Separation Between 25 kV Power and Insulated Neutral Cable

Exactly what is the specification for the power cable, especially its construction. I do hope that you're putting these cables in their own tray, totally segregated from any other cables.

As long as your Cat-6 cables are rated for 25 kV what's the problem?

power cables are 25 kv XPLE coming from 3 phase of one transformer and I make separation between cable 2times dia of cables centre to centre. I am planing to put 2-4/0 insulated neutral cables in same tray along with power cables which are coming from same transformer 25 kv secondary. I would be separation between power and neutral cable as per csa or NEC

If you stick with 2D, that will almost surely be conservative on account of the lesser voltage difference. (Assuming equivalent insulation.)

You need to remember the purpose for the separation. Cables are primarily separated for either voltage reasons or thermal reasons. The voltage reason doesn't apply in this case and since the neutral isn' t a current carrying conductor in a balanced 3-phase system with low harmonic content, the thermal reason doesn't apply either. So, separate the cables per the code requirements (which are based on thermal requirements) and put the neutral cable anywhere convenient in the same tray.

Is this non-metallic tray, Mildred, or are the cables required to be 2D from any earthed part too?

Good question, just to add the cleats are often aluminium although plastic versions are available. I've never used the plastic version so don't know much about them.

I've always run MV in aluminium trefoil cleats so find the 2D separation of conductors strange. Whatever cleat is used it must be able to withstand the magnetic repulsion during fault conditions.

Ft = (0.17 x Ip²)/S
Where
Ft = Maximum Force per unit length of cable (N)
Ip= Peak short circuit current (KA)
S = Centre to centre distance between neighboring conductors

http://www.youtube.com/watch?feature=player_detailpage&v=HmnrJa7Zk0A#t=84s

I'm startled that your Client's specification doesn't tell you that, Mildred. What a bummer of a contract!

NEC 2014

392.20 Cable and Conductor Installation. (B) Cables Rated Over 600 Volts.

Cables rated over 600 volts and those rated 600 volts or less installed in the same cable tray shall comply with either of the following:

1. The cables rated over 600 volts are Type MC.
2. The cables rated over 600 volts are separated from the cables rated 600 volts or less by a solid fixed barrier of a material compatible with the cable tray.

And what has your Site Manager and Project Manager to say about this spacing. Perhaps asking the client or the consultant maybe a better advantage than arbitrarily asking people on this site.

I have no doubt that you have no idea about installing the cables from the Tfx on cable trays whether flat formation, trefoil, spaced, touching and where you should install the other cables for control; Bucholtz, alarms, tap changer etc.

Approved design drawings provide all this detail, you really should look at them and clarify any anomalies with the designer and consultant. (If a new build, and on existing build, still check with the designer and discover what standards he is using and referring to for your local build/repair).

God help, your cable bend radius if you don't know the spacings.