This may sound like a flippant answer but it isn't. If you don't have that data you can't design the installation correctly. No doubt Sparx / KB / Blakey etc will be along in a moment to give you the full SP.
PEFC is to ensure the gear you use can take the PEFC
Ze to ensure the supply characteristics will allow disconection within the specified times (your supplier is required to provide a supply that meets this criteria, unless you have a TT system)
Assumed current demand is to ensure your intended installation will be man enough.
diconection times is because the regs require disconection of most circuits with 0.4secs, not just lighting circuits.
If I've left anything out I am sure some one will fill in the blanks!
basically, thats one of those hidden areas where your electrician earns his money, over the 'bang it in and if it works it's ok' DIY approach!
could someone please explain to me why i need the follwing information at the design stage of an istallation -
- Prospective earth fault current (I)
- The value of external impedance (Ze)
- Assumed current demand of the installation
- Disconnection times of certain light fittings
rosebery has summed up very well
, about the infomation you have asked about.
Moggy1968 has given a good answer , which i hope is of use 2 u.
with all this taken on board , the installation can be installed correctly , and not , b a fire hazard or shock risk .
Darn good answers all round guys!
Taken slightly further,
PEFC as Moggy states: if potential fault current exceeds say 6kA, then Isolators, MCB's etc would have to be rated for next level up ie 10kA.
Most domestic devices are rated for 6kA.
Ze is required so that when designing a final circuit r1+r2 which can be calculated from length of circuit X cable resistance added to Ze will not exceed allowable Zs for the protective device you propose to use.
Assumed current demand must obviously be lower than main supply fuse,
this is an educated guesstimate based on potential full load allowing for diversity if any, all IET test sheets have a box for this.
(Reg. 311.1 says for economic & reliable design)
I think 'certain light fittings' is a bit of a red herring as all circuits must disconnect before over load/over temp. occurs,
N.B. All circuits must be designed so that later on in-service checks have information to check back with,
regards all, SPARX