The current transformer secondary circuit should not be opened under any circumstances if current is flowing in the primary circuit.

The output of the current transformer acts as a current source. As the load increases, the voltage output therefore increases (according to the relationship U = R x I) until saturation is reached. Above saturation, the peak voltage continues to rise with increasing distortion and reaches its maximum value with an infinitely large load, i.e. open secondary terminals. High voltage peaks can therefore occur with open transformers, which are a danger to people and can destroy the transformer and measurement device when reconnected.

This means that open operation must be avoided and unloaded transformers must be short-circuited.

Primary current overload: Primary current too high --> Saturation of the core material --> Accuracy drops massively.

Nominal power overload: Too many measurement devices or too long cables are connected to a transformer with its defined nominal power --> Saturation of the core material --> Accuracy drops massively.

Our transformers generally measure harmonics currents up to 2.5 kHz (50th harmonic) and many types also up to 3 kHz and even beyond. At higher frequencies, however, the eddy current losses and thus also the heating increase. If the harmonics content becomes too high, current transformers with thinner laminations must be used.

However, it is not possible to make a general statement about a limit value for the harmonics content, as the heating depends on the core size, transformer surface (cooling), ambient temperature, transmission ratio, etc.