Principles

safety:

Electrically powered equipment, such as hot plates, stirrers, vacuum pumps, electrophoresis apparatus, lasers, heating mantles, ultrasonicators, power supplies, and microwave ovens are essential elements of many laboratories. These devices can pose a significant hazard to laboratory workers, particularly when mishandled or not maintained. Many laboratory electrical devices have high voltage or high power requirements, carrying even more risk. Large capacitors found in many laser flash lamps and other systems are capable of storing lethal amounts of electrical energy and pose a serious danger even if the power source has been disconnected.
Accounts of incidents on campus that resulted in electrical shock, including a near fatal incident, are described in.

Electrical Hazards

The major hazards associated with electricity are electrical shock and fire. Electrical shock occurs when the body becomes part of the electric circuit, either when an individual comes in contact with both wires of an electrical circuit, one wire of an energized circuit and the ground, or a metallic part that has become energized by contact with an electrical conductor.
The severity and effects of an electrical shock depend on a number of factors, such as the pathway through the body, the amount of current, the length of time of the exposure, and whether the skin is wet or dry. Water is a great conductor of plugelectricity, allowing current to flow more easily in wet conditions and through wet skin. The effect of the shock may range from a slight tingle to severe burns to cardiac arrest. The chart below shows the general relationship between the degree of injury and amount of current for a 60-cycle hand-to-foot path of one second's duration of shock. While reading this chart, keep in mind that most electrical circuits can provide, under normal conditions, up to 20,000 milliamperes of current flow
 
Current Reaction
1 Milliampere Perception level
5 Milliamperes Slight shock felt; not painful but disturbing
6-30 Milliamperes Painful shock; "let-go" range
50-150 Milliamperes Extreme pain, respiratory arrest, severe muscular contraction
1000-4,300 Milliamperes Ventricular fibrillation
10,000+ Milliamperes Cardiac arrest, severe burns and probable death
In addition to the electrical shock hazards, sparks from electrical equipment can serve as an ignition source for flammable or explosive vapors or combustible materials. See Anecdotes.

Power Loss

Loss of electrical power can create hazardous situations. Flammable or toxic vapors may be released as a chemical warms when a refrigerator or freezer fails. Fume hoods may cease to operate, allowing vapors to be released into the laboratory. If magnetic or mechanical stirrers fail to operate, safe mixing of reagents may be compromised.

Preventing Electrical Hazards (top)

There are various ways of protecting people from the hazards caused by electricity, including insulation, guarding, grounding, and electrical protective devices. Laboratory workers can significantly reduce electrical hazards by following some basic precautions:
  • Inspect wiring of equipment before each use. Replace damaged or frayed electrical cords immediately.
  • Use safe work practices every time electrical equipment is used.
  • Know the location and how to operate shut-off switches and/or circuit breaker panels. Use these devices to shut off equipment in the event of a fire or electrocution.
  • Limit the use of extension cords. Use only for temporary operations and then only for short periods of time. In all other cases, request installation of a new electrical outlet.
  • Multi-plug adapters must have circuit breakers or fuses.
  • Place exposed electrical conductors (such as those sometimes used with electrophoresis devices) behind shields.
  • Minimize the potential for water or chemical spills on or near electrical equipment.

Insulation

All electrical cords should have sufficient insulation to prevent direct contact with wires. In a laboratory, it is particularly important to check all cords before each use, since corrosive chemicals or solvents may erode the insulation.
Damaged cords should be repaired or taken out of service immediately, especially in wet environments such as cold rooms and near water baths.

Guarding

Live parts of electric equipment operating at 50 volts or more (i.e., electrophoresis devices) must be guarded against accidental contact. Plexiglas shields may be used to protect against exposed live parts.

Grounding

plugOnly equipment with three-prong plugs should be used in the laboratory. The third prong provides a path to ground for internal electrical short circuits, thereby protecting the user from a potential electrical shock.

Circuit Protection Devices

circuit breakerCircuit protection devices are designed to automatically limit or shut off the flow of electricity in the event of a ground-fault, overload or short circuit in the wiring system. Ground-fault circuit interrupters, circuit breakers and fuses are three well-known examples of such devices.
Fuses and circuit breakers prevent over-heating of wires and components that might otherwise create fire hazards. They disconnect the circuit when it becomes overloaded. This overload protection is very useful for equipment that is left on for extended periods of time, such as stirrers, vacuum pumps, drying ovens, Variacs and other electrical equipment.
The ground-fault circuit interrupter, or GFCI, is designed to shutoff electric power if a ground fault is detected, protecting the user from a potential electrical shock. The GFCI is particularly useful near sinks and wet locations. Since GFCIs can cause equipment to shutdown unexpectedly, they may not be appropriate for certain apparatus. Portable GFCI adapters (available in most safety supply catalogs) may be used with a non-GFCI outlet.

