Distribution Equipment
High-Resistance Grounding
(HRG) is increasingly applied in both medium-and low-voltage distribution systems to limit ground fault energy. Medium-voltage sys-tems have long used low-impedance grounding systems to limit ground fault current. Limiting ground fault current to values of 25A or less in-creased system reliability by allowing ground faults to be detected and selectively cleared. These systems used either a large resistor or inductance (transformer primary with a shorted secondary or a relay coil in the secondary) connected between the neutral bushing of the service transformer and the system bonding jumper. Detection of a ground fault at certain levels caused a protective relay to open the circuit and clear the fault.
What is the Minimum Distance from Phase Bus to Closest Steel Member when amps are 5000 to 6000 and up?
12" and Wherever possible, maintain 12" (300 mm) to steel members and 6" (150 mm) to aluminum or nonmagnetic members. Neutral spacing can be 6" and 3" (150 mm and 75 mm), respectively.
What is the Minimum Distance from Phase Bus to Closest Steel Member when amps are 3000?
3" and Minimum Distance from Neutral Bus to Closest Steel Member 2"
What is the Minimum Distance from Phase Bus to Closest Steel Member when amps are 4000?
4" and Minimum Distance from Neutral Bus to Closest Steel Member 3"
What is a circuit breaker defined as?
A circuit breaker is defined as a mechanical switching device that is capable of making, carrying, and breaking currents under normal circuit conditions and also making, carrying (for a specified time), and breaking currents under specified abnormal cir-cuit conditions, such as a short circuit (according to IEEE).
Bus:
A conductor or group of conductors that serves as a common connection for two or more circuits in a switchgear assembly.
Duplex Switch Characteristics
A duplex switch is actually two switches, each in its own bay. The bays are mechanically connected and the switches are electrically connected on the load side. This switch may be used to supply power to a single load from two different sources.
Switchgear:
A general term covering switching or interrupting devices and any combination thereof with associated control, instrumentation, metering, protective, and regulating devices.
Switchboard:
A large single panel, frame, or assembly of panels on which switches, fuses, buses, and instruments are mounted.
Selector Switch Characteristics
A selector allows an MVL switch to have double-throw characteristics. The selector switch is a single switch with a load connected to the moving or switch mechanism. Throwing the switch to one side connects the load to one source, while throwing it the other way con-nects it to a second source. The selector switch will be interlocked with another switch to pre-vent the selector switch from interrupting cur-rent flow. The selector serves a purpose similar to the duplex switch. However, the selector switch is not an interrupter; it is a disconnect.
Feeder:
A set of conductors originating at a main distribution center that supply one or more sec-ondary distribution centers, one or more branch circuit distribution centers, or any combination of these two types of load.
Current transformer (CT):
A single-phase in-strument transformer connected in series in a line that carries the full-load current. The turns ratio is designed to produce a reduced current in the secondary suitable for the current coil of standard measuring instruments and in propor-tion to the load current.
Potential transformer (PT):
A special trans-former designed for use in measuring high volt-age; normally, the secondary voltage is 120V.
Sluggish Operation of switch
A switch that is operating sluggishly hesitates on the opening cycle. This contrasts with the normal snapping action. Observing the interrupter blade during the opening operation is the proper way to determine sluggish operation.
What is a power transformer usually rated for as compared to the distribution transformer?
A transformer that is rated at more than 500kVA and/or 34.5kV is generally a power transformer. A transformer rated below these values can be considered a distribution transformer. Remember, there is an overlap in kVA capacity and voltage depending on the system and power requirements.
Switchboard
According to the National Electrical Code®, the term switchboard may be defined as a large sin-gle panel, frame, or assembly of panels on which switches, overcurrent and other protective de-vices, buses, and instruments may be mounted, either on the face, on the back, or on both the face and back. Switchboards are generally accessible from both the rear and from the front and are not intended to be installed in cabinets.
Frequency (Hertz) -
All MVL switches may be used in either 50Hz or 60Hz power systems.
What does an Electrical Power System consist of?
An electrical power system consists of several subsystems on both the utility (supply) side and the customer (user) side.
Bushing
An insulating structure including a through conductor, or providing a passageway for such a conductor, for the purpose of insulat-ing the conductor from the barrier and conduct-ing from one side of the barrier to the other.
