TRANSFORMER DICTIONARYAUTOTRANSFORMER – A transformer that has only one winding per phase; part of the winding is shared
between the primary and secondary
CONDITION
New - All materials and components are new.
Remanufactured - Transformer coils have been rewound; this may include all primary and secondary coils or
primary coils only; even if secondary coils are not rewound they will carry the same warranty as the rewoundprimary coils
Reconditioned - Transformer has passed all standard tests; if needed, broken or missing parts are replaced
Rental - Transformer condition can vary but all rental units will be in good, working condition
CONVENTIONAL VS CSP (POLE MOUNT)Conventional - A pole mounted transformers with two primary bushings is sometimes referred to as "conventional".
They can be used individually to provide single-phase power or they can be banked with two like transformers toprovide three-phase power.
Conventional - Pole mounted transformers with one primary bushing are utility type transformers and are the typemost commonly used in residential applications. The primary bushing is labeled "H1" and a grounding nut on thetank is labeled "H2". This transformer is connected between a phase and neutral. Some of these transformers are
CSP (completely self-protected). They usually include a primary fuse, a primary arrester, and a secondary breakerwith weak-link.
DEAD FRONT – Dead front refers to terminations where energized parts are enclosed in molded rubber. A typicaldead front connection consists of three parts - a bushing well (installed at the factory), a bushing insert, and an
elbow. Incoming (and outgoing) cables are inserted into the elbow to prevent any exposed wire or terminals. See ourTransformer Components page for more information.
DELTA – A Delta connection has the windings of three single-phase transformers connected in series with eachother to form a closed circuit; on a Delta connection of a three-phase transformer, the transformer will typically havethree (3) terminations or bushings.

This document is intended as a general informational guide only. Before performing any work or purchasing any equipment, you should
consult your electrician or electrical engineer. Alfa Transformer is not responsible for misinterpretation or misuse of this information.

6107 South Zero Street • Fort Smith AR 72903 • Phone 479-646-1668 • Fax 479-646-4101
www.alfatransformer.com • E-Mail: sales@alfatransformer.com
9/21/2023

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DRAIN VALVE – A drain valve provides a means of removing fluid from a transformer. Many drain valves also
have a port where samples of the fluid can be taken and sent to a laboratory for analysis.
DRIVE ISOLATION TRANSFORMER – SCR drives generate voltage spikes and transient feedback. Drive
isolation transformers are designed to withstand mechanical forces associated with SCR drives and to isolate the linefrom these destructive electrical conditions. Drive isolation transformers have different ratings than general purposetransformers. They are sized based on drive horsepower. For example, a 550 KVA drive isolation transformer
would be used to provide power to a 500 HP drive. The voltages on a drive isolation transformer are also different
than those on general purpose transformers. Standard voltages for general purpose transformers are 208V, 240V,
and 480V. Standard drive isolation transformer voltages are 230V and 460V.
DRY TYPE TRANSFORMER – A transformer that is cooled solely by air and does not contain any cooling
fluid for cooling or insulation.
ELECTROSTATIC SHIELD – An electrostatic shield suppresses common mode noise. The grounded shieldprovides a low resistance path to ground by the effect of capacitive coupling which prevents high frequency signals
present in the source voltage from reaching the secondary windings of the transformer and subsequently theconnected load.
ENCLOSURE – There are many types of enclosures for dry type transformers. These enclosures are designed toprovide a degree of protection to personnel against access to hazardous parts and to provide a degree of protection to
the transformer from contaminants such as moisture, dust, metallic dust, and chemicals. These enclosures are
typically specified by a NEMA rating. The most common dry type transformer enclosures are listed in the tablebelow.
NEMA RATINGDESCRIPTION

1

For indoor use wherever oil, dust or water is not a problem

2

Used indoors to exclude falling moisture and dirt

3R

Provides protection against rain, sleet and snow

4

Needed when subject to great amount of water from any angle,
such as areas which are repeatedly hosed down

4X

Same as NEMA 4, except stainless steel

12

Used for excluding oil, coolant, flying dust, lint, etc.

ENVIRONMENT – Certain environments require certain types of transformers or specific materials. Using thewrong type of enclosure or protective materials can result in an unwanted transformer failure or explosion.
This document is intended as a general informational guide only. Before performing any work or purchasing any equipment, you should
consult your electrician or electrical engineer. Alfa Transformer is not responsible for misinterpretation or misuse of this information.

