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Instrument Transformers

Metering Current Transformers | IEE C57.13 IEC 60044 Class CTs
Large Frame Current Transformers | Rectangle CTs | Split-Core Current Transformers
Relay Class Protection Current Transformers | Three Phase Current Transformers
Miniature Current Transformers | PC mount 50 to 400Hz Current Transformers
PC mount 2OkHz to 2OOkHz Current | Switchgear Style CTs | Bushing CTs | Generator Style CTs
Medium Voltage Instrument Transformers | Oil insulated CTs & PTs | Potential Transformers
Medium Voltage Potential Transformers | Rogowski Sensors / CTs

Instrument transformers comprise a large category of current and potential transformers for various voltage, frequency and physical size ranges. We have broken them up into several different groupings: low voltage, which are system voltages under 15kV; high frequency, operating frequency over 1kHz; and size ranges from board mount parts up to current transformers with window sizes of 254mm by 610mm. Read through the different types we supply below and use our Instrument Transformer RFQ Form to request a specific part to meet your requirements.

Metering Current Transformers

Hill Technical offers commercial grade low cost current transformers with center window type (donut style) constructions. CTs are available in many different sizes with capabilities for measuring 50-400HZ currents of 5A to 15000A with secondaries of 0.1A, 1A and 5A (special secondary currents are available). Burden: B 0.1 through 1.8 (2.5VA to 50 VA) with Accuracy class: 0.2 to class 5.0 as per IEC 185 or class 0.3, 0.6 or 1.2 as per ANSI C 57.13. Inside diameters of up to 8.00". Many models are available as U.L. recognized devices. They can be specified with or without mounting feet and the secondary can be leads, terminals or per customer requirement.

Applications include:

  • UPS systems
  • Transfer switches
  • Motor-generator sets
  • Commercial sub-metering,
  • 3 CT 's in one package for 3-phase metering
  • Accurate measuring for metering/WATT/VAR
  • Current sensing, recording, monitoring & control
  • Control panels and drives
  • Standard CT used as measuring standard for comparison
  • Winding temperature indicator (WTI) for power transformers
  • Summation current transformers.

IEE C57.13 IEC 60044 Class CTs
IEE C57.13 IEC 60044 Class CTs

These current transformers are designed to meet the requirement of ANSI IEEE C57.13 standard or IEC 60044 standard. CTs are typically used for metering and relaying applications such as switchgear, MCCs, switchboards, etc. Single ratio or multi ratio models are available with various secondary termination options (leads, terminals etc.). These CTs come with wide primary current range. Usually secondary current is 5 Amp or 1 Amp.

Large Frame Current Transformers

For measuring 50-400HZ currents in bus bar and other large conductor systems. Typical configuration is 400A to 12000A primary current with secondary of 1A or 5A (special secondary currents are also available).

Inside areas as small as 3.00" X 7.00" and as large as 7.00" X 27.00" and 10.00" X 24.00". All models are available with optional mounting plates for "bulk-head" mounting. Some models are U.L. recognized devices.


Rectangle CTs

Hill Technical has an extensive range of rectangle current transformers which are typically used for metering and relaying applications. Rectangle window of the CTs can fit around primary bus bar or cables in applications where physical size and placement are very important. The secondary can be lead wires or terminals

Split-Core Current Transformers

This type of current transformer is available to measure AC currents from 100A to 600A, at 50 to 400HZ. They are very popular in sub-metering applications where existing systems are being upgraded and it is impractical to isolate the primary conductor. It is even possible to install this type of transformer while the conductor is energized; however it is paramount that certain safety precautions be followed under such conditions. Rectangular in shape, standard split-core models are available with window dimensions up to 4.00" X 7.50". Even larger, custom designed sizes are available by special order. Secondary ratings of 5A, 1A, and 100ma are all common in split-core current transformers.

Two model groups are available, SP and SPS. The former is provided with a stainless steel screw-clamp band securing the two core halves; the latter has a UV resistant nylon band. All ratios are available in either type. Electrical and magnetic performance is identical for the two groups.


Relay Class Protection Current Transformers
Trans Resinmolded Ct

This type of CT includes oil-immersed bussing and Resin molded versions. Primary current range from 5 Amp to 5000 Amp with secondary current 5A, 1A , or 01.A. Typical Burden B 0.1 through B 4.0 (2.5VA to 50 VA & more) and Accuracy Class As per ANSI C 57.13 and IEC 185.

