MV short circuit values above 10ka

Status
Not open for further replies.
mbrooke

mbrooke

Batteries Included
Location
United States
Occupation
Technician
What if any extra precautions need to be taken with over head MV exceeding 10ka? Is extra PPE or distance required? Value I have in mind is 16-20ka.
 
W

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
I don't believe the fault current has any impact on working with the exception of arc flash hazard. It is possible that with the higher fault current the incident energy levels may be low based on relay clearing times.

Not knowing the voltage level or type of work, it makes providing a definitive answer impossible. For example, are you gloving or hotsticking? Or just operating a gang operated switch from the ground?
 
mbrooke

mbrooke

Batteries Included
Location
United States
Occupation
Technician
wbdvt said:
I don't believe the fault current has any impact on working with the exception of arc flash hazard. It is possible that with the higher fault current the incident energy levels may be low based on relay clearing times.

Not knowing the voltage level or type of work, it makes providing a definitive answer impossible. For example, are you gloving or hotsticking? Or just operating a gang operated switch from the ground?
Click to expand...

23kv multi grounded neutral- gloving and hot sticking yes.
 
R

RumRunner

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
mbrooke said:
What if any extra precautions need to be taken with over head MV exceeding 10ka? Is extra PPE or distance required? Value I have in mind is 16-20ka.
Click to expand...

Yo, BRO(oke) :)

Are you looking for an ARC expert?
I NOAH guy.

Bantering aside, why would it be a point of concern if the value of current is higher (or lower) and using it as a yardstick in determining the efficacy of the Personal Protective Equipment. (PPE)

I understand that a 45 volt power supply of say, a small welder could create an arc at say 250 amps as opposed to a unit that can deliver a 20 amp current at identical voltage.

In either case, we can safely (although I don't recommend) touch either level of current without subjecting ourselves to potential harm. Think car battery. . . you can see a spark from 100 Ah car battery, but you won't see a spark from a 12 volt garage door opener battery that can only deliver a few milliamps of power.

You would see spark at the input to the spark plug after raising the voltage from the high voltage coil between 6000 to 20000 volts. So, that's what keep our engine running.

Nicola Tesla's premise in his much-touted tesla coil-- involves a high-ratio primary to secondary winding to create a high voltage output to create an arc-- reminiscent of an artificial lightning.

Having said that, it is the voltage that create the arc (at a certain level) not the current.

Re-educate me if I've been out- of- the- loop. :)

Don't get me wrong. The AFCI breaker-protection (the curse of electricians) was based on this principle that is supposedly a “godsend”. I tried them on my circuits but it created more problems than it solves.

So, I ran back to Home Depot to get my refund.
If there is anyone who is happy with AFCI please share your pleasure will you?

Now, let's go back to your “what precaution should be taken” at 23 kV in terms of personal protective
equipment.

CAL-OSHA (California OSHA) recommends the following minimum stand-off clearances when working on MV and HV power lines:


15 kV---------------------------------2 ft
15.1 kV to 35 kV--------------------2 ft -4in (this is where your voltage range is)
46.1 kV to 72.5 kV------------------3 ft -0in
72.6 kV to 121 kV-------------------3 ft-4in
138 kV to 145 kV -------------------3 ft-6in
230 kV to 242 kV--------------------5 ft
500kV to 552 kV--------------------11 ft.
700 kV to 765 Kv –-----------------15 ft


The above guidelines are being used safely and successfully.

Your locality may require greater stand-off clearance for super high voltage (the last one on the list.)
Some may require a minimum of 18 ft.


As a side note:


There are other precautions to safely conduct maintenance on high voltage lines.

Washing high voltage insulators while a 500kV transmission line is live ( hot) is a regular routine for transmission workers at CAL-Edison. . . the primary supplier of electricity to Southern California.

They use purified water to remove contaminants like bird droppings or pollutants that build up on those insulating bells.

If you have driven in Southern CAL during May and October, you will see those helicopters shooting pressurized (stream) of water to clean those insulators.

Workers doing this kind of work are called “pea-shooters”.

Some “shooters” do this from the foot of the tower on a boom.

So, the stand-off clearances promulgated by OSHA is thrown out the window. . . in a way. :)
 
mbrooke

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Huge help! But what is the amps assumed at that voltage? Typical stick and glove line work to my knowledge assumes 12.5ka and below. I know the jumper size doubles above that, at least per POCO policy.
 
T

topgone

Senior Member
mbrooke said:
Huge help! But what is the amps assumed at that voltage? Typical stick and glove line work to my knowledge assumes 12.5ka and below. I know the jumper size doubles above that, at least per POCO policy.
Click to expand...
I have not seen any documentation that says about current rating regarding hot stick/gloves! They are rated in voltage withstand. It only needs a small leakage current to fry a person, IMHO.
 
mbrooke

mbrooke

Batteries Included
Location
United States
Occupation
Technician
topgone said:
I have not seen any documentation that says about current rating regarding hot stick/gloves! They are rated in voltage withstand. It only needs a small leakage current to fry a person, IMHO.
Click to expand...



