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Discuss 3 Phase Supply, Need A Seperate Earth in the Commercial Electrical Advice area at ElectriciansForums.net

K

KNIPEX

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Hi guys im after a bit of help please.

if im running a 4core 25mm armoured to a three phase distboard do i need a seperate earth to go with it or can i use the armourings???

i thought you had to run a seperate earth half the size of the cable so a 16mm earth.

all cable calcs say u need a seperate earth so im confused

is there any regs on this please???

thanks in advance.

no replies :(
 
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W

wayne

  • Thread Starter Thread Starter
  • #3
i usually do ,but just done a large job of 10 db,s swa as earth (client spec)so no help here:confused:

section543.2 gives "types of protective conductors,have a read ,post your thoughts
 
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wayne

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  • #4
further to my last post ,our technical dept just did a calc for me ,turns out it needed an earth cable due to length
i would suggest further you have a look at the whole of chapter 54 if you want to rely on the swa as a cpc
 

ian.settle1

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Mentor
Arms
further to my last post ,our technical dept just did a calc for me ,turns out it needed an earth cable due to length
i would suggest further you have a look at the whole of chapter 54 if you want to rely on the swa as a cpc
Wayne do you have a different posting as I can not see any mention of the length of the cable run.:confused:
 
B

Bane

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  • #6
Maybe he meant size?

Anyway, **** that. I answered a question right!!!
 
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wayne

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  • #7
ian ,
this thread doesnt mention cable run ,i was posting about a cable i was sizing up,sorry if i confused you
what the tech dept probably found out was the r1+r2values were too high and the easiest solution was a seperate earth
 
S

Shakey

  • Thread Starter Thread Starter
  • #9
I ripped out 4 core 25mm SWA the other week feeding a 3 phase sub board

probably about 60 metres, they were certainly using the armour as the cpc.....

Wayne, so what is the value of R2 for the armour of SWA? (and i'm not being 'funny' that is a genuine question)

I think this would have more to do with the adiabatic fourmula, because R1+R2 would only be relevant if you knew the breeaker (and its corresponding Zs) protecting the SWA

Now, just thinking logically here chaps, if you have TNS at your house, chances are its being feed from SWA, with the armour being used as the earth........:rolleyes:
 
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wayne

  • Thread Starter Thread Starter
  • #10
both mccb and zs are known
resistance values for swa can be obtained from cable manufacturers
 
S

Shakey

  • Thread Starter Thread Starter
  • #11
both mccb and zs are known
resistance values for swa can be obtained from cable manufacturers
Sorry Wayne, we are bouncing back and 'fore between your job and this chaps job:p

of course I know how to get the R2 for SWA,I was asking what the actual value being used was

this is all getting very confusing:p
 
W

wayne

  • Thread Starter Thread Starter
  • #12
largely my fault ,sorry
dont know the values ,i,m just installing and the tech dept are far more intelligent than me
all i do know is because the run was longer than originally measured an additional cpc was required
 

benji

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Arms
you can get them upto 95mm,don't no why companies can't read cataloges .iv'e just put a 70mm 4 core in with 6.0cpc (computer say's so)computers can't read cat.either
yours benji
 
R

rumrunner

  • Thread Starter Thread Starter
  • #16
you can get them upto 95mm,don't no why companies can't read cataloges .iv'e just put a 70mm 4 core in with 6.0cpc (computer say's so)computers can't read cat.either
yours benji
6.0 mm seems a bit on the lite side ,are you sure its correct:confused:
 
W

wayne

  • Thread Starter Thread Starter
  • #17
looked at the cleveland cable site ,yes they do 25x5 core but its got a 25 mm earth
running a seperate 16mm earth would be cheaper
 

benji

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Arms
yes 6.0mm does look small ,but comp. calc. prog.say,s its ok,it looks crap but we have elec. designers who do all the calcs. for us?


i would sooner install a 5c25mm than a 4c +16.0cpc ie only one cable to pull &only one to g&t

yours benji
 
W

wayne

  • Thread Starter Thread Starter
  • #20
the installation costs would be the same as you can generally get away with installing the swa and earth at the same time
that just leaves the price per metre of 25mm 5c swa versus 25mm 4c swa & 16mm earth
 

ian.settle1

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Mentor
Arms
the installation costs would be the same as you can generally get away with installing the swa and earth at the same time
that just leaves the price per metre of 25mm 5c swa versus 25mm 4c swa & 16mm earth
Thats what I ment it will cost more to run a 5-core swa than 4-core and seperate 16mm cpc
 
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wayne

  • Thread Starter Thread Starter
  • #22
thats what i figure too, 25mm 5c isnt commonly used
in fact i've never seen it used
 
W

wayne

  • Thread Starter Thread Starter
  • #24
I think it may be one that you have to specially order therfore cost goes through the roof
amen to ian(enoch powell)settle ;)
 
R

Rory166

  • Thread Starter Thread Starter
  • #25
Without knowing the full facts I see that no mention has been made of making the remote supply TT. This has the advantage of not requiring a CPC. The Swa is broken on entry to the remote building and is bonded at source. RCD protection is of course required and an earth stake.
 
well scots sparky as you know, if a job's worth doing its worth dragging out! (unless on price)

:D
 

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