CT for ANSI & IEC Applications

"Plastic case current transformers are so versitile and simple to install" is a comment customers often tell us. Now its even more enticing, with the devaluation of sterling against a basket of currencies has made this range, even more, cost-competitive leading it to become one of our the best runners for our switchgear and control panel customers.

Designed specifically for IEEE /ANSI C57.13 & IEC 61869 standards, Single Ratio or Multi-Ratio variants complete with integral mounting points. Suitable for Protection/Relaying or Measuring Classes up to 6000A with a 5A secondary makes this robust current transformer makes this a popular choice for our customers.

Secure your order today for this amazing current transformer.


Switchgear Protection & Measurement

M78x CT

Protecting Your Ass-ets

Protective Current Transformers are designed to measure the actual currents in power systems and to produce proportional currents in their secondary windings which are isolated from the main power circuit. These replica currents are used as inputs to protective relays which will automatically isolate part of a power circuit.

Satisfactory operation of protective relays can depend on accurate representation of currents ranging from small leakage currents to very high overcurrent's, requiring the protective current transformer to be linear, and therefore below magnetic saturation at values up to perhaps 30 times full load current.

This wide operating range means that protective current transformers require to be constructed with larger cross-sections resulting in heavier cores than equivalent current transformers used for measuring duties. For space and economy reasons, equipment designers should, however, avoid over-specifying protective current transformers ITL technical staff are always prepared to assist in specifying protective CT's but require some or all of the following information.

  • Protected equipment and type of protection.
  • Maximum fault level for stability.
  • Sensitivity required.
  • Type of relay and likely setting.
  • Pilot wire resistance, or length of run and pilot wire used.
  • Primary conductor diameter or busbar dimensions.
  • System voltage level.


Characterisation of a protective current transformer class is as follows:

Class P & PR  (A current transformer to meet the composite error requirements of a short-circuit current under symmetrical steady-state conditions).

Class PX & PXR (A current transformer by specifying its magnetising characteristic).

Class TPX, TPY & TPZ (A current transformer to meet the transient error requirements under the conditions of an asymmetrical short-circuit current).

Rated Output: The burden including relay and pilot wires generally follow standard burdens 2.5, 5, 7.5, 10, 15 and 30VA.

The accuracy designation "P" (which stands for protection)  uses the highest percentage composite error followed by the Accuracy Limit Factor (ALF).

Standard Protection Accuracy Classes are shown as 5P & 10P with Accuracy Limit Factors 5, 10, 15, 20, 30

Therefore the electrical requirement of a protection current transformer can be defined as:


For example, 1600/5A, 15VA 5P10.

Switchgear Protection

Switchgear Protection

Accuracy limit Factor is defined as the multiple of rated primary current up to which the transformer will comply with the requirements of 'Composite Error'. Composite Error is the deviation from an ideal CT (as in Current Error) but takes account of harmonics in the secondary current caused by non-linear magnetic conditions through the cycle at higher flux densities.

Selection of Accuracy Class & Limit Factor.

Class 5P and 10P protective current transformers are generally used in overcurrent and unrestricted earth leakage protection. With the exception of simple trip relays, the protective device usually has an intentional time delay, thereby ensuring that the severe effect of transients has passed before the relay is called to operate. Protection Current Transformers used for such


In some systems, it may be sufficient to simply detect a fault and isolate that  circuit. However, in more discriminating schemes, it is necessary to ensure that a phase-to-phase fault does not operate the earth fault relay.


Need a Protection Current Transformer, reach out to us and let us help you in your next project.


Accelerated Loss of Mains Programme

National Grid ESO and GB Distribution Network Operators (DNOs) / Independent Distribution Network Operators (IDNO) are undertaking electricity transmission and distribution industry-led initiative managed on behalf of the Distribution Code Review Panel.

It is a requirement of the "Distribution Code" that all owners of generation installed before February 2018, and where the generation equipment is not type-tested, to comply with new setting requirements for the interface protection per EREC G59. Owners of generation have to comply with the Distribution Code and have until May 2022 to comply with these modified interface protection requirements. After that date, owners who have not made the change will be the subject of an enforcement programme.

The purpose of the initiative is to consider Loss of Main (LoM) protection and their settings. The aim is to reduce the risk of inadvertent tripping and reduce system balancing issues by giving National Grid ESO more considerable latitude with regards to system Rate of Change of Frequency (RoCoF) limits.

To help owners of generation make the necessary changes, through ENA, National Grid Electricity System Operator (NGESO), the Distribution Network Operators (DNO) and Independent Distribution Network Operators (IDNO), have set up a database to allow generators to register their intention to make settings changes and to facilitate the development of a prioritised delivery programme. This database is available to generators from early-May 2019. It is intended that generators who make the settings changes will be recompensed by the DNO / IDNO they are connected to according to the degree of work involved in making the changes.