6107 South Zero Street • Fort Smith AR 72903 • Phone 479-646-1668 • Fax 479-646-4101
www.alfatransformer.com • E-Mail: sales@alfatransformer.com
9/21/2023

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DRY TYPE – Ventilated dry type transformers operate best in an indoor environment with no airborne
contaminants such as dust, metal dust, corrosive chemicals, salt water, etc. Dry type transformer enclosures can
include weathershielding to protect from typical weather conditions such as rain so they can be placed outdoors.
OIL FILLED – Oil filled transformers can operate in typical indoor or outdoor environments. Coastal areas withhigh moisture and salt content can quickly destroy the exterior of a transformer. Corrosive chemicals can also
destroy paint and metal. The presence of explosive gases can be very dangerous around transformers.
FANS – Fans are used to reduce the operating temperature of a transformer.
Dry Type Transformers – Fans can be optionally installed on many dry type. Generally, fans are not installed on
small transformers because it is cost-prohibitive. Cooling fan packages can include fans, temperature gauge/fancontroller, or other options.
Pad Mounted Transformers – Cooling fans are not available on pad mounted transformers. Fans negate thetamper-resistant rating of the transformer. If you require fans, please let us know so we can make a
recommendation.
Substation Transformers – Fans can be optionally installed on many substation transformers as a means of
reducing the operating temperature of the transformer. Cooling fan packages can include fans, temperature gauge,
auxiliary wiring compartment, mounting hardware, or other options.
Pole Mounted Transformers – Cooling fans are not available on pole mounted transformers.
FEED-THRU INSERTS – A feed-thru insert is a simple way to convert a radial feed primary configuration to aloop feed primary configuration. The 200-amp loadbreak insert is unscrewed from the bushing well and replacedwith the feed-thru insert. The feed-thru inserts provide two connection points - one for the incoming primary cables
and one for the outgoing primary cables that will loop to another transformer.
FLUID – There are basically two types of dielectric fluids used in transformers for electrical insulating and cooling
purposes. Mineral oil and fluids that are less flammable for use indoors or near buildings where fire could spread.
Below is information on the most common fluids.
Mineral Oil – This is a highly refined mineral oil that remains stable at high temperatures and has excellent
electrical insulating properties.
RTEmp (discontinued) – RTEmp fluid was discontinued by Cooper Power Systems in 2005. This high flash point
fluid was replaced by FR3.
Ester-Based Fluids – Ester-based fluids are a food-grade oil extracted from plants. It has been declared non-toxic
and has the highest flash and fire points of all transformer fluids. Ester-based fluids are miscible with mineral oil and
RTEmp so you can fill your existing transformers with Ester-based fluids. It is not miscible with silicone fluid.
Some product names are FR3, BIOTEMP, and VG100.
Silicone – Silicone is becoming more rare and it is not miscible with other fluids. Silicone is significantly more
expensive than Ester-based fluids.

This document is intended as a general informational guide only. Before performing any work or purchasing any equipment, you should
consult your electrician or electrical engineer. Alfa Transformer is not responsible for misinterpretation or misuse of this information.

6107 South Zero Street • Fort Smith AR 72903 • Phone 479-646-1668 • Fax 479-646-4101
www.alfatransformer.com • E-Mail: sales@alfatransformer.com
9/21/2023

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FREQUENCY – The number of times an alternating current voltage will alternate from positive to negative andback again within a specified period of time. It is typically expressed in cycles per second and identified as Hertz(Hz). The Western Hemisphere primarily operates on 60 Hz systems. Some 60 Hz systems exist in the Eastern
Hemisphere but this region primarily uses 50 Hz systems. A transformer designed for 50 Hz can operate on a 60 Hz
supply. A transformer designed for 60 Hz should not be connected to a 50 Hz supply.
FUSING – The most common type of fusing for pad mounted transformers is Bayonet fuses. These fuses are
externally removable and the fuse cartridges are replaceable. An optional ELSP backup current-limiting fuse isavailable to minimize the effects of high fault current stresses on equipment and the distribution system. Many othertypes of fusing that are installed in transformers have been discontinued or can be costly compared to Bayonet
fusing.
GAUGES – See Liquid Level Gauge, Pressure/Vacuum Gauge, and Temperature Gauge
KVA – Kilo-Volt-Amperes; designates the output which a transformer can deliver at rated voltage and frequency
without exceeding a specified temperature rise.
LIGHTNING ARRESTERS – See Surge Arresters
LIQUID LEVEL GAUGE – A liquid level gauge is used to determine if the fluid level inside the transformer iswithin an acceptable range. There are typically three levels shown on the gauge: Low, 25C, and High. The optimum
fluid level is 25C. However, this can vary based on the temperature of the fluid.
The arm that connects the indicator needle and the float is very short so small changes in fluid level can result in low
or high readings. If you believe your transformer may be low on fluid, you should have qualified personnel checkthe actual fluid level. Low fluid levels can result in transformer failure and/or fire.
A special liquid level gauge with contacts can be installed to send signals to relays or alarms for monitoring
purposes.
LIVE FRONT – Live front refers to terminations with exposed "live" parts. On pad mounted transformers, thesebushings are typically porcelain with an eyebolt terminal or spade terminal. Incoming primary cables are strippedand inserted into the eyebolt terminal or lugs.
LOADBREAK ELBOWS – A 200-amp loadbreak elbow is a dead front component that connects to a primarycable and plugs into a 200-amp loadbreak insert. This completes the connection between the incoming cables andthe transformer. Special 600-amp elbows are required to connect to 600-amp one-piece integral bushings used forhigher current applications. Three important pieces of information are required to properly size the elbow for an
application:
1. Insulation thickness – usually expressed in a percentage (100%, 133%, 150%, etc.)
2. Cable size (and quantity) – #4, 2/0, 500MCM, etc.
This document is intended as a general informational guide only. Before performing any work or purchasing any equipment, you should
consult your electrician or electrical engineer. Alfa Transformer is not responsible for misinterpretation or misuse of this information.

