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Andy Wade

wrote:

1mm2 T&E CPC will not have any difficulty at all blowing rewirable
fuses. Its a long way from marginal.

That's not the case at all if we're talking about the 30 A rewireable
fuse (BS 3036). If you superimpose the adiabatic line for a 1 mm^2
conductor with a k value of 115 over the relevant fuse characteristic
you discover that the former lies wholly below the latter. In other
words the fuse can't be relied on to protect the 1 mm^2 CPC at any fault
current. Hence unfused spurs in ring circuits in many old installations
were never compliant in respect of thermal protection of the CPC.
That's why the cable standard (BS 6004) was changed in 1984 to require a
1.5 mm^2 CPC.

Common sense might also hint of a problem in the above case: the fuse
wire is only about 10 thou' smaller than the conductor it's trying to
protect.

What can be marginal is its compliance with disconnect times for new
wiring regulations

That's more a general design matter (coordination), rather than
something related specifically to the fuse or cable. Of course the 0.4
s disconnection time requirement hasn't changed for TN systems (but does
now have wider scope). What has changed is that the old 'alternative
method' (limitation of fault voltage drop) has gone from the 17th ed.,
replaced by a requirement for local supplementary bonding where 0.4 s
disconnection can't be achieved. 30 A rings protected by a rewireable
fuse in TN-S earthed systems do rely on the alternative method if the Ze
value is high, and hence might not comply with the 17th ed.

In general you can say that although rewireable fuses are still
permitted it's now harder than before to ensure full compliance when
using them. Standard circuits - per Table 7.1 in the previous OSGs -
are not necessarily compliant with the 17th ed. It will be interesting
to see how this table looks in the new edition, when it finally comes out.

--
Andy