How do critters blow fuses?


  #1  
Old 07-19-09, 03:28 PM
G
Member
Thread Starter
Join Date: Nov 2007
Location: MI
Posts: 2,653
Upvotes: 0
Received 1 Upvote on 1 Post
How do critters blow fuses?

Lost a phase at work & looking skyward I noticed a blown fuse hanging down from the utility pole insulator. The power company came out & replaced the fuse & pointed out a fried bird on the ground beneath.
OK ignoring the question of how a small bird could possibly bridge across this wire & another or a ground my main question is this:
How can a bird blow a 100A fuse on a 13,200V feed?? That's 1.3MW minimum to blow the fuse--through a tiny bird?? Happens all the time but I don't understand how.
 
  #2  
Old 07-19-09, 05:35 PM
Tolyn Ironhand's Avatar
Group Moderator
Join Date: Nov 2007
Location: United States
Posts: 15,115
Received 660 Upvotes on 560 Posts
When bird, or any other animal for that matter, touches two lines they cause a direct short. Electricity travels very fast. In a direct short current skyrockets well past 100 amps. If you look at a plain home circuit breaker you will find it has a rating of 10,000 amps. That would be a case of a direct short at 120 volts. So yes, there was a crap load of current going through the bird.

You like dark or black meat?
 
  #3  
Old 07-19-09, 08:37 PM
T
Member
Join Date: Oct 2005
Location: USA
Posts: 1,792
Received 71 Upvotes on 66 Posts
not a bad question

It takes the equivalent of around a hundred ohms or so to blow that circuit. I would challenge anyone to find a hundred ohms of resistance from wing to wing on a bird, or any two points a foot apart on almost any animal that could get up there. I suspect the critter actually allows a plasma/arc to form with the voltage available. This plasma then sets up an ionic path for the current, with far less equivalent ohmage that just the bird. Kind of like a negative resistance mode, like when a gas discharge lamp ignites (neon, mercury vapor, etc)
 
  #4  
Old 07-20-09, 12:54 AM
F
Member
Join Date: Mar 2006
Location: NE Wis / Paris France{ In France for now }
Posts: 4,807
Upvotes: 0
Received 2 Upvotes on 2 Posts
With the 13.2KV system the phase conductors are spaced apart min of 18 inches normally typically 24 inches or more depending on the grid pattern is.{ that for bare conductors but for insluated conductors they can be far much closer depending on which type of insulating materals it used }

To get the arc start jumping on 13.2 KV all you need a couple inches gap to just lit it off but once it get started it will increase the current until the OCPD kick it out.

For the amout of short circuit amp it will depending on the transfomer size and the distance from the transfomer so like example at the substation if the fuse blow on 13.2 KV line next to the substation it will sound like 155 MM gun going off but get to far end of the distubation line it will go PFFTTTT.

Let you know most electricians will clasify two diffrent type of short circuits

Bolted fault { AKA direct short circuit }

Arc fault { aka air gap fault not direct bolt fault }

Both will do serious amout of damage if not have proper OCPD in the place.

The bolted fault will draw the current the highest at begaing and stay constant until OCPD kick out

The Arc fault will start current draw on low side and ramp up until the OCPD kick out or arc out { blow out }

Most common Arc fault mode useally found with critter get between the two or on windy day when the conductors breifly touch each other or very close to the point it start the arc across the gap.

The bolted fault is cause a direct short circuit like conductors are actally touching each others and they will build up alot of heat very quick.

Both type of fault will work like souped up arc welder they will keep going until something will give away.

I work on Med Voltage system pretty often and I do know how serious it will make a arc flash it don't take much to do it.

Merci,Marc
 
  #5  
Old 07-20-09, 08:59 PM
W
Member
Join Date: Mar 2008
Location: Cat Spring, Texas
Posts: 41
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by telecom guy View Post
It takes the equivalent of around a hundred ohms or so to blow that circuit. I would challenge anyone to find a hundred ohms of resistance from wing to wing on a bird, or any two points a foot apart on almost any animal that could get up there. I suspect the critter actually allows a plasma/arc to form with the voltage available. This plasma then sets up an ionic path for the current, with far less equivalent ohmage that just the bird. Kind of like a negative resistance mode, like when a gas discharge lamp ignites (neon, mercury vapor, etc)
I put an ohmmeter across the carcass of a recently fried bird and the resistance was quite high. Of course, he was considerably drier at that point in time (having been exposed to the arc-equivalent temperature of the sun) than when he started the arc.

Wet feathers, fur, etc. all make a really good conductor to establish the arc that, as you say, continues thru the ionized air. Sadly, they're often still alive when we find them. (Point to remember...given the opportunity, opt for the death penalty in a state with lethal injection rather than the electric chair ).

Snakes, buzzards and squirrels are all excellent conductors. I won't try to prove it with any particular electrical theorem, but I can send about 5 carcasses a week to anyone interested in doing the forensic studies.
 
  #6  
Old 07-22-09, 01:17 PM
G
Member
Thread Starter
Join Date: Nov 2007
Location: MI
Posts: 2,653
Upvotes: 0
Received 1 Upvote on 1 Post
Wild. So it actually starts out with the bird's high-resistance body forming a low-current path until the high voltage strikes an arc. At this point his guts vaporize and the arc leaves his body to sustain itself until the OCPD blows.
Cool.

Uhhh...I mean...poor little thing.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: