What is temporary personal protective grounding?
Sometimes, additional measures are desirable to provide an extra margin of safety assurance. Temporary personal protective grounds are used when working on de-energized electrical conductors to minimize the possibility of accidental re-energization from unexpected sources. Sometimes these are called safety grounds or equipotential grounding.
Induced voltages, capacitive recharging, and accidental contact with other circuits can occur. Depending on the electrical energy available, these occurrences could cause injury or death.
More often, however, they only cause reßexive actions. For example, although most induced voltages will not normally cause serious injury themselves, they could cause a person to jump backward suddenly, possibly tripping against something or falling to the floor.
Temporary protective grounding devices should be applied where such conditions might occur. Temporary personal protective grounds should be applied at possible points of re-energization. They can also be applied in such a way as to establish a zone of equipotential around a person.
When these grounds are used, they shall be connected tightly, since they establish a deliberate fault point in the circuit. If current does somehow get onto the circuit, the grounds shall stay connected securely until a protective device clears the circuit.
It is difficult to set firm criteria for when temporary personal protective grounds are needed. Blanket requirements are usually established. Many times, it is a decision made in the field by the person performing the work.
When there is uncertainty about exposure, it is wise to add this extra protection. Many industrial facilities and utilities require temporary personal protective grounding for all aerial power line work and for all work on power systems over 600 V because of the increased exposure these systems often have due to their length and location.
Temporary personal protective grounding can also be used as the additional safety measure required when hazardous electrical energy control must be performed using a tag only. Temporary personal protective grounding devices should meet the specifications in ASTM F855-96 and should be sized for the maximum available current of any possible event.
Temporary personal protective grounds should only be installed after all other conditions of an electrically safe work condition have been established. Because the unexpected can happen at any time, however, the installation and removal of temporary grounding devices should be performed, by procedure, as the conductors are energized.
When installed inside equipment enclosures, temporary grounds should be lengthy enough to extend outside of the equipment so that they can be easily seen. If they cannot extend out, they should be made highly visible. Brightly colored tapes are helpful identifiers. Once they are installed, bare-hand work could be permitted.
It should be quite obvious that all personal protective grounds must be removed prior to reenergization. Identification and accountability controls may be necessary on large construction or maintenance jobs. The installation and removal of these grounding devices can be controlled by permit in order to avoid re-energizing equipment into a faulted condition.
The integrity of personal protective grounds should be maintained through the use of periodic inspection and testing. It is a good idea to document this inspection and testing.
Showing posts with label Grounding. Show all posts
Showing posts with label Grounding. Show all posts
SOLID GROUNDING OF POWER SYSTEM BASIC INFORMATION AND TUTORIALS
What is a solidly grounded system?
Solid grounding refers to the connection of the neutral of a generator, power transformer, or grounding transformer directly to the station ground or to the earth. Because of the reactance of the grounded generator or transformer in series with the neutral circuit, a solid ground connection does not provide a zero-impedance neutral circuit.
If the reactance of the system zero-sequence circuit is too great with respect to the system positive-sequence reactance, the objectives sought in grounding, principally freedom from transient overvoltages, may not be achieved.
This is rarely a problem in typical industrial and commercial power systems. The zero-sequence impedance of most generators used in these systems is much lower than the positive-sequence impedance of these generators.
The zero-sequence impedance of a delta-wye transformer will not exceed the transformer's positive sequence impedance. There are, however, conditions under which relatively high zero-sequence impedance may occur.
One of these conditions is a power system fed by several generators and/or transformers in parallel. If the neutral of only one source is grounded, it is possible for the zero-sequence impedance of the grounded source to exceed the effective positive-sequence impedance of the several sources in parallel.
Another such condition may occur where power is distributed to remote facilities by an overhead line without a metallic ground return path. In this case, the return path for ground-fault current is through the earth, and, even though both the neutral of the source and the nonconducting parts at the load may be grounded with well-made electrodes, the ground return path includes the impedance of both of these ground electrodes.
This impedance may be significant. Another significant source of zero sequence impedance is the large line-to-ground spacing of the overhead line.
To ensure the benefits of solid grounding, it is necessary to determine the degree of grounding provided in the system. A good guide in answering this question is the magnitude of ground-fault current as compared to the system threephase fault current.
