Rise of voltage of local earth when a large current flows through an earth grid impedance
In electrical engineering, earth potential rise (EPR), also called ground potential rise (GPR), occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance from the source. Ground potential rise is a concern in the design of electrical substations because the high potential may be a hazard to people or equipment.
The change of voltage over distance (potential gradient) may be so high that a person could be injured due to the voltage developed between two feet, or between the ground on which the person is standing and a metal object. Any conducting object connected to the substation earth ground, such as telephone wires, rails, fences, or metallic piping, may also be energized at the ground potential in the substation. This transferred potential is a hazard to people and equipment outside the substation.
A computer calculation of the voltage gradient around a small substation. Where the voltage gradient is steep, a hazard of electric shock is present for passers-by.
Causes
Earth potential rise (EPR) is caused by electrical faults that occur at electrical substations, power plants, or high-voltage transmission lines. Short-circuit current flows through the plant structure and equipment and into the grounding electrode.[1] The resistance of the Earth is non-zero, so current injected into the earth at the grounding electrode produces a potential rise with respect to a distant reference point. The resulting potential rise can cause hazardous voltage, many hundreds of metres away from the actual fault location. Many factors determine the level of hazard, including: available fault current, soil type, soil moisture, temperature, underlying rock layers, and clearing time to interrupt a fault.
Earth potential rise is a safety issue in the coordination of power and telecommunications services. An EPR event at a site such as an electrical distribution substation may expose personnel, users or structures to hazardous voltages.
Step, touch, and mesh voltages
"Step voltage" is the voltage between the feet of a person standing near an energized grounded object. It is equal to the difference in voltage, given by the voltage distribution curve, between two points at different distances from the "electrode". A person could be at risk of injury during a fault simply by standing near the grounding point.
"Touch voltage" is the voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the object and a point some distance away. The touch voltage could be nearly the full voltage across the grounded object if that object is grounded at a point remote from the place where the person is in contact with it. For example, a crane that was grounded to the system neutral and that contacted an energized line would expose any person in contact with the crane or its uninsulated load line to a touch voltage nearly equal to the full fault voltage.
"Mesh voltage" is a factor calculated or measured when a grid of grounding conductors is installed. Mesh voltage is the largest potential difference between metallic objects connected to the grid, and soil within the grid, under worst-case fault conditions. It is significant because a person may be standing inside the grid at a point with a large voltage relative to the grid itself.
Mitigation
An engineering analysis of the power system under fault conditions can be used to determine whether or not hazardous step and touch voltages will develop. The result of this analysis can show the need for protective measures and can guide the selection of appropriate precautions.
Several methods may be used to protect employees from hazardous ground-potential gradients, including equipotential zones, insulating equipment, and restricted work areas.
The creation of an equipotential zone will protect a worker standing within it from hazardous step and touch voltages. Such a zone can be produced through the use of a metal mat connected to the grounded object. Usually this metal mat (or ground mesh) is connected to buried ground rods to increase contact with the earth and effectively reduce grid impedance.[2] In some cases, a grounding grid can be used to equalize the voltage within the grid. Equipotential zones will not, however, protect employees who are either wholly or partially outside the protected area. Bonding conductive objects in the immediate work area can also be used to minimize the voltage between the objects and between each object and ground. (Bonding an object outside the work area can increase the touch voltage to that object in some cases, however.)
The use of insulating personal protective equipment, such as rubber gloves, can protect employees handling grounded equipment and conductors from hazardous touch voltages. The insulating equipment must be rated for the highest voltage that can be impressed on the grounded objects under fault conditions (rather than for the full system voltage).
Workers may be protected from hazardous step or touch voltages by prohibiting access to areas where dangerous voltages may occur, such as within substation boundaries or areas near transmission towers. Workers required to handle conductors or equipment connected to a grounding system may require protective gloves or other measures to protect them from accidentally energized conductors.
In electrical substations, the surface may be covered with a high-resistivity layer of crushed stone or asphalt. The surface layer provides a high resistance between feet and the ground grid, and is an effective method to reduce the step and touch voltage hazard.
Calculations
In principle, the potential of the earth grid Vgrid can be calculated using Ohm's law if the fault current (If) and resistance of the grid (Zgrid) are known.
While the fault current from a distribution or transmission system can usually be calculated or estimated with precision, calculation of the earth grid resistance is more complicated. Difficulties in calculation arise from the extended and irregular shape of practical ground grids, and the varying resistivity of soil at different depths.
At points outside the earth grid, the potential rise decreases. The simplest case of the potential at a distance is the analysis of a driven rod electrode in homogeneous earth. The voltage profile is given by the following equation.
where
is the distance of a point from the center of the earth grid (in meters).
is the voltage at distance from the earth grid, in volts.