Source: Energy Networks Association.

Accelerated Loss of Mains Programme

Achilles – UVDB Registration

The utility market is fast moving, subject to ever more stringent regulation and new technologies. UVDB is the utility industry pre-qualification system used across the UK. Working closely with key buying organisations in the sector, this community helps them achieve the highest standards of supply chain assurance.

ITL is proud to continue its qualified status registration and contributing its part in the high-quality supply chain.



3.5.20 Transformers - Current

3.5.21 Transformers - Voltage

3.5.99 other Transformers

4.99.5 Testing & Analysis Services (Utilities)



UVDB Achilles Community Certificate

Transformer Accuracy

Transformer Accuracy Principles

Measuring instruments, such as ammeters, voltmeters, kilowatt-hour meters, etc , whether electromechanical or electronic, meet insuperable design problems if faced with high voltages or high currents commonly used in power systems.

Furthermore, the range of currents employed throughout is such that it would not be practical to manufacture instruments on a mass production scale to meet the wide variety of current ranges required.

Current transformers are therefore used with the measuring instruments to:

(a)  Isolate the instruments from the power circuits.

(b) Standardise the instruments, usually at 5 amps or 1 amp.

(The scale of the instrument (according to the CT ratio), then becomes the only non-standard feature of the instrument)

Accuracy classes for various types of measurement are set out in the relevant IEEE(ANSI), CAN/CSA, AS or in our case BSEN /IEC 61869.

It will be seen that the class designation is an approximate measure of the accuracy, eg. Class 1 current transformers have ratio error within 1% of rated current. The phase difference is important when power measurements are involved, i.e. when using wattmeter's, kilowatt‑hour meters, VAr meters and Power Factor meters.


Importance of Accuracy

Transformer Accuracy

The table below details the limits of error for current transformers for special applications and having a secondary current of 5A.


Design Considerations:

As in all transformers, errors arise due to a proportion of the primary input current being used to magnetise the core and not transferred to the secondary winding. The proportion of the primary current used for this purpose determines the amount of error.

The essence of good design of measuring current transformers is to ensure that the magnetising current is low enough to ensure that the error specified for the accuracy class is not exceeded. This is achieved by selecting suitable core materials and the appropriate cross-sectional area of the core. Frequently in measuring currents of 50A and upwards, it is convenient and technically sound for the primary winding of a CT to have one turn only.

In these most common cases the CT is supplied with a secondary winding only, the primary being the cable or busbar of the main conductor which is passed through the CT aperture in the case of ring CTs  (i .e. single primary turn) it should be noted that the lower the rated primary current the more difficult it is (and the more expensive it is) to achieve a given accuracy.

Considering a core of certain fixed dimensions and magnetic materials with a secondary winding of say 200 turns (current ratio 200/1 turns ratio 1/200) and say it takes 2 amperes of the 200A primary current to magnetise the core, the error is therefore only 1% approximately. However, considering a 50/1 CT with 50 secondary turns on the same core it still takes 2 amperes to magnetise to the core. The error is then 4% approximately, to obtain a 1% accuracy on the 50/1 ring CT a much larger core and/or expensive core material is required.

Still not sure & need help? Feel free to reach out to us with your enquiry where we will be happy to help.


CT12-100 MV CT Launch

New 12kV Medium Voltage CT by ITL

ITL proud to announce the launch of its updated medium voltage current transformer CT12-100. Always listening to customer feedback, an area of their frustration was the desire was to allow for changes in the primary ratio rather than only on the secondary. With many such customers requesting the same the die was cast for ITL’s R & D team to improve on existing models to meet their customer's yearning.

In any development, it is important to weigh up the costs and benefit whilst getting the maximum performance from the equipment and at the same time achieve improvements in manufacturing productivity. ITL’s highly skilled engineering & production teams have achieved both in the CT12-100 range.

“The CT12-100 is the first in an updated line of indoor MV current & voltage transformers in development at ITL” Paul Munro (ITL’s Marketing Director) said.

With the Highest System Voltage 12kV, primary ratios from 100A to 1200A designed with both tariff measurement & protection class accuracy in mind ensures that ITL continues to provide their global customer base with the right product for the right job and at the right price.

Got Questions?

Drop our technical team an email to discuss or jump over to our contact form and complete, we would love to hear from you.
New MV Current Transformer


Defining CT Characteristics

Current Transformer Terminology

Current transformers convert an alternating current usually high into a proportional lower one, depending on their use. Measurement type CTs are required to transform the primary current, at various classes of accuracy, as specified by the class designation, over a current range from 1 to 120 per cent of its rated primary ratio.


