High voltage: what does this mean?

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Jhr

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High voltage: what does this mean?

Reading the article in the January 2006 issue of EC&M on page 14 Under the Forensic Casebook, about the unfortunate death of an electrical foreman in one paragraph, sub heading Safety Protocol it states
the electrical crew did not de-energize and ground the terminations on which they were working on
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The Bold part is what I don't understand and what procedure is done to accomplish this.

[ February 16, 2006, 10:35 AM: Message edited by: Jhr ]
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

Section 120.1 of the NFPA 70E identifies the ways in which a system can be made an "electrically safe work condition".

Part (6) states that the phase conductors should be grounded in case of stored or induced voltages or if the possibility of contact with other energized conductors or equipment may take place.

I have seen this procedure done with PPE similar to vehicle jumper cables.
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

I did not read that article. But I can say that the term "high voltage" means different things to different members of the industry. In the power transmission and distribution world, "low voltage" might mean under 600, "medium voltage" might mean from 601 to 25,000, and "high voltage" might mean over 25,000. But to a specialist in fire alarm and security systems, "high voltage" might mean 120 and above, and "low voltage" might mean under 24.
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

There are cable and clamp assemblies made specifically for grounding lines and cables to be worked on.

And, as Bryan said, they do look somewhat like jumper cables.
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

But I can say that the term "high voltage" means different things to different members of the industry. In the power transmission and distribution world
Click to expand...
Acorrding to the article the voltage the electricians were working on was
The Electricians were tasked with installing cable connections between the new 138/13.8kV transformer and the distribution system control room
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Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

Back before LO/TO I used to make guys place shorts and grounds on 480 in some situations.

Paul
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

Utility work rules often require that a line be considered energized unless it has a visible air gap isolating it from the source, and grounds installed on the line side, or many times both sides of the work point.

These grounds are accomplished with the wiring mentioned above if overhead, or with grounding stands if underground with separable connectors.

An underground medium voltage cable can store a serious charge after it has been disconnected, due to its construction.

Jim T
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

Typically medium voltage and higher voltage cables are Spear Tested prior to being cut. This reliably ensures that they are de-energized. Shielded cables can be difficult to test with induction testing equipment so spearing is a highly effective method. One such spearing device is shown below. This allow you to spear the cable from a safe distance.


pg37a.gif
pg37b.gif
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

An underground medium voltage cable can store a serious charge after it has been disconnected, due to its construction.
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So you are saying there is still like a static charge present only with more POW! WOW that is serious stuff, when the grounds are placed how is this done, is there like a surge arrestor to supress this charge or what happens to this "serious charge".
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

quote:
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An underground medium voltage cable can store a serious charge after it has been disconnected, due to its construction.


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So the underground is like a large capacitor, never heard of this.
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

Originally posted by Paul B: So the underground is like a large capacitor, never heard of this.
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Any two metallic objects that are not electrically connected will form a capacitor. The pen on my desk and the watch on your wrist form a capacitor. But due to the separation distance, the effect is a bit tough to notice. However, a medium voltage cable has a large and long central metal core (the conductor), a layer or two of insulation materials, and a large and long metal sheath around the outside. I shouldn't be surprised if the capacitive effect between these two metal objects is significant enough to cause a safety hazard.
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

Charlie is correct. Modern UG cables are large capacitors. They store significant amounts of energy if they are disconnected at the proper point on the sine wave.

There are major resonance problems with ug lines and certain three phase transformer configurations due to the inductance of the transformers in series with the capacitance of the cables.

Beyond that, most utility work rules require circuits to be treated as energized unless they are (1)disconnected and (2)grounded. Without grounds, most circuits, overhead or ug, must be treated as energized .

Jim T
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

Obtain,study NFPA 70E Standard for Electrical in the Workplace. Also suggest an 8 hr.practical seminar to refresh everyone what arc fault can do.

Bob O.84,Pa.
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

We have a 3 mile underground 22KV lead cable that sound like a shotgun going of when we ground it after being de-energized.
 
Re: High voltage: what does this mean?

Re: High voltage: what does this mean?

Yes I understand about capacitance but after shorts and grounds wouldn't this be eliminated?

edit: I should have read the rest of the replies bedore shooting off my mouth.

[ February 22, 2006, 08:46 AM: Message edited by: Paul B ]
 
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