17th Edition question


Postby livewire » Wed Jun 17, 2009 2:05 pm

hi
A domestic consumer unit on a TN-C-S system uses a 15A Type 2 mcb, to BS3036 to protect a radial circuit wired in 2.5mm^2 two-core pvc insulated cable with a 1.5mm^2 protective conductor clipped directly to a surface. If this circuit supplies socket outlets for hand held equipment, the maximum length of run is:

a. 35m
b. 43m
c. 54m
d. 58m

The answer from the paper gives a correct answer of 35m.



My assumptions are:

Best way to tackle this problem is to rearrange the formula for voltage drop i.e.

V_drop = [(mV/A/m)*(I_b)*L]/1000

hence

L = [1000*(V_drop)] /[(mV/A/m)*(I_b)]


Problem is the question doesn't give me a value for the design current: I_b.

So I've used a hypothetical value of 14A, since 14A < 15A satisfying I_b < I_n.

Using table 4D5 from the regs I get a voltage drop of 18V (mV/A/m) for installation ref method C clipped direct for 2.5 twin + earth cable.


Using these values in the above formula gives:

L_max = (1000*18)/(18*14) = 71.42m


Some questions then,

Am I missing something?

Am I ok to use a design current of 14A?



help please...






:? :?
livewire
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Joined: Wed Apr 15, 2009 4:06 pm

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Postby kbrownie » Thu Jun 18, 2009 11:08 am

your circuit is peotected by 15A, as it's a socket circuit and load can only be assumed, I'd calculate Ib at 15A.
KB
kbrownie
Posts: 1757
Joined: Mon Mar 05, 2007 9:36 pm


Postby livewire » Fri Jun 19, 2009 10:21 am

[quote="kbrownie"]your circuit is peotected by 15A, as it's a socket circuit and load can only be assumed, I'd calculate Ib at 15A.
KB[/quote]


that's interesting, thanks KB
livewire
Posts: 22
Joined: Wed Apr 15, 2009 4:06 pm


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