Twin and Earth American style

[Mark Wright]

Can anyone tell me more about Twin and Earth American style?

What's it's full name? NM?
Is it always flat?
Do you add a sheath to the ground/earth cable?
Thanks

 

[Lucien Nunes]

Unlikely to be silk, which was only used for very fine coil winding wires due to cost. Before PVC, the normal insulation for British conduit cables was VIR, vulcanised rubber covered with an impregnated cotton braid. The rubber provided the insulation, the cotton provided the strength and abrasion resistance, and the impregnation protected it against rot. Other woven coverings for power cables included polybutyl jute and cambric.

 

[Mark Wright]

Thanks. I'll see if I have some pictures but I don't think there was any rubber so I suppose it could of just perished?

 

[Mark Wright]

Probably does have rubber. So annoying had some put aside to look at closer and think it's been binned.
Here's a picture not great.

 

[Lucien Nunes]

That's ordinary VIR conduit cable, I can't see the conductor to say what size it is. Rubber extruded over the conductor (sometimes in two layers) and the braided cotton serving over the top. There's thousands of miles of this stuff still in use, decreasing rapidly now but not all gone yet. The same is true of the nearest USA equivalent. They used a version with a heavier cotton braid but basically the same concept.

When the rubber-insulated cores were used inside a rubber or lead sheath, instead of the woven cotton braid they were usually lapped in cotton tape.

 

[Megawatt]

Can anyone tell me more about Twin and Earth American style?

View attachment 107858
What's it's full name? NM?
Is it always flat?
Do you add a sheath to the ground/earth cable?
Thanks

NM or Romex is flat for 14/2, 12/2, or 10/2 or 3 conductor. Starting at # 8 or # 6 is normally round. I still can’t understand why we have naked grounds but it’s easy to purchase shrink tubing that can be installed on the naked wire. Be sure and mark it green.

 

[Mark Wright]

Thanks for the info.

NM or Romex is flat for 14/2, 12/2, or 10/2 or 3 conductor. Starting at # 8 or # 6 is normally round. I still can’t understand why we have naked grounds but it’s easy to purchase shrink tubing that can be installed on the naked wire. Be sure and mark it green.

Thanks, I assume it's naked like us for cost? but also the Earth/Ground is normally of a smaller size/diameter than the Live and Neutral.

But I would be interested in there is some other thinking behind the ground being bare. I mean I have known people in the UK say it's for safety incase a nail is driven into the cable.

 

[westward10]

Love it, naked ground.

 

[Megawatt]

Thanks for the info.

Thanks, I assume it's naked like us for cost? but also the Earth/Ground is normally of a smaller size/diameter than the Live and Neutral.

But I would be interested in there is some other thinking behind the ground being bare. I mean I have known people in the UK say it's for safety incase a nail is driven into the cable.

The ground is the same size as the conductors because you can’t derate the ground until you reach 40 amps or more.

 

[pc1966]

Thanks, I assume it's naked like us for cost? but also the Earth/Ground is normally of a smaller size/diameter than the Live and Neutral.

The ground size depends on the adiabatic limit and for that the USA has had less demanding specs than the UK for some time, so as @Megawatt says reduced CPC size is only for quite big cables. Similar to the EU in that respect.

The UK has long favoured fuses as OCPD and they are very good at limiting the I2t let-through, also our design requierments on Zs to guarantee the disconnection time also is a factor in keeping I2t down due to the short 't' value and they are rather more onerous than EU/USA.

But I would be interested in there is some other thinking behind the ground being bare. I mean I have known people in the UK say it's for safety incase a nail is driven into the cable.

I think it is partly cost (not just the PVC needed, but the resulting size and weight for shipping), and partly to make the cable smaller so easier to work with.

If you nail a cable it is pot-luck as to what you hit anyway. I think the bigger safety aspect is the E/CPC between the live conductors so less likely to get L-N fault and so more chance of RCD picking up damage due to rodent chewing, etc. But that is more speculation.

 

[Dustydazzler]

Like most things , I imagine the bare copper earth / cpc / ground is purely and simply a Cost thing

 

[Mark Wright]

earth / cpc / ground

All bases covered there

 

[Mark Wright]

In Ireland the CPC is now insulated and the same size - I believe.

 

[Dustydazzler]

In Ireland the CPC is now insulated and the same size - I believe.

And in Australia and New Zealand they have Twin and Earth with a fully insulated earth / cpc / ground

What is interesting in America / Canada they leave the copper ground wire bare , where as we over sleeve it

 

[Mark Wright]

I mean it is kind of worksite speculation but there is a little bit of truth that it adds safety. If the Live or Neutral insulation are damaged they automatically come into contact with the bare CPC. Metal enclosures are kind of automatically earthed.

But I agree and have always been taught it's because of cost.

 

[loz2754]

I mean it is kind of worksite speculation but there is a little bit of truth that it adds safety. If the Live or Neutral insulation are damaged they automatically come into contact with the bare CPC. Metal enclosures are kind of automatically earthed.

But I agree and have always been taught it's because of cost.

This kind of speculation has led some people to believe that it's a requirement for all cables to contain a CPC. (Even on this forum!)
As you say, the only real design advantage is the reduced cost.