Can you use a LV CT in MV & HV Environment?

Can a Low Voltage Current Transformer (CT) be used in a Medium or High Voltage application? Surprisingly, it is a question we get a few times a year and thought it would be beneficial to put this out.

Firstly we must understand the configuration of the switchgear. For example, if the switchgear has a system voltage of 12kV and the CT will be installed on the bare busbars in the medium voltage section. Then yes you will need a CT that's fully rated, and generally, that's where the thought & decision process ends.

But let us dig a little deeper. If the CT is to be installed directly on medium voltage busbars, accordingly it must have an insulation level at least equal to that of the system/cubicle. However, if installing a CT on the medium voltage cable (which is already insulated), in this case, the CT can be a low voltage type (e.g. insulation level 0.72kV) as the cable provides the medium voltage insulation. It is this element that is missed by design engineers and as a result, adds unnecessary cost to the switchgear.

Another scenario is a neutral current transformer, typically on outdoor installations where the CT needs to be installed on an MV or HV system but on the neutral circuit. As such, it will not see the higher MV & HV voltages, so it's possible to use a lower insulation rating for the current transformer. It is critical to note on an MV or HV system were the CT will see "line voltage". Then a fully rated insulated current transformer will be required.

Got more questions about your specific requirement or maybe just a general inquiry, reach out to us at where one of our highly skilled engineers will assist or check-in on chat at our website or directly with the link

We look forward to being of service to you soon.

The Importance of Transformer Insulation Materials

Typically trаnѕfоrmеrѕ аrе сlаѕѕifiеd intо diffеrеnt tуреѕ such аѕ power transformer, рulѕе trаnѕfоrmеr, instrument trаnѕfоrmеr, audio transformer, RF transformer. It iѕ сlаѕѕifiеd into different ways bу itѕ роwеr capacity, frеԛuеnсу rаngе, vоltаgе сlаѕѕ, сооling type, аррliсаtiоn, рurроѕе, accuracy class, burden аnd bу itѕ winding turnѕ. It iѕ соnѕtruсtеd uѕing аir соrеѕ, tоrоidаl соrеѕ, ѕоlid cores and lаminаtеd steel cores. Aссоrding tо itѕ mеthоd оf mounting it iѕ сlаѕѕifiеd intо pole and рlаtfоrm, ѕubwау, switchgear and it iѕ сlаѕѕifiеd intо single and роlу-рhаѕе bу itѕ numbеr оf рhаѕеѕ. Depending on its рurроѕе it can be classified into Current аnd Cоnѕtаnt current, variable аnd соnѕtаnt vоltаgе.

Insulation iѕ оnе оf thе mоѕt important qualities thаt a trаnѕfоrmеr hаѕ, often under-appreciated. In fасt, the durability and ѕtаbilitу of a trаnѕfоrmеr highlу depend upon the рrореr application and utilisation оf the insulation mаtеriаlѕ рrеѕеnt within it. This mеаnѕ thаt other than mеtаlliс components likе ѕiliсоn ѕtееl аnd сорреr, don't suffer a mechanical or electrical breakdown thus aiding decades of service life.

A Power Transformer inѕulаtiоn ѕуѕtеm соnѕiѕts either liquid or gas combined with ѕоlid materials. Petroleum-based оilѕ have been uѕеd tо insulate роwеr trаnѕfоrmеrѕ ѕinсе 1886 and thе mid-1970ѕ whеn the flammability оf minеrаl oil wаѕ a соnсеrn since then been соmрlеtеlу phased оut of trаnѕfоrmеr рrоduсtiоn due to еnvirоnmеntаl concerns. It hаѕ bееn rерlасеd bу аnу оf a widе vаriеtу оf high-flash-point fluidѕ (ѕiliсоnеѕ, high-flash-point hуdrосаrbоnѕ, chlorinated bеnzеnеѕ, оr chlorofluorocarbons).

