You’re forgetting I’ve been in this for 35 years...
I’m talking about tripling or quintupling the capacity.
A 15 shouldn’t be fused for 75kVA..
Blown Transformer Cutout
- Thread starter mbrooke
- Start date
- Status
You’re forgetting I’ve been in this for 35 years...
I’m talking about tripling or quintupling the capacity.
A 15 shouldn’t be fused for 75kVA..
mivey
Senior Member
and don't matter
You’re fooling yourself if you really believe lightning arrestors really stop or “arrest” lightning.
Do you want the engineering or real world answer? The engineering answer is this turns into live line bare hands.
In practice you are what’s known as a floating object. You are at a voltage between ground and line voltage induced by the surrounding field. When you make contact that voltage equals the line voltage AFTER the capacitive charge in your body drains away, causing a spark to jump. Depending on your potential and the voltages involved it can be a little spark or a huge one. It can be lethal. Live line workers wear chain mail suits and discharge with a tool so the arc flows over their bodies instead of through them. The same happens again when breaking loose and can be worse than first contact. There are some versions of IEEE 519 strongly recommending things like never be between two insulation systems which kind of bans bucket truck work. Obviously that’s stupid so it was removed in the next version. But even something as innocent as passing tools via rope from lineman to ground man can involve this effect.
Linemen even use this to their advantage. Above 10 kv they “buzz” Aline with any conductive tool to test for voltage. Just hold say a wrench close to the line and it vibrates.
No tic needed. Of course it’s not an “engineered” wrench so not allowed by most work rules.
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
Then why do they put them on transformers and underground cable transitions?
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
From Mivey I get the idea that at 15kv its not even painful without the mesh suit in a bucket truck (live line bare hands).
Dumb approach- including the fact you could drive into an ungrounded 5kv segment thinking you are still on 15kv.
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
They do in an OR/SICU.
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
In relation to what though? Remember that 15-50kva pole pigs are often overloaded to 200-300% capacity during peaks.
Fuses reflect that, and their time current curves start at around 200% of the link rating.
Nothing stop the oil from getting hot- fuse pops- then linemen sends it in with hot oil.
You've never seen CSP units with their red signal lights lit up all over town when the units are under 10 years old? The bulb burns out before anything else.
They don’t “stop” it. A surge arrester is a voltage variable resistor. As voltage exceeds the MCOV it quickly changes from megaohms to ohms or less. BUT a much bigger factor is cable inductance which increases impedance way more than the arrester. So they definitely work but the idea that if you use say a 15 kV arrester on a 13.5 kV line then you will not get above say 20 kV is not true. There is a way to calculate if I have a surge of say X Volts the resulting surge is Y. On top of that the probability of getting a strike of X Volts or larger is well known. Each individual strike can vary a lot but we know the likelihood of how bad it might get. Voltage is unlimited just that higher voltage strikes become less likely.
Plus a lot of grounds are not as low impedance as we think they are. The rod to ground coupling might be 1 ohm but the souil resistance also plays a factor. In some parts of the country it doesn’t matter how good your ground grid is. You can ground every pole and have arresters on every pole but if the ground conductivity is garbage it doesn’t matter.
mivey
Senior Member
To give a path to earth for nature's current. The voltage dropped across the arrestor and leads is less than the BIL insulating rating of the equipment and that keeps the equipment from flashing over and becoming part of the main path and getting damaged.
mivey
Senior Member
that is what I am telling you.
What are you even talking about? That makes no sense.
mivey
Senior Member
I'm sure the line crew will keep that in mind the next time they haul sensitive surgical instruments up in the bucket to perform surgery at 50 ft in the air.
mivey
Senior Member
we try to limit temporary overloading to 20% in summer an 40% in winter. Doesn"t really matter though because at normal loading the oil can be hot.
It is the sudden rise in pressure that blows the lid off and the oil temp has little to do with it.
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
But as I understand it hot oil increases the odds of flames.
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
Right. So why does lightning still kill transformers?
mivey
Senior Member
not really. The arc will ignite cold oil too. Adding water/steam makes the situation worse. Fighting an already flaming open tank of oil with water is not so smart.
this shows lack of experience and a bigger lack of “listen” to what we are “telling” you...
mivey
Senior Member
Then don't do it. Simple. Use what works. Don't use it if it don't. I know you can buzz 7.2 kV.
I really don't remember about 2.4 kV but I doubt it. It would just lay on the ground and burn so not very strong.
- Status