A ring main feeds load from two directions, so a fault on one section can be fed from either side. Ordinary non-directional overcurrent relays cannot tell which way the fault current is flowing, and they would trip healthy sections along with the faulty one. Directional overcurrent relays solve this: each one trips only when fault power flows away from its own bus. This post explains the ring-main scheme and its time grading.

Figure 7.2: Ring main protection (A–B–C–D–A) with directional O/C relays. A fault on section B–C trips only R3 and R4. (Schematic, not to scale.)

The scheme

The ring A–B–C–D–A is fed from the grid at bus A. Each section has a directional overcurrent relay (device 67) at each end, looking outward — away from its local bus:

• A directional relay trips only when fault power flows away from its local bus. Current flowing into the bus, or in the reverse direction, restrains it.
• The exception is the source bus A: relays R1 and R9 at A are non-directional (device 51), because at the source there is only one direction fault current can flow — outward from the grid.

Time grading — two directions, two ladders

Because fault current can reach any section from either way round the ring, the relays are graded in both directions, with the longest time at the source and the shortest at the far end:

Direction	Grading (relay: time)
Clockwise (A→B→C→D→A)	R7: 0.5 s → R5: 0.9 s → R3: 1.3 s → R1: 1.7 s
Counter-clockwise (A→D→C→B→A)	R2: 0.1 s → R4: 0.5 s → R6: 0.9 s → R8: 1.3 s → R9: 1.7 s

Each step adds about 0.4 s of grading margin, so the relay nearest the fault always trips first and the relays further back act as backup.

Worked example: fault on section B–C

Suppose a fault occurs on section B–C. Fault current flows into the fault from both directions: clockwise through R3, and counter-clockwise through R4. Both relays see fault power flowing away from their local buses, so only R3 (1.3 s) and R4 (0.5 s) trip, isolating exactly the faulty section. Every other relay either sees current flowing toward its bus (and is restrained by its directional element) or has a longer time setting and never gets the chance to operate. The rest of the ring — A–B, C–D and D–A — stays in service, and the loads at C and D are still fed from the other side of the ring.

Key points to remember

• Ring A–B–C–D–A fed at A; directional 67 relays trip only when fault power flows away from the local bus.
• Source bus A uses non-directional 51 relays (R1, R9) — at the source, fault current can only flow outward.
• Clockwise grading: R7 0.5 s → R5 0.9 s → R3 1.3 s → R1 1.7 s. Counter-clockwise: R2 0.1 s → R4 0.5 s → R6 0.9 s → R8 1.3 s → R9 1.7 s.
• A fault on B–C trips only R3 and R4 — the faulty section alone is isolated and the rest of the ring stays live.

Based on Power System Protection & Switchgear, Complete Examination Solution, Exam 07 May 2026, Q7(c).