Flash Overs

[Wontdothatagain]

As far as i know, the majority of faulty service heads that flash over are caused by overload/heating of the service.

Should they not be designed such that no serious damage would be done to the cutout at currents that did not cause the fuse to operate?

Kind Regards, John

If you consider an installation, say a school, originally it was running fine on 3 phase 100A supply, probably drawing about 40A per phase.

Then a nursery is added to the school as well as a heated indoor swimming pool.

Then the school has to expand and they add 4 large portacabins. The council scrimped on the design and they are quite cold, so the school go out and buy a load of 2KW fan floor heaters but the electrician put all the sockets on 1 phase

Then because of the added children, the kitchen has to be expanded and modernized.

Because of the added children, more staff are employed and a teachers lounge is added.

A meter operator goes out one evening to exchange the meter and as part of his risk assement touches the L1 fuse holder and it is red hot, tails were clamped and L1 was sucking 109A, all the load is removed and the installation is allowed to cool down for 20 minutes, L1 fuse is firmly stuck and annealed into the holder.

DNO are called out and a hole is dug, cable cut, service head disassembled and found that all the pitch had leaked out of the cast iron head and the paper insulation has started to burn away. Meter operator is told he is very very lucky that a flash over had not acured, if he had tried harder and tugged on the fuse a bit harder, it probably would of given way and he would of got a big bang.

Now you may blame the electricians who added and added to the installation, you could blame the staff who stuck in a load of heaters, the head master for not stopping them, but this was all done over a period of 5 years and acording to them "we have never had a problem until you turned up".

 

[Wontdothatagain]

this was 1 week before half term, the school was shut for the week, new cable, service head, CT metering and wiring altered.

 

[westie101]

It's fair to say that not every over-loaded cut-out will suffer damage and fail in the ways seen. So the design is seen as adequate and for the occasional failure the PPE option is deemed acceptable.
It is fair to say that 100% reliability is not really viable without unacceptable results

 

[Wontdothatagain]

Translated, you cant design anything stupid proof.

 

[JohnW2]

If you consider an installation, say a school, originally it was running fine on 3 phase 100A supply, probably drawing about 40A per phase. ...<lots of loads added> ... A meter operator goes out one evening to exchange the meter and as part of his risk assement touches the L1 fuse holder and it is red hot, tails were clamped and L1 was sucking 109A....

Are you not making my point for me? When you say a '3 phase 100A supply' do you mean a supply with 3 x 100A fuses (each presumably able to pass 200A for a substantial period of time). If so, then what you describe is the antithesis of what I was suggesting, since you have a fuse that could allow 200A for a substantial time getting red hot at 109A ! I was talking about a cutout designed so that it didn't get red hot before the fuse operated - wouldn't that make sense?

Now you may blame the electricians who added and added to the installation, you could blame the staff who stuck in a load of heaters, the head master for not stopping them, but this was all done over a period of 5 years and acording to them "we have never had a problem until you turned up".

No, I would blame the cutout design, since it got red hot not only before the fuse operated, but seemingly at a current far less than would cause the fuse to operate.

Kind Regards, John

 

[JohnW2]

It's fair to say that not every over-loaded cut-out will suffer damage and fail in the ways seen. So the design is seen as adequate and for the occasional failure the PPE option is deemed acceptable.

... but is what Wontdothatagain has just described also 'deemed acceptable' - a cutout that became 'red hot' in service at a current far lower than that which would cause its fuse to operate?

Kind REgards, John

 

[westie101]

a cutout that became 'red hot' in service at a current far lower than that which would cause its fuse to operate?

Visibly red hot or figuratively red hot I wonder? As with cable it would probably have a max temperature rating of 70deg C which is very hot to the touch is it not?

 

[SimonH2]

It is very common in the states for each house or two to have there own tranney

Interesting. Is that because they tend to be widely separated and/or because of the currents required with 110/120V supplies?

I suspect that having split-phase supplies is also a significant consideration. If you are keeping neutral at anything like earth potential then you can't just tap off split phase supplies from one set of 3 phase cabling (which would itself need 6 cables).

So that logically suggests a lot of small transformers tapped across two phases on the primary, and with a split phase secondary - each supplying a small number of properties.

 

[JohnW2]

Visibly red hot or figuratively red hot I wonder? As with cable it would probably have a max temperature rating of 70deg C which is very hot to the touch is it not?

I presumed not literally (visibly) red hot (hence I put it in quotes), but my concern remains. To get very hot to the touch at a current which is not much more than half that which would cause the fuse to operate in an hour or so strikes me as being unsatisfactory.

Kind Regards, John

 

[Porque223]

This is equivalent to running your face into a wall at seven feet per second

Only if your mass is the same as a small car.

I think having your face hit by a hard, extremely heavy, moving mass from which your head would crush slightly and then recoil is the same as moving your head into a wall.
A person watching a film of this with no clues in the background would not be able to tell which was moving.

The main thing causing injury is how quickly your face decelerates, the same principle as the 'crumple zone' in the front of a car to provide a predictable constant deceleration in case of a head on crash.

