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 Ipn

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

Rated secondary current Isn

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 × Ipn and 1.2 × Ipn

Class 3

Coarse measurement
Current error ±3 % at 0.5 × Ipn and 1.2 × Ipn

Rated power Pn

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 Um

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 Ith

The rated short-time thermal current Ith 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 Idyn

The rated impulse current Idyn 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.

Rated primary current

A

50

60

75

100

150

200

250

Service

VA

2.5

5

Primary current to be measured

Number of required pin windings

A

Class 3

Class 1

4NC51 used as pin-wound transformer

5

10

–

10

6

15

4

20

3

25

2

8

30

40

50

75