6107 South Zero Street • Fort Smith AR 72903 • Phone 479-646-1668 • Fax 479-646-4101
www.alfatransformer.com • E-Mail: sales@alfatransformer.com
9/21/2023

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Page 5 of 9

Concentric neutral or shielded?

LOADBREAK INSERTS – A typical dead front connection consists of three parts - a bushing well (installed at
the factory), a loadbreak bushing insert, and a loadbreak elbow. The insert is cone-shaped with a threaded-hole in
one end that screws into the bushing well. The other end has a smooth hole that will accept the probe of a 200-amp
loadbreak elbow. Please note that 600-amp dead front bushings are one piece and have a threaded hole in the front toaccept a 600-amp elbow that has a threaded stud rather than a smooth probe. 200-amp components and 600-amp
components are not compatible.
LOADBREAK SWITCH – Loadbreak switches are installed in pad mounted transformers to allow power to thetransformer to be shut off or to route the incoming power to other places.
Two-Position – This is the most common loadbreak switch. The two positions are "Closed" (on) and "Open" (off).
Multiple two-position loadbreak switches can be used in place of a single four-position switch. (see below)
Four-Position – This is sometimes called a "sectionalizing switch". This type of switch is installed on transformers
with a loop feed configuration. The four positions are:
1. Open – Transformer and loop are off;
2. A – Transformer is on, loop is off;
3. B – Transformer is off, loop is on;
4. A & B – Transformer and loop are on;
Two (2) Two-Position – This is an alternative to a four-position switch. This is installed on transformers with a loop
feed configuration. One switch is connected between the incoming power and the transformer. The other switch isconnected between the incoming power and the loop feed bushings.
LOOP FEED – Loop feed indicates the incoming primary of a transformer can be fed through (looped) to anothertransformer of the same voltage and vector. Loop feed transformers have two sets of primary bushings - one set for
the incoming feed and one for the outgoing feed. In some installations, the second set of bushings is not used forlooping to another transformer but for dead front surge arresters.
PAD MOUNTED TRANSFORMER – A transformer with an enclosed cabinet that is installed on a pad.
Conduit openings in the bottom of the cabinet allow incoming and outgoing cables to be brought into the cabinet
from underground; the compartment is accessible through one or two locked doors that give the transformer atamper-resistant rating. Almost all pad mounted transformers are oil filled type. They are typically finished with agreen color to blend in with the surroundings.

This document is intended as a general informational guide only. Before performing any work or purchasing any equipment, you should
consult your electrician or electrical engineer. Alfa Transformer is not responsible for misinterpretation or misuse of this information.

