Choosing a current transformer in 4 steps
Four parameters decide which current transformer fits: the primary current with the secondary output (the ratio), the type for your conductor, the accuracy class and the rated output in VA for the burden. This page walks through the choice in exactly this order.
CELSA manufactures all the types mentioned here in Römerberg, Germany, to IEC 61869 – every version with article number, datasheet and dimensional drawing. If you prefer to be guided, use the transformer finder: it asks for the same parameters and ends with real article numbers.
Step 1: Define primary current and ratio
The primary current is the rated current of the conductor you want to measure. Choose it as close as possible to the actual load – a heavily oversized transformer is less accurate at partial load.
The secondary output depends on the connected device: 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. Together they give the ratio: with a 250/5 A transformer, 5 A flow on the secondary side when 250 A flow on the primary side.
Step 2: Choose the type for conductor and installation
Cable diameter or busbar size define the window, the installation site defines the mounting – on DIN rail, on the busbar or directly on the cable.
New installations: plug-on current transformers (ALO) are the standard type for low-voltage main distribution boards and control cabinets, 50/5 A to 4000/5 A, with the short-circuit bridge in the rotary cover. The conductor passes through the window, so the circuit has to be opened for installation.
Retrofit without shutdown: the KALO split-core transformer (100/5 A to 3000/5 A, class 0.5) as well as cable and busbar conversion transformers are placed around the conductor; the primary circuit stays closed.
Special cases: summation current transformers add the secondary currents of 2 to 6 main CTs for one common meter (SSR for plug-on, SSRK for split-core CTs). Protection current transformers saturate late so that fault currents reach the protection relay correctly. Rogowski coils (RALO) suit very high currents up to 10,000 A and beyond, and confined spaces.
Step 3: Determine the accuracy class
CELSA current transformers are available in accuracy classes 0.2s, 0.5, 1 and 3. Billing and conformity-assessed metering require class 0.5 or 0.2s with transformers approved for it; for display and monitoring tasks class 1 or 3 is usually sufficient.
For official calibration there are calibrated plug-on current transformers (ALO-E). The class comparison shows the permissible current error and phase displacement of every class to IEC 61869-2 – including partial load, where the differences are largest.
Step 4: Calculate burden and rated output in VA
The burden is everything the transformer has to drive in the secondary circuit: the consumption of the connected instruments, meters or relays plus the losses of the secondary wiring. The rated output of the transformer in VA must cover this burden and should not be much higher, otherwise accuracy suffers.
If the burden exceeds the rated output, the transformer saturates and reads low. The VA calculator determines the demand from cable length, cross-section and the connected devices; the cross-section calculator shows the other way round which cable cross-section suits the rated output.
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FAQ
Which parameters do I need to choose a current transformer?
Four: the primary current with the secondary output (1 A, 5 A or 333 mV), the type matching conductor and installation, the accuracy class and the rated output in VA for the burden. With these, the transformer finder shows the matching CELSA versions with article numbers.
1 A or 5 A secondary current – which one?
5 A is common for classic instruments and short secondary leads. For longer leads 1 A is the better choice: wiring losses rise with the square of the current, so at 1 A they are one twenty-fifth of the losses at 5 A.
What happens if the current transformer is oversized?
A heavily oversized primary current makes the measurement less accurate at partial load. A rated output far above the actual burden also reduces accuracy – so choose primary current and VA to suit the installation.
Which accuracy class do I need for billing?
Billing and conformity-assessed metering require class 0.5 or 0.2s with transformers approved for it. For display and monitoring tasks class 1 or 3 is usually sufficient.
Can I retrofit a current transformer without shutting down the installation?
Yes – with the KALO split-core transformer or with cable and busbar conversion transformers. The core is placed around the conductor; the primary circuit stays closed.