pg 11-15 Electrical Inspection, Testing and Documentation – Module 6 Key Points

Flashcards covering ring final circuit continuity tests (steps, relationships, reasons) and insulation resistance requirements per BS 7671.

In a 2.5 mm²/1.5 mm² 2-core + CPC ring, what should R2 equal?

R2 should equal R1 × 1.67.

For a correctly wired ring circuit, what is the expected relationship between R1 and Rn?

R1 should equal Rn.

If the expected R1/Rn/R2 relationships are not achieved, what does this suggest?

Either the conductors are incorrectly identified or there is a continuity problem at one or more accessories.

During Step 1 of ring continuity testing, what is measured and recorded?

The resistance of each complete loop of phase (R1), neutral (Rn) and CPC (R2).

When the CPC has the same CSA as the live conductors, how should its measured resistance compare with R1 and Rn?

The CPC resistance should be the same as the live conductors’ resistance.

If the CPC is 1.5 mm² while the live conductors are 2.5 mm², what correction factor is applied?

The measured CPC resistance should be 1.67 × the live-conductor resistance.

What is done in Step 2 of ring testing and what reading is expected?

Line and neutral are cross-connected to form a figure-of-eight; the resistance measured at the cross-connection should be roughly half of R1 + Rn (i.e. R1 + Rn ⁄ 4).

At each socket during Step 2, how should the line-to-neutral resistance compare with the cross-connection value?

Each socket reading should be essentially the same as the value obtained at the cross-connection.

What is cross-connected in Step 3 of ring testing, and what value is sought?

Line and CPC are cross-connected; the phase-to-earth resistance at each socket should be approximately R1 + R2 ⁄ 4.

Why is a higher resistance expected at a socket wired as a spur during Step 3?

Because the spur adds extra conductor length, giving the highest R1 + R2 value, which is recorded as the circuit’s maximum.

How can the maximum R1 + R2 reading obtained be used after testing?

To calculate the loop impedance Zs of the circuit for verification against regulations.

Why is continuity of ring final circuit conductors so important?

It ensures every outlet is supplied by two conductors; if a break occurs, the ring becomes two radials whose 27 A capacity may be overloaded by a 30 / 32 A protective device, risking overheating and fire.

What does an unintentional spur do to resistance readings and compliance?

It raises the resistance at affected sockets and means the layout does not comply with BS 7671 Appendix 15.

What fundamental rule is addressed by insulation resistance testing according to BS 7671 Chapter 41?

Hazardous live parts must not be accessible and accessible conductive parts must not become live, in normal or single-fault conditions.

What test voltage is typically applied during insulation resistance testing of general circuits?

500 V DC.

Name three types of equipment that should be disconnected before performing a 500 V insulation resistance test.

Examples: dimmer switches, electronic timers/controls, RCDs or RCBOs (also indicator lamps, capacitors, emergency lighting modules, electronic fluorescent starters).

Under BS 7671 416.1, what is ‘basic insulation’ of live parts?

Live parts completely covered with insulation that can only be removed by destruction, or protected by barriers/enclosures meeting the relevant equipment standard.