The higher the ground-fault current in relation to the three-phase fault current the greater the degree of grounding in the system.
Effectively grounded systems will have a line-to-ground short circuit current of at least 60% of the three-phase short-circuit value. In terms of resistance and reactance, effective grounding of a system is accomplished only when R0</=X1 and X0 </= 3X1 and such relationships exist at any point in the system.
The X1 component used in the above relation is the Thevenin equivalent positive-sequence reactance of the complete system including the subtransient reactance of all rotating machines.
Application of surge arresters for grounded-neutral service requires that the system be effectively grounded.
REACTANCE GROUNDING OF POWER SYSTEM BASIC INFORMATION
What is reactance grounding? How reactance grounding is beneficial?
The term reactance grounding describes the case in which a reactor is connected between the system neutral and ground.
Since the ground-fault that may flow in a reactance-grounded system is a function of the neutral reactance, the magnitude of the ground-fault current is often used as a criterion for describing the degree of grounding.
In a reactance-grounded system, the available ground-fault current should be at least 25% and preferably 60% of the threephase fault current to prevent serious transient overvoltages (X0 </=X1).
This is considerably higher than the level of fault current desirable in a resistance-grounded system, and therefore reactance grounding is usually not considered an alternative to resistance grounding.
In most generators, solid grounding, that is, grounding without external impedance, may permit the maximum ground fault current from the generator to exceed the maximum three-phase fault current that the generator can deliver and for which its windings are braced.
Consequently, neutral-grounded generators should be grounded through a low-value reactor that will limit the ground-fault current to a value no greater than the generator three-phase fault current.
In the case of three-phase four-wire systems, the limitation of ground-fault current to 100% of the three-phase fault current is usually practical without interfering with normal four-wire operation.
In practice, reactance grounding is generally used only in this case and to ground substation transformers with similar characteristics.
The term reactance grounding describes the case in which a reactor is connected between the system neutral and ground.
Since the ground-fault that may flow in a reactance-grounded system is a function of the neutral reactance, the magnitude of the ground-fault current is often used as a criterion for describing the degree of grounding.
In a reactance-grounded system, the available ground-fault current should be at least 25% and preferably 60% of the threephase fault current to prevent serious transient overvoltages (X0 </=X1).
This is considerably higher than the level of fault current desirable in a resistance-grounded system, and therefore reactance grounding is usually not considered an alternative to resistance grounding.
In most generators, solid grounding, that is, grounding without external impedance, may permit the maximum ground fault current from the generator to exceed the maximum three-phase fault current that the generator can deliver and for which its windings are braced.
Consequently, neutral-grounded generators should be grounded through a low-value reactor that will limit the ground-fault current to a value no greater than the generator three-phase fault current.
In the case of three-phase four-wire systems, the limitation of ground-fault current to 100% of the three-phase fault current is usually practical without interfering with normal four-wire operation.
In practice, reactance grounding is generally used only in this case and to ground substation transformers with similar characteristics.
LOW VOLTAGE SYSTEM EARTHING BASIC INFORMATION AND TUTORIALS
For many years the Regulations required
that each l.v. system should be solidly connected to earth at only
one point, that being the neutral of the source transformer. Special
permission was necessary to earth at more than one point.
The Regulations also required that
cables buried in the highway must have a metallic sheath. Systems
earthed at only one point require the neutral conductor to be
electrically separate and are now known as SNE (separate neutral and
earth).
It was, and still is, the
responsibility of each consumer to provide the earth connection for
his own installation. This was commonly achieved by connection to a
metallic pipe water main.
The growing use of PVC water mains
makes this impossible for new installations and causes problems with
existing ones when water mains are replaced. Gradually, supply
companies developed a practice of providing consumers with an earth
terminal connected to the sheath of their service cable.
This is, of course, a very satisfactory
arrangement but it is not universally practical as many cables laid
in the 1920s or earlier are still in use and many of these are not
bonded across at joints. The arrangement is not practical on most
overhead systems.
In Germany and elsewhere in Europe an
earthing system known as ‘nulling’ grew up. This employed the
principle of earthing the neutral at as many points as possible.
