Balanced loads-harmonic effects.

[Rich408]

My first Post....

My thought and would welcome others.
- My recent experience made think how can this be correct...

Install type that was in place - 3 phase -ali shaped cores - copper armoured - no neutral conductor - Using the Copper as n/e combined.
CSA-185mm- 3 poles.
CSA - 50mm- N/E.

My point is- high ampere rating of The 3 poles - infact Ali shaped is good - As for the Harmonic Effects which the N/E may be to much for the CSA-
As one phase goes into different Harmonics then the Heat generated would impact the rated value of N/E.
When this happens surely the 2 other poles would suffer,so how could it correctly deal with a Fault.

I can say this if the install starts at a 2500 KVA sub-Hv>LV with up to 12 feeds to DB's Each one demanding underload 700 ampere.The cables Bunched/trenched/ connected to the DB's located in High humid/Temps get high to 80 odd the Impacted heat would also have an impact on the rated value of cable.

I do understand the % of load may differ - Small amount allowed-what happens when the EOL Units connected go into fault-
The effects would have a massive impact.

 

[Richard Burns]

I did not know they made copper armoured cable, are you sure it is not concentric!
However the minimum neutral csa you could get away with (without harmonics) is half the size of the line conductors and with a copper equivalent of 110mm² line you do not have half the size on the "armour", with harmonics messing things up you would need an equivalent csa the same as the line.

 

[Marvo]

The combined N&E on a copper armour does sound a little odd to me as well. I'd need to dig out some info to be sure but i think if it's a split concentric the neutral has a higher current rating than the individual phases... It's not a cable we often come across TBH.

 

[Engineer54]

Never seen or heard of such a cable make-up?? Any chance of a photo or two??

As has been correctly stated, reduced neutral multi core cable will be half the phase/line conductor size. You don't see them used as much these days, as the increase in IT equipment and other detrimental loads, precludes their use. The old regional electricity boards used to used them extensively, and are now paying the price on initially saving a few pennies in the past... lol!!

 

[Knobhead]

Rich, you need to give the specification of the cable. Without further information no one can help.

I’ve run rather a lot of 185 3½core, I can assure you the ½ core is 95mm².

 

[Chr!s]

My first Post....

My thought and would welcome others.
- My recent experience made think how can this be correct...

Install type that was in place - 3 phase -ali shaped cores - copper armoured - no neutral conductor - Using the Copper as n/e combined.
CSA-185mm- 3 poles.
CSA - 50mm- N/E.

Are you sure you have a TNC-S (PEN/CNE), is this UK?

My point is- high ampere rating of The 3 poles - infact Ali shaped is good - As for the Harmonic Effects which the N/E may be to much for the CSA-
As one phase goes into different Harmonics then the Heat generated would impact the rated value of N/E.
When this happens surely the 2 other poles would suffer,so how could it correctly deal with a Fault.

You stated balanced load, so all the phases will be producing the same level oh harmonic distortion as the others?

What harmonics are present?

I can say this if the install starts at a 2500 KVA sub-Hv>LV with up to 12 feeds to DB's Each one demanding underload 700 ampere.The cables Bunched/trenched/ connected to the DB's located in High humid/Temps get high to 80 odd the Impacted heat would also have an impact on the rated value of cable.

It all depends upon the Harmonics present, Frequency effects Inductive reactance, then you have to consider the skin effect and the proximity effect.

I do understand the % of load may differ - Small amount allowed-what happens when the EOL Units connected go into fault-
The effects would have a massive impact.

I don't really understand what your asking, and your extremely vague with something which is complex, you will not get the answer i'm afraid.

 

[Bobster]

I did not know they made copper armoured cable, are you sure it is not concentric!
However the minimum neutral csa you could get away with (without harmonics) is half the size of the line conductors and with a copper equivalent of 110mm² line you do not have half the size on the "armour", with harmonics messing things up you would need an equivalent csa the same as the line.

I've seen the armour of a cable to be 1/3 copper, was used quite often in quarries. Never seen a full copper armoured cable though, do you have any photos?