Installation Guide for Current Transformers

  • Installation Orientation

    To identify the correct installation orientation for a current transformer, determine the direction of energy flow in the cable you are measuring. P1 indicates the side where the current source is located, while P2 indicates the load side.

  • Fig.: Installation orientation

Terminals S1/S2 (k/l)

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

Accidentally swapping the S1/S2 terminals can produce erroneous measurements and cause incorrect control behavior in Emax and PFC systems.

  • Fig.: Installation orientation of current transformers

  • Fig.: Direction of energy flow

  • Line Length and Cross-Section

    The power consumption (in watts) caused by line losses can be calculated as follows:

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

Brief Overview of 5 A & 1 A Power Consumption in Copper Line:

Note that 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

You can calculate consumption using this equation:

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

  • Operation in Parallel / Summation Current Transformer

    When carrying out measurement 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 ratio of 1,000/5 A. The total measurement is carried out using a summation transformer 5+5 / 5 A.

    The UMG setup should be:
    Primary current: 1,000 A + 1,000 A = 2,000 A
    Secondary current: 5 A

  • 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. For low voltage, grounding can be omitted if the current transformers do not have large metal contact surfaces. However, it's common practice to ground low voltage transformers too, typically on S1. Grounding can also take place on the S1(k) or S2(k) terminal. Important: Always ground on the same side!

  • Fig.: Connection example UMG 96RM-EL

Use of Protection Current Transformers

If you are retrofitting a measuring device and only have a protective core available, we recommend using a winding current transformer 5/5 for decoupling the protection core. This ensures accurate measurements while preserving the safety functionality of the protection transformer.