Toroidal transformers for residual current devices

Working principles: The toroidal transformer is installed between the source and the load. The system works on the current balance principle. In a correct installation the vector sum of the currents is zero and the relay will not trip. In case of an insulation fault on the circuit a leakage current flows to earth. Now the vector sum of the current passing through the transformer is not zero, this imbalance is detected by the transformer, which induces a current in the secondary winding which is connected to the relay. If the fault level is higher than the selected sensitivity, and when the trip time delay has elapsed, the relay trips and actuates on the shunt trip of a circuit breaker or the coil of a contactor interrupting the supply to the load. The dimensioning of the toroidal transformer depends on the diameter of all active wires (not earth conductors) put through the transformers.

The toroidal current transformers TO consist of laminations of high quality and heat treated by special techniques and allow the detection of even the slightest fault currents.

Toroidal Transformer in general

Doughnut shaped toroidal transformers are used to save space compared to EI cores, and sometimes to reduce external magnetic field. These use a ring shaped core, copper windings wrapped round this ring (and thus threaded through the ring during winding), and tape for insulation.
Toroidal transformers compared to EI core transformers:

  • Lower external magnetic field
  • Smaller for a given power rating
  • Higher cost in most cases, as winding requires more complex and slower equipment
  • Less robust
  • Central fixing is either
    • bolt, large metal washers and rubber pads
    • bolt and potting resin
  • Overtightening the central fixing bolt may short the windings
  • Greater inrush current at switch-on