What are the activities/ works that requires work permit?
The main types of permit and the work to be covered by each are identified below. Appendix 6.4 illustrates the essential elements of a permit form with supporting notes on its operation.
General permit
The general permit should be used for work such as:
S alterations to or overhaul of plant or machinery where mechanical, toxic or electrical hazards may arise
S work on or near overhead crane tracks
S work on pipelines with hazardous contents
S work with asbestos-based materials
S work involving ionising radiation
S work at height where there are exceptionally high risks
S excavations to avoid underground services.
Confined space permit
Confined spaces include chambers, tanks (sealed and open-top), vessels, furnaces, ducts, sewers, manholes, pits, flues, excavations, boilers, reactors and ovens.
Many fatal accidents have occurred where inadequate precautions were taken before and during work involving entry into confined spaces. The two main hazards are the potential presence of toxic or other dangerous substances and the absence of adequate oxygen.
In addition, there may be mechanical hazards (entanglement on agitators) and raised temperatures. The work to be carried out may itself be especially hazardous when done in a confined space, for example, cleaning using solvents, cutting/welding work.
Should the person working in a confined space get into difficulties for whatever reason, getting help in and
getting the individual out may prove difficult and dangerous.
Stringent preparation, isolation, air testing and other precautions are therefore essential and experience shows that the use of a confined space entry permit is essential to confirm that all the appropriate precautions
have been taken.
Work on high voltage apparatus (including testing)
Work on high voltage apparatus (over about 600 volts) is potentially high risk. Hazards include:
S possibly fatal electric shock/burns to the people doing the work
S electrical fires/explosions
S consequential danger from disruption of power supply to safety-critical plant and equipment.
In view of the risk, this work must only be done by suitably trained and competent people acting under the terms of a high voltage permit.
Hot work
Hot work is potentially hazardous as a:
S source of ignition in any plant in which highly flammable materials are handled
S cause of fires in all locations, regardless of whether highly flammable materials are present.
Hot work includes cutting, welding, brazing, soldering and any process involving the application of a naked
flame. Drilling and grinding should also be included where a flammable atmosphere is potentially present.
In high risk areas hot work may also involve any equipment or procedure that produces a spark of sufficient energy to ignite highly flammable substances.
Hot work should therefore be done under the terms of a hot work permit, the only exception being where hot work is done in a designated area suitable for the purpose.
PROJECT RISK CONTROL - HIERARCHY OF RISK CONTROLS BASIC INFORMATION AND TUTORIALS
When assessing the adequacy of existing controls or introducing new controls, a hierarchy of risk controls should be considered. The Management of Health and Safety at Work Regulations 1999 Schedule 1 specifies the general principles of prevention which are set out in the European Council Directive.
These principles are:
1. avoiding risks
2. evaluating the risks which cannot be avoided
3. combating the risks at source
4. adapting the work to the individual, especially as regards the design of the workplace, the choice
of work equipment and the choice of working and production methods, with a view, in particular, to alleviating monotonous work and work at a predetermined work-rate and to reducing their effects on health
5. adapting to technical progress
6. replacing the dangerous by the non-dangerous or the less dangerous
7. developing a coherent overall prevention policy which covers technology, organization of work, working conditions, social relationships and the influence of factors relating to the working environment
8. giving collective protective measures priority over individual protective measures and
9. giving appropriate instruction to employees.
These principles are not exactly a hierarchy but must be considered alongside the usual hierarchy of risk control which is as follows:
S elimination
S substitution
S engineering controls (e.g. isolation, insulation and
ventilation)
S reduced or limited time exposure
S good housekeeping
S safe systems of work
S training and information
S personal protective equipment
S welfare
S monitoring and supervision
S reviews.
These principles are:
1. avoiding risks
2. evaluating the risks which cannot be avoided
3. combating the risks at source
4. adapting the work to the individual, especially as regards the design of the workplace, the choice
of work equipment and the choice of working and production methods, with a view, in particular, to alleviating monotonous work and work at a predetermined work-rate and to reducing their effects on health
5. adapting to technical progress
6. replacing the dangerous by the non-dangerous or the less dangerous
7. developing a coherent overall prevention policy which covers technology, organization of work, working conditions, social relationships and the influence of factors relating to the working environment
8. giving collective protective measures priority over individual protective measures and
9. giving appropriate instruction to employees.
These principles are not exactly a hierarchy but must be considered alongside the usual hierarchy of risk control which is as follows:
S elimination
S substitution
S engineering controls (e.g. isolation, insulation and
ventilation)
S reduced or limited time exposure
S good housekeeping
S safe systems of work
S training and information
S personal protective equipment
S welfare
S monitoring and supervision
S reviews.
POWER CIRCUIT BREAKER TYPES FOR SAFETY INFORMATION BASICS
The five general types of high-voltage circuit breakers are as follows.
1 Oil circuit breakers use standard transformer oil, an effective medium for quenching the arc and providing an open break after current has dropped to zero. There are two general types of oil circuit breakers: dead-tank for the higher voltage ranges and live-tank for lower voltages.
Oil circuit breakers have been improved by adding such features as oil-tight joints, vents, and separate chambers to prevent the escape of oil.
Also, improved operating mechanisms prevent gas pressure from reclosing the contacts, making them reliable for system voltages up to 362 kV. However, above 230 kV, oil-less breakers are more economical.
2 Air-blast circuit breakers were developed as alternatives to oil circuit breakers as voltages increased. They depend on the good insulating and arc-quenching properties of dry and clean compressed air injected into the contact region.
3 Magnetic-air circuit breakers use a combination of strong magnetic field with a special arc chute to lengthen the arc until the system voltage is unable to maintain the arc any longer. They are used principally in power distribution systems.
