Single Point or Solid Bonding query on 20kv U/G supplies

at 50Hz, depending on how the singles are laid you will get perhaps (400A * 400m * geometry factor)  maybe 10 volts along the length of the armour - imagine unbolting one gland and measuring voltage across the open gap ( the rather unknown geometry factor means you could see  between  lowest maybe  5v   if you had the cores as  a tight and rotating trefoil, maybe as much as 20v if phase cables are widely spaced on ladder and form a large area air cored loop). Just imagine as a thought experiment, please  do not actually undo a live gland  !!!


We know the resistance of the armour, and the parallel CPC path, and we can calculate the amps that will flow round the loop from that driving voltage. This tells you how much heat is lost in pushing current in an endless loop of armour.  Compare that to the expected loss in the cables from current times resistive voltage drop, and you can see what a fraction it is. You will also see why too thin metal gland plates can run hot from this loop current, as well as steel ones running hot from magnetic loss.

To run cool the cross section of the metal between glands needs to match cable cross -section,  so a fairly chunky gland plate or tails are needed.

The ratings for AWA trerfoils  in the annex of BS7671 are for the both ends solid bonded case. Moving to a single end bond and flying CPC allows you to up the core ratings a bit.
this nexan data (see page 11 for example) also has the note  'solid bond' in the top row of cable layouts for the various ratings where this applies


If you get stuck I am happy to help review/ check  sums when I have some free time. With  the caveat I'm as likely to make an error as anyone but the chances of both of us making the same one is reduced a bit.


M.
wiring matters article

more background intro

at 50Hz, depending on how the singles are laid you will get perhaps (400A * 400m * geometry factor)  maybe 10 volts along the length of the armour - imagine unbolting one gland and measuring voltage across the open gap ( the rather unknown geometry factor means you could see  between  lowest maybe  5v   if you had the cores as  a tight and rotating trefoil, maybe as much as 20v if phase cables are widely spaced on ladder and form a large area air cored loop). Just imagine as a thought experiment, please  do not actually undo a live gland  !!!


We know the resistance of the armour, and the parallel CPC path, and we can calculate the amps that will flow round the loop from that driving voltage. This tells you how much heat is lost in pushing current in an endless loop of armour.  Compare that to the expected loss in the cables from current times resistive voltage drop, and you can see what a fraction it is. You will also see why too thin metal gland plates can run hot from this loop current, as well as steel ones running hot from magnetic loss.

To run cool the cross section of the metal between glands needs to match cable cross -section,  so a fairly chunky gland plate or tails are needed.

The ratings for AWA trerfoils  in the annex of BS7671 are for the both ends solid bonded case. Moving to a single end bond and flying CPC allows you to up the core ratings a bit.
this nexan data (see page 11 for example) also has the note  'solid bond' in the top row of cable layouts for the various ratings where this applies


If you get stuck I am happy to help review/ check  sums when I have some free time. With  the caveat I'm as likely to make an error as anyone but the chances of both of us making the same one is reduced a bit.


M.
wiring matters article

more background intro