Paper 3

Question 1

State the earth return path, external to the installation, for each of the following systems.

i) TN-C-S.
ii) TN-S.
iii) TT.

Answer 1

PEN conductor
Separate Metallic conductor
General mass of earth

Question 2

3b State what happens to the value of insulation resistance when

i) a parallel conductor is provided
ii) the conductor length is decreased
iii) a distribution board is measured as a whole instead of the individual circuits.
iv) the cross sectional area of conductors is increased

Answer 2

i. Decrease
ii. Increases
iii. Decrease
iv. Stays the same

Question 3

when undertaking insulation resistance testing as part of a periodic inspection what does GN3 identify as normal practice

Answer 3

For most installations a test between live conductors connected together and earth is satisfactory
OR; testing between live conductors is not generally required

Question 4

3d State, from the formula Zs = Ze + (R1+R2), what is represented by

i) Zs
ii) Ze
iii) R1+R2.

Answer 4

i. System or total EFLZ
ii. External EFLZ
iii. Resistance of line and cpc conductors

Question 5

State the

i) purpose of the IP coding system
ii) level of protection provided by IPXXB
iii) level of protection provided by IP4X.

Answer 5

i. Level of protection against solids and moisture.
ii. BS finger to a depth of80 mm
iii. 1 mm wire