Window-type current transformers

The conductor to be measured (busbar or cable) is passed through the window opening and creates the primary circuit of the window-type current transformer.

Pin-wound transformers: An economical solution especially for small primary currents of 5 A to 75 A is achieved when the conductor to be measured is pin-wound several times.

Primary rated operational current I_pn

Current transformers can be continuously loaded with 1.3 times the primary rated current (I_pn).

Rated secondary current I_sn

1 A

Particularly suitable for longer measuring leads. Cable losses of only 4 % in contrast to 5 A current transformers.

5 A

5 A current transformers create 25 times the power losses on measuring leads as compared with 1 A current transformers. These stray losses result in higher power in the case of long cables. Only recommended for use with short measuring leads.

Accuracy class

Class 1

Operation measurement, internal metering
Current error ±1 % at 1 ? I_pn and 1.2 ? I_pn

Class 3

Coarse measurement
Current error ±3 % at 0.5 ? I_pn and 1.2 ? I_pn

Rated power P_n

The rated power of transformers is specified in VA. The actual load rating should be similar to the rated power; a lower actual load rating (underburden) increases the overcurrent factor and measuring instruments may be damaged in case of a short-circuit, a higher actual load rating (overburden) has a negative effect on the accuracy.

With a frequency of 60 Hz the rated power increases to 1.2 times. With 16²/₃ Hz the output power decreases to ¹/₃ of the rated power.

Maximum voltage for equipment U_m

This is the rms value of the maximum voltage between the conductors of a network. For this voltage the insulation must be rated at normal operating conditions.

4NC5 current transformers are suitable for 720 V.

Overcurrent limiting factor FS

The overcurrent limiting factor is expressed using the characters FS and a factor, e.g. FS5 or FS10.

When a short-circuit current flows through the primary winding of a current transformer, the load on the measuring instruments connected to the current transformer is the lower the smaller the overcurrent limiting factor is.

Rated short-time thermal current I_th

The rated short-time thermal current I_th is the rms value of the primary current with a duration of one second, whose heat effect the current transformer can resist without being damaged in the event of a short-circuited secondary winding.

Rated impulse current I_dyn

The rated impulse current I_dyn is the highest instantaneous value of the current after a short-circuit whose force the current transformer can resist without being damaged.

The rated impulse current is specified as peak value.