Twin and Earth CPC

[Cookie]

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?

 

[Cookie]

I love that video, but I will still call BS on that third contactor 'till the day I die. I've watched it a number of times and the contactor certainly doesn't appear to move at all.

I'll have to watch it again. I love your avi btw!

 

[pc1966]

but seriously while were roughly on the subject another maybe stupid question from me - how can the minimum recommend size of Earthing conductor for a TT be 2.5mm ?

Basically you are very unlikely to have an earth rod with an Ra below a couple of ohms, more likely tens of ohms, so under fault conditions the maximum current is in the tens to low hundred Ampere range and for a fraction of a second for the breaker to clear it that is OK. Also most TT installations have an incomer RCD that trips at levels of 100mA to 300mA usually.

So basically you won't see a large enough I2t to overload a 2.5mm conductor.

Having said that, personally I would not use anything below 4mm in that case for mechanical strength even for the protected cases.

 

[davesparks]

how can the minimum recommend size of Earthing conductor for a TT be 2.5mm ? this is not the main earthing conductor? this is just an out building or something?

Where is this recommendation?

Unless I'm mistaken an earthing conductor is subject to a minimum size of 6mm. And if it is the conductor which connects to the earth rod it is also subject to a minimum size requirement if any part of it is buried.

 

[pc1966]

Where is this recommendation?

Table 4.4(iii) of the OSG has that for "Protected against corrosion and mechanical damage" cases.

I am not sure I would call it a recommendation though, more of an absolute minimum!

 

[Mark Wright]

Yeah I might be totally off but OSG 4.4 see table 4.4(iii) for TT "Protected against corrosion and mech damage" 2.5 - also iirc 54.1? BS7671.

Post automatically merged: Jun 13, 2020

Table 4.4(iii) of the OSG has that for "Protected against corrosion and mechanical damage" cases.

I am not sure I would call it a recommendation though, more of an absolute minimum!

Yeah Can't really see this being adequate but maybe for a car charger or something TT it might be ok.

 

[davesparks]

Yeah I might be totally off but OSG 4.4 see table 4.4(iii) for TT "Protected against corrosion and mech damage" 2.5 - also iirc 54.1? BS7671.

That is not a recommendation, that is the minimum permitted.

Post automatically merged: Jun 13, 2020

Table 4.4(iii) of the OSG has that for "Protected against corrosion and mechanical damage" cases.

I am not sure I would call it a recommendation though, more of an absolute minimum!

Which is my point exactly, it is not a recommendation, merely the minimum permitted.

 

[Cookie]

Basically you are very unlikely to have an earth rod with an Ra below a couple of ohms, more likely tens of ohms, so under fault conditions the maximum current is in the tens to low hundred Ampere range and for a fraction of a second for the breaker to clear it that is OK. Also most TT installations have an incomer RCD that trips at levels of 100mA to 300mA usually.

So basically you won't see a large enough I2t to overload a 2.5mm conductor.

Having said that, personally I would not use anything below 4mm in that case for mechanical strength even for the protected cases.

So what if metal water piping or CATV gets added latter lowering the fault loop path?

 

[Julie.]

So what if metal water piping or CATV gets added latter lowering the fault loop path?

That would be great, now the majority of the fault current would pass via the pipe rather than the 2.5mm^2 cable to the earth rod.

 

[davesparks]

So what if metal water piping or CATV gets added latter lowering the fault loop path?

The chances are pretty slim when all new underground pipes to a house will be plastic these days.

What is CATV?

 

[pc1966]

What is CATV?

Cable TV

But I can't imagine the coax cable has much current carrying capacity! Long term that will go fibre so isolated eventually, but existing infrastructure can be expected to be there for decades.

 

[Cookie]

The chances are pretty slim when all new underground pipes to a house will be plastic these days.

What is CATV?

Coaxial cable, or cable television.

Also phone lines in the US have a shield which are required to be bonded to the service. This provides parellel paths which lower earth Z substantially.

 

[Cookie]

A bit late but if curious here is how US homes are served outside major metropolitan areas:

1-7 homes on a single transformer. Primary neutral ties in with the secondary neutral. Any phone or CATV is also tied to the secondary neutral at the pole and again at the home.

 

[LastManOnline]

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

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.

This would appear to be the logic behind it.I can appreciate both sides of the argument. Reducing the size of the CPC and not using Insulation for it has obvious economic benefits. On the other side of the equation, a full sized CPC lowers the Fli and allows us to extend the length of the supply cable for cicuits. A full sized CPC will also lower the "touch voltage" appreciably. From an installation point of view the adjustment to full size CPC in T&e cables has been a PAINas as it has knock on effects on the number of cables we can now fit into the standard wavin pipes

 

[Cookie]

What about larger circuits in the UK? Circuits over 32 amps? 63amps? 100 amps? 225 amps?

 

[pc1966]

What about larger circuits in the UK? Circuits over 32 amps? 63amps? 100 amps? 225 amps?

Regulation 543.1.3 suggests the use of the adiabatic equation to compute the minimum size.

But if that is not easy to apply (for example, if the I2t of the source OCPD is unknown) then regulation 543.1.4 has a table which basically has the same earth conductor as the live conductors to 16mm, 16mm for live conductors to 35mm, and then half the live size for conductors above 35mm (rounded up to the next standard size). To illustrate by example:

• 32A supply on 6mm cable would have a 6mm CPC (larger than UK style T&E which is based on adiabatic limits for standard fuse/MCB, but the norm for 3-core round cables)
• 100A supply on 25mm or 35mm cable would have 16mm CPC
• 225A supply on 95mm cable would have 50mm CPC