TheScullster
wibbled on Thursday 17 June 2010 09:28


"Tim Watts" wrote

"Normal" runs are Ok, but it's worth asking if you live in a 10 bedroom
mansion and the main CU is in the garage beyond the stables(!!) You get
the idea...



Thanks again Tim

The CU is at the other end of the building to the kitchen (not quite 10
bedroomed mansion though).
The cable runs to get to the boundary of the kitchen loft space are about
12m.
The kitchen is 5m x 3.6m and, at a rough estimate, this gives a total
cable length of approx 50m.

I have managed to cadge a copy of Niceic's 17th Edition Toolbox Guide from
our electrical department (don't think they knew they had it!).
It seems pretty similar to the 17th Edition OSG, so that's a bonus. The
problem with a lot of it though can be interpretation and application,
which is difficult I find when you are DIYing rather than immersed in this
stuff daily.

Phil

Hi,

OK - 50m...


OnSite Guide summarises your max cable lengths as follows:

6mm2 @ 32A = 49m
10mm2 @ 32A = 81m

That is with a BS88-2, BS88-6 fuse or Type B MCB *only*. What do you have?



For all earthing systems:

Your permitted L-E loop impedance including the supply for a Type B 32A
breaker is 1.16 Ohms for a TN* earthing system

You must either assume for TN-S, the supply impedance (called Ze) is 0.8
Ohms or TN-C-S (PEN) 0.35 Ohms *unless* you measure it[1] and prove it is
lower.


[1] With a proper tester not a multimeter(!)

So your max circuit loop resistance (it is a DC test) is TN-S = 1.16-0.8 =
0.36 Ohms, TN-C-S = 1.16-0.35= 0.81 Ohms

You could test that with a decent very low ranging multimeter (and allow the
worst case stated meter error to be safe). Connect L-E at the CU of your
cooker circuit (obviously connected to nothing else) with a chock block,
then measure the resistance between L-E at the cooker connection point. If
the resistance is lower than the stated values you are OK.

Note: for a TN-S earthing system, the L-E resistance limit (called R1+R2)
coincides more of less with the voltage drop limit. If you have a TN-C-S
system you might get away with a slightly longer cable run without danger[1]
but your voltage drops will go outside of tolerance.

[1] The safety factory of R1+R2, combined with your Ze is making sure the
MCB/fuse trips in =0.4 seconds on a dead L-E short at the furthest end of
the circuit (cooker end).

Even if you have a TT (rod) earth and master TT RCD, it's best to use these
limits on cable lengths (unless you do a more in depth calculation) as you
also need the breaker to trip correctly for a L-N fault.

Conclusion
==========

You're on the limit with 6mm2 cable for 50 odd metres. And you should
absolutely measure your circuit run IMO, for the resultant L-E resistance
after it's all wired in bar the CU connection.

Do you have access to a suitable meter?

I would do a more detailed length measurement. If it is 49m, and if you
don't have access to a low resolution ohmmeter, it might be necessary to
switch upto 10mm2 cable. You'll be pretty bombproof then.

HTH

Tim

--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.