It simplified the problem of earthing
in high resistance areas and by combining the sheath with the neutral
conductor permitted a cheaper cable construction. These benefits were
attractive and during the 1960s the official attitude in the UK
gradually changed to permit and then encourage a similar system known
as PME (protective multiple earthing).
Blanket approvals for the use of this
system, and the required conditions to be met, were finally given to
all area boards in 1974. In BS 7671 – the 16th edition of the IEE
Wiring Regulations this system is classified as TN-C-S.
Providing the consumer with an earth
terminal which is connected to the neutral conductor ensures that
there is a low impedance path for the return of fault currents, but
without additional safeguards there are possibilities of dangerous
situations arising under certain circumstances.
If the neutral conductor becomes
disconnected from the source of supply then the earthed metalwork in
the consumer’s premises would be connected via any load to the live
conductor and thus present an electric shock hazard from any
metalwork not bonded to it, but which has some connection with earth.
In order to eliminate this rare potential hazard the Secretary of
State, in his official Regulations, requires that all accessible
metalwork should be bonded together as specified in the IEEE Wiring Regulations and so render the
consumer’s premises a ‘Faraday cage’. This is the reason for
the more stringent bonding regulations associated with PME.
Under the extremely rare circumstances
of a broken service neutral and intact phase conductor, there may be
a danger of electric shock on the perimeter of the ‘cage’ to
someone using an earthed metal appliance in a garden, even though the
appliance may be protected by an RCD (residual current device) in
accordance with the IEE Wiring Regulations. For the same reason metal
external meter cabinets are undesirable.
In order to eliminate as far as
possible the chance of a completely separated neutral, a number of
precautions are taken. First, all cables must be of an approved type
with a concentric neutral, either solid or stranded, of sufficient
current carrying capacity.
Secondly the neutral conductor of a
spur end on the system is connected to an earth electrode if more
than four consumers’ installations are connected to the spur, or if
the length of the spur connection from the furthest connected
consumer to the distributing main exceeds 40 metres.
Where reasonably practicable, cable
neutrals are joined together to form duplicate earth connections. A
faulty or broken neutral will give an indication of its presence by
causing supply voltages to fluctuate, which, of course, should be
reported to the local DNO as soon as possible. All these measures
contribute to a system which is as safe as practicable and
self-monitoring.
It is the declared intention of the EI
in the UK to provide earth terminals wherever required and
practicable within the foreseeable future. The local DNO should be
contacted regarding their requirements for the use of PME earth
terminals for TN-C-S systems.
THE DANGERS OF ASBESTOS - BASIC INFORMATION AND TUTORIALS
What are the dangers of inhaling asbestos in construction?
Inhaling asbestos dust has been shown to cause the following diseases:
• asbestosis
• lung cancer
• mesothelioma (cancer of the lining of the chest and/or abdomen).
Asbestosis is a disease of the lungs caused by scar tissue forming around ve ry small asbestos fibres deposited deep in the lungs. As the amount of scar tissue increases, the ability of the lungs to expand and contract decreases, causing shortness of breath and a heavier wo rkload on the heart.
Ultimately, asbestosis can be fatal.
Lung cancer appears quite frequently in people exposed to asbestos dust.While science and medicine have not yet been able to explain precisely why or how asbestos causes lung cancer to develop, it is clear that exposure to asbestos dust can increase the risk of contracting this disease.
Studies of asbestos wo rkers have shown that the risk is roughly five times greater than for people who are not exposed to asbestos.
Cigarette smoking, another cause of lung cancer, multiplies this risk . Research has shown that the risk of developing cancer is fifty times higher for asbestos workers who smoke than for workers who neither smoke nor work with asbestos.
Mesothelioma is a relatively rare cancer of the lining of the chest and/or abdomen.While this disease is seldom observed in the general population, it appears frequently in groups exposed to asbestos.
Other illnesses—There is also some evidence of an increased risk of cancer of the stomach, rectum, and larynx. However, the link between asbestos exposure and the development of these illnesses is not as clear as with lung cancer or mesothelioma.
The diseases described above do not respond well to current medical treatment and, as a result, are often fatal.
Inhaling asbestos dust has been shown to cause the following diseases:
• asbestosis
• lung cancer
• mesothelioma (cancer of the lining of the chest and/or abdomen).