The fist of a trained fighter can go 30 MPH and the fighter's mass is behind a correctly thrown punch, and supposedly the energy expended in a handshake is the same as that of .45 slug.

Figuring the foot-lbs of energy of each and how fast that energy is delivered should correlate well to injury.

If I'm wrong my teachers owe some bucks!

 

[ban-all-sheds]

You said

I think having your face hit by a hard, extremely heavy, moving mass from which your head would crush slightly and then recoil is the same as moving your head into a wall.

Presumably because you don't think the masses involved are relevant.

But then almost immediately you also wrote

Figuring the foot-lbs of energy of each and how fast that energy is delivered should correlate well to injury.

Ho hum.

supposedly the energy expended in a handshake is the same as that of .45 slug.

The muzzle energy of a .45ACP round is in the region of 400 ft-lbf.

When I think of the times I've tightened wheel nuts on a car using a wrench about 1' long, I'm inclined to disagree that the energy in a handshake is anywhere near 400 ft-lbf.

If I'm wrong my teachers owe some bucks!

I don't think that the problem lies with them.

 

[JohnW2]

I think having your face hit by a hard, extremely heavy, moving mass from which your head would crush slightly and then recoil is the same as moving your head into a wall. A person watching a film of this with no clues in the background would not be able to tell which was moving.

Whilst the impact velocity will be the same regardless of which is moving, the amount of kinetic energy which gets turned into work (hence damage) when the 'crumpling' following impact occurs, depends upon the mass of the object which is 'actually moving'.

In the absence of background information, a film of a tennis ball hitting a 1 ton block of concrete at 5mph would not enable you tell whether one, the other or both were moving. However, I presume that you would not expect the tennis ball to suffer the same amount of damage if it were dropped at 5mph onto the concrete block as it would if the concrete block were dropped at 5mph onto the ball - or would you?!

If I'm wrong my teachers owe some bucks!

Maybe

Kind Regards, John

 

[SimonH2]

In the absence of background information, a film of a tennis ball hitting a 1 ton block of concrete at 5mph would not enable you tell whether one, the other or both were moving. However, I presume that you would not expect the tennis ball to suffer the same amount of damage if it were dropped at 5mph onto the concrete block as it would if the concrete block were dropped at 5mph onto the ball - or would you?!

I'd expect the same effect either way if the masses were moving horizontally. By introducing that the concrete block is falling, you've very significantly altered the situation - you not only have to consider inertia/momentum, but also gravity which does make a different depending which object is falling.

The damage is down to the rate of change of velocity (ie acceleration) and duration of the event. So lets look at the two situations :

a) Hard thing is moving. So car hits head at 5mph. Given that mass of car is >> than mass of head, then head is accelerated from 0 to 5mph, and duration of the event is determined by the "squishiness" of head (assuming no deformation of car).

b) Squishy thing is moving. Head is moving at 5mph and hits stationary car. Again, since mass of car >> mass of head, we can assume the car doesn't move so head is accelerated from 5mph to 0mph.

In both cases, the acceleration of the head is rearwards. The change in velocity is 5mph. And I'd expect the event duration (and hence force imposed) to be about the same.


If you want to think about it a bit more, consider two people having a (very narrow) game of tennis in a railway wagon or carriage. Say one player is able to hit the ball at (say) 60mph at the end of the carriage - from a reference position inside the carriage you'd see no difference whether the train was stationary or moving at 60mph such that the ball was stationary (relative to the outside world) while in flight. From a reference point outside the train, the difference would be between the ball hitting the stationary carriage wall at 60mph (train not moving) and the carriage wall hitting a stationary ball (train moving).

 

[Porque223]

When I think of the times I've tightened wheel nuts on a car using a wrench about 1' long, I'm inclined to disagree that the energy in a handshake is anywhere near 400 ft-lbf.

Energy is measured in ft-lbs, torque in lb-ft.
This much torque is usually for crankshaft nuts.

Look these things up, calculate energy and how fast it is dumped and decide for yourself.
The 45 ft-lbs is also supposed to be the energy of a fatal dose of radiation.

BTW, you can probably put out 1 hp for a half minute or 2/3 hp for one minute.
If you don't have an exercise machine, run up several flights of stairs (ft) while timing yourself (sec) , get your weight (lbs) and then it's hp = (ft-lb)/(sec-550).
You can also measure your car's hp using the same method while driving up a hill.

 

[JohnW2]

In the absence of background information, a film of a tennis ball hitting a 1 ton block of concrete at 5mph would not enable you tell whether one, the other or both were moving. However, I presume that you would not expect the tennis ball to suffer the same amount of damage if it were dropped at 5mph onto the concrete block as it would if the concrete block were dropped at 5mph onto the ball - or would you?!

I'd expect the same effect either way if the masses were moving horizontally. By introducing that the concrete block is falling, you've very significantly altered the situation - you not only have to consider inertia/momentum, but also gravity which does make a different depending which object is falling.

Maybe my wording wasn't totally clear. When I said 'dropped at 5mph' I meant 'dropped such that the velocity at the moment of impact was 5mph' - thereby taking gravity out of the equation. I could just as easily have talked about a horizontal 5mph impact. I'm pondering the rest of what you wrote.

Kind Regards, John