Grounding
Any sections of ground bus that were previously disconnected for shipping should be reconnected when the units are in-stalled. In addition, the system must be bonded at this time. The ground bus should be con-nected to the system ground with as direct a connection as possible. If the system ground is to be run in metal conduit, bonding to the conduit is required. The ground connection is necessary for all switchgear and should be sized per the NEC®.
transformer nameplate ratings and classifications:
BIL - Phase - Frequency - Class - Temperature rise Capacity - Impedance - Phasor diagrams
How can the ground fault system be tested.
By pressing the test and reset button and a green light appears stating that the system is working properly.
What is the meaning of transformer nameplate ratings and classification Capacity?
Capacity - The capacity of a transformer to transfer energy is related to its ability to dissipate the heat produced in the windings. The capacity rating is the product of the rated voltage and the current that can be carried at that voltage without exceeding the temperature rise limitation.
What is an Air Circuit Breaker?
Circuit breakers in which the interruption occurs in air.
What is the meaning of transformer nameplate ratings and classification Class?
Class - Transformers are classified by the type of cooling they employ.
Class B insulation specifications
Class B or Class H insulation is used for the windings. Class B insulation may be op-erated safely at a hot-spot temperature of 130°C (266°F).
Class H insulation systems
Class H insulation may be operated safely at a hot-spot temperature of 180°C (356°F)
What's the conventional bus size in the US?
Conventional bus sizing in the United States is 0.25" x 2" through 0.375" x 7". Copper provides an ampacity of 1,000A/in2 of cross-sectional area. When using aluminum, the ampacity is 750A/in2.
Distribution Transformer
Distribution transformers are designed to handle larger cur-rents at lower voltage levels. Distribution trans-formers have smaller kVA ratings and are phys-ically much smaller than power transformers. Power transformers often have an auxiliary means of cooling, such as fans and radiators. Distribution transformers are usually self-cooled, using no fans or other cooling methods.
Wire harnessing is generally used within the switchboard with the following restrictions:
Each bundle or cable of wires must be run in a vertical or horizontal direction, securing the harness by means of plastic cable ties or cable clips. • Plastic wire cable clamps shall be placed at strategic locations along the harnessing to hold the harness firmly in place to prevent interfer-ence with the control components' required electrical, mechanical, and arcing clearances. • Wire ties must be applied to the harnessed wiring every 3" to 4" (75 to 100 mm) with self-adhesive cable ties spaced at every 12" (300 mm).
Service-entrance equipment:
Equipment lo-cated at the service entrance of a given build-ing that provides overcurrent protection to the feeder and service conductors and also provides a means of disconnecting the feeders from the energized service equipment.
What is the meaning of transformer nameplate ratings and classification Frequency?
Frequency - The frequency rating of a transformer is the normal operating system frequency. When a transformer is operated at a lower frequency, the reactance of the primary winding decreases. This causes a higher exciting current and an increase in flux density. In addition, there is an increase in core loss, which results in overall heating.
Bolted Pressure Switches
Frequently on service-entrance disconnects and feeders in switchgear. They are often used instead of circuit breakers because they are inexpensive and can handle a higher available fault current. Bolted pressure switches can be manually operated, shunt-trip operated, or motor operated. However, unlike a circuit breaker, they are tripped by an external signal such as a ground fault relay, phase failure relay, or a blown fuse detector.
Ground Fault
Ground faults exist when an unintended current path is established between an ungrounded con-ductor and ground on a solidly grounded service. These faults can occur due to deteriorated insulation, moisture, dirt, rodents, foreign objects (such as tools), and careless installation. When a ground fault occurs, this sensed current increases, eventually reaching the ground fault relay pickup point and causing the bolted pressure switch to trip.
Sealed dry transformers
Hermetically sealed dry transformers are con-structed in large sizes for voltages above 15kV. They are used for installations in buildings and other locations where oil-filled transformers would be a serious fire hazard, but they may also be used for lower voltages and kVA ratings and for water-submersible transformers in locations subject to floods. Nitrogen is typically used for the insulation and cooling of sealed dry trans-formers.
Phase Failure
If a phase failure relay is installed, it will cause a trip of the bolted pressure switch if a phase is lost. This could occur if a tree limb knocks a line down. Under this condition, the phase failure relay will sense the lost phase and trip the bolted pressure switch, preventing a single-phasing condition.