I know, but I am referring to when a person is gloving the line and something goes bang- the arc flash is another matter even if the gloves are fully insulated.
 
Sahib

Sahib

Senior Member
Location
India
mbrooke said:
I know, but I am referring to when a person is gloving the line and something goes bang- the arc flash is another matter even if the gloves are fully insulated.
Click to expand...
A proper PPE fully rated per risk assessment is a must per OSHA. One recommendation: employees must wear arc-rated face shields rated at a minimum of 12 cal/cm2.

 
W

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
Sahib said:
A proper PPE fully rated per risk assessment is a must per OSHA. One recommendation: employees must wear arc-rated face shields rated at a minimum of 12 cal/cm2.
Click to expand...

Not entirely accurate. The wearing and rating of the arc rated faceshield depends on whether it is a single phase open air exposure and the cal/cm2 hazard level. Refer to OSHA 1910.269 Appendix E, where there is a table for this.
 
W

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
R

RumRunner

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
wbdvt said:
For three phase work, a faceshield is required for 5-8 cal/cm^2 and a arc rated hood or faceshield with balaclava for 9 cal/cm^2 and higher. Since no faceshield was used, this would indicate that the arc flash hazard is below 5 cal/cm^2. This was most likely discussed in the tailboard briefing.
Click to expand...


OP is referring to (hot) live work on high voltage line. Not all high voltage line work involves the likelihood of creating or exposure to arc faults.

Hence, some workers can be seen without face shield.

Breaking and making contacts create arcs. (did I say Noah?) :)

Arc fault events and the accompanying destructive components have two elements.

Arc Flash and Arc Blast.

Arc Flash can melt and vaporize metal, that will often show evidence of melted copper and charred surroundings like enclosures etc.

The Arc Blast could linger even after the initial arc fault event-- especially when certain metal burns.

This is what happened when a substation transformer in New York not too long ago-- had an arc flash that melted and burned aluminum components in the transformer/substation.
Aluminum burns with a bluish hue that was visible from a distance. A bluish night for New Yorkers. :)

In an instantaneous arc blast, it can throw a worker away. . . and no amount of PPE (face shield or whatever) could save a worker from being thrown against a wall for example. A hard hat could save him from concussion though.

Extremely important to have workers properly trained and made aware of the hazards involved.
 
kingpb

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
PPE is not intended to prevent injury. It is intended to keep you from non-recoverable injury. It is rated in Cal/cm^2. Not voltage or current.

Utilities per se do not have to follow NEC 70E, but OSHA would definitely stick their nose in. I wouldn't want to be the guy on the stand being asked the question:

"So, one of your workers was killed, correct"
Yes.
"Was he wearing proper PPE"
Well, I don't know. We hadn't done an arc flash assessment.
"Why is that?"
We are a utility, and arn't governed by the NEC
"So, you had them work on hot lines, without proper safe guards?"
Well, we told them to be careful.
Your honor we request our clients family be awarded $300 Million Dollars; Granted.
 
mbrooke

mbrooke

Batteries Included
Location
United States
Occupation
Technician
myspark said:
OP is referring to (hot) live work on high voltage line. Not all high voltage line work involves the likelihood of creating or exposure to arc faults.

Hence, some workers can be seen without face shield.

Breaking and making contacts create arcs. (did I say Noah?) :)

Arc fault events and the accompanying destructive components have two elements.

Arc Flash and Arc Blast.

Arc Flash can melt and vaporize metal, that will often show evidence of melted copper and charred surroundings like enclosures etc.

The Arc Blast could linger even after the initial arc fault event-- especially when certain metal burns.
Click to expand...

Agreed.

This is what happened when a substation transformer in New York not too long ago-- had an arc flash that melted and burned aluminum components in the transformer/substation.
Aluminum burns with a bluish hue that was visible from a distance. A bluish night for New Yorkers. :)
Click to expand...


You've been watching to much mainstream media! :D What happened was a small Capacitive coupled voltage divider device failed creating an arc from the busbar to ground that relaying failed to clear- the blue is from the nitrogen in the air. NYPD mistakenly called it a transformer explosion and the media ran with it. People now imagine a giant unit the size of a house burning with oil and copper windings giving off that blue light, not that case.


FWIW here are similar incidents, happens semi often in developing counties:


https://www.youtube.com/watch?v=vidFKw8MhBE


https://www.youtube.com/watch?v=mo1-1Zuse7c


Out on a line, less short circuit current thus less intense arc:

https://www.youtube.com/watch?v=oNOIVD_4rWc


In an instantaneous arc blast, it can throw a worker away. . . and no amount of PPE (face shield or whatever) could save a worker from being thrown against a wall for example. A hard hat could save him from concussion though.

Extremely important to have workers properly trained and made aware of the hazards involved.
Click to expand...


As they say open casket vs closed casket.
 
Status
Not open for further replies.
Top