Motors

In laboratories where volatile flammable materials are used, motor-driven electrical equipment should be equipped with non-sparking induction motors or air motors. These motors must meet National Electric Safety Code (US DOC, 1993) Class 1, Division 2, Group C-D explosion resistance specifications. Many stirrers, Variacs, outlet strips, ovens, heat tape, hot plates and heat guns do not conform to these code requirements.
Avoid series-wound motors, such as those generally found in some vacuum pumps, rotary evaporators and stirrers. Series-wound motors are also usually found in household appliances such as blenders, mixers, vacuum cleaners and power drills. These appliances should not be used unless flammable vapors are adequately controlled.
Although some newer equipment have spark-free induction motors, the on-off switches and speed controls may be able to produce a spark when they are adjusted because they have exposed contacts. One solution is to remove any switches located on the device and insert a switch on the cord near the plug end.

Safe Work Practices 

The following practices may reduce risk of injury or fire when working with electrical equipment:
  • Avoid contact with energized electrical circuits.
  • Use guarding around exposed circuits and sources of live electricity.
  • Disconnect the power source before servicing or repairing electrical equipment.
  • When it is necessary to handle equipment that is plugged in, be sure hands are dry and, when possible, wear nonconductive gloves and shoes with insulated soles.
  • If it is safe to do so, work with only one hand, keeping the other hand at your side or in your pocket, away from all conductive material. This precaution reduces the likelihood of accidents that result in current passing through the chest cavity.
  • Minimize the use of electrical equipment in cold rooms or other areas where condensation is likely. If equipment must be used in such areas, mount the equipment on a wall or vertical panel.
  • If water or a chemical is spilled onto equipment, shut off power at the main switch or circuit breaker and unplug the equipment.
  • If an individual comes in contact with a live electrical conductor, do not touch the equipment, cord or person. Disconnect the power source from the circuit breaker or pull out the plug using a leather belt.

    High Voltage or Current 

    Repairs of high voltage or high current equipment should be performed only by trained electricians. Laboratory workers who are experienced in such tasks and would like to perform such work on their own laboratory equipment must first receive specialized electrical safety related work practices training by EHS staff. Contact the University Safety Engineer at 258-5294 for more information.

    Altering Building Wiring and Utilities 

    Any modifications to existing electrical service in a laboratory or building must be completed or approved by either the building facility manager, an engineer from the Facilities department or the building's Special Facilities staff. All modifications must meet both safety standards and Facilities Engineering design requirements.
    Any unapproved laboratory facilities modifications discovered during laboratory surveys or other activities are reviewed by EHS and facility staff to determine whether they meet design specifications.

Kirchhoff's circuit laws are two equalities that deal with the conservation of charge and energy in electrical circuits, and were first described in 1847 by Gustav Kirchhoff. Widely used in electrical engineering, they are also called Kirchhoff's rules or simply Kirchhoff's laws (see also Kirchhoff's laws for other meanings of that term).
Both circuit rules can be directly derived from Maxwell's equations, but Kirchhoff preceded Maxwell and instead generalized work by Georg Ohm.
This law is also called Kirchhoff's first law, Kirchhoff's point rule, Kirchhoff's junction rule (or nodal rule), and Kirchhoff's first rule.
The principle of conservation of electric charge implies that:

At any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node.
or
The algebraic sum of currents in a network of conductors meeting at a point is zero.
Recalling that current is a signed (positive or negative) quantity reflecting direction towards or away from a node, this principle can be stated as:
\sum_{k=1}^n {I}_k = 0
n is the total number of branches with currents flowing towards or away from the node.
This formula is valid for complex currents:
\sum_{k=1}^n \tilde{I}_k = 0
The law is based on the conservation of charge whereby the charge (measured in coulombs) is the product of the current (in amperes) and the time (in seconds).

