selection and erection of wiring systems

Question 1

how can we avoid ferromagnetic effect

Answer 1

single core cables entering the same whole

Question 2

what should the circuit arrangement be

Answer 2

the line and neutral conductors for each circuit need to be seperate from those of other circuits. NO BORROWED NEUTRALS

Question 3

installation of cables -

we have multiple types of cables , e.g twin and earth which has an outer sheathing ( the grey wraps around the brown and blue conductors ) which means you can clip these cables etc. other cables however don’t have outer sheathing which offers mechanical protection

Answer 3

mechanical protection is protection against impact.

single core conductors that don’t have outer sheathing need to be installed in enclosures that offer ilxxd or IP4x

Question 4

wiring systems should be supported so that they wont be liable to premature collapse in the event of a fire by :

Answer 4

clips keep cables in a straight line and stop them sagging. conductors should be in metal trunking rather than plastic that would melt that are less likely to melt. if u have plastic trunking then inside the trunking you should put steel clips / bands and screwing the cables through the trunking and in the ceiling so their held up with steel band so if the trunking were to melt cables would be intact.

Question 5

what are the 3 external influences that can effect selection of a cable.

section 522 of BS7671. pg138-142

we need to take these factors into account when selecting a cable.

Answer 5

1. environment. ( temp , water wind etc )
2. the utilisation. ( who is using it , children , disabled people ?
3. the building its installed in. is that building made of combustible materials ?

Question 6

protection against impact - cables installed under floors or in walls at a depth of less than 50 mm must be :

” damage to conductors “

Answer 6

1. run in positions not liable to be damaged
2. in safe zones
3. buried 4. requires an RCD (30ma if a domestic property)

Question 7

what may affect the current carrying capacity of a conductor.

table 52.1 for thermal insulation correction factor (Ci)

Answer 7

1. cable operating temperature. if our conductor is od a sufficient size , it wont overheat to a point where its dangerous.
2. groups of circuits
3. thermal insulation
   4 . ambient temperature

Question 8

cross sectional areas of conductors - 524 of bs7671

Answer 8

the neutral should always be the same size as the line.

use table 52.3 to make sure what the minimum csa a conductor can be.

Question 9

voltage drop. appendix 4 pg383

Answer 9

you’re allowed to lose 3% of the voltage on a lighting circuit. anything more than 3% would start to have faults . e.g be dim.

all other circuits 5%

3% of 230 is 6.5V. -
5% of 230 is 11.5V. -

Question 10

electrical connections.

we need to join cables to one another. if a cable gets damaged we repair it with a electrical joint. all connections need to be accessible except :

Answer 10

526. 3 of bs7671
527. a joint buried in the ground

etc

Question 11

proximity of wiring systems to other services.

Answer 11

band 1 ( 0-50v ELV ) and and band 2 ( 50-1000v) circuits shall be kept separate unless one of the conductors from 528.1