"Tim Watts" wrote

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

Sorry Tim

I think my response to your last post was misleading...
It's the power circuit that totals 50m - comprising 12m to the kitchen
boundary from the CU and 12m return leg, with the remaining 26m used in the
lap around and the drops down the kitchen walls.

The cooker cable will go to 2 points - length to point 1 approx 14m,
additional length point 1 to point 2 approx 7m.
The earth provision is TN-C-S and all ciruits have Type B MCBs - all power
ciruits are currently protected by a single RCD (split box setup).

Yes I have access to a suitable meter.
The resistance test you describe for the cooker cable will presumably also
prove good connections at any junction boxes/face plate terminals along the
route.

Phil