Blown Main Fuse Detect
If one of the in-line main fuses were to blow, the blown main fuse detector would detect it and cause a trip of the bolted pressure switch. The trip signal generated comes from a capacitor trip unit. This ensures that power is always available to trip the switch.
Switchgear handling -
Immediately upon receipt of switchgear, an inspection for damage during transit should be performed. If any damage is noted, the transportation company should be notified immediately.
What is the meaning of transformer nameplate ratings and classification Impendance?
Impedance - Impedance identifies the opposition of a transformer to the passage of short circuit current.
Switchgear storage
Indoor switchgear that is not being installed right away should be stored in a clean, dry location. The equip-ment should be level and protected from the environment if construction is proceeding.
Switchgear rigging
Instructions for switchgear should be found in the manufacturer's instruc-tion books and drawings. Verify that the rig-ging is suitable for the size and weight of the equipment.
Some precautions to be observed when wir-ing the switchboard electrical components are as follows:
Keep control wires at least 1⁄2" (13 mm) from moving parts. • Avoid running wires across sharp metal edges. To protect the wiring from mechanical dam-age, use approved cable protectors, such as a nylon clip cable guard, a wire guard for edge protection, or special edge protection molding. • Wires must not touch exposed bare electrical parts of opposite polarity. • Wires must not interfere with the adjustment or replacement of components. • Wires should be as straight and as short as pos-sible. • Wires shall not be spliced unless in a listed wireway. • To eliminate possible strain on the control wire, a certain amount of slack should be given to the individual or harnessed conductor ter-minated at a component connection. • The equipment ground busbar shall not be used as a portion of the control or metering circuits. • Do not use pliers for bending control wiring. Use your hands or an approved wire bending device.
What are the grounding requirements for low voltage and medium voltage equipment?
Low-voltage equipment with only a single source of supply usually does not require temporary grounding for safety. Low-voltage equipment with dual supply and medium-voltage equipment should be grounded at the bus.
What are low voltage power circuit breakers rated for?
Low-voltage power circuit breaker switchgear, for example, may be rated up to 1,000VAC or 3,200VDC.
Fused/Unfused Switch Characteristics
MVL switches are available in both fused and unfused models. If equipped with fuses, the entire MVL switch has the fault interrupting capacity of the fuse and therefore provides fault protection. Either current-limit-ing or boric acid fuses may be used in the MVL switch.
Dry Transformers (Air-Cooled)
Many transformers do not use an insulating liq-uid to immerse the core and windings. Dry or air-cooled transformers are used for many jobs where small, low-kVA transformers are required. Large distribution transformers are usually oil-filled for better cooling and insulating. However, for instal-lations in buildings and other locations where the oil in oil-filled transformers would be a seri-ous fire hazard, dry transformers are used.
What are metal-clad or metal enclosed switchgear rated for?
Metal-clad or metal-enclosed switchgear is applied at voltages over 1,000VAC up to a maximum of 35,000VAC.
Who Recommends best practices for Electrical Equipment Maintenance?
NFPA 70E®, "Standard for Electrical Safety in the Workplace" and NFPA 70B, "Recommended Practice for Electrical Equipment Maintenance" both require that temporary grounds be applied before the circuit or equipment is considered safe. In fact, standard practice in overhead line construction and within open substations is that any conductor without a temporary ground connection is considered energized.
What does NEC stands for?
National Electrical Code
Where are switchboards used?
One location for switchboards is typically where the main power enters the building. In this location, the switchboard is referred to as service-entrance equipment. The other location common for switchboards is downstream from the service-entrance equipment. In the downstream location, the switchboard is commonly referred to as distribution system equipment.
What is the meaning of transformer nameplate ratings and classification Phase?
Phase - The phase information indicates the number of phase windings contained in a transformer tank.
What is the meaning of transformer nameplate ratings and classification Phasor Diagrams?
Phasor diagrams - Phasor diagrams show phase and polarity relationships of the high and low windings. They can be used with the schematic connection diagram to provide test connection points and to provide proper external system connections.
Power transformer
Power transformers handle large amounts of power and step down from transmission voltages to distribution voltages. Power transformers are always freestanding.
What are the Circuit breaker ratings are given on the breaker's nameplate?
Rated voltage Rated current. Interrupting rating
Protective Grounding
Remember that even after a circuit has been isolated, de-energized, locked out, and verified without voltage, it still may not be safe to work on. This is because of the possibility that a circuit or conductor may be inadvertently re-energized.