Applications include:

  • Protection relays/Relay panels
  • Earth fault protection
  • Bussing type, oil-immersed CT in power transformer
  • Control panes and switch boards
  • Air/Gas circuit breakers
  • Motor control cubicles
  • Power control centers
  • Bus bar protection systems
  • Differential protection systems
Trans Bushing Ct

Three Phase CTs

Three phase current transformer is made with three single phase current transformers packaged together as one unit. There are usually encased in plastic or resin housing. They are typically used in motor control applications with three phase metering and overload protection application. These CTs are compact and easier to install compared to three single phase CTs.


Miniature Current Transformers

These are constructed using one of the following methods: Plastic casing, Resin casted, Resin dipped, Tape insulated,. Typical turns ratio: 4000:1 to 500:1 and Accuracy: Class 0.1 to Class 1.0. Applications include:

  • Energy meters for accurate current measurement
  • Current control
  • Current signature of motors
  • Load sensing
  • Ground fault sensing
  • Monitoring of process parameters
  • AC level to logic conversation & bar graph
  • As a transducer in instrumentation

PC mount 50 to 400Hz Current Transformers

These offer a small footprint for the design engineer looking to sensor current on board. They can also be used for Metering Class (Burden from B O.1 to B 1.8 with accuracy class from 0.3 to 2.4 as per customer requirement. (As per ANSI C 57.13 and IEC 185) and for Relay Class Burden from B 1.0 to B 4.0 and relay voltage class from C 10 to C 400 or T200 as per customer requirement. (As per ANSI C 57.13 and IEC 185) Secondary current range from 0.1 to 5 amp. Typical constructions are plastic casing or resin molded.

Applications include:

  • Sensing current overload
  • Ground fault detection
  • Metering

PC mount 2OkHz to 2OOkHz Current Transformers

These are used for measuring high frequency primary currents up to 15 Amps with primary to secondary isolated to 2500 VAC and have optimum performance over designated current and frequency ranges.

Applications include:

  • Isolated current feed-back signal in switch mode power supplies
  • Motor current load/overload
  • Lighting
  • Switch controls
  • Ultra-sound current
  • High resolution sonar current
  • Isolated bi-directional current sensor with full wave bridge

Switchgear Style CTs

We offer a wide range of current transformers designed for specific mounting applications on circuit breakers used in medium voltage switchgears. These switchgear CTs can be slipped over primary bushings in MV switchgears, and can be bolted using corner mounting holes. They are available in ANSI or IEC accuracy classes for relaying and metering applications.


Bushing CTs

Bushing current transformers for dry or oil-immersed applications. Bushing CTs are manufactured for many different applications such as motor controls, circuit breakers, mega-VA power transformers etc. These are typically manufactured with tape wound constructions and with leads. Various outer encapsulation methods are used depending on application – paper (crepe paper, craft paper) insulation, varnished or un-varnished cotton tape insulation etc. Polyester taped construction is also used for some applications. Bushing CTs can be manufactured to meet high relaying and metering accuracies. They can be made for single or multi ratio applications.

Generator style CTs

Generator style CTs are designed specifically for high current applications inside large generators. They are mounted on output bushings and can have current rating of 5000 to 30000 Amps. These CTs typically have tape insulated outer construction. Special mounting boards are used for floor mountable or wall mountable applications. Hill Tech also provides generator style CTs with cast resin type construction for generator applications. These CTs are constructed with epoxy or polyurethane resin construction, usually mounted around the Generators bus bars with round or rectangle shape. Mounting arrangement can be provided as per customer requirement. These generator style CTs usually have terminal box on top to meet customer requirement. Both tape wound and cast resin CTs can be provided with shielded winding to reduce the effects of magnetic flux interference from adjacent conductors.


Medium Voltage Current Transformers




Medium voltage current transformers from 5kV to 35.5 kV Class system voltage. CTs are encapsulated with epoxy or polyurethane resin cast construction and are able to withstand heavy fault conditions but are not made for exposure to sunlight. The core and coil assemblies are vacuum molded to ensure homogenous, bubble-free construction. The medium voltage CT construction provides high dielectric and BIL 4.5 to 125 full wave crest kV.

The MV CTs can be supplied with round or rectangle window in the center. This provides easy installation for customer as they can use their primary cable or bus bar through the CT ID. Usually these CTs come with equi-potential shield around the ID surface. This equi-potential screen is attached with a pig tail (lead with connector) which comes out of the ID surface. This arrangement allows uniform distribution of charge (non-concentrated) around the surface which further ensures that the CTs withstand high current and high voltage environment in long term use.

We also provide wound primary medium voltage CTs where primary and secondary terminals are usually taken out on top surface of the CTs. Customers can do the primary connections using these terminals. The bus bar type MV CTs construction usually have resin cast CT with bur bar built into the construction. Various primary bar arrangements can be supplied to suite most applications. Medium voltage CTs come with our without mounting bracket.