4 Gas circuit breakers take advantage of the excellent arc-quenching and insulating properties of sulfur hexafluoride (SF6) gas. These outdoor breakers can interrupt system voltages up to 800 kV.
These circuit breakers are typically included in gasinsulated substations (GISs) that offer space-saving and environmental advantages over conventional outdoor substations. Gas (SF6) circuit breakers are made with ratings up to 800 kV and continuous cur rent up to 4000 A.
They are alternatives to oil and vacuum breakers for metal-clad and metal-enclosed switchgear up to 38 kV.
5 Vacuum circuit breakers, more accurately termed vacuum-bottle interrupters, are generally used for voltages up to 38 kV and continuous current ratings to 3000 A.
They are used for higher system voltage, current, and interrupting ratings, and are typically specified for metal-clad and metal-enclosed switchgear in distribution systems.
1 Oil circuit breakers use standard transformer oil, an effective medium for quenching the arc and providing an open break after current has dropped to zero. There are two general types of oil circuit breakers: dead-tank for the higher voltage ranges and live-tank for lower voltages.
Oil circuit breakers have been improved by adding such features as oil-tight joints, vents, and separate chambers to prevent the escape of oil.
Also, improved operating mechanisms prevent gas pressure from reclosing the contacts, making them reliable for system voltages up to 362 kV. However, above 230 kV, oil-less breakers are more economical.
2 Air-blast circuit breakers were developed as alternatives to oil circuit breakers as voltages increased. They depend on the good insulating and arc-quenching properties of dry and clean compressed air injected into the contact region.
3 Magnetic-air circuit breakers use a combination of strong magnetic field with a special arc chute to lengthen the arc until the system voltage is unable to maintain the arc any longer. They are used principally in power distribution systems.
4 Gas circuit breakers take advantage of the excellent arc-quenching and insulating properties of sulfur hexafluoride (SF6) gas. These outdoor breakers can interrupt system voltages up to 800 kV.
These circuit breakers are typically included in gasinsulated substations (GISs) that offer space-saving and environmental advantages over conventional outdoor substations. Gas (SF6) circuit breakers are made with ratings up to 800 kV and continuous cur rent up to 4000 A.
They are alternatives to oil and vacuum breakers for metal-clad and metal-enclosed switchgear up to 38 kV.
5 Vacuum circuit breakers, more accurately termed vacuum-bottle interrupters, are generally used for voltages up to 38 kV and continuous current ratings to 3000 A.
They are used for higher system voltage, current, and interrupting ratings, and are typically specified for metal-clad and metal-enclosed switchgear in distribution systems.
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.
BUCK BOOST AUTOTRANSFORMERS BASIC INFORMATION AND TUTORIALS
The buck-boost transformer is a simple and economical means for raising a voltage that is too low or decreasing a voltage that is too high. This transformer can raise or lower voltage being supplied to the load more than ±5 percent, to improve the efficiency of the device or system.
Buck-boost transformers are small single-phase transformers designed to reduce (buck) or raise (boost) line voltage from 5 to 20 percent. A common application is boosting 208 V to 230 or 240 V AC.
For example, there might be a requirement to power the motor in an air conditioner with a 230- or 240-V AC motor from the 208-V AC supply line. This can be done with a buckboost transformer.
Buck-boost transformers are standard distribution transformers with ratings ranging from 50 VA to 10 kVA. Commercial units are made with primary voltages of 120, 240, or 480 V AC.
They can also power low-voltage circuits for control or lighting applications requiring 12, 16, 24, 32, or 48 V AC. Schematics of buck-boost transformers that can transform 120 and 240 V AC to 12 and 24 V AC are shown in the figure below.
When the primary and secondary lead wires of buck-boost transformers are connected together electrically in a recommended bucking or boosting connection, they become autotransformers. Some typical connection diagrams for these transformers in autotransformer arrangements for single-phase systems are shown below.
Buck-boost transformers have four windings for versatility. Their two primary and two secondary windings can be connected eight different ways to provide many different voltage and kVA outputs.
Because their output voltage is a function of input voltage, they cannot be used as voltage stabilizers. Output voltage will vary by the same percentage as the input voltage.
These transformers can also function in three-phase systems. Two or three units can be used to buck or boost three-phase voltage. The number of units needed in a three-phase installation depends on the number of wires in the supply line.
Buck-boost transformers are small single-phase transformers designed to reduce (buck) or raise (boost) line voltage from 5 to 20 percent. A common application is boosting 208 V to 230 or 240 V AC.
For example, there might be a requirement to power the motor in an air conditioner with a 230- or 240-V AC motor from the 208-V AC supply line. This can be done with a buckboost transformer.
Buck-boost transformers are standard distribution transformers with ratings ranging from 50 VA to 10 kVA. Commercial units are made with primary voltages of 120, 240, or 480 V AC.
They can also power low-voltage circuits for control or lighting applications requiring 12, 16, 24, 32, or 48 V AC. Schematics of buck-boost transformers that can transform 120 and 240 V AC to 12 and 24 V AC are shown in the figure below.
When the primary and secondary lead wires of buck-boost transformers are connected together electrically in a recommended bucking or boosting connection, they become autotransformers. Some typical connection diagrams for these transformers in autotransformer arrangements for single-phase systems are shown below.
Buck-boost transformers have four windings for versatility. Their two primary and two secondary windings can be connected eight different ways to provide many different voltage and kVA outputs.
Because their output voltage is a function of input voltage, they cannot be used as voltage stabilizers. Output voltage will vary by the same percentage as the input voltage.
These transformers can also function in three-phase systems. Two or three units can be used to buck or boost three-phase voltage. The number of units needed in a three-phase installation depends on the number of wires in the supply line.
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