Visual Tool 1
Zones of Protection Diagram
Single-line diagram showing overlapping protection zones (busbars, feeders, transformers) with CT locations. The call-out highlights the blind-spot anomaly where a fault between a CT and circuit breaker requires intertripping or zone extension.
Zone A - Busbar Protection
Zone B - Feeder Protection
Zone C - Transformer Protection
Fault point F (blind spot)
Blind Spot: When CTs are located exclusively on the circuit/feeder side of a breaker, a fault between the CT and the breaker is seen by the busbar zone (which trips its breakers) but fault current continues to be fed from the remote feeder end. This requires intertripping the remote end or extending the protection zone to cover this gap.
Visual Tool 2
Fault Clearance Time vs. System Stability (Load Power)
Inverse-relationship curves showing maximum load power sustainable against fault clearance time for different fault types. Phase faults demand far faster clearance to maintain the same power transfer as earth faults.
Key insight: The three-phase curve is steepest and lowest - even a small increase in clearance time causes a large drop in the permissible load. Phase faults severely stress the network and demand high-speed unit protection (typically <80 ms) on EHV systems and generators.
Visual Tool 3
Tripping Circuit Logic - Comparative Schematic
Three-panel comparison of relay tripping circuit configurations: series sealing, shunt reinforcing, and shunt reinforcement with sealing (contact-bounce mitigation).
Panel 2 - Shunt Reinforcing
Panel 3 - Shunt + Sealing
Panel 1: The series contactor coil carries the full trip current; its parallel contact then relieves the PR contact of sustained duty.
Panel 2: One PR contact trips the CB directly; a second simultaneously energises an auxiliary unit that reinforces the connection.
Panel 3: Adds a retaining contact on the auxiliary unit to defeat contact bounce. An external provision (e.g. CB auxiliary contact) must release the sealing circuit after the trip is complete.
Visual Tool 4
Trip Circuit Supervision - 'H5' Scheme
Continuous monitoring circuit for a circuit breaker's trip coil using time-delayed relays A, B, C, with supervision valid in both open and closed breaker states. An independent alarm supply is separated from the tripping supply.
H5 Operation: When the supervision circuit is healthy, relays A, B, or both remain energised, which in turn keeps relay C energised - no alarm. If continuity is lost at any point in the trip coil circuit, A or B de-energises; if the condition persists beyond the time delay, relay C drops off and raises an alarm. Series resistors are sized so that even if a relay short-circuits, the resulting current is insufficient to trip the circuit breaker - preventing spurious operation while maintaining supervision integrity. The alarm supply is fully independent of the tripping DC supply.