500 amp mains intake

[westie101]

Earthing them at both ends leads to some interesting circulating currents, it also serves to interconnect the HV steelwork earth with the LV earthing. Something that we routinely do but resist if we do not have control of all the connections

 

[JohnW2]

If, as Westie suggests, the waveforms are earthed at just one end, no fault current would flow. Will see I guess....

I'm tempted to suggest that one "can't have it both ways" - if no fault current can flow through those G/Y's, what purpose are they serving?

Kind Regards, John

 

[westie101]

if no fault current can flow through those G/Y's, what purpose are they serving?

As the cables are part of the DNO system they are ensuring compliance with ESQCR

 

[JohnW2]

if no fault current can flow through those G/Y's, what purpose are they serving?

As the cables are part of the DNO system they are ensuring compliance with ESQCR

Fair enough - but how does that translate into their 'purpose''. As I said, I'm pretty ignorant about anything non-domestic!

Kind Regards, John

 

[westie101]

but how does that translate into their 'purpose''

By ensuring that the live cores are surrounded by an earthed metallic sheath.

Protective screens

13. (1) Underground cables and associated equipment which contain conductors not connected with earth shall be protected in accordance with paragraph (2).

(2) The protection referred to in paragraph (1) shall comprise—

(a)in respect of joints or terminations of a conductor in a low voltage system, some form of mechanical protection; and

(b)in respect of any other part of any conductor, an electrically continuous metallic screen connected with earth,

so placed as to ensure that, so far as is reasonably practicable, any tool or device likely to be used in the vicinity will make contact with that protection or screen before it can make contact with any conductors not connected with earth.

In reality the sheaths could be connected to the HV metalwork earth but that would introduce two separate earthing systems into the customer's switchgear (though one is not connected)

 

[JohnW2]

but how does that translate into their 'purpose''

By ensuring that the live cores are surrounded by an earthed metallic sheath.

I fear we're going around in circles here Needless to say, that's what I assumed, but is it not then implicit that it can, in some circumstances, carry a 'full fault current'? For example, per your quote from ESQCR:

...so placed as to ensure that, so far as is reasonably practicable, any tool or device likely to be used in the vicinity will make contact with that protection or screen before it can make contact with any conductors not connected with earth.

If the cable were pentrated by a tool, which first came into contact with that sheath (whch is earthed via those very modest G/Ys) and then went on to also contact the line conductor within, would not a 'very large' fault current then flow (or try to flow) via the sheath and hence to earth through those 'G/Y tiddlers'?

Kind Regards, John

 

[westie101]

would not a 'very large' fault current then flow (or try to flow) via the sheath and hence to earth through those 'G/Y tiddlers'?

Given the possible fault currents, the size of the bonding conductors would be huge and pretty much impossible to connect to the glands.
The issues are well understood and if anyone were to damage one of these with a hand tool they would have circumvented a number of other safety barriers!
Cables such as these are always installed in a very controlled environment where

I'll let you work the possible currents out, but the protection for a 6 + 1 set of singles would possibly be a set of 86A fuses at 11kV. A fault to earth would need to be seen as Ph - Ph at HV to operate the protection!

 

[JohnW2]

would not a 'very large' fault current then flow (or try to flow) via the sheath and hence to earth through those 'G/Y tiddlers'?

Given the possible fault currents, the size of the bonding conductors would be huge and pretty much impossible to connect to the glands. The issues are well understood and if anyone were to damage one of these with a hand tool they would have circumvented a number of other safety barriers! Cables such as these are always installed in a very controlled environment where!

The issues may be "well understood" by some, but they continue to be mysterious to me. Is is not possible to explain to me, in simple English (preferably short words ), what 'purpose' (other than 'satisfying ESCQR's requirement for an earthed {until the earth conductor melts} metallic sheath) these small G/Y cables serve, given that we seem to be aggreeing that they are far too small to carry the fault currents which theoretically could arise?

Kind Regards, John

 

[westie101]

But you've got it, compliance with ESQCR is all they serve!

 

[westie101]

The reality of contact with one of the cables would probably be the destruction of the tool before the protection would operate, the operative, if a hand tool, possibly suffering serious injury at the least.

 

[JohnW2]

But you've got it, compliance with ESQCR is all they serve!

Oh dear - on a par with bonding plastic pipes, then!

Kind Regards, John