Using Current Transformers with Current Sensing Relays

Posted: 5/2/15 | Current Sensing Relays


How do I use a current transformer with one of your current sensing relays?


The Macromatic line of current sensing relays come with a variety of current ranges. Since these ranges are not always large enough to work on most applications, an external transformer can be used to extend the range above the maximum value of each product. Follow these steps to properly select and apply a current transformer relay with our current sensing relays:

  • Typically, a standard 5A secondary current transformer is selected.
  • Select a mounting style that fits your application.
  • Make sure the ID (inner diameter) of the current transformer is large enough to accommodate the size of the wire and the number of turns that will be run through it.
  • The nameplate ratio of the current transformer is based on passing the main conductor through the transformer ID once. This rating can be reduced by looping the conductor through the ID two or more times, assuming the ID is large enough to accommodate multiple loops. A transformer with a 200:5 ratio will be changed to 100:5 if two loops or turns are made with the conductor being sensed.
  • Connect the secondary terminals of the current transformer to the current sensing relay per the wiring diagram provided with the relay.

NOTE: Terminal markings are shown for 8 pin SPDT plug-in products and should be used for reference only.


Without External Current Transformer With External Current Transformer
Schematic 1: Selecting and applying a current transformer relay with a Macromatic Current Sensing Relay. Schematic 2: Applying a current transformer with a Macromatic Current Sensing Monitor