The design of this type of transformer requires a "steel" core and winding wire, typically copper which will when connected to its rated burden (load); perform within the limits of error as indicated by the standard’s specification e.g. IEC 61869 or IEEE C57.13. It is an advantage for a measurement type transformer to saturate above this range, which provides a protection against damage to instruments by limiting the secondary current when surge currents or faults appear in the primary circuit.


What Is:

Measuring transformer:

A current transformer intended to supply indicating instruments integrated meter, relay and similar apparatus.

Current transformer:

An instrument transformer in which the secondary current, in the normal condition of use, is substantially proportional to the primary current and differs in phase it by an angle which is approximately zero for an appropriate direction of connections.

Rated primary current:

The value of primary current which appears in the designation of the transformer and on which the performance of the current transformer is based.

Rated secondary current:

- The value of secondary current which appears in the designation of the transformer and on which the performance of the current transformer is based.

Rated transformation ratio:

The ratio of the rated primary current to the rated secondary current.

Current error (ratio error):

The error with a transformer introduces into the measurement of a current and which arises from the fact that actual transformation ratio is not equal to the rated transformer ratio. The current error expressed in percentage is given by the formula:
Current error, percent = (Ka.Is-Ip) x 100 / Ip
Where Ka= rated transformation ratio
Ip= actual primary current
Is= actual secondary current when Ip is flowing under the conditions of measurement

Phase displacement:

The difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for the perfect transformer. The phase displacement is said to be positive when the secondary current vector leads the primary current vector. It is usually expressed in minutes.

Accuracy class:

A designation assigned to a current transformer the errors of which remain within specified limit under prescribed conditions of use.


The impedance of the secondary circuit in ohms and power factor.

Rated burden:

The impedance of the secondary circuit on which the accuracy requirements are based. It is usually expressed as apparent power (in VA), at the rated secondary current and at a specified power factor.

Rated output:

The value of the apparent power (in volt-amperes at a specified power (factor) which the current transformer is intended to supply to the secondary circuit at the rated secondary current and with rated burden connected to it.

Highest system voltage:

- The highest rms line to line voltage which can be sustained under normal operating conditions at any time and at any point on the system. It excludes temporary voltage variations due to fault condition and the sudden disconnection of large loads.

Rated insulation level:

That combination of voltage values (power frequency and lightning impulse, or where applicable, lightning and switching impulse) which characterizes the insulation of a transformer with regard to its capability to withstand by dielectric stresses. For low voltage transformer the test voltage 4kV, at power-frequency, applied for 1 minute.

Rated short-time thermal current (Ith):

The rms value of the primary current which the
current transformer will withstand for a rated time, with their secondary winding shortcircuited
without suffering harmful effects.

Rated dynamic current (Idyn):

The peak value of the primary current which a current transformer will withstand, without being damaged electrically for mechanically by the resulting electromagnetic forces, the secondary winding being short-circuited.

Rated continuous thermal current (Un):

The value of current which can be permitted to flow continuously in the primary winding, the secondary windings being connected to the rated burdens, without the temperature rise exceeding the specified values.

Instrument security factor (ISF or Fs):

The ratio of rated instrument limit primary current to the rated primary current. The times that the primary current must be higher than the rated value, for the composite error of a measuring current transformer to be equal to or greater than 10%, the secondary burden is equal to the rated burden. The lower this number is, the more protected the connected instrument is against.

Routine test:

Tests carried out on each current transformer to check requirements likely to vary during production.
Depending on which standard the transformer is to meet e.g. IEC or IEEE/ANSI. In general, the following tests apply to each individual transformer:
1. Verification of terminal markings
2. Power-frequency withstands test primary winding.
3. Partial discharge measurement.
4. Power-frequency withstand test on secondary windings.
5. Power-frequency withstand test, between sections.
6. Inter-turn overvoltage test
7. Determination of errors.
The order of the tests is not standardised, but the determination of error shall be performed after the other test.

Type test:

Tests carried out to prove the general qualities and design of a given type of current transformer in accordance with the requirements of the applicable standers Tests may be carried out on a prototype which may incorporate special arrangements for the measurements required by the applicable standard.
The following tests are type test:
1. short time current test
2. temperature rise test
3. lightning impulse test
4. switching impulse test
5. wet test for outdoor type transformer
6. determination of errors
7. radio interference voltage measurement (RIV) (As specified in IEC 61869-1 for 123kV and above)
All the dielectric type test should be carried out on the same transformer unless otherwise specified.

Special tests / optional tests:

Chargeable testing which may be in the nature of type tests or routine tests, and are carried out only by agreement between ITL & the customer.

Got Questions?