Kirchhoff's voltage law (KVL)

http://upload.wikimedia.org/wikipedia/commons/thumb/4/40/Kirchhoff_voltage_law.svg/200px-Kirchhoff_voltage_law.svg.png
http://bits.wikimedia.org/skins-1.17/common/images/magnify-clip.png
The sum of all the voltages around the loop is equal to zero. v1 + v2 + v3 - v4 = 0
This law is also called Kirchhoff's second law, Kirchhoff's loop (or mesh) rule, and Kirchhoff's second rule.
The principle of conservation of energy implies that
The directed sum of the electrical potential differences (voltage) around any closed circuit is zero.
or
More simply, the sum of the emfs in any closed loop is equivalent to the sum of the potential drops in that loop.
or
The algebraic sum of the products of the resistances of the conductors and the currents in them in a closed loop is equal to the total emf available in that loop.
Similarly to KCL, it can be stated as:
\sum_{k=1}^n V_k = 0
Here, n is the total number of voltages measured. The voltages may also be complex:
\sum_{k=1}^n \tilde{V}_k = 0
This law is based on the conservation of "energy given/taken by potential field" (not including energy taken by dissipation). Given a voltage potential, a charge which has completed a closed loop doesn't gain or lose energy as it has gone back to initial potential level.
This law holds true even when resistance (which causes dissipation of energy) is present in a circuit. The validity of this law in this case can be understood if one realizes that a charge in fact doesn't go back to its starting point, due to dissipation of energy. A charge will just terminate at the negative terminal, instead of positive terminal. This means all the energy given by the potential difference has been fully consumed by resistance which in turn loses the energy as heat dissipation.
To summarize, Kirchhoff's voltage law has nothing to do with gain or loss of energy by electronic components (resistors, capacitors, etc.). It is a law referring to the potential field generated by voltage sources. In this potential field, regardless of what electronic components are present, the gain or loss in "energy given by the potential field" must be zero when a charge completes a closed loop. Electric field and electric potential
Kirchhoff's voltage law could be viewed as a consequence of the principle of conservation of energy. Otherwise, it would be possible to build a perpetual motion machine that passed a current in a circle around the circuit.
Considering that electric potential is defined as a line integral over an electric field, Kirchhoff's voltage law can be expressed equivalently as

which states that the line integral of the electric field around closed loop C is zero.
In order to return to the more special form, this integral can be "cut in pieces" in order to get the voltage at specific components.

protective devices:
In an  electrical power station, when anything becomes abnormal, it become necessary to isolate the abnormal conditions instantaneously or in some cases after a predetermined time delay. 
Equipment applied to electric power systems to detect abnormal and intolerable conditions and to initiate appropriate corrective actions. These devices include lightning arresters, surge protectors, fuses, and relays with associated circuit breakers, reclosers, and so forth.
From time to time, disturbances in the normal operation of a power system occur. These may be caused by natural phenomena, such as lightning, wind, or snow; by falling objects such as trees; by animal contacts or chewing; by accidental means traceable to reckless drivers, inadvertent acts by plant maintenance personnel, or other acts of humans; or by conditions produced in the system itself, such as switching surges, load swings, or equipment failures. Protective devices must therefore be installed on power systems to ensure continuity of electrical service, to limit injury to people, and to limit damage to equipment when problem situations develop. Protective devices are applied commensurately with the degree of protection desired or felt necessary for the particular system
protective

These are compact analog or digital networks, connected to various points of an electrical system, to detect abnormal conditions occurring within their assigned areas. They initiate disconnection of the trouble area by circuit breakers. These relays range from the simple overload unit on house circuit breakers to complex systems used to protect extrahigh-voltage power transmission lines. They operate on voltage, current, current direction, power factor, power, impedance, temperature. In all cases there must be a measurable difference between the normal or tolerable operation and the intolerable or unwanted condition. System faults for which the relays respond are generally short circuits between the phase conductors, or between the phases and grounds. Some relays operate on unbalances between the phases, such as an open or reversed phase. A fault in one part of the system affects all other parts. Therefore relays and fuses throughout the power system must be coordinated to ensure the best quality of service to the loads and to avoid operation in the nonfaulted areas unless the trouble is not adequately cleared in a specified time. See also Fuse (electricity); Relay.
Zone protection
For the purpose of applying protection, the electric power system is divided into five major protection zones: generators; transformers; buses; transmission and distribution lines; and motors (see illustration). Each block represents a set of protective relays and associated equipment selected to initiate correction or isolation of that area for all anticipated intolerable conditions or trouble. The detection is done by protective relays with a circuit breaker used to physically disconnect the equipment. For other areas of protection See also Grounding; Uninterruptible power system.
Zones of protection on simple power system.
Zones of protection on simple power system.
Fault detection
Fault detection is accomplished by a number of techniques, including the detection of changes in electric current or voltage levels, power direction, ratio of voltage to current, temperature, and comparison of the electrical quantities flowing into a protected area with the quantities flowing out, also known as differential protection.