This case is a simplified system; practical earthing systems are more complex than a single rod, and the soil will have varying resistivity. It can, however, reliably be said that the resistance of a ground grid is inversely proportional to the area it covers; this rule can be used to quickly assess the degree of difficulty for a particular site. Programs running on desktop personal computers can model ground resistance effects and produce detailed calculations of ground potential rise, using various techniques including the finite element method.
Standards and regulations
The US Occupational Safety and Health Administration (OSHA) has designated EPR as a "known hazard" and has issued regulations governing the elimination of this hazard in the work place.[3]
Protection and isolation equipment is made to national and international standards described by IEEE, National Electrical Codes (UL/CSA),FCC, and Telcordia.
IEEE Std. 80-2000 is a standard that addresses the calculation and mitigation of step and touch voltages to acceptable levels around electrical substations.
High-voltage protection of telecommunication circuits
In itself, a ground potential rise is not harmful to any equipment or person bonded to the same ground potential. However, where a conductor (such as a metallic telecommunication line) bonded to a remote ground potential (such as the Central Office/Exchange) enters the area subject to GPR, the conflicting difference of potentials can create significant risks. High voltage can damage equipment and present a danger to personnel. To protect wired communication and control circuits in sub stations, protective devices must be applied. Isolation devices prevent the transfer of potential into or out of the GPR area. This protects equipment and personnel who might otherwise be exposed simultaneously to both ground potentials, and also prevents high voltages and currents propagating towards the telephone company's central office or other users connected to the same network. Circuits may be isolated by transformers or by non-conductive fiber optic couplings. (Surge arresting devices such as carbon blocks or gas-tube shunts to ground do not isolate the circuit but divert high voltage currents from the protected circuit to the local ground. This type of protection will not fully protect against the hazards of GPR where the danger is from a remote ground on the same circuit.)
Telecommunication standards define a "zone of influence" around a substation, inside of which, equipment and circuits must be protected from the effect of ground potential rise. In North American practice, the zone of influence is considered to be bounded by the "300 volt point", which is the point along a telecommunications circuit at which the GPR reaches 300 volts with respect to distant earth.[4] The 300 volt point defining a zone of influence around a sub station is dependent on the ground resistivity and the amount of fault current. It will define a boundary a certain distance from the ground grid of the sub station. Each sub station has its own zone of influence since the variables explained above are different for each location.[5]
In the UK, any site subject to a Rise-of-Earth-Potential (ROEP) is referred to as a 'Hot-Site'. The Zone-of-Influence was historically measured as anywhere within 100m of the boundary of the high-voltage compound at a Hot-Site. Depending on the size of the overall site this may mean that parts of a larger site may not need to be classed as 'Hot', or (conversely) the influence of small sites may extend into areas outside of the land owner's control. Since 2007, it is allowable to use the Energy-Networks-Association (ENA) Recommendation S34[6] ('A Guide for Assessing the Rise of Earth Potential at Substation Sites') to calculate the Hot-Zone. This is now defined as a contour-line marking where the ROEP exceeds 430V for normal-reliability power lines, or 650V for high-reliability lines. The 'Zone' extends in a radius from any bonded metalwork, such as the site earth electrode system or boundary fence. This may effectively reduce the overall size of the Hot-Zone compared to the previous definition. However, strip earth electrodes, and any non-effectively insulated metallic-sheath/armouring of power cables which extend out of this zone would continue to be considered as 'hot' for a distance of 100m from the boundary, encompassing a width of two meters on either side of the conductor. It is the responsibility of the owning Electrical-Supply-Industry (ESI) to calculate the Hot-Zone.
Openreach (a BT Group company tasked with installing and maintaining a significant majority of the physical telephone network in the UK) maintains a Hot-Site Register, updated every 12 months by voluntarily supplied information from the ESI companies in the UK. Any Openreach engineer visiting a site on the register must be Hot-Site trained. Certain working practices and planning considerations must be followed, such as not using armoured telephone cables, fully sealing cable joints to prevent access, over-sleeving of individual pairs of wires beyond the end of the cable sheath, and isolating (outside the Hot-Zone) any line to be worked on. It is assumed to be the responsibility of the party ordering the initial installation of a service to cover the cost of providing isolation links, service isolating devices, and clearly marked trunking for running cables, and all should be part of the planning process.
In some circumstances (such as when a 'cold' site is upgraded to 'hot' status), the Zone of Influence may encompass residential or commercial property which is not within the property of the Electrical Supply Industry. In these cases the cost of retroactively protecting each telephone circuit may be prohibitively high, so a drainage electrode may be supplied to effectively bring the local Earth Potential back to safe levels.
