Interpretation of test results, defect classification and client communication
Read the results, do not just copy them
Check polarity at every point. Check ring continuity against the expected ratios. Compare R1+R2 with Zs and Ze. Compare insulation resistance with the size and age of the installation. Compare RCD times with the required maxima. Look for inconsistency: a 60 m circuit with a Zs of 0.2 Ω means parallel paths or a recording error.
Classify and explain
Every observation gets a code, and every code needs a clear sentence: what is wrong, where, why it matters, and what should be done. C1 or C2 makes the report unsatisfactory. Tell the client face to face where possible, put it in writing, and be explicit about urgency - a C1 condition should be made safe before you leave, with their agreement, and recorded.
Use satisfactory or unsatisfactory, never pass or fail; the installation has not been examined for compliance with every clause of BS 7671, only for safety.
Trap: a high Zs on a circuit that also has 30 mA RCD protection is a different risk from a high Zs with no RCD - classify on the actual danger and say why.
Trap: a low insulation resistance that has not been investigated before being reported as a defect.
Rule of thumb: describe the danger in plain English, then give the code.