The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is important because it is directly linked to the reliability of the electricity supply.
The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications
Substations
Typically, switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. The switchgear located on the low voltage side of the transformers in distribution type substations, now are typically located in what is called a Power Distribution Center (PDC). Inside this building are typically smaller, medium-voltage (~15kV) circuit breakers feeding the distribution system. Also contained inside these Power Control Centers are various relays, meters, and other communication equipment allowing for intelligent control of the substation.
For industrial applications, a transformer and switchgear (Load Breaking Switch Fuse Unit) line-up may be combined in one housing, called a unitized substation or USS.
Housing
Switchgear for low voltages may be entirely enclosed within a building. For transmission levels of voltage (high voltages over 66 kV), often switchgear will be mounted outdoors and insulated by air, though this requires a large amount of space. Gas insulated switchgear used for transmission-level voltages saves space compared with air-insulated equipment, although it has a higher equipment cost. Oil insulated switchgear presents an oil spill hazard.
At small substations, switches may be manually operated, but at important switching stations on the transmission network all devices have motor operators to allow for remote control.
Types
A piece of switchgear may be a simple open-air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is gas insulated switchgear (GIS), where the conductors and contacts are insulated by pressurized sulfur hexafluoride gas (SF6). Other common types are oil or vacuum insulated switchgear.
The combination of equipment within the switchgear enclosure allows them to interrupt fault currents of thousands of amps. A circuit breaker (within a switchgear enclosure) is the primary component that interrupts fault currents. The quenching of the arc when the circuit breaker pulls apart the contacts open (disconnects the circuit) requires careful design. Circuit breakers fall into these four types:
  • Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.
  • Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 to quench the stretched arc.
  • Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (<2–3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts.
  • Air circuit breakers may use compressed air (puff) to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.
Circuit breakers are usually able to terminate all current flow very quickly: typically between 30 ms and 150 ms depending upon the age and construction of the device.
Several different classifications of switchgear can be made
  • By the current rating.
  • By interrupting rating (maximum short circuit current that the device can safely interrupt)
    • Circuit breakers can open and close on fault currents
    • Load-break/Load-make switches can switch normal system load currents
    • Isolators may only be operated while the circuit is dead, or the load current is very small.
  • By voltage class:
    • Low voltage (less than 1,000 volts AC)
    • Medium voltage (1,000–35,000 volts AC)
    • High voltage (more than 35,000 volts AC)
  • By insulating medium:
    • Air
    • Gas (SF6 or mixtures)
    • Oil
    • Vacuum
  • By construction type:
    • Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure type)
    • Outdoor
    • Industrial
    • Utility
    • Marine
    • Draw-out elements (removable without many tools)
    • Fixed elements (bolted fasteners)
    • Live-front
    • Dead-front
    • Open
    • Metal-enclosed
    • Metal-clad
    • Metal enclose & Metal clad
    • Arc-resistant
  • By IEC degree of internal separation
    • No Separation (Form 1)
    • Busbars separated from functional units (Form 2a, 2b, 3a, 3b, 4a, 4b)
    • Terminals for external conductors separated from busbars (Form 2b, 3b, 4a, 4b)
    • Terminals for external conductors separated from functional units but not from each other (Form 3a, 3b)
    • Functional units separated from each other (Form 3a, 3b, 4a, 4b)
    • Terminals for external conductors separated from each other (Form 4a, 4b)
    • Terminals for external conductors separate from their associated functional unit (Form 4b)
  • By interrupting device:
    • Fuses
    • Air Blast Circuit Breaker
    • Minimum Oil Circuit Breaker
    • Oil Circuit Breaker
    • Vacuum Circuit Breaker
    • Gas (SF6) Circuit breaker
  • By operating method:
    • Manually-operated
    • Motor-operated
    • Solenoid/stored energy operated
  • By type of current:
    • Alternating current
    • Direct current
  • By application:
    • Transmission system
    • Distribution
  • By purpose
    • Isolating switches (disconnectors)
    • Load-break switches.
    • Grounding (earthing) switches
A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles.
Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards are used, much of the rest of the world uses IEC standards, sometimes with local national derivatives or variations.
Functions
One of the basic functions of switchgear is protection, which is interruption of short-circuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear is also used to enhance system availability by allowing more than one source to feed a load

