CTs – Frequently Asked Questions about Current Transformers

Why is it critically important never to leave the CT secondary open under load, and what happens if it does?
It is absolutely essential never to leave the secondary of a current transformer (CT) with standard current outputs of 5 A or 1 A open while current is flowing in the primary. An open secondary can induce dangerously high peak voltages. This can damage the CT itself, any connected measuring instrument, and it also poses a safety risk to operating personnel. For this reason, whenever disconnecting the load (meter), the secondary must first be short-circuited. Special short-circuiting terminals or test blocks are recommended for this purpose.

Do the same rules apply for split-core and clamp-on CTs with 5 A and 1 A outputs?
Yes, the same rules apply to CTs with 5 A and 1 A outputs, whether they are split-core or clamp-on. In both cases, the secondary must never be opened under load. When disconnecting the measuring device, the secondary must first be short-circuited, and single-point earthing of the secondary is mandatory.

What are the rules for earthing the CT secondary, and why is earthing important?
The secondary of a CT with 5 A and 1 A outputs must be earthed at ONE single point in the secondary circuit, usually at terminal S2/l. This earthing serves several purposes: it eliminates capacitive couplings that could affect measurement accuracy, and it limits hazardous touch voltages, thereby improving safety. It is strongly recommended to ensure there is only one earthing point along the entire path.

What is the recommended procedure for CT installation and maintenance?
The recommended procedure includes several steps to ensure safety and proper operation. Short-circuiting terminals should be installed at the meter. When disconnecting the instrument, the short-circuit must be closed first before disconnecting the wiring. One pole of the secondary (S2/l) should be earthed at a single point in the switchboard. For cable runs up to 2 m with a conductor cross-section of 2.5 mm², shorter runs reduce line losses (VA), making it easier to meet the rated burden.

How does copper resistivity and conductivity vary with temperature?
Copper resistivity and conductivity change with temperature. At 0 °C, copper resistivity is 0.0178 Ω·mm²/m and conductivity is 56.18 m/(Ω·mm²). At 25 °C, resistivity increases to 0.020826 Ω·mm²/m and conductivity decreases to 48.02 m/(Ω·mm²). These values are essential for accurate calculations of resistance and line losses. The temperature coefficient of copper is 0.0068 K⁻¹.

How are line resistance and losses calculated for CT wiring?
Line resistance and losses are calculated based on cable length, conductor cross-section, and copper resistivity. For example, for a 20 m run with 4 mm² conductors (at 25 °C, resistivity 0.020826 Ω·mm²/m), the line resistance is 0.2083 Ω. To calculate losses (burden), the formula P = I² * R is used. With a current of 5 A, the losses (burden) amount to 5.21 VA.

What does “suitable burden” mean in the context of CTs, and what are the lower limits?
“Suitable burden” refers to the load a CT can handle without causing unacceptable measurement errors or overheating. Sources indicate minimum burden levels, expressed as a percentage of rated burden. For instance, calculated losses of 5.21 VA correspond to 50% of rated burden. Another lower limit is given as 90% of rated burden, which equals 9.37 VA. These values are critical for correct CT dimensioning and reliable operation.

What is the main purpose of summarizing CT information?
The purpose of summarizing key CT information is to provide a comprehensive overview and recommendations for the installation, wiring, and maintenance of current transformers, with a focus on standard 5 A and 1 A outputs. The document highlights safety aspects, correct handling of the secondary, earthing, recommended work practices, and important copper-related calculations. The ultimate goal is to ensure safe, reliable, and efficient CT operation.