Instrument transformers

Instrument transformers reproduce a current or a voltage at a fixed ratio as values that meters, instruments and protection relays can process, and isolate the measuring circuit from the mains. There are two kinds: current transformers for current and voltage transformers for voltage. CELSA manufactures current transformers in Römerberg, Germany – for low voltage up to 720 V to IEC 61869 parts 1 and 2.

All types, ratios, accuracy classes, burden and wiring are explained on the main page Current transformers.

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CELSA ALO plug-on current transformers – low-voltage instrument transformers in all sizes

Two kinds of instrument transformers

Both work on the transformer principle but differ in measured quantity, connection and behaviour under fault. The ratio states the primary and secondary value, for example 400/5 A for a current transformer or 20 000/100 V for a voltage transformer.

Current transformerVoltage transformer
Measured quantitycurrent in the primary conductorvoltage between two conductors or to earth
Secondary value5 A or 1 A, a voltage signal for Rogowski coils100 V or 110 V, divided by √3 when connected line to earth (IEC 61869-3)
Connectionprimary conductor through the window or on primary terminals, in series with the loadin parallel with the voltage, fused on the primary side
Secondary circuit in servicenever leave open – short-circuit it when working on the instrumentnever short-circuit
Uselow to high voltage: distribution, switchboards, meters, protectionmainly medium and high voltage; at low voltage the voltage is usually measured directly
At CELSAplug-on, wound, split-core, calibrated, protection and summation CTs, Rogowski coilsnot in the range

Current measuring transformer, measuring CT and current transformer mean the same device; voltage measuring transformer and voltage transformer likewise.

Current transformers: instrument transformers for current

For current measurement in low-voltage installations CELSA manufactures every common type. Each group has its own list with filters for ratio, aperture, class and output, and a datasheet for every version. All types, ratios, accuracy classes, burden and wiring are explained on the main page Current transformers.

Voltage transformers: instrument transformers for voltage

A voltage transformer is connected in parallel with the voltage and reproduces it as a standardised secondary value – to IEC 61869-3 usually 100 V or 110 V, divided by √3 when connected between line and earth. It is needed where the system voltage is too high to be applied directly to a meter or instrument: in medium- and high-voltage installations.

At low voltage – up to 1000 V AC – the voltage is normally measured directly. Meters, digital instruments, voltmeters and grid protection relays take the voltage straight from the mains; only the current passes through current transformers. CELSA therefore manufactures current transformers, not voltage transformers. For voltage measurement in distribution boards and switchboards the range offers digital instruments, analogue voltmeters and the grid and plant protection relay CEL-NA-S.

Instrument transformers for billing and protection

For energy measurement the accuracy class matters: CELSA manufactures measuring CTs in classes 0.5, 1 and 3 and calibrated CTs for billing in 0.2S, 0.2, 0.5S and 0.5. Which class a metering point needs is set by the grid or metering operator. Protection CTs carry protection classes from 5P5 to 10P10; they saturate deliberately late so that the relay also sees the short-circuit current correctly – the opposite of the measuring CT, which saturates early and so protects the instruments.

The classes in detail, the difference between 0.5 and 0.5S and the requirements for calibrated CTs are covered in the articles accuracy classes of current transformers, calibration and MID and protection current transformers.

Instrument transformer or transducer?

An instrument transformer is passive: it needs no auxiliary power and passes the current or voltage on reduced but in the same form – alternating current flows again on the secondary side. A transducer is an electronic device with auxiliary supply that turns the measured quantity into a standard signal such as 4–20 mA or 0–10 V for control and SCADA systems. In the measuring chain the current transformer comes before the transducer: the CT delivers 5 A or 1 A, the transducer turns it into the standard signal.

4–20 mA transducers in the knowledge base →

Instrument transformers for switchgear and distribution boards

In the low-voltage main distribution board and the switchboard the current transformer sits on the cable or busbar, with the meter or instrument next to it on the DIN rail. For new installations the plug-on CT is the rule, for retrofits in service the split-core CT, for several feeders on one meter the summation CT and for very high currents the Rogowski coil. The conditions of use apply to all CELSA current transformers:

Conditions of use of CELSA current transformers

Secondary current5 A or 1 A
Rated frequency50 – 60 Hz
Max. permissible operating voltage720 V
Test voltage3 kV / 1 min
Insulation classE to IEC 85
StandardsIEC 61869 parts 1 + 2 / DIN EN 42600

Values from the technical data on the product pages. Special versions on request: any primary and secondary rated currents, operating voltage series 1 (1.2 kV), resin encapsulation.

Frequently asked questions about instrument transformers

What is an instrument transformer?

An instrument transformer is a transformer for measuring purposes: it reproduces a high current or a high voltage at a fixed ratio as a small value that meters, instruments and protection relays can process, and isolates the measuring circuit from the mains. Current transformers measure current, voltage transformers measure voltage.

What is the difference between an instrument transformer and a current transformer?

Instrument transformer is the generic term, current transformer one of the two kinds. Every current transformer is an instrument transformer; voltage transformers belong to the group as well. In low-voltage practice “instrument transformer” usually means the current transformer, because voltage is measured directly there.

What is a current measuring transformer?

Another name for a current transformer. It reproduces the current in the primary conductor as 5 A or 1 A on the secondary side, for example 400/5 A. All types, ratios and classes are on the current transformers page.

What is a voltage measuring transformer?

A voltage transformer: it is connected in parallel with the system voltage and delivers a standardised secondary voltage, to IEC 61869-3 usually 100 V or 110 V. It is mainly needed in medium- and high-voltage installations; CELSA does not manufacture voltage transformers.

What is a low-voltage instrument transformer?

An instrument transformer for systems up to 1000 V AC. CELSA current transformers are rated for a maximum operating voltage of 720 V and tested at 3 kV for one minute; they are installed in main distribution boards, switchboards and metering panels.

Which instrument transformers does switchgear need?

In low-voltage switchgear a current transformer per feeder or incomer: plug-on CTs for new installations, split-core CTs for retrofits without shutdown, summation CTs for several feeders on one meter, protection CTs for protection relays. The voltage is taken directly to meter and instrument.

What distinguishes an instrument transformer from a transducer?

The instrument transformer works passively without auxiliary power and passes the current or voltage on reduced. The transducer is an electronic device that turns it into a standard signal such as 4–20 mA for control and SCADA systems. Often both are used in series: current transformer, then transducer.

May an instrument transformer be short-circuited?

A current transformer yes – and when working on the instrument it must be: an open secondary circuit under load produces dangerously high voltages at the terminals. For a voltage transformer the opposite applies – the secondary side must never be short-circuited.

Who manufactures instrument transformers in Germany?

CELSA Messgeräte has developed and manufactured current transformers since 1940 in Römerberg, Germany – plug-on, wound, split-core, calibrated, protection and summation CTs as well as Rogowski coils, every version with datasheet and dimensional drawing, special versions on request.

Which instrument transformer fits your installation? We will advise you – or let the transformer finder guide you.

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