Safety
To help ensure safe operation sequences of switchgear, trapped key interlocking provides predefined scenarios of operation. For example, if only one of two sources of supply are permitted to be connected at a given time, the interlock scheme may require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible.
Indoor switchgear can also be type tested for internal arc containment. This test is important for user safety as modern switchgear is capable of switching large currents.
Switchgear is often inspected using thermal imaging to assess the state of the system and predict failures before they occur.


Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them.
The mathematical equation that describes this relationship is:
I = \frac{V}{R}
where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.
The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire. He presented a slightly more complex equation than the one above (see History section below) to explain his experimental results. The above equation is the modern form of Ohm's law.
In physics, the term Ohm's law is also used to refer to various generalizations of the law originally formulated by Ohm. The simplest example of this is:
\boldsymbol{J} = \sigma \boldsymbol{E},
where J is the current density at a given location in a resistive material, E is the electric field at that location, and σ is a material dependent parameter called the conductivity. This reformulation of Ohm's law is due to.
 
protective device:


In an  electrical power station, when anything becomes abnormal, it become necessary to isolate the abnormal conditions instantaneously or in some cases after a predetermined time delay. 
Equipment applied to electric power systems to detect abnormal and intolerable conditions and to initiate appropriate corrective actions. These devices include lightning arresters, surge protectors, fuses, and relays with associated circuit breakers, reclosers, and so forth.
From time to time, disturbances in the normal operation of a power system occur. These may be caused by natural phenomena, such as lightning, wind, or snow; by falling objects such as trees; by animal contacts or chewing; by accidental means traceable to reckless drivers, inadvertent acts by plant maintenance personnel, or other acts of humans; or by conditions produced in the system itself, such as switching surges, load swings, or equipment failures. Protective devices must therefore be installed on power systems to ensure continuity of electrical service, to limit injury to people, and to limit damage to equipment when problem situations develop. Protective devices are applied commensurately with the degree of protection desired or felt necessary for the particular system
protective
These are compact analog or digital networks, connected to various points of an electrical system, to detect abnormal conditions occurring within their assigned areas. They initiate disconnection of the trouble area by circuit breakers. These relays range from the simple overload unit on house circuit breakers to complex systems used to protect extrahigh-voltage power transmission lines. They operate on voltage, current, current direction, power factor, power, impedance, temperature. In all cases there must be a measurable difference between the normal or tolerable operation and the intolerable or unwanted condition. System faults for which the relays respond are generally short circuits between the phase conductors, or between the phases and grounds. Some relays operate on unbalances between the phases, such as an open or reversed phase. A fault in one part of the system affects all other parts. Therefore relays and fuses throughout the power system must be coordinated to ensure the best quality of service to the loads and to avoid operation in the nonfaulted areas unless the trouble is not adequately cleared in a specified time. See also Fuse (electricity); Relay.
Zone protection
For the purpose of applying protection, the electric power system is divided into five major protection zones: generators; transformers; buses; transmission and distribution lines; and motors (see illustration). Each block represents a set of protective relays and associated equipment selected to initiate correction or isolation of that area for all anticipated intolerable conditions or trouble. The detection is done by protective relays with a circuit breaker used to physically disconnect the equipment. For other areas of protection See also Grounding; Uninterruptible power system.
Zones of protection on simple power system.
Zones of protection on simple power system.
Fault detection
Fault detection is accomplished by a number of techniques, including the detection of changes in electric current or voltage levels, power direction, ratio of voltage to current, temperature, and comparison of the electrical quantities flowing into a protected area with the quantities flowing out, also known as differential protection.


The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is important because it is directly linked to the reliability of the electricity supply.
The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications

Substations

Typically, switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. The switchgear located on the low voltage side of the transformers in distribution type substations, now are typically located in what is called a Power Distribution Center (PDC). Inside this building are typically smaller, medium-voltage (~15kV) circuit breakers feeding the distribution system. Also contained inside these Power Control Centers are various relays, meters, and other communication equipment allowing for intelligent control of the substation.
For industrial applications, a transformer and switchgear (Load Breaking Switch Fuse Unit) line-up may be combined in one housing, called a unitized substation or USS.

