Disconnection time of external bulk head...

[Sparky12021986]

i need your help please this question is going to be a long 1...

first let me explain the situation...

Electrical Supply

The electrical supply to the house is 230V, 50Hz, single phase and together with the installation forms part of a TN-C-S system. The electrical intake position is housed in a fibre glass enclosure adjacent to the front door, with external opening.

Electrical Installation

Distribution is by an 8-way consumer unit (C.C.U) which currently has 2 spare ways available. Existing overcurrent protection is provided by means of BS 3036, Semi enclosed (rewireable) fuses. The c.c.u is mounted at ceiling height on the inside wall directly above the electrical intake position.

The existing installation within the house and garage is wired in general purpose P.V.C insulated, P.V.C sheathed twin and earth and C.P.C multicore cables to BS 6004embedded within the fabric of the building. The extensio is to use the same wiring system and the 2 spar ways are to be used for dedicated lighting and power final circuits. Existing BS 3036 protective devices are to be replaced with Type B c.b.s to BS EN 60898.

All socket outlets are to BS 1363 and mounted 300 mm above finish floor level (f.f.l). All lighting switches are mounted at 1400 mm above f.f.l.

So after all that here is my question....

During the design stage, explain with reasons why you need to know the disconnection time of the external bulkhead type luminaire?

 

[telectrix]

you need to know disconnection times to calculate Zs values and hence ensure disconnection times are met. ( 0.4secs. for any circuit < 32A ).

 

[EastAnglian]

Strictly speaking its the disconnection time of the circuit supplying the external bulkhead luminaire. Sounds like an exam question!

 

[Sparky12021986]

you need to know disconnection times to calculate Zs values and hence ensure disconnection times are met. ( 0.4secs. for any circuit < 32A ).

my answer is not really to know what the disconnection time really is but why you need to know it... can you possible shed some light on the subject for me plz...

 

[Sparky12021986]

Strictly speaking its the disconnection time of the circuit supplying the external bulkhead luminaire. Sounds like an exam question!

not exam question the boss will chuck out a random question at us and the best answer that he gets from us gets a bonus!!!!!

 

[EastAnglian]

If you are happy with all the above explanations then the reason you should know the disconnection time is to demonstrate you are familiar with BS7671 and so that you can write it on the EIC!!!

If the question is why is it 0.4 secs as opposed to 0.2 or 5 seconds, that's a different question!

 

[telectrix]

say the disconnection time for the circuit is 0.4 secs. you refer to the time/current graphs in bs7671 to see what PFC will cause the breaker/fuse to operate in 0.4 secs. then you need to calculate/measure the Zs to see if it will.

 

[Deleted member 26818]

Knowing the disconnection time, will allow you to determine what size cable, and what type and size of CPD to use.
For instance, it used to be that lighting circuits only required a 5s disconnection time. This has been changed to 0.4 for TN, and 0.2 for TT.
So if you were replacing a CU, and the lighting circuits were all wired for 5s, then you may have to replace the wiring, install supplementary bonding, use a lower rated CPD, or use an RCD.

 

[Sparky12021986]

thank guys i got the question correct but unfortunatly 1 of the other boys beat me to it...