6107 South Zero Street • Fort Smith AR 72903 • Phone 479-646-1668 • Fax 479-646-4101
www.alfatransformer.com • E-Mail: sales@alfatransformer.com
9/21/2023

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PHASE – Classification of an alternating current circuit. The most common phases are single-phase and threephase. Typical transformers do not have mixed phases. If a transformer has a three-phase input, it will have a threephase output. The same is true of single-phase transformers. It is common to connect a single-phase transformer to athree-phase system to get a single-phase output.
PHASE CONVERTER – In many areas, three-phase power is not available or can be very expensive to obtain. A
phase converter can take available single-phase power and convert it to three-phase power.
POLE MOUNTED TRANSFORMER – A transformer that is mounted onto a pole with a hanger; occasionallythese transformers may be mounted on a platform between two poles or they may be installed on a concrete pad.
One-Bushing – Pole mounted transformers with one primary bushing are utility type transformers and are the typemost commonly used in residential applications. The primary bushing is labeled "H1" and a grounding nut on thetank is labeled "H2". This transformer is connected between a phase and neutral. Some of these transformers are
CSP (completely self-protected). They usually include a primary fuse, a primary arrester, and a secondary breakerwith weak-link.
Two-Bushing – Pole mounted transformers with two primary bushings are sometimes referred to as "conventional".
They can be used individually to provide single-phase power or they can be banked with two like transformers toprovide three-phase power.
PRESSURE RELIEF DEVICE (COVER-MOUNTED) – A cover-mounted device that activates when aspecified tank pressure is reached. Relieves pressure much more rapidly than a pressure relief valve. Some covermounted pressure relief devices have an optional semaphore to help you identify when the device has been
activated.
PRESSURE RELIEF VALVE – A device that provides pressure relief on transformers during overpressure
conditions, automatically resealing once pressure has fallen. A pull-ring allows the device to be operated manually.

This document is intended as a general informational guide only. Before performing any work or purchasing any equipment, you should
consult your electrician or electrical engineer. Alfa Transformer is not responsible for misinterpretation or misuse of this information.

6107 South Zero Street • Fort Smith AR 72903 • Phone 479-646-1668 • Fax 479-646-4101
www.alfatransformer.com • E-Mail: sales@alfatransformer.com
9/21/2023

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PRESSURE/VACUUM GAUGE – A pressure/vacuum gauge indicates the level of vacuum or pressure (in psi)
inside a transformer tank. On colder days or on a de-energized transformer, the gauge may indicate a vacuum. On
hotter days or when a transformer has a load on it, the gauge will likely show a positive pressure reading.
What you don't want to see is a reading of zero. This could indicate that the transformer is “breathing” due to a leak.
If you see a reading of zero on the pressure/vacuum gauge, check it under different temperature and/or loading
conditions to see if it changes or remains at zero.
A special pressure/vacuum gauge with contacts can be installed to send signals to relays or alarms for monitoring
purposes.
PRIMARY CURRENT – The amperage capacity of the primary winding of a transformer
PRIMARY VOLTAGE – The INPUT voltage of a transformer
PROTECTIVE CAPS – If you have a transformer with a dead front, loop feed primary configuration but you
don't want to loop to another transformer, you can install protective caps to insulate the set of bushings used forlooping. These caps plug into a 200-amp loadbreak insert.
RADIAL FEED – Radial feed means the incoming primary cables are connected to the transformer's primarybushings with no outgoing feed.
SECONDARY CURRENT – The amperage capacity of the secondary winding of a transformer
SECONDARY VOLTAGE – The OUTPUT voltage of a transformer
SUBSTATION TRANSFORMER – A transformer that is installed on a pad and may have exposed bushings orbushings that are enclosed in throats or air terminal chambers; substation transformers do NOT have a tamperresistant rating.
COVER – Utility type station transformers typically have cover-mounted bushings. They are exposed bushings on
the cover for easy connection to overhead power lines.
SIDE – Side-mounted bushings are typically connected to other equipment like switchgear. They are normallyenclosed in some type of box. The types of enclosing boxes are listed below.
FLANGE – A flange is a shallow frame that surrounds the bushings and can be bolted to an enclosing box.
THROAT – A throat is a deeper frame that may or may not fully cover the bushings. It can be bolted or welded tothe transformer. A throat can also be bolted to a box or enclosure of another piece of equipment like bus duct orswitchgear.
AIR TERMINAL CHAMBER – Air terminal chambers are boxes that connect to a flange or throat. Some air
terminal chambers are partial-height (they don't extend to the pad or floor) and some are full-height (they extend to
the pad or floor.
This document is intended as a general informational guide only. Before performing any work or purchasing any equipment, you should
consult your electrician or electrical engineer. Alfa Transformer is not responsible for misinterpretation or misuse of this information.

