The selection of the ac supply system voltage typically begins at the service entrance of the facility. In most commercial environments in the U. S., the utility supplies three-phase power at 480 Y/277 V (or 600 Y/347 V) or 208 Y/120 V.
In industrial environments, the utility may supply three-phase power at even higher voltages such as 4160 V, 13 800 V and higher. The magnitude of the voltage will typically depend on the size of the facility, the load conditions, and the voltage ratings of the utilization equipment in the facility.
In some cases, the facility owners may design, install, and maintain their own medium-voltage electrical distribution system.
Recommended practice is to provide distribution power in most facilities at 480 Y/277 V (or 600 Y/347 V) rather than at the actual utilization equipment level of most electronic load equipment (208 Y/120 V). Electrical distribution systems operating at 480 Y/277 V (or 600 Y/ 347 V) have the following benefits over 208 Y/120 V systems:
a) The source impedance of 480 Y/277 V systems are typically less than 208 Y/120 V systems. This characteristic provides a more stable source with better voltage regulation, and minimizes voltage distortion due to the nonlinear load currents.
b) 480 Y/277 V systems are less susceptible to on-premises generated disturbances. Step-down transformers (and other power enhancement devices) for 208 Y/120 V utilization equipment help attenuate disturbances originating on the 480 V system.
c) 480 Y/277 V systems distribute power at lower currents, which result in lower heat losses in feeders. 480 Y/277 V systems may also decrease material and labor costs associated with installing long feeder circuits.
Step-down transformers (and other power enhancement devices) may be located physically close to the electronic load equipment to minimize the buildup of common-mode voltage.
Delta-connected transformer primaries trap balanced triplen harmonic currents generated on the secondary side by nonlinear electronic load equipment. This action serves to reduce distortion of the voltage waveform at the 480 Y/277 V level.
It is not recommended practice to step-up the voDue to the generally lower impedance of 480 Y/277 V distribution systems, higher short-circuit currents may be available throughout the system. Overcurrent protective devices with higher interrupting capabilities and equipment with higher withstand ratings may be required.
In some situations, electrical distribution at 208 Y/120 V is unavoidable. This may be due to limitations of the utility or facility to provide higher voltages. As previously noted, nonlinear electronic load equipment may cause undesirable voltage distortion that can adversely affect the entire premises.
In these situations, a system analysis may be performed to determine proper mitigation techniques such as the installation of isolation transformers, and other power conditioning or filtering equipment located close to the electronic load equipment tage from the service entrance by means of a locally installed transformer in order to obtain a higher power system voltage for the electrical distribution system serving electronic load equipment.
Although this can be done in certain cases, it is also possible that less satisfactory results can occur than if the system voltage at the service entrance was used.
TYPES OF AC GENERATOR ROTORS BASIC INFORMATION AND TUTORIALS
Synchronous AC generators are fitted with one of two different rotor designs depending on their intended rotational speeds.
Round rotors are solid steel cylinders with the field winding inserted in slots milled into the surface or the rotor. They usually have two or four poles. Round rotors can withstand the stresses of high-speed rotation.
Salient-pole rotors have multiple pole pieces (typically six) mounted to the rotor structure, and the field winding is wound around the pole pieces. Because of their more complex construction and larger diameter-to-length ratios, salient-pole rotors cannot withstand the stresses of high-speed rotation.
Electric utility steam-turbine–driven generators designed for 50- or 60-Hz AC output voltage have round rotors with two poles because they can withstand the stresses of speeds of 3000 and 3600 rpm.
Hydroelectric, diesel, and natural-gas engines have far lower shaft speeds than steam turbines, so the generators they drive usually have six or more pole rotors, requirements usually met with more complex salient-pole rotors.
Three-phase AC generators have a winding that is made up of three separate stator windings, each displaced from the other two by 120 electrical degrees. The three windings can either be wye- or delta-connected. The wye connection is more common because it is better suited for direct high-voltage generation.
Round rotors are solid steel cylinders with the field winding inserted in slots milled into the surface or the rotor. They usually have two or four poles. Round rotors can withstand the stresses of high-speed rotation.
Salient-pole rotors have multiple pole pieces (typically six) mounted to the rotor structure, and the field winding is wound around the pole pieces. Because of their more complex construction and larger diameter-to-length ratios, salient-pole rotors cannot withstand the stresses of high-speed rotation.
Electric utility steam-turbine–driven generators designed for 50- or 60-Hz AC output voltage have round rotors with two poles because they can withstand the stresses of speeds of 3000 and 3600 rpm.
Hydroelectric, diesel, and natural-gas engines have far lower shaft speeds than steam turbines, so the generators they drive usually have six or more pole rotors, requirements usually met with more complex salient-pole rotors.
Three-phase AC generators have a winding that is made up of three separate stator windings, each displaced from the other two by 120 electrical degrees. The three windings can either be wye- or delta-connected. The wye connection is more common because it is better suited for direct high-voltage generation.
ELECTRICAL OVERLOADING - THE HAZARDS OF OVERLOADING BASIC INFORMATION
Overloads in an electrical system are hazardous because they can produce heat or arcing. Wires and other components in an electrical system or circuit have a maximum amount of current they can carry safely.
If too many devices are plugged into a circuit, the electrical current will heat the wires to a very high temperature. If any one tool uses too much current, the wires will heat up.
The temperature of the wires can be high enough to cause a fire. If their insulation melts, arcing may occur. Arcing can cause a fire in the area where the overload exists, even inside a wall.
In order to prevent too much current in a circuit, a circuit breaker or fuse is placed in the circuit. If there is too much current in the circuit, the breaker “trips” and opens like a switch.
If an overloaded circuit is equipped with a fuse, an internal part of the fuse melts, opening the circuit. Both breakers and fuses do the same thing: open the circuit to shut off the electrical current.
If the breakers or fuses are too big for the wires they are supposed to protect, an overload in the circuit will not be detected and the current will not be shut off. Overloading leads to overheating of circuit components (including wires) and may cause a fire.
You need to recognize that a circuit with improper overcurrent protection devices—or one with no overcurrent protection devices at all— is a hazard.
Overcurrent protection devices are built into the wiring of some electric motors, tools, and electronic devices. For example, if a tool draws too much current or if it overheats, the current will be shut off from within the device itself.
Damaged tools can overheat and cause a fire. You need to recognize that a damaged tool is a hazard.
If too many devices are plugged into a circuit, the electrical current will heat the wires to a very high temperature. If any one tool uses too much current, the wires will heat up.
The temperature of the wires can be high enough to cause a fire. If their insulation melts, arcing may occur. Arcing can cause a fire in the area where the overload exists, even inside a wall.
In order to prevent too much current in a circuit, a circuit breaker or fuse is placed in the circuit. If there is too much current in the circuit, the breaker “trips” and opens like a switch.
If an overloaded circuit is equipped with a fuse, an internal part of the fuse melts, opening the circuit. Both breakers and fuses do the same thing: open the circuit to shut off the electrical current.
If the breakers or fuses are too big for the wires they are supposed to protect, an overload in the circuit will not be detected and the current will not be shut off. Overloading leads to overheating of circuit components (including wires) and may cause a fire.
You need to recognize that a circuit with improper overcurrent protection devices—or one with no overcurrent protection devices at all— is a hazard.
Overcurrent protection devices are built into the wiring of some electric motors, tools, and electronic devices. For example, if a tool draws too much current or if it overheats, the current will be shut off from within the device itself.
Damaged tools can overheat and cause a fire. You need to recognize that a damaged tool is a hazard.
WORKS REQUIRING PERMITS BASIC INFORMATION AND TUTORIALS
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.
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.
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