Transformer Neutral Cable - Shielded or Unshielded ? 1

[sparky_007]

Hello,
For a transformer 13.8kV WYE-Neutral bushing to NGR (Low Resistance Grounded) cable or 69kV WYE-Neutral to Ground cable (Solidly Grounded) cables, is it shielded or unshielded?

Per NEC/CEC standards, any cable above 2kV should be shielded and by that requirement, neutral cable shall be shielded as well. However, the transformer manufacturer insists on using unshielded cable as the neutral is not a continuously current carrying conductor and will run currents only for 1 or 2 sec before the relay will trip.

Before I push back to vendor to use shielded cable, I wanted to know what others have done in this scenario.

 

[waross]

The voltage to ground seen by a solidly grounded neutral is close to zero and the cable need not be shielded.
For a low resistance grounded neutral the actual value of the low resistance ground must be considered.
If the transformer and the neutral grounding resistor are supplied as a pre-wired, approved unit, then it may not be your problem.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Is the cable accessible? 1 or 2 seconds of raised voltage could ruin someone's day. If guarded it should be okay unshielded. Where we have a similar setup, open wire is used guarded by height and physical guards per NESC.

 

[prc]

In my part of the world, this is brought down to ground through open bus bars supported on 11 or 33 kV pin insulators from tank body.

 

[sparky_007]

The 400A, 10sec NGR is mounted on top of the transformer. The X0 neutral bushing is also on top of the transformer. The 15kV insulated (unshielded) cable will run directly from the top mounted X0 bushing to NGR. The transformer has firewalls and fence to prevent unauthorized access. Height of transformer is more than 10ft. So I would say the cable is not easily accessible.

 

[jghrist]

We have normally used bare conductor or bus bar from the X0 bushing to the NGR when both are on the top of the transformer.

 

[stevenal]

If ever called on to run at 400A for 10s, I imagine things could get a bit warm. Bare conductor avoids the need to specify high temperature insulation.