4160V Motor Surge Capacitor Replacement

[deedo1124]

Replacing some PCB containing surge capacitors in a 4160V compressor motor termination box. Contractor reccomends replacing the short length of cable also. Here's our dilema the existing cables are non sheilded. If we replace this short length with sheilded cable as required by the current code what should we do with the sheilds? The existing motor feeds from the gear are un sheilded and are not being replaced. Picture is attached for reference

 

[waross]

This is not a new installation. This is a repair of an existing installation. I would simply replace the capacitor and re-insulate the existing cables.

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

 

[LionelHutz]

I don't think the code applies to that situation. It seems to me that code requirement is for cable that is external to enclosures, for example connecting the MCC and the motor. It makes no sense to use shielded for pieces of wire which are inside a box like that.

 

[electricpete]

I am a little curious to figure out what is in the picture.

I think what it shows is this:[ul]
[li]1- Supply current enters through three single phase round brown insulators or bushings at bottom of the picture. [/li]
[li]2 -Unshielded cable goes from insulators to a 3-phase rectangular grey surge cap unit. Motor T-leads (shown at top of picture) are also landed on the same surge cap unit and are unshielded (like all motor T-leads).[/li]
[li]3 -The insulator/bushing has an inside-TB terminal where this unshielded lead is connected, and another outside-TB terminal which connects to a cable that has been terminated with a stress cone. (?)[/li][/ul]
Are 1, 2, 3 correct?

At any rate, there are no stress cones inside the box and everything looks unshielded, so I don’t see how you could add any shielded cable (need a stress cone at each transition between shielded and unshielded).

Even if this were a new installation, what kind of code requirement would you to install require shielded cables if they run completely inside a term box? All such cables are unshielded in my experience (we rely on proper training of unshielded leads inside the terminal box to prevent stress concentration from close contact to sharp metal corners). Note if you had partial discharge pd coupling capacitors in the termbox, the tapoff cable for those would be unshielded as well.


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(2B)+(2B)' ?

 

[electricpete]

so I don't see how you could add any shielded cable

I guess maybe you were thinking shielded cable between insulator and surge cap with stress cone on each end? Even if there is enough room to do that (not sure there is), that just seems silly.

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(2B)+(2B)' ?

 

[electricpete]

I do see NEC 310.6 states:

310.6 Shielding
Solid dielectric insulated conductors operated above 2000 volts in permanent installations shall have ozone-resistant insulation and shall be shielded. All metallic insulation shields shall be grounded through an effective grounding path meeting the requirements of 250.4(A)(5) or 250.4(B)(4). Shielding shall be for the purpose of confining the voltage stresses to the insulation. Exception: Nonshielded insulated conductors listed by a qualified testing laboratory shall be permitted for use up to 2400 volts under the following conditions:
(a) Conductors shall have insulation resistant to electric
discharge and surface tracking, or the insulated conductor(s) shall be covered with a material resistant to ozone,electric discharge, and surface tracking.
(b) Where used in wet locations, the insulated conductor(s) shall have an overall nonmetallic jacket or a continuous metallic sheath.

And my Early’s Handbook states that the 2400v exception used to be 8kv exception.
So this is supposed to apply to jumpers inside terminal boxes (for new installations)? Has anyone seen a shielded jumper running completely inside a term box?


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[electricpete]

Looking at that insulator/bushing thing some more, it is not obvious how/where we would make up any connection to it. Is it a pass-through / draw-lead type of insulator/bushing ? In that case there is unshielded cable on the other side outside of the term box? That would be unusual.

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[VTer]

Pete,
What I think you are seeing at the bottom of the picture are older style spark-gap style surge arresters. The Surge Caps and Surge arresters are part of the surge pack that comes all pre-wired when you purchase the surge pack. I have even seen bare conductors/links between the cap and arrester - see attached photo. Maybe that can help clear up some questions. I would try to replace the entire surge pack and not just the caps, but in any case I do not see a reason for using shielded cable and would even doubt that you can with such short jumpers.


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[electricpete]

That makes more sense. We only use surge caps for motor surge protection in the terminal box. I have heard of people using gapless mox arresters in the terminal box, but never spark gaps right in the terminal box (they create their own steep wavefront, which surge cap may not completely attenuate). At any rate that makes a lot more sense than bushing… the only place I have seen bushings in a motor terminal box was on a hermetic compressor.

Sounds like unanimous opinion that it doesn’t make any sense to install shielded jumpers inside a terminal box.

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[deedo1124]

Thanks for the input guys. Vter you are correct the capacitor is in the center and the(3)arrestors are in front. There are bus bars in the other side of the box that the incoming 2-500MCM cables terminate to. The 2/0 that feeds the surge equipment is tapped of the bus bars and enters through a insulating plate behind the capacitor. The contractor has agreed to replace the capacitors and try to reuse the existing cable. If the cable insulation falls apart during this exercise they will replace the cable and the arrestors as required since it appears as though the existing cable is moulded to the arrestors. Any new cable will be the 2400V unsheilded type.