In electrical engineering, ground or earth may be a reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth.
Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two differently-charged objects when brought close together or when the dielectric between them breaks down, often creating a visible spark associated with the static electricity between the objects.
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by overcurrent. Its basic function is to interrupt current flow to protect equipment and to prevent the risk of fire. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset to resume normal operation.
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. They are a common component of the infrastructure. There are 55,000 substations in the United States.
Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line which supplies single-phase electric power from an electrical grid to remote areas at lowest cost. Its distinguishing feature is that the earth is used as the return path for the current, to avoid the need for a second wire to act as a return path.
Electrical wiring in North America follows the regulations and standards applicable at the installation location. It is also designed to provide proper function, and is also influenced by history and traditions of the location installation.
An earth-leakage circuit breaker (ELCB) is a safety device used in electrical installations with high Earth impedance to prevent shock. It detects small stray voltages on the metal enclosures of electrical equipment, and interrupts the circuit if a dangerous voltage is detected. Once widely used, more recent installations instead use residual-current devices which instead detect leakage current directly.
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons or to reduce transients and harmonics. Isolation transformers provide galvanic isolation; no conductive path is present between source and load. This isolation is used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings.
In electrical engineering, ground and neutral are circuit conductors used in alternating current (AC) electrical systems. The ground circuit is connected to earth, and neutral circuit is usually connected to ground. As the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equipment and structures. Current carried on a grounding conductor can result in objectionable or dangerous voltages appearing on equipment enclosures, so the installation of grounding conductors and neutral conductors is carefully defined in electrical regulations. Where a neutral conductor is used also to connect equipment enclosures to earth, care must be taken that the neutral conductor never rises to a high voltage with respect to local ground.
Electrical wiring in the United Kingdom is commonly understood to be an electrical installation for operation by end users within domestic, commercial, industrial, and other buildings, and also in special installations and locations, such as marinas or caravan parks. It does not normally cover the transmission or distribution of electricity to them.
High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, high voltage refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant special safety requirements and procedures.
A shielded cable or screened cable is an electrical cable that has a common conductive layer around its conductors for electromagnetic shielding. This shield is usually covered by an outermost layer of the cable. Common types of cable shielding can most broadly be categorized as foil type, contraspiralling wire strands or both. A longitudinal wire may be necessary with dielectric spiral foils to short out each turn.
An earthing system or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary among countries, though most follow the recommendations of the International Electrotechnical Commission (IEC). Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants.
Extra-low voltage (ELV) is an electricity supply voltage and is a part of the low-voltage band in a range which carries a low risk of dangerous electrical shock. There are various standards that define extra-low voltage. The International Electrotechnical Commission (IEC) and the UK IET define an ELV device or circuit as one in which the electrical potential between two conductors or between an electrical conductor and earth (ground) does not exceed 50V AC or 120V DC.
Electrical bonding is the practice of intentionally electrically connecting all exposed metal items not designed to carry electricity in a room or building as protection from electric shock. Bonding is also used to minimize electrical arcing between metal surfaces with electrical potential differences. If a failure of electrical insulation occurs, all bonded metal objects in the room will have substantially the same electrical potential, so that an occupant of the room cannot touch two objects with significantly different potentials. Even if the connection to a distant earth is lost, the occupant will be protected from dangerous potential differences.
An arc flash is the light and heat produced as part of an arc fault, a type of electrical explosion or discharge that results from a connection through air to ground or another voltage phase in an electrical system.
An electrical room is a room or space in a building dedicated to electrical equipment. Its size is usually proportional to the size of the building; large buildings may have a main electrical room and subsidiary electrical rooms. Electrical equipment may be for power distribution equipment, or for communications equipment.
Stray voltage is the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them. Small voltages often exist between two grounded objects in separate locations, due to normal current flow in the power system. Contact voltage is a better defined term when large voltage appear as a result of a fault. Contact voltage on the enclosure of electrical equipment can appear due to a fault in the electrical power system, such as a failure of insulation.
A lightning rod or lightning conductor is a metal rod mounted on a structure and intended to protect the structure from a lightning strike. If lightning hits the structure, it is most likely to strike the rod and be conducted to ground through a wire, rather passing through the structure, where it could start a fire or cause electrocution. Lightning rods are also called finials, air terminals, or strike termination devices.
In building wiring installed with separate neutral and protective ground bonding conductors, a bootleg ground is a connection between the neutral side of a receptacle or light fixture and the ground lug or enclosure of the wiring device.
[1] ACIF Working Committee CECRP/WC18, AS/ACIF S009:2006 Installation Requirements for Customer Cabling (Wiring Rules), Australian Communications Industry Forum, North Sydney, Australia (2006) ISBN1-74000-354-3
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