Things that make you go Hmmm.....

[securespark]

The donny cable is one of ours...

Oh well...

 

[ban-all-sheds]

What's the max ze for that I wonder?

dizz's negative one?

 

[kai]

It looks like a very messy install to me!

 

[ban-all-sheds]

Is your calendar broken, Kai?

 

[securespark]

Needs a new battery.

 

[AdrianUK]

An official conversion of a 400A TP factory supply to PME by Central Networks .....

 

[ban-all-sheds]

It it my eyes, or is that main earth conductor a tad small?

 

[AdrianUK]

I agree. I reckon that its a piece if 16mm sq ...which they have 'tapped' onto the main neutral tail. I'm not sure what size the neutral is ... it is definitely imperial & is smaller than the phase tails but I would still expect it to be something like 95mm sq.

 

[RF Lighting]

As they're VIR tails, they'll be single insulated, so an imperial equivalant of 70mm² to 95mm² looks about right.

With that size of supply, you are required to have 25mm² main bonding conductors, and my quick adibatic calculation requires a 25mm² main earth too.

 

[AdrianUK]

Thanks RF

The main bonds (added after this pic was taken) are in 50mm sq so should be ok .... I think that main N-E bond is too skinny.... did a quick clamp test on it the other day ... its carrying around 30A under normal conditions.

 

[JohnW2]

As they're VIR tails, they'll be single insulated, so an imperial equivalant of 70mm² to 95mm² looks about right. With that size of supply, you are required to have 25mm² main bonding conductors, and my quick adibatic calculation requires a 25mm² main earth too.

But that's per BS7671, which appears not to apply on the DNO's side of the 'DNO provided earth terminal' - DNOs seem to have their own set of rules and calculations which they would probably apply to that conductor from the neutral to the block, so maybe 16mm² (if that's what it is) may be OK for that bit, per their practices.

Kind Regards, John

 

[JohnW2]

I think that main N-E bond is too skinny.... did a quick clamp test on it the other day ... its carrying around 30A under normal conditions.

Do you mean that 30A of the neutral current is going down the sheath of the (presumably previously TN-S) incoming supply? Is that OK?

Kind Regards, John

 

[AdrianUK]

I think that main N-E bond is too skinny.... did a quick clamp test on it the other day ... its carrying around 30A under normal conditions.

Do you mean that 30A of the neutral current is going down the sheath of the (presumably previously TN-S) incoming supply? Is that OK?

Kind Regards, John

Yes, the sheath is carrying around 30A ... but then its going to carry a current... its in parallel with the neutral conductor.

The installation neutral current is usually around the 60-90A figure so I guess this proves that the cable sheath is still intact after all these years ... we reckon the supply was installed circa 1955

 

[RF Lighting]

I wonder why they converted the supply to TN-C-S if the cable sheath is still properly connected to earth?

 

[JohnW2]

Do you mean that 30A of the neutral current is going down the sheath of the (presumably previously TN-S) incoming supply? Is that OK?

Yes, the sheath is carrying around 30A ... but then its going to carry a current... its in parallel with the neutral conductor. The installation neutral current is usually around the 60-90A figure so I guess this proves that the cable sheath is still intact after all these years ... we reckon the supply was installed circa 1955

That's all very well, but that sheath was never intended to carry substantial currents under non-fault conditions. Having said that, I can't see what one can do to avoid the situation, since that sheath needs bonding to the PME earth - as an extraneous-c-p, if nothing else. This is a 'complication' of TN-S to TN-C-S conversions (with the sheath still intact) that I'd never really previously thought about!

Kind Regards, John