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Discharge of Short Circuit Current in Transformer Cable Box

10/27/2017 4:29 AM

Small jumpers are provided all over flanges for a Transformer Cable box. But, when the cable box flanges are fully painted and connected through nut and bolt, how is it possible to discharge full rated Short circuit current?

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#1

Re: Discharge of short circuit current in Transformer cable box

10/27/2017 4:49 AM

Each metallic part should be bonded by a jumper connected to a point where the paint has been removed. If corrosion protection is required, the area where the paint is removed (or more often the area masked before painting) may be treated with a suitable plating material.

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#2

Re: Discharge of Short Circuit Current in Transformer Cable Box

10/27/2017 11:18 AM

Paint is an electrical insulator, or semi-conductor at best.

See#1 above.

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#3

Re: Discharge of Short Circuit Current in Transformer Cable Box

10/27/2017 1:40 PM

"Small jumpers are provided all over flanges for a Transformer Cable box. But, when the cable box flanges are fully painted and connected through nut and bolt, how is it possible to discharge full rated Short circuit current?"

Short answer: It isn't.

Long answer: This system is improperly installed, paint needs to be scraped off and ground connections added, as stated in post #1, above.

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#5
In reply to #3

Re: Discharge of Short Circuit Current in Transformer Cable Box

10/28/2017 3:04 AM

Thank you. If the installation is not proper (paint removal at nut, bolt surfaces), can this be replaced by a suitable jumper. What should be the jumper size?

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#7
In reply to #5

Re: Discharge of Short Circuit Current in Transformer Cable Box

10/30/2017 8:56 AM

Short answer: No.

Long answer: You are asking for a 'workaround' to avoid doing the proper correction to the instillation. Scrape the paint off the flange and bolt head, unscrew the bolt, reinstall the bolt with a properly sized ground cable, and bond the other end of that ground cable to a suitable earth ground. Depending on the installation, you might be able to avoid scraping paint off by using locking 'star/gear' washers to 'bite' through the paint and engage the bare metal underneath, but you will need to consult with an electrician on-site to determine if this is a viable option.

As to the cable size, you will need to consult an electrician who can put eyeballs on the scene. That's the best way to get an accurate answer.

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#4

Re: Discharge of Short Circuit Current in Transformer Cable Box

10/27/2017 11:27 PM

You get a big fault current to the terminal box no amount of bonding will prevent damage to the terminal box

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#6

Re: Discharge of Short Circuit Current in Transformer Cable Box

10/30/2017 6:50 AM

The simplest solution is to insert star washers under the bolt head and under the nut. As the nut and bolt are tightened, the star washers scrape away any paint and surface grime, and "Robert is your mother's brother" - Anon.

Then, earthing testing is well within the domain of acceptance testing, carried out by the Commissioning Engineer, as is the record keeping associated therewith.

Then, why use metal boxes at all, when so many non-metallic enclosures are available these days (rhetorical question - NNTR)?

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#8
In reply to #6

Re: Discharge of Short Circuit Current in Transformer Cable Box

10/30/2017 9:07 AM

"Then, why use metal boxes at all, when so many non-metallic enclosures are available these days (rhetorical question - NNTR)?"

I'm going to be a smart-alec and answer anyway. The Chicago Electrical Code, for one, does not allow for non-metalic boxes for building electrical runs. All "building power" (anything 50V or above, but typically referring to 110VAC / 220VAC / 3-Phase power) must be run in Metallic boxes, connected with metallic conduit all the way back to the breaker panel/fuse box/distribution box, and that box must be bonded to a solid Earth Ground.

Flexible cables to equipment do not need to be in metal pipe, however, suck flexible cables, if carrying over 50V, must include a ground wire, unless the machine is bonded to ground in another fashion. Some large machines are bonded to an earth ground right at their installation site, by use of a long copper stake inserted into the ground near the machine, and a short cable connecting the stake to the machine chassis.

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#9
In reply to #8

Re: Discharge of Short Circuit Current in Transformer Cable Box

10/30/2017 9:58 AM

How quaint.

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#11
In reply to #9

Re: Discharge of Short Circuit Current in Transformer Cable Box

10/31/2017 1:01 PM

We Chicagoans had to deal with a bad fire(1), once.

Once.

We stay pretty strict about fire safety after that.

Notes:

  1. How bad was it? It was so bad the Chicago Tribune missed publication that day, the only day they missed publication(2) in their long history from their founding in 1847.
  2. And the big headline the next day? FIRE!(3)
  3. And that morning, when Chicagoans saw the headline, all over the city there was a resounding "NooOOOooo [poopie](4)!"
  4. Yes, I self-censor in public forums, most of you should be used to it by now.
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#10

Re: Discharge of Short Circuit Current in Transformer Cable Box

10/30/2017 10:05 AM

If a metal part is isolated from earth by a layer of paint, then <...discharge full rated Short circuit current...> will not happen immediately, and the part will become live until the paint is degraded into a partial conductor that is sufficiently conductive to operate an upstream circuit protective device, with associated hazards to nearby personnel during that period that include smoke, fire, and electric shock.

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