During commissioning of a ring final circuit, what must be verified about the two ring paths?

• A. That both paths have identical insulation resistance to earth
• B. That the supply voltage is exactly 230 V
• C. That the loop resistance between the two ends is equal and within tolerance
• D. That only one path is intact

Answer: (C)

In a ring final circuit there are two return paths around the ring. To ensure the circuit is intact and protective devices will operate correctly, you verify the loop resistance between the two ends in both directions and check that they are equal and within the specified tolerance. When both paths are continuous and have similar impedance, the fault-loop characteristics are predictable and protection trips reliably in a fault. If one path were damaged or much more resistive, the two directional loop readings would differ and the overall loop impedance could exceed limits, compromising safety. The other checks (insulation to earth, exact supply voltage, or only one path being intact) don’t assess the equality and acceptability of the two ring paths.