Current transformers

A current transformer maps a high primary current at a fixed ratio to a small, safely measurable secondary current – typically 1 A or 5 A. CELSA develops and manufactures current transformers in Römerberg, Germany: plug-on, wound, split-core and summation CTs, protection CTs and Rogowski coils.

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How a current transformer works

The primary conductor – a cable or busbar – passes through the core or is connected to the primary terminals. The alternating current creates an alternating magnetic field in the core, which induces a current in the secondary winding at the transformation ratio. With a 250/5 A transformer, 5 A flow on the secondary side when 250 A flow on the primary side.

Meters, energy meters or protection relays are connected to the secondary circuit. Their consumption and the wiring form the burden that the transformer must drive with its rated output in VA. If the burden is too high, the transformer saturates and reads low.

Important in operation: the secondary circuit of a current transformer must never be left open under load – dangerous voltages would arise at the terminals. CELSA ALO and KALO transformers therefore have an integrated short-circuit bridge.

Selecting the right current transformer

Five parameters determine the choice. The transformer finder asks for them one by one and shows the matching types with aperture and outer dimensions.

Primary current – the rated current of the conductor, as close as possible to the actual load. A heavily oversized transformer is less accurate at partial load.
Secondary output – 5 A for classic instruments and short leads, 1 A for longer secondary wiring, 333 mV for energy meters and monitoring systems with voltage input.
Accuracy class – 0.2s, 0.5, 1 or 3. Billing and conformity-assessed metering require class 0.5 or better. Read more →
Rated output in VA – must cover the consumption of the devices and the losses of the secondary wiring. The VA calculator determines the demand from cable length and cross-section. Read more →
Conductor and space – cable diameter or busbar size define the window; mounting on DIN rail, busbar or directly on the cable.

Standards, quality and manufacturing

CELSA current transformers are manufactured to IEC 61869 parts 1 and 2 and DIN EN 42600 and are available in accuracy classes 0.2s, 0.5, 1 and 3. Some series are approved for official calibration or conformity-assessed metering.

The housing is made of self-extinguishing polycarbonate V0 to UL 94. Development, winding and testing take place in Römerberg – every type is stored as a variant with article number, datasheet and dimension drawing. Custom manufacturing to customer requirements is possible.

Types at a glance

Plug-on current transformers (ALO)
The standard type for low-voltage main distribution boards and control cabinets: window for cable or busbar, DIN-rail mounting, integrated short-circuit bridge, 50/5 A to 4000/5 A.
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Wound current transformers
With their own primary winding, for a flexible primary side and individual ratios – for switchgear, machine building and energy infrastructure.
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Split-core transformers (KALO)
Openable core for retrofitting without interrupting the primary circuit, 100/5 A to 3000/5 A, accuracy class 0.5, with integrated short-circuit bridge.
Cable and busbar conversion transformers
Compact retrofit transformers placed around the cable and secured with UV-resistant cable ties – for energy monitoring in existing installations.
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Summation current transformers (SSR)
Capture the vector sum of several conductor currents – for differential and residual current monitoring in distribution boards and industrial plants.
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Protection current transformers
Deliberately saturate late so the protection signal is transmitted correctly even at high fault currents – for system protection with circuit breakers and relays.
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Rogowski coils (RALO)
Flexible air coils without iron core and without saturation – for very high currents up to 10,000 A and beyond, and for confined spaces.
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Frequently asked questions about current transformers

What does 250/5 A mean on a current transformer?

The transformation ratio: at 250 A in the primary conductor, 5 A flow in the secondary circuit. Connected instruments and meters scale back to the primary current with this ratio.

Which accuracy class do I need?

Billing-relevant and conformity-assessed metering requires class 0.5 or 0.2s with approved transformers; for pure display and monitoring tasks class 1 or 3 is usually sufficient. The class comparison shows the error limits of each class.

What is the difference between measuring and protection CTs?

Measuring transformers saturate quickly above their operating range and thus protect the connected devices. Protection transformers saturate far outside so that short-circuit currents are transmitted correctly to the protection relay.

Can I retrofit a current transformer without shutting down the system?

Yes – with split-core transformers (KALO) or cable conversion transformers. The core is opened, placed around the conductor and latched again; the primary circuit stays closed.

How many VA does my current transformer need?

As much as the device consumption plus the losses of the secondary wiring add up to – and not much more, otherwise accuracy suffers. The VA calculator determines the value from cable length, cross-section and the connected devices.

When is a Rogowski coil the better choice?

For very high currents, large conductor diameters or where there is no room for a core. The flexible coil has no saturation effect and can be fitted without interrupting the circuit; it delivers a voltage signal for devices with a Rogowski input.

Not sure which current transformer fits? We will advise you – or let the transformer finder guide you.

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