Gаѕ ѕуѕtеmѕ include nitrоgеn, аir, аnd fluоrоgаѕеѕ. Thе fluоrоgаѕеѕ are uѕеd tо аvоid combustibility аnd limit secondary effects of internal fаilurе. Sоmе trаnѕfоrmеrѕ have bееn соnѕtruсtеd using lоw bоiling-роint liԛuidѕ such as Frеоn whiсh аllоwѕ imрrоvеd heat trаnѕfеr uѕing a 2-рhаѕе cooling system.

Within thе соrе аnd соil аѕѕеmblу, inѕulаtiоn can be dividеd intо twо fundаmеntаl groups: mаjоr inѕulаtiоn and minоr insulation. Mаjоr insulation ѕераrаtеѕ thе high аnd lоw-vоltаgе windingѕ, аnd the windingѕ tо core. Minоr inѕulаtiоn mау bе uѕеd bеtwееn thе раrtѕ of individuаl соilѕ оr windingѕ dереnding on construction. Finаllу, inѕulаtiоn is applied to each ѕtrаnd of соnduсtоr and or grоuрѕ оf ѕtrаndѕ forming a single turn.

Understanding the transformer application is key to understanding what insulation materials to use. In Power Transformers insulating рареr аnd cardboard are the twо main insulating mаtеriаlѕ used for isolating рrimаrу and ѕесоndаrу windingѕ with thе mаin соrе. Whereas in some low and medium voltage applications Epoxy or Polyester Resin systems protect while in others a plastic case, polyester tape or crepe paper are most suited.

In the end, insulation plays a vital role in the operation of a transformer; getting it wrong can be expensive.

Need an Instrument Current Transformer or Voltage Transformer up to 36kV?

Drop our technical team an email to discuss your requirement or jump over to our contact form and complete, we would love to hear from you.

UL Recognised Components

Instrument Transformers Limited (ITL) is happy to announce our new range of UL recognised component, low voltage current transformers, and is yet another milestone of achievement in our expanding portfolio.

Tape Finish, Resin Cast & Plastic Case models with prefix part numbers 560, 561 & 562 will fall under our UL programme (File Number E502882) and highlights our ongoing commitment to providing the very best customer solutions. So if a UL requirement is what you need, just let us know we would be happy to help.

To understand the importance of UL and what it stands for we need to look at its' history: -

Underwriter Laboratories, or UL, is a global company with headquarter in USA, Illinois. Offering a variety of services to help business improve their products or services and demonstrates they are safe to use and comply with set quality standards and regulations. From a manufacturer point of view, the UL affixed label gives our customers additional confidence in the safety, quality, and reliability of our product.

In 1893 a fire at the World Columbian Exposition in Chicago Illinois, where sixteen people lost their lives. Prompted William Henry Merrill, Jr who would go on to become UL's founder. Proposed the idea of an independent electrical testing laboratory and in 1894 the Underwriter Electrical Bureau was born. Finally, after several name changes, in 1901 the name Underwriter Laboratories as we know it today got incorporated, and the business took off.

UL published the first standard in 1903 for a Tin Clad Fire Door and has since branched out in a variety of industries (e.g., building construction, aircraft, security, home appliances, resins, TV's and much more). In 1956 UL also expanded internationally and started to certify products made in Europe.

ITL continues its expansion into North America & the Petro-Chemical processing space, where electrical switchgear equipment manufacturers need UL approved components. 

Which is part of our ongoing drive to support our global customer base with the right products for the right application 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.

SAP Ariba – Good or Bad?

SAP Ariba is the leader in spend management. It began with a mission to help companies control spending and cut costs by making it easier for buyers and suppliers to work together. In 2012, Ariba was acquired by SAP, the world’s largest business software company, and in 2016 launched its new new brand identity.

While SAP Ariba is open to all systems and all types of goods and services, its unified name conveys to customers the value of combining solutions for procurement and supply chain collaboration with SAP’s expertise in enterprise application software and supply chain management.

