I think you mean, 4*70 mm^2 +ground [3 phases+1 neutral+ground]06/1kV .
In my opinion neutral could be only 35 mm^2.
For delta connection and for ungrounded supply system you don't need neutral wire.
The neutral is required to be well grounded -if the supply system is grounded.
The metallic motor enclosure has to be grounded.
IEC 60034-1 Rotating electrical machines –Part 1:Rating and performance
11.1 Protective earthing of machines:
"Machines shall be provided with an earthing terminal or another device to permit the
connection of a protective conductor or an earthing conductor"
In this paragraph are some exceptions -in which you don't need it.
If the live conductor is equal or more than 50 mm^2 the grounding conductor has to be 50% of this.
So what do you need is 3*70+35+35 conductors.
it's LV , 3phase +ground, the cable is armored
the earthing system is every where aroud the motor location.
i thought the use of the metallic armoris enough to use a torus.
Armour is not acceptable for grounding (There may be a few exceptions for short lengths).
After some time in service, the armour tends to not conduct across the overlaps between turns but the current tends to spiral down the armour.
What may have been an acceptable grounding path resistance becomes a much higher resistance with the addition of induction due to the spiral armour.
Motors don't often use neutrals unless they have the wye point brought out and are used for induction generators.
Bill
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"Why not the best?"
Jimmy Carter
Armour is widely accepted as an earth path over here in the UK, especially on smaller cables up to about 4C/35mm². Use of good quality cable glands is essential to keeping a good earth path in the long term; there are rather more suppliers of poor quality glands than there are good ones. I can think of about three or four in the latter category.
I agree with waross. According to NEMA MG1 ch.4.20 Grounding Means for Field Wiring recommends
a copper conductor of 1 awg [42 mm^2]. I have to agree with ScottyUK ,also.
In the same paragraph above mentioned in IEC 60034-1, it is stated, however:
"When an earthing terminal is provided in the terminal box, it shall be assumed that the
earthing conductor is made of the same metal as the live conductors.
When an earthing terminal is provided on the frame, the earthing conductor may, by
agreement, be made of another metal (for example, steel). In this case, in designing the
terminal, proper consideration shall be given to the conductivity of the conductor."
If the armor is steel two-tapes made of 0.5 mm thick 90 mm width [iec 60502-1] then the steel total section area will be 90 mm^2.In my opinion, this section correspond only to 15 mm^2 copper, but you need 35.
You want a low resistance path back to the source so that the fault will draw enough current to trip the over-current protection device.
In Canada, armour must be bonded to ground but is not allowed as the bonding/grounding conductor.
At one time it was allowed in lengths less than 3 feet.
Bill
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"Why not the best?"
Jimmy Carter
I also agree using the armoring of a cable for bonding is a no.. no.. we used to use the armoring of 6.6kV PILCA cable for bonding under ground in the Platinum / Gold mines because the ground copper conductor would always get stolen. Since then one can obtain XLPESWA cable with 1/3 ground conductors wound as part of the armoring.
goldel 123 LV Scenario = Lighting DB feeder a 4c/70mm2 + Sep Gnd - LV Motor supply 4c/70mm2 - VSD 3c/70mm2 + Sep 1c/35mm2 Gnd.
