decay time to safe touch voltage 3

[jimmy2times]

how would you calculate the time for a MV conductor to decay to voltage that is safe to touch, i.e. extra low voltage. i'm aware that all circuits must be earthed and proven dead before maintenance/work can take place. but if a feeder circuit breaker were to be opened and not its circuit earth not applied and at the remote end of circuit a person were lying in close proximity to an exposed conductor (which one would have to assume live). again before the barrage of abuse about my question and responses like it is fundamental to make area safe first, use of rescue hooks, earthing sticks etc i am aware. this is battle damage consideration and to some extent, people are expendable in such cases. are there any references on this, e.g. if it was a cable feeder to MV/LV transformer, and the circuit breaker at switchboard was tripped but circuit main earth not applied (due to lack of knowledge from rescue party in worst case scenario). Presume it is just a time constant thing with knowledge of R and C values in the circuit. I'm just wondering if any references or thumb guide rules on such a question (which is hypothetical to most i guess).

 

[cuky2000]

Not sure what are you asking:

* Q1: How to calculate the time to decay to safe to touch voltage in a MV feeder.?
A1: After CB open it safety procedure recommend to ground the feeder cable before touching it. Trapping charges in the cable should decay fairly quickly. Most engineers do not consider that an issue for MV applications.

* Q2: Safety Concern of earth (ground) conductor remains connected after circuit breaker open.
A2: Ground conductor on a de-energized feeder is not usually a safety concern since the grounding conductor remain connected to the earth at least in one end.

 

[Palletjack]

Scenario.
Your son or daughter is trapped in a car that is in a river, the car is sinking, a live power line is laying on the roof, and you have to move it to get them out.
Once the breaker is opened, you immediately grab it by hand and throw it out of the way and save them just in the nick of time...

That quick...

Any other time it should be tested and grounded.

 

[waross]

It depends on too many factors to give a definite answer.
What is the voltage?
Are there parallel energized circuits? If so it may not be safe at any time until grounds are installed.
How long is the line?
How dirty are the insulators?
Are there any line to neutral/ground connected loads?
Are there any capacitor banks on the line?
There may be other unintended consequences that I have not thought of.
Are you in a war zone?
If so the proper procedure is to order:
"Sargent. Take some men and quickly and safely clear that power line."

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

 

[jimmy2times]

thanks. yes I was setting myself up for some of those flippant remarks!!! ha!

extract of a paper i have, can't share sorry, suggests charge could stay trapped in capacitance of transformer winding for days. The short time constant set up by the body's resistance (at point in time, worst case, it did come into contact of conductor) however would lead to unpleasant discharge shock but not lethal.

 

[davidbeach]

Consider the open breaker a capacitor rather than an air gap. At transmission voltages the answer is very clearly that it never gets to level that wouldn’t knock you on your keaster (or worse) if you make contact with it. Hot line tools and visible grounds are the only safe approach.

 

[crshears]

What Bill said; it depends on far too many circumstances for any rules of thumb to be applied.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

If there is an induced voltage it never decays.
Stop looking for inappropriate short cuts when life safety is involved.

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