distant manufacturing building or other sites. Telecommunications cable damage can occur if grounding takes place across the potential difference caused by a power fault or lightning strike.

Note: Any type of cable such as a twisted pair telephone cable or coaxial cable and any type of cable plant construction such as aerial, buried or underground,can experience damage. Also, any type of service can be interrupted if proper protection is not provided.

In general, special protection equipment, corporate policies and procedures are necessary to ensure the safety of personnel and protection of equipment including cables, termination equipment, etc. The reliability of communications circuits is questionable when they are in an unprotected high voltage environment. Disturbances on electric power systems and lightning can cause GPR, voltage induction on telecommunications cables, power surges and high-voltage transients.

ervice Performance Objectives (SPO) or Quality of Service

Telecommunications service is expected to operate with a standard performance objective as outlined below:

* Class A - Service must be available before, during and after the fault event
* Class B – Service must function before and after, but not necessarily during, the fault event
* Class C – Interruptible, non-critical service

Levels of Protection

Standard protection devices are designed to work under the following GPR conditions:

Basic Protection Level I

* Cable stress below 300 Volts peak asymmetrical
* All SPO classes of service (A, B and C)

Basic Protection Level II

* Cable stress below 1000 Volts peak asymmetrical
* SPO Class B or Class C services

Basic Protection Level III

* Above 1000 Volts peak asymmetrical
* Should require a High Voltage Isolator (HVI)
* All SPO classes of service

Common Protection Devices

Protection devices listed below are designed to work under the following categories and levels:

Basic Protection Level I and II

* Carbon blocks
* Gas tubes
* Solid state protectors
* Solid state hybrid protectors
* Heat coils
* Spark gaps
* Mutual Drainage Reactors (MDR)

Protection devices listed below are designed to work under the following:

Basic Protection Level III

* Lightning arresters
* High dielectric cables
* Isolation transformers
* Neutralizing transformers (Old technology)
* Copper HV electronic isolators
* Fiber HV electronic isolators

Electric Power System Telecommunications Equipment and Applications

There are many types of protective relaying and system control equipment used in the electric power industry which require communications circuits to clear faults, operate the system, protect personnel and equipment, and maintain system reliability.

Advanced protective relaying schemes that require “Class A” communications circuits are commonly used by the electric power industry to open High Voltage (HV) circuit breakers at remote locations. Examples of this type of equipment are listed below:

Protective relaying communications

* Audio tones
* DC pilot wire relaying Applications
* Transfer trip
* Breaker failure
* Permissive overreaching
* Directional comparison

Electric power systems use “Class B” communications circuits for the following services:

* Supervisory Control and Data Acquisition (SCADA)
* Telephones for safety, metering and automation circuits

Ground Potential Rise (GPR)

:"Main article: Earth potential rise"The definition of GPR, according to IEEE Standards 487 and 367, is: “the product of a ground electrode impedance, referenced to remote earth, and the current that flows through that electrode impedance.” The total station GPR is equal to the product of the station ground grid impedance and that portion of the total fault current that flows through it. In other words, GPR follows basic Ohm's law calculations for example, voltage is equal to current times resistance. When a ground fault occurs, fault current will divide among all circuit paths back to the source including metallic, earth return, etc. Metallic return paths include overhead ground wires, multi-grounded neutrals, bonding conductors, station ground grids, messenger wires, metallic cable shields and other conducting materials.

Zone of Influence (ZOI) and the 300-Volt Point

GPR decays outward in an exponential manner. The ZOI is the area around the ground grid dissipating the energy. Depending on soil resistivity and the size of the ground grid, the distance to the 300-Volt point on the ZOI can be calculated.

Total Electrical Cable Stress

The total electrical stress on a communications cable is the vectorial sum of the GPR and the induced voltage on the cable under fault conditions. When a faulting electric power line parallels an overhead or underground telecommunications cable, a potentially dangerous voltage is electromagnetically induced on the communications conductors. Excessive induction, by itself, can damage equipment and indirectly injure personnel.

Power and Lightning Fault Current Distribution

GPR will occur at several locations simultaneously. Fault current will divide among all circuit paths back to the source (metallic and earth return, for example) and create GPRs in the process. Metallic return paths include overhead ground wires, multi-grounded neutrals, bonds, station ground grids, and other conducting materials.

General safety considerations

Hazardous voltages can appear suddenly as a result of power faults or lightning strikes. Conductive objects (metal, damp saline soils, etc.) can become energized or carry a harmful potential that, if not properly protected, can cause serious injury. Safety of personnel can be achieved through education, proper facility design, and approved and tested insulated safety equipment. Personnel should use approved and tested rubber gloves and/or insulating blankets when working on the highvoltage interface (HVI) equipment.

Touch potential is the difference between the voltage gradient that one is standing on and the voltage gradient one is touching. A significant difference in touch potential can be hazardous. Step potential is the difference in voltage gradients between a person’s two feet. Shoes, gloves, etc. can provide insulation to touch and step potentials.

Basic personal safety equipment

:"See warning on "Proper personal protection equipment, procedures and tools can help job safety and efficiency.

Rubber Gloves, Leather Protectors and Cotton Liners

All rubber gloves and other personal protection equipment have to meet safety standards established by national organizations like the American National Standards Institute (ANSI) and the American Society for Testing and Materials (ASTM).

Cell sites on electrical towers

Installing HVI equipment on cell sites is similar to power substation installations. Both copper and fiber optic high-voltage protection equipment are available for cell sites on regular and electric towers. Each situation needs to be carefully analyzed for possible GPR problems. Proper isolation is a primary consideration. Electric power service (i.e., 120/240 Vac) can introduce a remote ground hazard.

Conclusion

It is of utmost importance to always determine the proper protection scheme when dealing with telecommunication wire line circuits between Hydro sub stations, PCS towers, 911 communications centers and any location where high voltage and high current equipment is co-located with these circuits. The most effective protection will always utilize High Voltage Isolation (HVI) designs and is the most effective against high ground potential rise (GPR) and lightning strike scenarios. It is always best to design for the worst case situation, since more likely than not, this can happen. Protection against both High voltage and ground potential rise is the best precaution against equipment failures and human injury.


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