Question 1

Where BS 7671 show a protective device as having a maximum Zs value of 2.3O the maximum corrected value will be&#10;A) 2.3&#226;&#8222;&#166;&#10;B) 1.84&#226;&#8222;&#166;&#10;C) 1.72&#226;&#8222;&#166;&#10;D) 2.87&#226;&#8222;&#166;

Accepted Answer

BS 7671 requires the maximum Zs value to be corrected for the actual voltage at the point of installation, the type of protective device and the cable length. The correction factor for 230V, type B circuit breaker and 30m length is 0.8. Therefore, the maximum corrected Zs value will be 2.3/0.8 = 1.84Ω.

Question 2

The maximum tabulated Zs for a protective device as shown in BS 7671 is 1.44&#226;&#8222;&#166;, the maximum permissible measured Zs will be&#10;A) 1.44&#226;&#8222;&#166;&#10;B) 1.43&#226;&#8222;&#166;&#10;C) 1.15&#226;&#8222;&#166;&#10;D) 1.08&#226;&#8222;&#166;

Accepted Answer

The maximum tabulated Zs for a protective device is 1.44Ω, but the maximum permissible measured Zs should be less than or equal to 0.8 times the tabulated value for the protective device. Therefore, the maximum permissible measured Zs would be 1.15Ω (1.44 x 0.8 = 1.15). Option C is the only choice that corresponds with this calculation.

Question 3

The stated maximum Ze for a TN-S supply system is&#10;A) 0.35&#937;&#10;B) 0.21&#937;&#10;C) 0.8&#937;&#10;D) 0.85&#937;

Accepted Answer

C

Deck 9: Electrical-Installations