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A padmount or pad-mounted transformer is a ground-mounted electric power
distribution transformer
Single-phase distribution transformer in Canada
A distribution transformer or service transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the di ...
in a locked steel cabinet mounted on a concrete pad. Since all energized connection points are securely enclosed in a grounded metal housing, a padmount transformer can be installed in places that do not have room for a fenced enclosure. Padmount transformers are used with underground
electric power distribution
Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmissi ...
lines at
service drop
In electric power distribution, a service drop is an overhead electrical line running from a utility pole, to a customer's building or other premises. It is the point where electric utilities provide power to their customers.Carson Dunlop "El ...
s to step down the primary
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
on the line to the lower secondary voltage supplied to utility customers. A single transformer may serve one large building or many homes.
Pad-mounted transformers are made in power ratings from around 15 to around 5000
kVA and often include built-in fuses and switches. Primary power cables may be connected with elbow connectors, which can be operated when energized using a
hot stick
In the electric power distribution industry, a hot stick is an insulated pole, usually made of fiberglass, used by electric utility workers when engaged on live-line working on energized high-voltage electric power lines, to protect them from ...
and allows for flexibility in repair and maintenance.
Design
Pad-mount transformers are available in various electrical and mechanical configurations. Pad-mount transformers operate on medium-voltage distribution systems, up to about 35 kV. The low-voltage winding matches the customer requirement and may be single-phase or three-phase.
Pad-mount transformers are (nearly always)
oil-filled units and so must be mounted outdoors only. The core and coils are enclosed in a steel oil-filled tank, with terminals for the transformer accessible in an adjacent lockable wiring cabinet. The wiring cabinet has high and low-voltage wiring compartments. High and low-voltage underground cables from below enter the terminal compartments directly. The top of the tank has a cover secured with carriage bolt-nut assemblies. The wiring cabinet has
sidewall
A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide traction on the surface over which ...
s on two ends with doors that open sideways to expose the high and low voltage wiring compartments.
Pad-mount transformers have self-protecting
fuses
Fuse or FUSE may refer to:
Devices
* Fuse (electrical), a device used in electrical systems to protect against excessive current
** Fuse (automotive), a class of fuses for vehicles
* Fuse (hydraulic), a device used in hydraulic systems to protec ...
consisting of a
bayonet mount
A bayonet mount (mainly as a method of mechanical attachment, such as fitting a lens to a camera using a matching lens mount) or bayonet connector (for electrical use) is a fastening mechanism consisting of a cylindrical male side with one ...
fuse placed in a high voltage compartment, with a backup high energy
current limiting Current limiting is the practice of imposing a limit on the current that may be delivered to a load to protect the circuit generating or transmitting the current from harmful effects due to a short-circuit or overload. The term "current limiting" i ...
fuse in series to protect against secondary faults and transformer overload. The bayonet mount fuse protects against secondary faults and transformer overload and is a
field replaceable device. The backup current-limiting fuse operates only during transformer failure; therefore, it is not field replaceable. These transformers also serve the conventional low voltage fusing requirements.
The use of a polymeric cable and load break elbows enable switching and isolation to be carried out in the HV chamber in what is known as a "''dead front''" environment, i.e., all terminations are fully screened and watertight.
Single- and three-phase pad-mounted transformers are used in underground industrial and residential
power distribution
Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmissi ...
systems, where there is a need for safe, reliable, and aesthetically appealing transformer design. Their enclosed construction allows the installation of pad-mount transformers in public areas without protective fencing. Pad-mount transformers are usually located on the street
easement
An easement is a nonpossessory right to use and/or enter onto the real property of another without possessing it. It is "best typified in the right of way which one landowner, A, may enjoy over the land of another, B". An easement is a propert ...
s and supply multiple households in residential areas.
While most traditional pad-mount transformers are fixed on a concrete 'pad,' today, small single-phase designs are also available with the transformer already mounted on a 'polypad' base to be mounted on hard ground, connected, and switched on.
Standards
American National Standards Institute /
Institute of Electrical and Electronics Engineers (ANSI/IEEE)
* ANSI C57.12.00 – Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
* ANSI C57.12.22 – Standard for Transformers - Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers with High-Voltage Bushings, 2500 kVA and Smaller: High Voltage, 34,500Grd/19,920 Volts, and Below; Low-Voltage, 480 Volts, and Below - Requirements
* ANSI C57.12.26 (Withdrawn) – Standard for Transformers-Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers for Use with Separable Insulated High-Voltage Connectors, 34,500 Grd/19,920 Volts and Below; 2500 kVA and Smaller
* ANSI C57.12.28 – Standard for Switchgear and Transformers, Pad-Mounted Equipment – Enclosure Integrity
National Electrical Manufacturers Association
The National Electrical Manufacturers Association (NEMA) is the largest trade association of electrical equipment manufacturers in the United States. Founded in 1926, it advocates for the industry, and publishes standards for electrical product ...
(NEMA) Standards
* NEMA TR 1-1993 (R2000) – Transformers, Regulators and Reactors, Table 0-2 Audible Sound Levels for Liquid-Immersed Power Transformers.
* NEMA 260-1996 (2004) – Safety Labels for Pad-Mounted Switchgear and Transformers Sited in Public Areas
References
{{Electric transformers
Electric transformers