"Smart" consumer units?

[JohnW2]

However, it does lead to some conclusions which I find rather hard to believe. Can you really believe that, say, the IR of 1m length of this cable really is >2TΩ (i.e. >2,000,000 M&#937?

Why not ? If the cable is indeed uniform, then that is exactly what it means - regardless of how difficult it might be to measure.

No argument about that - it is certainly 'what it means'.

I think the only problem is a mental one - in that (in this context) there is a general attitude of "> xG == unmeasurable ≈ infinite" .... I think it's just a mental hangup from dealing with "numbers we can't get our head round".

Yes, they are difficult numbers to get one's head around, but that was not the basis of my statement. I did some mental arithmetic based on guesstimates, which resulted in my 'finding the figure hard to believe'. However, I've now done some 'scribbling arithmetic' which indicates that my mental attempt was seemingly appreciably wrong ....

The resistivity of PVC is around 10^15 Ω.cm. Per spec, the conductor diameter of Cat5e is about 0.5mm and the insulation thickness about 0.25mm, hence a totl 'insulated conductor diameter' is about 1mm. The very difficult thing is guessing the width of the area of contact (when everything is squashed together) between two of the insulated conductors, but I have wildly guessed at 0.5mm - which equates to a contact area of 5cm² for a 1 metre length. With an insulation thickness of 0.25mm that all works out at a resistance of about 5 TΩ (for 1 metre of cable). That's certainly compatible with, and in the same ballpark as, the figure calculated above (from the posted spec) of ">2 TΩ" ... so I have to concede that I probably should not have "found it hard to believe"!

Kind Regards, John

 

[ban-all-sheds]

I know they have split phase, but the highest voltage the insulation on the sensor cable would need to withstand would be 120V. Think about it ...

The highest PD between two points inside a DB would be 240V...

 

[ban-all-sheds]

Think what's it's like talking voltages to "the man in the street". We've got 'low voltage', like you get from a battery that won't kill you. We've got 'mains voltage' that will give you a belt and possibly kill you. And then there's 'everything more than mains voltage' and once you get there then the average man in the street would probably struggle to picture how (say) 132kV relates to 415V - both are "something more than my mains, and it'll kill".

Oh I don't know - I had a belt off a spark plug once, must have been 10's of kV.

Gave me a hell of a surprise, but don't think it killed me.

 

[SimonH2]

I know they have split phase, but the highest voltage the insulation on the sensor cable would need to withstand would be 120V. Think about it ...

The highest PD between two points inside a DB would be 240V...

I know, that too is a true statement, but doesn't negate what I said - I did say you might need to think about it.

Assuming the absence of multiple faults of course.

 

[JohnW2]

I know they have split phase, but the highest voltage the insulation on the sensor cable would need to withstand would be 120V. Think about it ...

The highest PD between two points inside a DB would be 240V...

I know, that too is a true statement, but doesn't negate what I said - I did say you might need to think about it. Assuming the absence of multiple faults of course.

I'm wondering what you mean by 'multiple faults'. As I think BAS was implying, there could be bare busbars within such enclosures as a DB, or 'exposed' terminals etc. within JBs etc. - any of which could be live at the opposite phase to whatever was within the Cat5 cable. Would you count that cable touching one of those bare conductors as a 'fault'?

Kind Regards, John

 

[SimonH2]

OK, with the assumption that you weren't planning to connect the cat 5 cable to any of the live parts, then it will (should) be at something close to around ground potential. Since neither of the phases should exceed 120V to ground then that's all the Cat 5 insulation should be exposed to.

IF you have a fault and connect something in the cat 5 to one of the phases then you will see 240V between that and some other parts - but you will also have fried the electronics board quite comprehensively as well.
For that 240V to be an issue, it would also require that the Cat 5 be in contact with bare metal on the other phase (second fault).

 

[ban-all-sheds]

Which could happen.

Which is why, perfectly sensibly, the requirements talk about the highest voltage present, not the highest voltage to earth.

 

[simpic]

Listerofsmeg,

Did you end up using this solution?

http://www.automatedhome.co.uk/vbulletin/showthread.php?1699-Sub-Meter-Electricity-Monitoring