Maintenance tasks for an MVL switch include the following:
Step 1 The MVL switch should be operated sev-eral times. Observe the mechanism and check for binding. Step 2 Inspect the interrupting and main blades every 100 operations for excessive wear or damage; replace as necessary. Also, in-spect the arc chutes for damage. Step 3 Clean the switch and its compartment thoroughly. Use a clean cloth and avoid solvents. Step 4 Lubricate the switch. The pivot points on the switch should be greased. The switch contacts should also be lubricated with a light film of grease after being cleaned. Step 5 Final maintenance checks include phase-to-ground and phase-to-phase megger testing. If the results are satisfactory, then a DC high-potential test is performed.
Ratings for switches include the following:
Switch kV Basic impulse insulation level (BIL) Frequency (Hertz) - Withstand (kV) - Capacitor switching (kVAR) - Fault close - Short time current - Continuous current (amps) Interrupting current (amps)
Distribution system equipment:
Switchboard equipment that is downstream from the service-entrance equipment.
Difference between Switchboard and Switchgear?
Switchgear has physical barriers between breakers, and between the breakers and the bus. Switchgear is more durable and fault resistant and is commonly selected for larger applications where low-voltage power circuit breakers and selective coordination are applied, such as computer data centers, manufacturing, and process facilities.
Switchgear
Switchgear is a general term used to describe switching and interrupting devices and assemblies of those devices containing control, metering, protective, and regulatory equipment, along with the associated interconnections and supporting structures.
How are switch gears classified as?
Switchgear is classified first by the type of construction and secondly by voltage rating.
Metal-enclosed switchgear:
Switchgear that is enclosed on all sides, with the exception of small ventilation and viewport openings.
What is the meaning of transformer nameplate ratings and classification Temperature rise?
Temperature rise - The temperature rise rating is the maximum elevation above ambient temperature that can be tolerated without causing insulation damage.
What does the NEC consider low voltage vs medium voltage and High voltage?
The NEC® considers low voltage as systems rated up to 1,000V, while medium voltage refers to systems rated above 1,000V and up to 35,000V (35kV). Above this volt-age level, cable is usually run on overhead power lines rather than in raceway or cable tray.
Continuous current (amps) -
The amount of cur-rent that the switch will carry continuously.
Short time current
The amount of current that the switch will carry for 10 seconds without sustaining any damage.
Switch kV
The design voltage for the switch. Of course, nominal system voltage is the nor-mal application method; thus, a 5kV switch may be used for nominal system voltages of 2.4kV or 4.16kV, etc.
Inverted switch
The inverted switch design has the terminals, jaws, and arc chutes located near the bottom of the cubicle. The hinge point is above the jaws and arc chutes. This type of switch is used primarily as a main switch to a lineup of other switches. Its handle operation is identical to that of an upright switch; to close the switch, the handle is moved up, and to open it, the handle is moved down.
Bus connections
The main bus that is usually removed during shipping should be recon-nected. Ensure that the contact surfaces are clean and pressure is applied in the correct manner. The conductivity of the joints is de-pendent on the applied pressure at the contact points. The manufacturer's torque instructions should be referenced.
Basic impulse insulation level (BIL):
The maxi-mum impulse voltage the winding insulation can withstand without failure.
Withstand (kV) -
The maximum 60Hz voltage that can be applied to the switch for one min-ute without causing insulation failure.
Interrupting current (amps)
The maximum amount of current that the switch will safely interrupt.
Capacitor switching (kVAR) -
The maximum capacitance expressed in kVAR that can be switched with the MVL.
Basic Impulse Insulation Level (BIL)
The maximum voltage pulse (in kilovolts) that the equipment will withstand.
Fault close -
The maximum, fully offset fault current that the switch can be closed into with-out sustaining damage. The term fully offset means that the fault current will have a delay-ing DC component in addition to the AC com-ponent.
Capacity:
The rated load-carrying ability, ex-pressed in kilovolt-amperes or kilowatts, of gen-erating equipment or other electric apparatus.
What are the station-type cubicle switchgear consists of?
The station-type cubicle switchgear consists of indoor and outdoor types with power circuit breakers rated from 14.4kV to 34.5kV, 1,200A to 5,000A, and 1,500kVA with 2,500kVA interrupting capacity.
