2391 exam questions

[ta-electrical]

Hi. i got a few questions i'm having trouble with and hoping someone can help answer them.

14) An electrical installation is to be carried out in a barn used for wintering livestock on a farm. State the requirements of BS7671 regarding the:
a) disconnection time for 230V socket outlet circuits on a TN system
b) method of providing supplementry protection against direct contact for socket outlet circuits
c) condition which must be achieved for the Zs in respect to l.n and voltage where protection is by use of an RCD

24) Give two reasons why a designer would use a larger conductor cross-sectional area than those normally used.

Thanks in advance.

 

[Paul.M]

14 a.705.553.1
B. Read thru 705
C.706.410.3.10 (iii) b.... I think.
As for the other question, I haven't looked into it but I bet it will do with length of run and volt drop. Also csa of cpc with the cables being in the ground min .6m and 1.0m and the extra mechanical protection that is required. Hope this points you in the right direction for you to find a complete answer.

 

[Sintra]

a) 705.411.1 In circuits, whatever the type of earthing system, the following disconnection devices shall be provided:
(i) In final circuits supplying socket-outlets with rated current not exceeding 32A, an RCD with a rated residual operating current not exceeding 30 mA
(ii) In final circuits supplying socket-outlets with rated current more than 32 A, an RCD with a rated residual operating current not exceeding 100 mA
(iii) In all other circuits, RCDs with a rated residual operating current not exceeding 300 mA.

b) 705.415.2.1 Additional protection: Supplementary equipotential bonding
In locations intended for livestock, supplementary bonding shall connect all exposed-conductive-parts and extraneous- conductive-parts that can be touched by livestock. Where a metal grid is laid in the floor, it shall be included within the supplementary bonding of the location. Extraneous-conductive-parts in, or on, the floor, e.g. concrete reinforcement in general or reinforcement of cellars for liquid manure, shall be connected to the supplementary equipotential bonding.
It is recommended that spaced floors made of prefabricated concrete elements be part of the supplementary equipotential bonding. The supplementary equipotential bonding and the metal grid, if any, shall be erected so that it is durably protected against mechanical stresses and corrosion.
NOTE: Where a metal grid is not laid in the floor a TN-C-S supply is not recommended.

c) Where an RCD is used for earth fault protection, the following conditions are to be fulfilled for a TT installation:
The disconnection time must satisfy Regulation 411.3.2.2 or 411.3.2.4, and RA x I∆n ≤ 50 V
where:
RA is the sum of the resistances of the earth electrode and the protective conductor connecting it to the exposed-conductive-parts (in ohms)
I∆n is the rated residual operating current of the RCD.

 

[Paul.M]

Take it as gospel from Sintra and not my half ***** answer. At least I had a go, I've just woke up so don't shout to loud if any if mine is wrong but please inform if it is.