Electrical hazards to tree workers around bucket trucks.

[fast]

The bucket may be non conductive but the lift part is not. It would be easy to measure the voltage from chassis to earth and flash a horn and light if a voltage is detected. Electrical workers wear insulating gloves and shoes.

Tree trimmers could wear rubber boots around the truck.

If the truck itself is grounded and it contacts a power line the current would be huge, a short circuit. I Imagine there could be explosive results.

Novice question:
Suppose the lift is touching the power line such that I would be injured if I grounded the circuit by standing barefoot on moist grass and grabbed a conductive part of the ungrounded truck. I haven’t touched the truck yet, and you’re there in quality insulated boots. Clearly, if I touch the truck, there’s going to be high voltage, and voltage is something we can measure, but if you touch the truck, is there anything (voltage or otherwise) to measure?

I figure, either the voltage is there and measurable and just not immediately dangerous because I have not touched the truck or more likely the voltage is not there and thus not measurable but very present and dangerous should I touch the truck.

I suppose that’s why you suggested grounding the truck since there’s no alarm if no voltage, but gee, do we really want to ground the truck? That would guarentee voltage should a power line hit the lift. That’s if no ground suggests no voltage.

I’m guessing.

 

[steve_bank]

The bucket may be non conductive but the lift part is not. It would be easy to measure the voltage from chassis to earth and flash a horn and light if a voltage is detected. Electrical workers wear insulating gloves and shoes.

Tree trimmers could wear rubber boots around the truck.

If the truck itself is grounded and it contacts a power line the current would be huge, a short circuit. I Imagine there could be explosive results.

Novice question:
Suppose the lift is touching the power line such that I would be injured if I grounded the circuit by standing barefoot on moist grass and grabbed a conductive part of the ungrounded truck. I haven’t touched the truck yet, and you’re there in quality insulated boots. Clearly, if I touch the truck, there’s going to be high voltage, and voltage is something we can measure, but if you touch the truck, is there anything (voltage or otherwise) to measure?

I figure, either the voltage is there and measurable and just not immediately dangerous because I have not touched the truck or more likely the voltage is not there and thus not measurable but very present and dangerous should I touch the truck.

I suppose that’s why you suggested grounding the truck since there’s no alarm if no voltage, but gee, do we really want to ground the truck? That would guarentee voltage should a power line hit the lift. That’s if no ground suggests no voltage.

I’m guessing.

ctualy grounding the truck intentialy iscould be a bad idea. A lot of current with poyentialy expl;osive results.

If the truck is isolated by say tires and toucjes a wire those on the truck are safe/

If you touch the truck you get hammered.

The power system is referenced to earth potential at the generator. Although the term is used in common electronic circuits to mean a common voltage reference point, technically ground means a metal stake in the ground. If you put a voltmeter between a power wire and the earth you will read a voltage. The earth resistance back to the generator or substation may be high, but it only takes on the order of 10-20 milliamps to stop your heart.

Believe it or not. There have been cases of cows being shocked and electrocuted from power line ground currents due to bad utility wiring and connections. When you are caught in the open in a lightning storm you are better off standing with feet together than apart. That is part of the reason why cows are susceptible. The currents through the erth restance devloops a voltage/meter.

https://www.sandiegocounty.gov/dplu...ortant-Factor-in-Electromagnetic-Exposure.pdf

 

[fast]

The bucket may be non conductive but the lift part is not. It would be easy to measure the voltage from chassis to earth and flash a horn and light if a voltage is detected. Electrical workers wear insulating gloves and shoes.

Tree trimmers could wear rubber boots around the truck.

If the truck itself is grounded and it contacts a power line the current would be huge, a short circuit. I Imagine there could be explosive results.

Novice question:
Suppose the lift is touching the power line such that I would be injured if I grounded the circuit by standing barefoot on moist grass and grabbed a conductive part of the ungrounded truck. I haven’t touched the truck yet, and you’re there in quality insulated boots. Clearly, if I touch the truck, there’s going to be high voltage, and voltage is something we can measure, but if you touch the truck, is there anything (voltage or otherwise) to measure?

I figure, either the voltage is there and measurable and just not immediately dangerous because I have not touched the truck or more likely the voltage is not there and thus not measurable but very present and dangerous should I touch the truck.

I suppose that’s why you suggested grounding the truck since there’s no alarm if no voltage, but gee, do we really want to ground the truck? That would guarentee voltage should a power line hit the lift. That’s if no ground suggests no voltage.

I’m guessing.

ctualy grounding the truck intentialy iscould be a bad idea. A lot of current with poyentialy expl;osive results.

If the truck is isolated by say tires and toucjes a wire those on the truck are safe/

If you touch the truck you get hammered.

The power system is referenced to earth potential at the generator. Although the term is used in common electronic circuits to mean a common voltage reference point, technically ground means a metal stake in the ground. If you put a voltmeter between a power wire and the earth you will read a voltage. The earth resistance back to the generator or substation may be high, but it only takes on the order of 10-20 milliamps to stop your heart.

Believe it or not. There have been cases of cows being shocked and electrocuted from power line ground currents due to bad utility wiring and connections. When you are caught in the open in a lightning storm you are better off standing with feet together than apart. That is part of the reason why cows are susceptible. The currents through the erth restance devloops a voltage/meter.

https://www.sandiegocounty.gov/dplu...ortant-Factor-in-Electromagnetic-Exposure.pdf

Green situation: no wires touch truck.

