Installation of current transformers

Installation orientation

Determine the flow direction of the energy in the cable that you wish to measure. P1 indicates the side on which the current source is located, whilst P2 indicates the load side.

Fig.: Installation orientation
Fig.: Installation orientation

Terminals S1/S2 (k/l)

The connections of the primary winding are designated "K" and "L" or "P1" and "P2", and the connections of the secondary winding are designated "k" and "l" or "S1" and "S2". The polarity must be established such that the "flow direction of the energy" runs from K to L.

Inadvertently swapping the terminals S1/S2 leads to erroneous measurement results and can also cause incorrect control behaviour with Emax and PFC systems.

Fig.: Installation orientation of current transformers
Fig.: Installation orientation of current transformers
Fig.: Direction of energy flow
Fig.: Direction of energy flow

Line length and cross-section

The power consumption (in W) caused by the line losses is calculated as follows:

  • specific resistance
    for CU: 0.0175 Ohm *mm2 / m for AI: 0.0278 Ohm * mm2 / m
    L = Line length in m (outward and return line) I = Current in Amperes
    A = Line cross-section in mm2

Brief overview (power consumption copper line) for 5 A and 1 A:

With every temperature change of 10 °C the power consumed by the cables increases by 4 %.

Serial connection of measuring devices to a current transformer

Pv = UMG 1 + UMG 2 +....+ PLine + PTerminals ....?

Operation in parallel / summation current transformer

If the current measurement is carried out via two current transformers, the overall transformer ratio of the current transformers must be programmed into the measuring device.

Example: Both current transformers have a transformer ratio of 1,000/5A. The total measurement is carried out using a summation current transformer 5+5 / 5 A.

The UMG must then be set up as follows:

Primary current: 1,000 A + 1,000 A = 2,000 A
Secondary current: 5 A

Fig.: UMG 508 Current measurement via summa- tion transformer
Fig.: UMG 508 Current measurement via summa- tion transformer

Grounding of current transformers

According to VDE 0414, current and voltage transformers should be secondary grounded from a series voltage of 3.6 kV. With low voltage it is possible to dispense with grounding if the current transformers do not possess large metal contact surfaces. However, common practice is to ground low voltage transformers too. Customary is grounding on S1. However, grounding can also take place on the S1(k) terminal or S2(k) terminals. Important: Always ground on the same side!

Fig.: Connection example UMG 96RM-EL
Fig.: Connection example UMG 96RM-EL

Use of protection current transformers

In the event of retrofitting a measuring device and the exclusive availability of a protective core, we recommend the use of a winding current transformer 5/5 for decoupling the protective core.