Asbestosis is a disease of the lungs caused by scar tissue forming around ve ry small asbestos fibres deposited deep in the lungs. As the amount of scar tissue increases, the ability of the lungs to expand and contract decreases, causing shortness of breath and a heavier wo rkload on the heart.
Ultimately, asbestosis can be fatal.
Lung cancer appears quite frequently in people exposed to asbestos dust.While science and medicine have not yet been able to explain precisely why or how asbestos causes lung cancer to develop, it is clear that exposure to asbestos dust can increase the risk of contracting this disease.
Studies of asbestos wo rkers have shown that the risk is roughly five times greater than for people who are not exposed to asbestos.
Cigarette smoking, another cause of lung cancer, multiplies this risk . Research has shown that the risk of developing cancer is fifty times higher for asbestos workers who smoke than for workers who neither smoke nor work with asbestos.
Mesothelioma is a relatively rare cancer of the lining of the chest and/or abdomen.While this disease is seldom observed in the general population, it appears frequently in groups exposed to asbestos.
Other illnesses—There is also some evidence of an increased risk of cancer of the stomach, rectum, and larynx. However, the link between asbestos exposure and the development of these illnesses is not as clear as with lung cancer or mesothelioma.
The diseases described above do not respond well to current medical treatment and, as a result, are often fatal.
TRANSFORMER TERMS GLOSSARY BASIC INFORMATION AND TUTORIALS
The following technical terms apply to transformers.
BIL: An abbreviation for basic impulse level, a dielectric strength test. Transformer BIL is determined by applying a high-frequency square-wave voltage with a steep leading edge between the windings and between the windings and ground.
The BIL rating provides the maximum input kV rating that a transformer can withstand without causing insulation breakdown. The transformer must also be protected against natural or man-made electrical surges. The NEMA standard BIL rating is 10 kV.
Exciting current: In transformers, the current in amperes required for excitation. This current consists of two components: (1) real in the form of losses (no load watts) and (2) reactive power in kvar. Exciting current varies inversely with kVA rating from approximately 10 percent at 1 kVA to as low as 0.5 percent at 750 kVA.
Eddy-current losses: Contiguous energy losses caused when a varying magnetic flux sets up undesired eddy currents circulating in a ferromagnetic transformer core.
Hysteresis losses: Continuous energy losses in a ferromagnetic transformer core when it is taken through the complete magnetization cycle at the input frequency.
Insulating transformer: A term synonymous with isolating transformer, to describe the insulation or isolation between the primary and secondary windings. The only transformers that are not insulating or isolating are autotransformers.
Insulation system temperature: The maximum temperature in degrees Celsius at the hottest point in the winding.
Isolating transformer: See insulating transformer.
Shielded-winding transformer: A transformer with a conductive metal shield between the primary and secondary windings to attenuate transient noise.
Taps: Connections made to transformer windings other than at its terminals. They are provided on the input side of some high-voltage transformers to correct for high or low voltages so that the secondary terminals can deliver their full rated output voltages.
Temperature rise: The incremental temperature rise of the windings and insulation above the ambient
temperature.
Transformer impedance: The current-limiting characteristic of a transformer expressed as a percentage. It is used in determining the interrupting capacity of a circuit breaker or fuse that will protect the transformer primary.
Transformer voltage regulation: The difference between the no-load and full-load voltages expressed as a percentage. A transformer that delivers 200 V at no load and 190 V at full load has a regulation of 5 percent.
BIL: An abbreviation for basic impulse level, a dielectric strength test. Transformer BIL is determined by applying a high-frequency square-wave voltage with a steep leading edge between the windings and between the windings and ground.
The BIL rating provides the maximum input kV rating that a transformer can withstand without causing insulation breakdown. The transformer must also be protected against natural or man-made electrical surges. The NEMA standard BIL rating is 10 kV.
Exciting current: In transformers, the current in amperes required for excitation. This current consists of two components: (1) real in the form of losses (no load watts) and (2) reactive power in kvar. Exciting current varies inversely with kVA rating from approximately 10 percent at 1 kVA to as low as 0.5 percent at 750 kVA.
