6mm flat twin NO earth?

There certainly were twin without earth cables in the old days - but more usually seen in smaller sizes - typically on lighting circuits - where earthing to all-insulated or out-of-reach accessories wasn't considered necessary. The other possibility is a T&E cable that has had the c.p.c. neatly cut back.

For the likes of a cooker, it was likely that earthing would have been needed from the outset - but that may well have been provided by separate earth conductor (often bare) alongside the twin cable, or even just to a local water pipe.

I'd have to lookup the imperial equivalent of 2.5mm² T&E - I've a feeling the 7-strand L&N were slightly larger than 2.5mm² but the 3-strand c.p.c. was a little smaller than 1.5mm². I've a vague recollection that early metric (solid core) 2.5s had a 1.0mm² c.p.c. (which often doesn't meet modern adiabatic requirements).

   - Andy.

The imperial equivalent of 2.5mm T&E was 0.29" diameter strands: 7 strands for L/N,  3 for E, giving 2.9 / 1.27 mm²

I meant 0.029" !

Yes the lighting circuits for this property have no earth but I can explain that.

When I looked at the cooker cable my first thought was that it was cut or broken out of sight however after investigation there is no CPC at the CU end. The cooker circuit has been extended and must have a join somewhere, the switch/socket has a CPC but I dont know where it is conncected too! 

Twin cable without earth was common in the days of rubber insulation.

PVC or polythene cables without an earth did exist but were rare. Plastic insulated cables started to come into general use in the early 1960s, but earthing of lighting circuits was not a requirement until 1968 IIRC. And enforcement was patchy for some years.

A cooker wired in twin cable hopefully had either a separate earth wire or a connection to a water pipe. 

It is not likely to be exactly 6mm2 either - but there certainly was 2 core cooker cable, I took some out at my parents. The earth ran as 7 strands of  bare copper nailed to the joists back to the under stairs cupboard where the consumer unit was.

The problem of the separate earth in thin strands like that is that sometimes it gets broken or disconnected by accident, so modern practice for lone earth wires requires mechanical protection or something quite chunky.

You do not say if this old cable is rubber or PVC - the conductor sizes metricated and then changed a bit (~ 1970) not long after PVC became dominant over rubber (mid 1960s). The temperature to which you can safely flash rubber for a short period while blowing a fuse is quite a bit higher, so the copper core sizes needed to meet what we now call the adiabatic requirement were not really at the forefront of thinking in the way they are today.

If it is a plastic cable it will be a very early one.

Mike

The CPC at the cooker switch is connected to something/somewhere as I get a Zs of 0.36 ohms

I don't really have any experience of rubber cables, i assume they are obviously rubber ie very bendy?

The cable feels farely stiff and is branded Ashathene, it is 7 strand tinned L+N.

IIRC that cable had rubber insulated cores and a plastic outer sheath.

The rubber insulation tends to deteriorate where exposed at the ends. 

Ashathene was a type  of polythene - but not like modern XLPE,  just plain PE, more like sandwich bags and  the filler in the cladding at Grenfell. It has a slightly slippery feel, and burns rather well..

There may have been more than one variant of hybrid rubber/plastic cables, all sorts of things were tried as cable makers modified their production processes to the new materials.

Rubber would look and feel a bit like bicycle tyre if in good condition, and goes all flaky and disappointing  once perished - which by now most of it has, though a few samples in cool dry conduits are still OK in service.

Mike.