Drop our technical team an email to discuss or jump over to our contact form and complete, we would love to hear from you.
Defining a Current Transformer

Measurement CT

Agent Appointed – PAKISTAN

Following our international growth plan, ITL is proud to announce our new agency partner in Pakistan. Ittehad Energy Systems (IES). Based Karachi with a planned office in Lahore within the next 12 months this dynamic company already has a number of experienced Electrical Sales Engineers who be promoting & supplying Instrument Transformers Limited's wide range of current transformers and voltage transformers to the Industrial & Electrical Utility markets.

With our increased business activity in the region, now was the right time for us to formalise our position in the country. Ittehad was born out of former employees of our Saudi Arabian agent whom we have worked with for over 30 years. This previous experience was of particular interest to ITL and vastly reduces the product learning curve and accelerates market awareness making it a quick win for both companies.

Critical to success is a field support function. ITL has dedicated Engineering & Sales Team members who regularly engage with and provide assistance to our international agent network ensuring we understand the techno-commercial aspect of any project. In addition, regular sharing of market intelligence data allows ITL to adapt its marketing strategy or deploy extra resources to maximise local market opportunities.

Our product line is often hidden from view but is an essential element in our every day lives. Without electricity in some form, economies cannot grow beyond local bartering on what you can hand make. Electricity gives us industrialisation, innovation and the world we live in. Why wouldn't you want to contribute to that?

Next time you need a CT or PT and are based in Pakistan, then speak to Ittehad on +92-336-2337482 or email rubnawaz.khan@ittehadenergy.com where they will be more than happy to assist you in your project.

If you would like to be considered as an independent sales agent for Instrument Transformers Limited send an email to Office.ROW@itl-uk.com

Agent Certificate for Pakistan

Agent -Certificate

ITL Renews Russian Connection

ITL has renewed its connection with the Russian, Kazakhstan, Belarus, Armenia & Kyrgyzstan markets with an updated Current Transformer & Voltage Transformer product lines that comply with the EAC requirements of TR CU 020/2011 (Electromagnetic compatibility of technical means) & TR CU 004/2011 (Safety of low-voltage equipment). About 70% of products exported to Russia / the Customs Union require mandatory EAC certification system to pass the customs clearance. The EAC is replacing the former national GOST certification in Russia, Belarus, Kazakhstan, Armenia and Kyrgyzstan. At the moment, there are 36 Technical Regulations in effect and 20 more will be adopted in the future.

Ministry pre-qualification was undertaken and once accepted, any goods exported to Russia / The Customs Union must be clearly marked by the EAC logo. The logo has to be on each individual unit, packaging and any documentation and if possible, next to the manufacturer’s trademark.

The EAC conforming units are specially designed to work in environments down to -50 Degrees Celcius. These products have been manufactured to the most recent IEC standards for instrument current and voltage transformers, using the latest technology and a host of materials suited for low-temperature environments.

Paul Munro, Marketing Director for Instrument Transformers Limited said: "Always listening to customer feedback, there was a particular pain point experienced in the Eurasian market. Whilst ITL was not directly affected, we took the lead in assisting our customers to remove potential barriers to market by having a speciality product line developed to meet the newest TR CU requirements. Now completed, we are supporting a number of customers in this initiative, whilst actively rolling out an awareness program to our wider global customer base."

This is one of the many projects that Instrument Transformers Limited has undertaken to ensure that they timeously adapt to an ever-changing & challenging marketplace.

Need a CT or PT, then you need to be speaking to ITL at sales@itl-uk.com

EurAsEC Customs Union

EAC Certification Logo

Alive & Kicking after 28 years

At ITL we always promote the reliability and quality of our transformers, and from a marketing perspective, it can be seen as spin in front of the customer. It is great to have supporting feedback from the field and today this CT was part of a 12 off manufactured for Centre Switchgear (latterly amalgamated into ABB in the 90's to become ABB Centre Switchgear Limited). The project was for Bishopsgate Incoming Main Power and has been faithfully serving for 28 years. 

Normally in system upgrades the CT is removed and replaced, not this one, the customer is connecting upgraded monitoring equipment and we expect that this CT will continue to provide steadfast performance.

We stand by the quality & reliability of our products and are proud of our achievements over the past 45 years. Always welcoming new challenges from whatever our customers throw at us and whilst we may not be the cheapest in the market, without a doubt we can say that our customers definitely get a positive return on investment when installing an Instrument Transformers Current Transformers or Voltage Transformers.

If you are not using our transformers, firstly I would cheekily say WHY, but I'm sure you have your reasons. None the less if you haven't already joined our global customer base where our goal is to exceed your expectations and help you build your brand as a quality manufacturer of switchgear & electrical equipment then speak to us now.

Reach out today on using our chat page, email our sales & marketing teams or do it the old-fashioned way and give us a call on +44(0)1355236057 where one of our highly qualified engineers will assist with your requirements.

Main Incoming Current Transformer

Main Incoming CT