Eddy-current losses: Contiguous energy losses caused when a varying magnetic flux sets up undesired eddy currents circulating in a ferromagnetic transformer core.
Hysteresis losses: Continuous energy losses in a ferromagnetic transformer core when it is taken through the complete magnetization cycle at the input frequency.
Insulating transformer: A term synonymous with isolating transformer, to describe the insulation or isolation between the primary and secondary windings. The only transformers that are not insulating or isolating are autotransformers.
Insulation system temperature: The maximum temperature in degrees Celsius at the hottest point in the winding.
Isolating transformer: See insulating transformer.
Shielded-winding transformer: A transformer with a conductive metal shield between the primary and secondary windings to attenuate transient noise.
Taps: Connections made to transformer windings other than at its terminals. They are provided on the input side of some high-voltage transformers to correct for high or low voltages so that the secondary terminals can deliver their full rated output voltages.
Temperature rise: The incremental temperature rise of the windings and insulation above the ambient
temperature.
Transformer impedance: The current-limiting characteristic of a transformer expressed as a percentage. It is used in determining the interrupting capacity of a circuit breaker or fuse that will protect the transformer primary.
Transformer voltage regulation: The difference between the no-load and full-load voltages expressed as a percentage. A transformer that delivers 200 V at no load and 190 V at full load has a regulation of 5 percent.
GROUNDING OF ELECTRICAL EQUIPMENT SYSTEM FOR SAFETY
Grounded electrical systems are required to be connected to earth in such a way as to limit any voltages imposed by lightning, line surges, or unintentional contact with higher voltage lines. Electrical systems are also grounded to stabilize the voltage to earth during normal operation.
If, for example, the neutral of a 120/240 V, wye-connected secondary of a transformer were not grounded, instead of being 120 V to ground, the voltage could reach several hundred volts to ground. A wye-connected electrical system becomes very unstable if it is not properly grounded.
OSHA 1910.304(f)(7)(iii) Grounding of equipment. All non-current-carrying metal parts of portable equipment and fixed equipment including their associated fences, housings, enclosures, and supporting structures shall be grounded.
However, equipment that is guarded by location and isolated from ground need not be grounded. Additionally, polemounted distribution apparatus at a height exceeding 8 feet above ground or grade level need not be grounded.
In 29 CFR 1910.303, “General Requirements,” OSHA states under “(b) Examination, installation, and use of equipment (1) Examination” that “Electrical equipment shall be free from recognized hazards that are likely to cause death or serious physical harm to employees.”
This section continues with “other factors which contribute to the practical safeguarding of employees using or likely to come in contact with the equipment.” One of these “other factors” is proper grounding.
If the non-current-carrying metal parts of electric equipment are not properly grounded and these parts become energized, then any employee “using or likely to come in contact with the equipment” is at risk of an electrical shock that may or may not be fatal. This is a risk that must not be taken.
Proper grounding can effectively eliminate this shock hazard by providing a permanent and continuous low impedance path
If, for example, the neutral of a 120/240 V, wye-connected secondary of a transformer were not grounded, instead of being 120 V to ground, the voltage could reach several hundred volts to ground. A wye-connected electrical system becomes very unstable if it is not properly grounded.
OSHA 1910.304(f)(7)(iii) Grounding of equipment. All non-current-carrying metal parts of portable equipment and fixed equipment including their associated fences, housings, enclosures, and supporting structures shall be grounded.
However, equipment that is guarded by location and isolated from ground need not be grounded. Additionally, polemounted distribution apparatus at a height exceeding 8 feet above ground or grade level need not be grounded.
In 29 CFR 1910.303, “General Requirements,” OSHA states under “(b) Examination, installation, and use of equipment (1) Examination” that “Electrical equipment shall be free from recognized hazards that are likely to cause death or serious physical harm to employees.”
This section continues with “other factors which contribute to the practical safeguarding of employees using or likely to come in contact with the equipment.” One of these “other factors” is proper grounding.
If the non-current-carrying metal parts of electric equipment are not properly grounded and these parts become energized, then any employee “using or likely to come in contact with the equipment” is at risk of an electrical shock that may or may not be fatal. This is a risk that must not be taken.
Proper grounding can effectively eliminate this shock hazard by providing a permanent and continuous low impedance path
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