Twin and Earth CPC

[pc1966]

We in the UK used to use SWG, also wires would be given by strands/diameter (e.g. 3/0.029 for 3 strands of 0.029" diameter each, about 1.3mm2 CAS typically for light circuits, etc).

These days it is all the CSA that is specified, which helps a lot for sizing circuits!

 

[Julie.]

IEC 60364-5-54 states that phase conductors 16mm2 and smaller require a CPC of the same size, over 16 to 35mm2 a 16mm2 CPC and over 35mm2 a half size CPC is required.

My questions is why am I seeing catalogs with harmonized twin and earth with reduced size CPCs over 2.5mm2? And why doesn't the half size rule kick in when wire is over 4mm2 instead of 16mm2?

It doesn't - the cross sectional area is determined by calculation in accordance with 543.1.3, however if you don't want to calculate it then you can refer to 543.1.4 and table 54.7 in which case your summary is correct.

Did you just look at the table rather than reading the standard?

BTW IEC 60364-5-54 is chapter 54 in BS7671 other than the 9 clearly marked exceptions

 

[pc1966]

In reality few electricians have to use the adiabatic equation (though they should know how to) as the IET's On Site Guide has some useful tables that incorporate the information for circuit design. For example this table is for BS88 fuses and shows the maximum Zs values for different CPC sizes:

For example, if you have 10mm T&E cable with a 4mm CPC used as a sub-main feed so you could allow 5s disconnection, you might have a 63A fuse for short circuit protection only, and then the downstream DB can use a mix of MCBs up to 32A in order to provide overload protection with a reasonable chance of selectivity. Looking at the above table you see you max measured Zs is 0.49 ohms, so your final test at the nice new DB would be to confirm this is met.

Also you see the value is 0.62 ohms in all the larger CPC sizes - they are time-limited for the fuse action, where as at 4mm it is adiabatically limited (hence lower Zs for a shorter fault disconnection time).

 

[Gavin John Hyde]

slight variation on the theme, I had a roll of twin and earth recently and the cpc was insulated in green/yellow, made a change... I checked and turns out this is apparently standard in Ireland. suppliers had received wrong pallet and not realised. i think if it was put on sale over here it would sale. I would buy it as it saves sleeving.

 

[Mark Wright]

Some people always say non insulated CPC is safer/better because if there's a break in insulation or nail hammered in or something it will come into contact with the CPC easier, although I don't think this is in any official books you hear it alot.

 

[pc1966]

Some people always say non insulated CPC is safer/better because if there's a break in insulation or nail hammered in or something it will come into contact with the CPC easier, although I don't think this is in any official books you hear it alot.

That might be a factor, but I suspect it was originally driven by the desire to save cost of extra PVC and smaller overall size of cable to make installation easier.

 

[Mark Wright]

and what's the name of the IEC publication? is it free? or where can you buy it?

what country puts in the most input to the IEC regulations?

 

[Dustydazzler]

I was always told by older sparks that Rings and reduced cpcs were due to copper shortages during the war....

 

[Julie.]

and what's the name of the IEC publication? is it free? or where can you buy it?

what country puts in the most input to the IEC regulations?

and what's the name of the IEC publication? is it free? or where can you buy it?

what country puts in the most input to the IEC regulations?

IEC 60364-5-54:2011
Low-voltage electrical installations - Part 5-54: Selection and erection of electrical equipment - Earthing arrangements and protective conductors

Not free - Don't know the cost - I just buy the UK version which includes all sections and is around £76 - amazon, IET... loads of places.

Most of europe - UK is a big contributor

 

[Mark Wright]

IEC 60364-5-54:2011
Low-voltage electrical installations - Part 5-54: Selection and erection of electrical equipment - Earthing arrangements and protective conductors

Not free - Don't know the cost - I just buy the UK version which includes all sections and is around £76 - amazon, IET... loads of places.

Most of europe - UK is a big contributor

Thanks so the IEC is broken down into quite small publications where as BS7671 includes the whole lot in one publication.