Twin and Earth CPC

[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.
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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.
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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.