3 phase help

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Mornin` all.

I`ve got a small job, connecting up a paint spraying and baking booth. Customer `phoned last week, as his mate, who works for DNO, had offered him cable. I`d calculated 10mm, and his mate offered 25mm. Cool.

So while I`m elsewhere, they ran 90m of this cable down to the D.B.

I return, call in to check on installation, and find cable to be 3 core. Whoops!

Booth uses 240 for lights and control circuits, and 415 for blowers. DNO guy thought we could use armour; forgetting that that`s a no-no for our side of things.

Is there any other option here than pulling cable out, and re-laying 4 core? Your thoughts would be appreciated. Thanks

 

Mr Sworld

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Hmmmmm....

Afraid not mate, without the neutral inside the armour you'll start getting eddy currents forming in the armour. :(

Rip it out and take it to the scrap yard! At least you'll get a bit of dosh for it. ]:)

 
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:_|

OMG Mate. Did you tell them that they needed 4 core? (But there again, surley the DNO guy would have known this?).

I wonder how the client will take that news. :(

Good Luck with it KME - I mean that.

 
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:_| OMG Mate. Did you tell them that they needed 4 core? (But there again, surley the DNO guy would have known this?).

I wonder how the client will take that news. :(

Good Luck with it KME - I mean that.
Thanks guys.

DNO bloke was thinkin g of his side of things i.e. pme, and thought I could use armour for N+E combined. Pillock.

Client already knows m8.

 

Mr Sworld

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Did you sit back and look wise while all this was going on? ]:)

At least the bloke who installed it was doing a mate a favour and not charging him the going rate! :^O

 
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Probably get shot for even thinking of this as a solution - what about a length of single core double insulated cable (25mm) and simply run it along side of the installed 3 core armoured cable? or is it possible to get some single core armoured cable at all?

If I am wrong gentlemen please say so I am still learning on this big stuff (3phase cable etc)

Andy? :|

 

steptoe

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Probably get shot for even thinking of this as a solution - what about a length of single core double insulated cable (25mm) and simply run it along side of the installed 3 core armoured cable? or is it possible to get some single core armoured cable at all?If I am wrong gentlemen please say so I am still learning on this big stuff (3phase cable etc)

Andy? :|
where is M107 when he's needed,,,,, ]:)

 

steptoe

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Probably get shot for even thinking of this as a solution - what about a length of single core double insulated cable (25mm) and simply run it along side of the installed 3 core armoured cable? or is it possible to get some single core armoured cable at all?If I am wrong gentlemen please say so I am still learning on this big stuff (3phase cable etc)

Andy? :|
not even considering the protection side of things Z, its all down to harmonics etc.

 

steptoe

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Thanks Steptoe its nice to have a reply sent to me that both informative and not ripping my head off for being incorrect. Thank you again :) Andy
sorry was a bit short zee,

basically without the N bunched you can get EMF in the cable, can cause interference etc in other equipment.

then you also get(as you mentioned) the factor of armouring etc.

its not really viable (I wouldnt think) in this sort of situation,

 

steptoe

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No its cool Steptoe, thank you honestly, its just that I have meet some people in screwball that are very condascending is all, unlike yourself :) thanks again anyways, always nice to know on here that I can be wrong and have someone say where and why I have gone wrong.Andy
well to be honest, I think thats why most of us are here.

 
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well to be honest, I think thats why most of us are here.
Couldn`t agree more sir!

Thanks for all thoughts. Must admit Mr Sworld, It was hard not to laugh at the crestfallen look when he realised he WOULD have to actually pay for the cable, unless Someone who does more industrial than I knew of a trick I didn`t.

Confirmed my thinking really, thanks. Rip it up. Nice thought with the inverter Steptoe, but I don`t think, financially, there`d be much in it.

 
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Hmmmmm....Afraid not mate, without the neutral inside the armour you'll start getting eddy currents forming in the armour. :(

Rip it out and take it to the scrap yard! At least you'll get a bit of dosh for it. ]:)
That is completely wrong.I know of loads of three core, 3 phase installs with no neutral. However this can't be bodged up with an external neutral so it will have to be changed.

 
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Slips, m8, I think Mr S. was saying that if the Neutral was required, and was installed as a seperate entity, the eddy current issue would arise. Not that you`ll have eddy currents on a three phase (no neutral) supply.

Thanks for your input. Oh, yes, Good evening.

 

Mr Sworld

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That is completely wrong.I know of loads of three core, 3 phase installs with no neutral. However this can't be bodged up with an external neutral so it will have to be changed.
Yes, if they are supplying motors, or other balanced loads, then the phases are balanced and no neutral is needed. However if you read the original posting then you will realise that the load will not be balanced.

So a neutral is needed to take the current caused by the unbalanced 3-phase. When it is within the sheath the magnetic field caused by current flowing in the neutral cancels out the field caused by the unbalanced current flowing in the phases. Therefore no overall magnetic field is there to induce currents into the metal armour.

With the neutral outside the sheath the phase's fields will not be balanced out. And therefore induce a eddy current in the sheath.

Take a chill pill dude! :)

 
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Yes, if they are supplying motors, or other balanced loads, then the phases are balanced and no neutral is needed. However if you read the original posting then you will realise that the load will not be balanced.So a neutral is needed to take the current caused by the unbalanced 3-phase. When it is within the sheath the magnetic field caused by current flowing in the neutral cancels out the field caused by the unbalanced current flowing in the phases. Therefore no overall magnetic field is there to induce currents into the metal armour.

With the neutral outside the sheath the phase's fields will not be balanced out. And therefore induce a eddy current in the sheath.

Take a chill pill dude! :)
That's more like it. Your original reply said there would be Eddie currents just because there was no neutral, This was before the mention of a possible external neutral being run. Your above post is much easier to read ; \ I read the original as wrong because without a neutral there can not be an imbalance. :x

 
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That's more like it. Your original reply said there would be Eddie currents just because there was no neutral, This was before the mention of a possible external neutral being run. I read the original as wrong because without a neutral there can not be an imbalance. :x
It`s ok. I`m not really actually posting to you two. It`s all in my imagination!!

 
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