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Participant

Join Date: Oct 2017
Posts: 2

NGR Grounding

10/25/2017 10:42 PM

Which type of cable should I use to connect 25 kv X0 of transformer to NGR. I checked in CSA and its mentioned insulated cable which should be rated of system voltage. Which type of insulation jacket or type of cable I should use where high resistance NGR'S 12A and 1080 ohm N-G. Could I use bare copper conductor?

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Guru
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#1

Re: NGR Grounding

10/25/2017 10:52 PM

Use system rated and insulated cable similar to the rest of the installation, use insulated earth cable for the earth connection, make sure the cable is rated for the particular application.

This is only a rough guide. If in doubt or still confused please consult a consultant or local Electrical Engineer for local site and standard-specific advice.

Could I use bare copper conductor?

Doubtful. Copper bus (insulated on the HV end, bare on the earth side) may (may mind you) be acceptable, but it really depends on the local site standards and application.

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Participant

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#3
In reply to #1

Re: NGR Grounding

10/25/2017 10:59 PM

Thanks for your reply. Do u know any manufacturer part number of cable I could use. Could I use XLPE cable? I checked and 25 kv XLPE cable are with concentric neural. Could I use it?

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Guru
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#4
In reply to #3

Re: NGR Grounding

10/25/2017 11:27 PM

XLPE concentric neutral type cable would probably work but it really depends on the site, the standards and application. Details of one type of cable suitable for one site offered may not necessarily be suitable for your site.

Do you have the details of the existing associated 22kV(?) transformer cable you can use as a guide, the cable will likely be the same type and hence a good starting point for selection. After that a quick internet search for cable manufacturers with local agents.

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Guru

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#5
In reply to #3

Re: NGR Grounding

10/26/2017 8:11 AM

Folk cannot give a cable type number because you gave no details of the rating of plant or the fault level/protection & settings or where the you are geographically for relevant standard.

Speaking generally, you do not want this cable to fail - if it shorts you no longer have a very limited earth fault current to limit damage. Also, a cable to just carry 12 amps will probably be blown apart if the NGR fails short. My experience was that cables are sealed-up against damp & trouble-free, while busbars and their enclosure are complicated+expensive technical drawing & parts list jobs and flashed over if the ventilation/sealing or anti-condensation heaters were not adequate or failed [or were not connected/properly installed!].

So...

  1. You want the cable to be rated for 25 kV, not just 25/√3, the normal working voltage, a concentric outer conductor is a good idea with outer earthed, because on cable insulation failure a flashover to unknown nearby plant is avoided.
  2. The cable should be rated for the short circuit circuit current at its location with the NGR shorted and the operate time of the back-up protection which detects that, plus the operate time of generator field suppression or similar, & current decrement of the power source which has to be shut down. That will ensure the cable localises the fault and avoids external damage.
  3. Equipped with this information, you should speak to the technical department of potential cable suppliers in your country, explain the purpose and proposed location of the cable and ask their advise on the correct cable to use & how to install it. Delivery time and availability of cable may be more important than apparent minimum price & size - watch the cable bend radius versus space available - if you are already buying cable of this voltage rating/size for the project, some meters more, fixings & glands for NGR are at bulk price & avoid another order/consignment/customs problem (so long as you do not want the NGR cable before the rest!).
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Guru

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#2

Re: NGR Grounding

10/25/2017 10:56 PM

Google said:

IEEE Standard 141-1993, “Recommended Practice for Electrical Power Distribution for Industrial Plants” (Red Book)

IEEE Standard 142-1991, “Recommended Practice for Grounding of Industrial and Commercial Power Systems” (Green Book)

I too, am clueless. I call an experienced electrical technician when I have these questions.

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Guru

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#6

Re: NGR Grounding

10/26/2017 2:01 PM

Unless local codes require it, the X0 bushing of your transformer should be able to be connected to the NGR with bare conductor (bus bar or cable), as long as any support insulators needed between X0 and the NGR terminal are rated for 25kV or greater. Also make sure that it is protected from physical access same as you would for the bushings themselves (physical clearances for safety purposes, electrical clearances phase-to-ground, etc.). That might mean some sort of enclosure or not, depending on the installation. But electrically, there is not a need for the conductor to be literally insulated as long as it is isolated from earth.

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Guru

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#7

Re: NGR Grounding

10/28/2017 8:44 AM

If you intend to use bare conductor it is recommended to read:

NESC/2007 Rule163. Isolation:

All nonshielded insulated conductors of more than 2500 V to ground and bare conductors of more than 150 V to ground shall be isolated by elevation or guarded in accordance with Rule 124.

Rule124. Guarding live parts

Table 124-1

25 kV vertical above ground: 9 ft 3inches horizontal: 3 ft 7 inch.

Rule 124 C 3:

If the vertical distance in Table 124-1 cannot be obtained, railings or fences may be used

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#8

Re: NGR Grounding

10/29/2017 2:49 PM

If you've got to ask you shouldn't be doing it, get a professional to do it.

Bazzer

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