Housing

Switchgear for low voltages may be entirely enclosed within a building. For transmission levels of voltage (high voltages over 66 kV), often switchgear will be mounted outdoors and insulated by air, though this requires a large amount of space. Gas insulated switchgear used for transmission-level voltages saves space compared with air-insulated equipment, although it has a higher equipment cost. Oil insulated switchgear presents an oil spill hazard.
At small substations, switches may be manually operated, but at important switching stations on the transmission network all devices have motor operators to allow for remote control.

Types

A piece of switchgear may be a simple open-air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is gas insulated switchgear (GIS), where the conductors and contacts are insulated by pressurized sulfur hexafluoride gas (SF6). Other common types are oil or vacuum insulated switchgear.
The combination of equipment within the switchgear enclosure allows them to interrupt fault currents of thousands of amps. A circuit breaker (within a switchgear enclosure) is the primary component that interrupts fault currents. The quenching of the arc when the circuit breaker pulls apart the contacts open (disconnects the circuit) requires careful design. Circuit breakers fall into these four types:
  • Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.
  • Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 to quench the stretched arc.
  • Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (<2–3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts.
  • Air circuit breakers may use compressed air (puff) to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.
Circuit breakers are usually able to terminate all current flow very quickly: typically between 30 ms and 150 ms depending upon the age and construction of the device.
Several different classifications of switchgear can be made
  • By the current rating.
  • By interrupting rating (maximum short circuit current that the device can safely interrupt)
    • Circuit breakers can open and close on fault currents
    • Load-break/Load-make switches can switch normal system load currents
    • Isolators may only be operated while the circuit is dead, or the load current is very small.
  • By voltage class:
    • Low voltage (less than 1,000 volts AC)
    • Medium voltage (1,000–35,000 volts AC)
    • High voltage (more than 35,000 volts AC)
  • By insulating medium:
    • Air
    • Gas (SF6 or mixtures)
    • Oil
    • Vacuum
  • By construction type:
    • Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure type)
    • Outdoor
    • Industrial
    • Utility
    • Marine
    • Draw-out elements (removable without many tools)
    • Fixed elements (bolted fasteners)
    • Live-front
    • Dead-front
    • Open
    • Metal-enclosed
    • Metal-clad
    • Metal enclose & Metal clad
    • Arc-resistant
  • By IEC degree of internal separation
    • No Separation (Form 1)
    • Busbars separated from functional units (Form 2a, 2b, 3a, 3b, 4a, 4b)
    • Terminals for external conductors separated from busbars (Form 2b, 3b, 4a, 4b)
    • Terminals for external conductors separated from functional units but not from each other (Form 3a, 3b)
    • Functional units separated from each other (Form 3a, 3b, 4a, 4b)
    • Terminals for external conductors separated from each other (Form 4a, 4b)
    • Terminals for external conductors separate from their associated functional unit (Form 4b)
  • By interrupting device:
    • Fuses
    • Air Blast Circuit Breaker
    • Minimum Oil Circuit Breaker
    • Oil Circuit Breaker
    • Vacuum Circuit Breaker
    • Gas (SF6) Circuit breaker
  • By operating method:
    • Manually-operated
    • Motor-operated
    • Solenoid/stored energy operated
  • By type of current:
    • Alternating current
    • Direct current
  • By application:
    • Transmission system
    • Distribution
  • By purpose
    • Isolating switches (disconnectors)
    • Load-break switches.
    • Grounding (earthing) switches
A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles.
Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards are used, much of the rest of the world uses IEC standards, sometimes with local national derivatives or variations.

Functions

One of the basic functions of switchgear is protection, which is interruption of short-circuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear is also used to enhance system availability by allowing more than one source to feed a load

Safety

To help ensure safe operation sequences of switchgear, trapped key interlocking provides predefined scenarios of operation. For example, if only one of two sources of supply are permitted to be connected at a given time, the interlock scheme may require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible.
Indoor switchgear can also be type tested for internal arc containment. This test is important for user safety as modern switchgear is capable of switching large currents.
Switchgear is often inspected using thermal imaging to assess the state of the system and predict failures before they occur.
Subscribe to: Posts (Atom)