Expert Systems 9501 Tinker Court Burke , VA 22015
BE ALERT TO DANGER POSED BY GROUND POTENTIAL RISE
Reprinted from AMERICAS NETWORK, May 15, 1995
An ADVANSTAR Publication Printed in U.S.A.
Ernest M. Duckworth Jr., P.E. and W.G. Petersen
Electrical damage each year from ground potential rise (GPR) is costing industry many millions of dollars, yet few engineers or their managers are even aware of the phenomenon. The damage is passed off as the result of last nights thunderstorm.
Electrical damage from a GPR occurs when there is a high-voltage difference between a grounded communications site and a distant site connected by wire-line communication circuits.
Ohm's law dictates that a voltage potential will result when current of any magnitude or frequency flows through a grounding impedance. Power substation GPR is the result of a 60Hz earth return fault current flowing through a substation ground grid impedance. With lightning, a high-frequency l000Hz to l00MHz current flows through a grounding system structure and generates a high-frequency GPR. Either high-current source can damage communication equipment connected to a remote location via wire-line cable facilities.
Damage occurs when a GPR voltage appears on one side of a communications device (or service technician) within an elevated ground reference location during the instant it naturally is seeking a lower ground reference at a remote location. This is the special case of transferred potential defined in IEEE standard 80-1986(IEEE Search Database). People and equipment most susceptible to GPR are located at power substations, power generating facilities, tall grounded radio towers such as emergency 9-1-1 locations, cellular radio sites and microwave communication buildings.
There are recorded cases where 60Hz GPR lasted more than 15 minutes, causing millions of dollars in damage. But a lightning strike GPR may last a very short time (50 nanoseconds to 10 milliseconds). These strikes cause most of the electronic circuit damage on wire-line facilities at radio tower sites. They may not destroy every circuit immediately, but may weaken individual components that will fail later on. Lightning causes much latent damage to electrical equipment.
How are GPR damage problems solved? A series of field-proven national standards provides methods for protecting people and equipment from GPR. These documents have existed for years, but most field engineers and technicians seem unaware of the valuable information provided. The most important and useful standards include:
NFPA 70-1992-National Electrical Code (NEC); NFPA Home
ANSI/IEEE Standard 80-1986-Guide for Safety in AC Substation Grounding;IEEE Home (IEEE members Search Database)
ANSI/IEEE Standard 367-1987-Recommended Practice for Determining the Electric Power Station Ground Potential and Induced Voltage from a Power Fault; and
ANSI/IEEE Standard 487-1992-Guide for the Protection of Wire-Line Communication Facilities Serving Electric Power Stations.
Although IEEE 487-1992 addresses protection from GPR due to 60Hz fault currents, GPR from lightning strike energy may also be applied. Both currents generate a GPR and have potential to harm personnel and damage or destroy communication facilities.
The most important consideration when protecting equipment from a GPR is the ineffectiveness of standard electrical protection methods using gas tubes, metal oxide varistors (MOVs), silicon-controlled rectifiers (SCRs) and SASs (four-layer semiconductors). These 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 site and remote ground.
Today's standards limit the use of basic electrical protection devices that may be used at power substation locations. The voltage level defined in IEEE 487-1992 is below the level where standard station protectors do not fuse, explode or cause a fire hazard. The limit of GPR was established at less than 1000V-Peak-Asymmetrical and applies only to a small percentage of today's substation locations. In addition, due to the nature of lightning-induced GPR, the chances of an average 30kVA strike exceeding this 1000V limit are very high.
So how should one protect personnel and equipment from GPR at dangerous levels above 1000V-Peak-Asymmetrical? Follow existing national codes and IEEE standard installation procedures while using special high-voltage protection (HVP) devices-such as isolation transformers, optical couplers and fiber optics intended to protect against GPR. These devices and related facilities will provide a high dielectric strength barrier between grounding potentials on a full-time basis and isolate communication facilities from transient voltage damage before, during and after a GPR incident.
Communications engineers and service managers should not turn a blind eye to GPR damage because special high-voltage protection devices are more expensive than standard gas tubes. Consider ongoing costs for continuously replacing damaged equipment year after year. Also, labor costs for one repair can equal the cost of any properly protected GPR site the first time. And don't forget personal safety issues: employees working in, on or around communications equipment connected to a remote ground potential are at risk if the equipment is not isolated from the GPR.
Properly protected GPR locations designed and maintained by trained employees will reduce overall costs, improve productivity and increase circuit reliability over any time period. Ignoring GPR today will result in costly repairs and legal bills tomorrow!
GROUND POTENTIAL RISE
Damage occurs when a high GPR voltage appears on one side of a communications device within an elevated ground reference location during the instant it naturally is seeking a lower ground reference at a remote location.
The most important consideration when protecting equipment from a GPR is the ineffectiveness of standard electrical protection methods using ground shunting devices such as gas tubes, carbon block, solid state, etc.
To protect personnel and equipment, follow codes and IEEE standard installation procedures while installing and maintaining special high-voltage protection equipment.
Properly protected GPR locations designed and maintained by trained employees
will reduce overall costs, improve productivity and increase circuit reliability. Ignoring
GPR can result in costly repair, possible harm to employees and legal bills.
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