Protection circuit for a three-phase power supply system

An electrical circuit (1) comprising:

• a medium-voltage power supply system (4), the power supply system having a first phase line (L1), a second phase line (L2) and a third phase line (L3),
• a circuit breaker (8) configured to switch a current on the first phase line (L1), on the second phase line (L2) or on the third phase line (L3), and
• a protection circuit (2) comprising:
• a first bushing current transformer (10) having a primary side coupled to the first phase line, and a secondary side,
• a second bushing current transformer (12) having a primary side coupled to the second phase line, and a secondary side,
• a third bushing current transformer (14) having a primary side coupled to the third phase line, and a secondary side,
• a first overcurrent current transformer (16) having a primary side connected to the secondary side of the first bushing current transformer (10), and a secondary side,
• a second overcurrent current transformer (18) having a primary side connected to the secondary side of the third bushing current transformer (14), and a secondary side,
• an earth fault current transformer (20) having a primary side connected to the secondary side of the second bushing current transformer (12), and a secondary side, and
• a first fuse (28A) connected in parallel to the primary side of the first overcurrent current transformer (16), and a second fuse (28B) connected in parallel to the primary side of the second overcurrent current transformer (16),
• wherein the primary side of the first overcurrent current transformer (16) comprises a downstream terminal (30), the primary side of the second overcurrent current transformer (18) comprises a downstream terminal (32), the primary side of earth fault current transformer (20) comprises an upstream terminal (34);
• the downstream terminal (30) of the primary side of the first overcurrent current transformer is connected the downstream terminal (32) of the primary side of the second overcurrent current transformer and to the upstream terminal (34) of the primary side of earth fault current transformer, at a second connection point (35) and in a star configuration.