Medium-voltage limiting switch.
The switches are single-throw devices designed for use on 1,001V to 35kV systems. MVL switches may provide both switching and overcurrent protection. MVL switches are commonly used as a service disconnect in unit substations and for sectionalizing medium-volt-age feeder systems.`
Upright Switch
The upright switch design is the most common type. The upright construction locates the service entry, jaws, and arc chutes near the top of the cubicle. The hinge point is below the jaws and arc chutes.
What are the residential electrical inputs?
This may range from the common 200A, 120/240V residential service to thousands of amps at voltages from 480V/277V to 69kV in an industrial facility.
Motor-operated Switch Characteristics
This type of switch is most commonly used as the major component in an automatic transfer scheme. It can also be used when open and close functions are to be initi-ated from remote locations.
Transformers are used to
Transformers are used to step voltage up and down in the power transmission and distribution system. A transformer is an electrical device that uses the process of electromagnetic induction to change the levels of voltage and current in an AC circuit without changing the frequency and with very little loss of power.
Distribution transformers:
Transformers that are used for transferring electric energy from a primary distribution circuit to a secondary dis-tribution circuit. Distribution transformers are usually rated between 5kVA and 500kVA.
What are the six main types of switches suitable for unique applications?
Upright - Inverted - Fused/unfused - Duplex - Selector - Motor-operated -
Cable connections
When making cable con-nections, verify the phasing of each cable. This procedure is done in accordance with the con-nection diagrams and the cable tags. When forming and mounting cables, ensure that the cables are tightened per the manufacturer's instructions.
What does DLRO stands for?
digital low-resis-tance ohmmeter
Transformers can be divided into two main categories
power transformers and distribution transformers.
The four general classifications of circuit breakers are?
• Air circuit breakers • Oil circuit breakers • Vacuum circuit breakers • Gas circuit breakers
This principle is known as Lenz's law:
• An induced EMF always has such a direction as to oppose the action that produced it. • For this reason, the EMF induced is also known as a counter-electromotive force (CEMF).
What is the meaning of transformer nameplate ratings and classification BIL?
• BIL - This identifies the maximum impulse voltage the winding insulation can withstand without failure.
Typical switchboard components includes?
• Circuit breakers • Fuses • Motor starters • Ground fault systems • Instrument transformers • Switchboard metering • Control power transformers • Busbars
Possibility that a circuit or conductor may be inadvertently re-energized for any one of the following reasons:
• Induced voltages from other energized conduc-tors • Static buildup from wind on outdoor conduc-tors • High voltage from lightning strikes • Any condition that might bring an energized conductor into contact with the de-energized circuit • Switching errors causing re-energizing of the circuit • Capacitive charges in equipment or conductors
Interrupting Rating
• Interrupting rating - This is the maximum value of current at rated voltage that the circuit breaker is required to successfully interrupt for a limited number of operations under specified conditions. The term is usually applied to ab-normal or emergency conditions.
Switchgear can be classified as follows:
• Metal-enclosed switchgear (low voltage) • Metal-clad switchgear (low and medium voltage) • Metal-enclosed interrupters • Unit substations
Transformer wind-ings are rated as follows:
• Phase-to-phase and phase-to-neutral for wye windings, such as 480Y/277VAC • Phase-to-phase for delta windings, such as 480VAC • Dual-voltage windings, such as 480VAC x 240VAC
What are the two basic functions that a switchgear perform?
• Provides a means of switching or disconnect-ing power system apparatus • Provides power system protection by automati-cally isolating faulty components
Rated current
• Rated current - This is the continuous current that the circuit breaker can carry without exceeding a standard temperature rise (usually 55°C or 131°F).
Rated voltage
• Rated voltage - The rated voltage is the maxi-mum voltage for which the circuit breaker is designed.
When inspecting the inside of a dry-type, air-cooled transformer case, look for the following:
• Temporary shipping supports or guards • Bent, broken, or loose parts • Debris on the floor or in the coils • Corrosion of any part • Worn or frayed insulation • Shifted core members • Damaged tap changer mounts or mechanisms • Misaligned core spacers and loose coil elements • Broken or loose blocking
The most common types of faults in power sys-tems are as follows:
• Three-phase (balanced) • Phase-to-phase • Phase-to-phase-to-ground • Phase-to-ground
