pg 15-20 Electrical Inspection, Testing & Documentation – Module 6

20 question-and-answer flashcards covering insulation resistance testing, polarity verification, earth fault loop impedance and related BS 7671 requirements from Module 6.

Why must all electrical loads be removed or disconnected before an insulation resistance test?

To prevent damage to sensitive equipment and to avoid their influence on the insulation-resistance measurement.

When loads cannot be removed, how should live conductors be arranged for insulation resistance testing?

Connect line and neutral conductors together and test between the joined conductors and the earthing arrangement.

What must be done to the incoming neutral conductor prior to insulation resistance testing?

It must be disconnected so there is no connection with earth from the incoming supply.

What capability must an insulation resistance tester have according to BS 7671?

It must supply the appropriate DC test voltage at a short-circuit current of 1 mA.

List the two main actions in "Test 1" before insulation resistance testing.

(1) Securely isolate the circuit, including the neutral. (2) Disconnect or remove all current-using equipment and ensure switches controlling any non-removable equipment are open.

Name three checks included in "Test 2" before taking insulation resistance readings.

Disconnect vulnerable equipment, verify instrument/leads condition and battery state, select the correct test voltage/range.

For circuits NOT vulnerable to the test voltage, which additional insulation resistance readings must be taken?

Line-to-earth and neutral-to-earth.

What minimum insulation resistance value is generally acceptable for a 230 V system?

Greater than 1 MΩ.

Below what insulation resistance value (at 230 V) should further investigation normally be considered?

Less than 2 MΩ.

Why are insulation resistance tests important?

They reveal the condition of insulation, helping protect against electric shock, short-circuits, earth faults, leakage currents and potential fire hazards.

Give three examples of specialist tests mentioned that may be required for certain installations.

Protection by SELV/PELV or electrical separation, earth electrode resistance, prospective fault current (others include phase sequence, voltage drop, etc.).

What does BS 7671 Regulation 643.6 require regarding fuses and single-pole devices in polarity testing?

Every fuse and single-pole control or protective device must be connected in the line (live) conductor only.

How must bayonet and Edison screw lamp holders be connected in circuits with an earthed neutral conductor?

Their outer or screwed contacts must be connected to the neutral conductor.

Which combined continuity test often removes the need for a separate polarity test?

The R1 + R2 test method.

What is the difference between dead and live polarity testing?

Dead testing checks wiring with the installation isolated; live testing at the origin confirms that the distributor’s service cables have correct polarity before energising the installation.

What does Ze represent in an electrical installation?

The external earth fault loop impedance of the supply.

Where is Ze measured and why must the earthing conductor be disconnected during this test?

At the origin of the installation; the earthing conductor is separated to remove parallel earth paths and prove the intended means of earthing.

How can earth fault loop impedance be established if direct measurement is impractical?

By calculation or enquiry from the supply provider.

Write the formula that relates Ze, R1, R2 and Zs.

Ze + R1 + R2 = Zs (the total earth fault loop impedance of a circuit).

In a TN-C-S system, what does PEN stand for and what does it signify?

Protective Earth and Neutral—a single combined conductor in the supply that is later separated into PE and N within the installation.