MV CTs are designed to meet IEEE C57.13 or IEC 60044 standards. Partial discharge test is carried out as per Canadian CAN3-C13-M83 or as per IEC 60044. Standard burdens are used to measure accuracies (ratio and phase angle error) on calibrated accuracy test bridges. Hi-pot tests are carried out as per the required standards.Medium-Voltage-current-transformers-1

Applications include:

  • Metering and Relaying
  • Energy meter panels
  • Medium voltage switch gears and control panels
  • Medium voltage circuit breakers
  • Motor Control Panels

Oil Insulated CTs & PTs


Oil insulated CTs & PTs products are designed for outdoor use. They can be manufactured for up-to 36 kV system voltage. Besides standalone CT and PT, we can manufacture a combination units comprising of both. For example, a metering unit, consisting of 3 CTs and 3 VTs (3 phase 4 wire application) with accuracy class up to 0.2 (IEC) and 0.15 class (IEEE/ANSI). For 3 phase 3 wire application, Hill Tech also offers the combination unit of 2 CTs and 1 three phase VT also. These units are strongly built internally to withstand electrical and mechanical stresses. Units are provided with nitrogen cushioning protection and are duly pressure tested. Hermetically sealed units can be provided depending on customer requirements as well as CTs with Live Tank/Dead Tank design for system voltage of 25 kV and above. Pole or pad mounted tanks are usually processed with either hot dip galvanizing or outdoor paint on it. Primary terminals can be provided with either stud type arrangement or with attachment of connectors suitable for A.C.S.R .(Aluminum conductor steel reinforced) conductors. Oil level indicator, pressure release device, oil filling and draining plugs are standard features. Most of these units are custom designed to meet exact application and requirement. These CT and PT units are suitable for all metering, relaying and differential protection applications. They are 100% tested to meet the required IEC or IEEE/ANSI or relevant test standards.

Potential Transformers

Used primarily in a step down environment to monitor voltage. They are designed for connection line-to-line or line-to-neutral in the same manner as ordinary voltmeters. The secondary voltage bears a fixed relation with the primary voltage so that any change in potential in the primary circuit will be accurately reflected in the meters or other devices connected across the secondary terminals. Potential transformers can be used with voltmeters for voltage measurements or they can be used in combination with current transformers for watt-meter or watt-hour meter measurements. They are used also to operate protective relays and devices, and for many other applications, Since they are used in a monitoring capacity, they generally require much greater accuracy in design.

potential trans

Medium Voltage Potential Transformers

Medium voltage potential transformers for up to 25kV class system voltage. PTs are encapsulated with epoxy or polyurethane resin cast construction. The core and coil assemblies are vacuum molded to ensure homogenous, bubble-free construction. PT construction provides high dielectric and BIL strength.

Medium voltage PTs are constructed in various configurations – double pole or single pole design. Pts can be supplied with two fuses, one fuse or no fuse. MV PTs with fuse clips can be provided, if required by the customer. Switchgear style design is also available. Usually these PTs are supplied with the metal mounting bracket.

We also offer medium voltage PTs with three phase construction. Three phase MV PTs designed in a single package are typically used for the application where physical size and location are very important. These PTs can be supplied with or without fuses. Terminal locations are clearly marked for proper connections.

MV PTs are designed to meet IEEE C57.13 or IEC 60044 standards. Partial discharge test is carried out as per Canadian CAN3-C13-M83 or as per IEC 60044. Standard burdens are used to measure accuracies (ratio and phase angle error) on calibrated accuracy test bridges. Hi-pot tests are carried out as per the required standards

Rogowski Sensors/CTs

Rogowski sensor is an air-core – usually toroidal or rectangle coil placed around the conductor. Rogowski sensors do not use magnetically permeable core. Since there is no magnetic core to saturate, one of the most important properties of these coils is that they produce extremely linear output. Rogowsi sensors are very popular for the application requiring excellent linearity for very high current range.

Amran also manufactures combination of Rogowski CTs and CTs with magnetic core. Both these are combined into one construction. These combination CTs are typically used for protection application where magnetic core CT can operate for low current range and then, once it saturates, Rogowski CT picks up signal and provides appropriate output.

It is not easy to wind Rogowski sensors. We have developed equipment and expertise to wind these products uniformly so that the coil does not pick up any magnetic signals from adjacent circuitry. Special machines are used for accurate winding and state of the art in-house test facilities to test Rogowski sensors for required parameters.



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