SAP Ariba is how companies connect to get business done. On the Ariba Network, trading partners from more than 3.6 million businesses, operating in more than 190 countries, discover new opportunities, collaborate on transactions, and deepen their relationships.

  • Large, midsize, and growing companies use an end-to-end, automated system that simplifies the management of everything from sourcing to payments, all in one place.
  • With intelligent spend management tools and network-generated insights, customers establish sustainable, trusted connections with partners while ensuring efficient, error-free transactions.
  • Buyers can manage the entire purchasing process as they control spending, find new sources of savings on both direct and indirect goods, and build healthy, ethical supply chains
  • Suppliers can connect with profitable customers and efficiently scale existing relationships, simplifying sales cycles and improving cash control along the way.

The Ariba Network makes it easy for buyers and suppliers to collaborate on transactions, strengthen their relationships, and discover new business opportunities. Buyers can manage the entire procurement process from source to settle, while controlling spending, finding new sources of savings, and building a healthy, ethical supply chain. Suppliers can help buyers achieve their procurement transformation goals, while boosting customer satisfaction, simplifying the sales cycle, and improving cash flow.

Content Source:

In the end it is the choice of the supplier to either join or not. Some of your customers may insist while others are not on the platform, that is all fine at least you are able to make an informed judgement. It might be sales related or foresight to where you want to see your business in the future to join. Our experience over the past years has been good and are able to engage and participate in number of projects easily.

Find Instrument Transformers Limited on SAP Ariba

Need an Instrument Current Transformer or Voltage Transformer?

Drop our technical team an email to discuss your requirement or jump over to our contact form and complete, we would love to hear from you.

G59 Installation Loss of Mains Changes & How it Affects You!

Loss of Mains Change Programme Logo

If you have installed renewable generation before 1st February 2018 in accordance with EREC G59 requirements, then it is likely you will need to undertake an update.

The "Accelerated Loss of Mains Programme (ALoMCP)" is being led by National Grid and run by Distributed Network Organisations (DNO's) & Independent Distribution Network Organisations (IDNO's). 

Covering the United Kingdom but excluding Northern Ireland; it aims to reduce the risk of spurious tripping of the protection system. For renewable electricity generators, this means realigning the RoCoF limits (rate-of-change-of-frequency) in their generation equipment.

If you are a domestic generator of power, then thankfully this does not affect you. However, if you generate & sell back to the grid, using wind generators, hydro or solar PV, means you have until 31st August 2022 to implement the necessary changes. Failure to do so may open you up to an enforcement programme.

Why the change?

RoCoF or Vector Shift (VS) relays usually provides Loss of Mains Protection to prevent "power islanding." RoCof relays were programmed to trip when sensing changes of frequency as low as 0.125Hz. Both these and Vector Shift relays have found to respond to transmission faults that do not cause power islanding leading to low demand disconnections.

Who is responsible for the change implementation?

It's intended that the generators will undertake the settings changes themselves through their qualified contractors. Where a generator doesn't have these facilities, contractors such as ourselves are here to help. Instrument Transformers Limited in partnership with Switchgear Services & Solutions Limited offer changes & complete testing in accordance with the Accelerated LoM programme. Or where necessary, removal of existing LoM relay and install a new relay that is compliant with the new LoM requirements.

Generators can recover the cost of making these changes directly via an online portal located on the Distribution Network Operators (DNO), Independent Distribution Network Operators (IDNO), Energy Networks Association (ENA) & National Grid (NG) websites.

The rates paid for the work will vary from:

£1,000 to £1,500 for change in settings.

£2,500 to £4,000 for a relay change.

Payments will be fixed-rate rather than individual circumstances and are on a first-come, first-serve basis.

Still got questions?


or check out more information on the ENA Website Loss of Main Change Programme.

Loss of Mains process at a glance

LoM process at a glance

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.