Zero Sequence And Residual Current Monitoring

The Principle

Pass all three phases, and the neutral where there is one, through a single core. Under healthy conditions the currents sum to zero and the secondary sees nothing. When current escapes to earth the sum is no longer zero and the core produces an output proportional to the residual current.

Why Core Material Matters

The signal being measured is a small imbalance between large currents, so the core needs very high permeability and very low remanence. High magnetic permeability nickel steel and nanocrystalline materials give both. Ordinary silicon steel does not, which is why a standard measuring core makes a poor residual core.

Protection Grade Bodies

For ground fault protection rather than monitoring, the cast resin zero sequence range is rated 50 A to 600 A primary with a 40 kA one second thermal current, 63 kA dynamic current and a 12/42/75 kV insulation level, to IEC 60044, IEC 61869 and ANSI/IEEE C57.13.

Open Or Closed

• Closed body for new switchgear, where the cable can be threaded during the build
• Open body, the K suffix in the LSY and LSF ranges, for retrofit onto cables already terminated
• Cast resin split core for medium voltage ring main units

Installation Detail That Decides Everything

Every live conductor of the circuit, including the neutral, must pass through the core, and the cable screen must be brought back through it if it is earthed at the far end. Miss either and the core will report a fault that is not there.