6107 South Zero Street • Fort Smith AR 72903 • Phone 479-646-1668 • Fax 479-646-4101
www.alfatransformer.com • E-Mail: sales@alfatransformer.com
9/21/2023

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SURGE ARRESTERS – Surge arresters are devices that help reduce the risk of damage due to lightning or othersurges. These arresters provide a low-resistance path to ground to route over voltages away from your transformer.
There are many types of surge arresters and specifying the correct one is important. Consult your engineer orelectrical contractor if you are unsure of how to choose the correct type and rating of arrester.
TAPS – Taps are used to adjust voltage. They are normally located on the high voltage windings. By changing taps,
you can compensate for an incoming voltage that is too low or too high. Taps are typically rated in a percentage ofthe nominal tap (the rated voltage of the transformer) or by the actual voltage rating of each tap. Taps should bechanged when the transformer is in a de-energized state only. One of the more common tap arrangements fordry type transformers is "2(+) 2(-) at 2.5%". This may be written several ways including "2FCAN, 2FCBN" whichmeans 2 Full Capacity Taps Above Nominal and 2 Full Capacity Taps Below Nominal. On dry type transformers,
taps are typically adjusted by connecting a jumper wire between two terminals on the high voltage winding.
TEMPERATURE GAUGE – A temperature gauge indicates the temperature of the cooling fluid. It may include amanually-resettable maximum temperature indicator (red needle) as shown in the photograph.
Most oil filled transformers are designed to operate at 65°C above an average ambient temperature of 30°C with amaximum ambient temperature of 40°C. Excessive heating can cause insulation breakdown and failure.
A special temperature gauge with contacts can be installed to send signals to relays or alarms for monitoring
purposes.
TEMPERATURE RISE – Temperature rise is the increase in temperature over the ambient temperature due toloading. These ratings are typically based on an average ambient temperature of 30°C (86°F) over any 24-hour
period with a maximum ambient temperature of 40°C (104°F). Transformers with lower temperature rise ratings
operate at a lower temperature than an otherwise equal, higher temperature rise transformer. However, a lower
temperature rise rating is essentially a lower KVA rating. For example, a dry type transformer that is rated 2000
KVA at 150°C is rated for only 1500 KVA at 80°C.
DRY TYPE – The standard temperature rise for dry type transformers is 150°C. Optional temperature rises are
115°C and 80°C.
OIL FILLED – The standard temperature rise for oil filled transformers is 65°C. Optional temperature rise is 55°C.
TRANSFORMER – A device that transforms electrical power using electromagnetic coils and core; the voltage
and current are generally changed during this transformation.
VECTOR
Vector – Three phase transformers should have a vector (connection type) indicated along with their voltage.

This document is intended as a general informational guide only. Before performing any work or purchasing any equipment, you should
consult your electrician or electrical engineer. Alfa Transformer is not responsible for misinterpretation or misuse of this information.

6107 South Zero Street • Fort Smith AR 72903 • Phone 479-646-1668 • Fax 479-646-4101
www.alfatransformer.com • E-Mail: sales@alfatransformer.com
9/21/2023

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Delta – A three-phase connection in which each phase winding is connected in series to form a closed loop;
on a Delta connection of a three-phase transformer, the transformer will have three (3) terminations or
bushings; commonly denoted using the Greek Delta symbol – Δ
2. Wye – A three-phase connection in which similar ends of each phase winding are connected together at acommon point which forms the electrical neutral and is often grounded; on a Wye connection of a threephase transformer, the transformer will typically have four (4) terminations or bushings
Single-Phase Vector – Single-phase transformers may also have a Wye vector designation to indicate how they will
be connected in a three-phase bank, i.e., "7200/12470Y-277/480Y". In this example, the primary may be connected
as 7200 Delta or 12470Y/7200. If the voltage is written as "12470-277/480Y" is assumed to mean the primary will
be connected as 12470 Delta and the secondary will be connected as 480Y/277.
1.

VOLTAGE
Primary Voltage – The INPUT voltage of a transformer
Secondary Voltage – The OUTPUT voltage of a transformer
WINDING MATERIAL – The primary and secondary windings of the transformer can either be copper oraluminum, or one can be copper and the other aluminum. The industry standard, since the early 1970's, has beenaluminum. You can find more information on copper and aluminum on our web site using the Downloads link on
our home page.
WYE – A three-phase connection in which similar ends of each phase winding are connected together at a common
point which forms the electrical neutral and is often grounded; on a Wye connection of a three-phase transformer,
the transformer will typically have four (4) terminations or bushings.

This document is intended as a general informational guide only. Before performing any work or purchasing any equipment, you should
consult your electrician or electrical engineer. Alfa Transformer is not responsible for misinterpretation or misuse of this information.

6107 South Zero Street • Fort Smith AR 72903 • Phone 479-646-1668 • Fax 479-646-4101
www.alfatransformer.com • E-Mail: sales@alfatransformer.com
9/21/2023

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