Trefoiling of single core cables for 3 Phase AC power

Never heard of this. Who advised you?

The trefoil arrangement [or twisting in a 3 phase group] of cables is undertaken with large current carrying conductors and is done to minimise the effects of self inductance and losses. If sheathed or armoured cables are in use, they should be bonded and grounded at the trefoil point.

If you refer to passing single core cables through a metal plate [e.g. entry into a switchboard] the preferred option is to use a non-ferrous material such as brass or aluminium plate. If steel is used it will be necessary to cut a slot say no more that 0.5mm between each hole cut for the cable gland. Doing this will eliminate current being induced into the plate, that would otherwise circulate in the plate around the cable and cause heat rise.

Greg Lynch

The big power companies don't bother they use side by side conductors without this

Trefoiling, in-fact they deliberately avoid it. Ever seen a big underground cable with trefoil-ed conductors? What about the overhead cables and when singles are drawn through conduits. This is new to me where can I look up more info. What are the codes. UK/US.

Trefoiling(occupying 120° of 360° degrees ofthe cable's cross-section) THE 3 conductors of a 3-phase cable -- and sheathing/armouring OVER all of 3 phases- brings overall Reactance/stray fields/net transmission loss to MINIMUM-POSSIBLE .

Good for feeding a 3 phase heavy motor or a balanced L.T. -415/240V - load .

Limitation is manufacturing capacity.

Hardly available in the Market. That is why 99% applications use 3 of single-phase cables tied together as NEXT-BEST.

But what about the increased capacitance?

All known Loads are lagging--any way.

More Capacitance-should be welcome.

Will at least partly compensate the LAGGING P.F.

In instrument cabling, it is done to minimize RF interference with the sensitive cable signal.

One of the challenges in electricity delivery for the environmentally friendly power station invention was the elimination of both EMI and RFI radiation from transmission cabling. There have been medical reports of the statistically higher occurrence of brain cancer in children living in close proximity to transmission corridors. Debating the accuracy of those findings is a future challenge for the engineering/medical community. But meanwhile, the existence of both EMF and RF radiation from the lines is not in question.

To eliminate both, the stations' delivery strategy calls for MU metal trefoiling of all cabling as well as wrapping it in RF caging with periodic grounds. In addition, all electricity delivery, including capacitance stations, is to be located below ground with the same protection. In additon to the advantages already mentioned by Greg Lynch, the MU metal trefoiling and below-ground corridors, sub-stations and conduits will largely eliminate potential danger from EMI noise, and the grounded RF caging around the cabling will eliminate all stray RF.

Monitoring of the transmission corridors is done by creating deliberate small breaks in the insulation from time to time. EMI and RF monitoring positions along the corridors measure both the breaks (datum) and the lines, looking for unwanted changes.

Other environmental protections offered by the stations can be read about on the bits of Executive Summary posted at http://ecofriendlypower.wetpaint.com . The power staion patent requires all such stations to be cabled in this fashion all the way from the genset to the end-users.

Mark

A single core cable creates concentric magnetic fields radiating outward from the conductor, when this magnetic field encroaches on adjacent ferrous metal local heating is produced as a result of Eddy Currents. I was once called to a customers premises where the 300A meter tails passed through individual drilled holes in the metal trunking at the mains switchgear. With the switch room in darkness it was possible to see that the trunking was glowing a cherry red colour with the heat. This is why single core cable must not be passed through separate holes in metalwork, all cables must pass through the same slot or hole so that the resulting magnetic fields around the phases/neutral conductor cancel each other out. Multi core cables are not a problem because all the magnetic fields from the adjacent conductors are cancelled out within the cable.

In the UK this requirement is covered by BS 7671 - IEE Regs

The main reason for the trefoil formation in heavy current applications is to reduce local heating caused by magnetic Eddy Currents which set up around single core cables.

If three single cables are laid side by side the magnetic fields from the middle cable will to a certain extent be cancelled out by the magnetic field from the two outer cables.

The magnetic field from the outer cables however, have no no magnetic field on their outer side to cancel out and therefore these magnetic fields can react with steelwork to produce local heating.

When cables are laid in trefoil formation this is usually done by spacer blocks to prevent cables actually touching, the concentric magnetic fields around each conductor all intersect equally and especially if all cable are equally loaded then there is no imbalance and therefore no magnetic field being produced.

Obviously having three cables placed together in contact is not a good idea and this would require a design current reduction in the cable. That is why trefoil formation cable should always be used with spacers. In bygone days these were a wooden block with three holes drilled in them in a triangle formation, then the block was sliced through the holes to allow them to be located on the cables

I tried to insert a drawing but have not figured out how to do that yet

Hello

I have answered your Post/comments,

Generally best to use the Cable Maker of your particular cable, but try:

http://relemaccables.com/

If you are still needing help, reply with

Kind Regards....

would you get overheating if singe pyro was used as a 3phase and neutral supply

When single conductor cables are installed, two type losses take place in the cable support system. One is magnetic hysteresis loss and another is eddy current loss. Hysteresis loss can be avoided by installing non-ferrous materials for cable support system. The eddy current loss can be minimized by putting 3-single conductor cable in trefoil formation and by supporting it by using trefoil cable clits. Generally trefoil cable clit is made of stain less steel (non ferrous). This kind of support system also helps to prevent cable damage from mechanical forces developed during the short current flowing through the cable.