For low values of 3Io , the relay automatically switches from a product to a balancing algorithm (3Io–Rct In ).
This allows it to detect internal faults when the low-side transformer breaker is open, as in Figure 14.
A ratiomatching auxiliary CT (Rct) is provided as part of the software algorithm in digital relays, as opposed to an actual CT, as it was in EM technology. This scheme provides excellent security against misoperations for external, high-magnitude faults, even for cases where the phase CTs saturate.
There are, however, some digital relay manufacturers that try to employ only a simple differential ( In -3Io ).
This method is prone to misoperate during high-magnitude through faults.