Yellow situation: wires touch truck. People on truck unharmed.

Red situation: I touch truck. I’m harmed.

During the red situation, the volt meter registers and we secure a measurement.

What fascinates me is the yellow situation. Sure, there’s no harm to those on the truck, but there is a hazard for me to avoid that isn’t present with the green situation.

Does the volt meter register differently when the situation changes from green to yellow? This is a pre-red question.

What I’m speculating is that the voltage of the truck is no different regardless of whether it’s a green or yellow situation. If that’s true, how can we measure the hazard that is present during a yellow situation without making it a red situation?

 

[fast]

ETA: dang that’s not many amps. Like less than 1! Geez that sounds low
ETA2: reminder to self: keep feet close.

 

[skepticalbip]

ctualy grounding the truck intentialy iscould be a bad idea. A lot of current with poyentialy expl;osive results.

If the truck is isolated by say tires and toucjes a wire those on the truck are safe/

If you touch the truck you get hammered.

The power system is referenced to earth potential at the generator. Although the term is used in common electronic circuits to mean a common voltage reference point, technically ground means a metal stake in the ground. If you put a voltmeter between a power wire and the earth you will read a voltage. The earth resistance back to the generator or substation may be high, but it only takes on the order of 10-20 milliamps to stop your heart.

Believe it or not. There have been cases of cows being shocked and electrocuted from power line ground currents due to bad utility wiring and connections. When you are caught in the open in a lightning storm you are better off standing with feet together than apart. That is part of the reason why cows are susceptible. The currents through the erth restance devloops a voltage/meter.

https://www.sandiegocounty.gov/dplu...ortant-Factor-in-Electromagnetic-Exposure.pdf

Green situation: no wires touch truck.

Yellow situation: wires touch truck. People on truck unharmed.

Red situation: I touch truck. I’m harmed.

During the red situation, the volt meter registers and we secure a measurement.

What fascinates me is the yellow situation. Sure, there’s no harm to those on the truck, but there is a hazard for me to avoid that isn’t present with the green situation.

Does the volt meter register differently when the situation changes from green to yellow? This is a pre-red question.

What I’m speculating is that the voltage of the truck is no different regardless of whether it’s a green or yellow situation. If that’s true, how can we measure the hazard that is present during a yellow situation without making it a red situation?

A shock hazard only exists when there is a potential difference present.

.. Green situation: The truck is at ground potential.

.. Yellow situation: The truck and the people in it are all at the potential of the power line. No potential difference, no shock hazard.

.. Red situation: You are at ground potential and the truck is at the potential of the power line. There is a large potential difference between you and the truck so touching it is a severe shock hazard.

ETA:
The shock hazard from ground currents (such as from a lightning strike) exists because of the I^2 X R losses of the ground current. This is why keeping your feet together helps reduce the risk - the shorter distance the current travels the lower the potential difference (voltage) will be.

 

[fromderinside]

Tree workers are an unruly lot.

Tell me about it. We have about an acre facing south 300 yards from the ocean six miles south of the westernmost continental US. That's southern Oregon. We have about 20 people in town whose primary employment is cleaning up storm caused tree problems.

In our seventeen years here we've call for services twelve times. Seven times for spring clean up after bad winters, three time for fallen trees, and two times to remove hazards after a storm. We interviewed 25 different persons for these jobs. Only seven had insurance and legal status while two we used claimed such without providing evidence. We include this latter category because although they didn't cause or recieve harm while working on our place they did have accidents for which they received no injury compensation because they had no insurance or licence.

One of those we did hire admitted he didn't have current insurance or licence because of no insurance. We arranged to pay for his licence and insurance which he presented when he came to do his first job with us. We've used him three times. Sometime you see competent honest people and you find a way to make it work. At least here in secondpoorest county in the state.

No way these people will wear rubber for safety.

Of all our jobs at our property only one was injured when he was topping a pitchy tree. Three weeks off work. A high price to pay.

All our electrical line near tree work is carried out by our electricity provider with professional tree service companies who follow strict safety regulations with both electrical and tree personnel on site during entire work period.

 

[steve_bank]

You can look up the safety levels for current on the net.

Imagine you have a high voltage battery. You run wires from the terminals 100 miles. At the battery you connect the negative terminal ta metal pipe that goes 40 feet in the ground.

There is a wiring diagram for the neighborhood transformer in the link. Neutral is connected to earth at the transformer. At the house the chassis of a refrigerator is connected to earth at the house through the third wire on the plug. Ideally zero current flows in the earth ground. What GFI does is detect current flowing through the earth ground.

https://electronics.stackexchange.com/questions/232734/how-does-ground-mains-work

 

[fast]

Awe, so the alarm needs to sound when the potential changes!

Situation green:
I’m walking unprotected towards the truck which is at ground potential. No big deal since both the truck and I are at ground potential. No difference in potential! Everyone is safe no mater how you slice it. No need for caution, especially if the truck isn’t close to the power lines.

Situation yellow:
I haven’t arrived at the truck yet, but some dummy not paying attention inadvertently brushes the conductive metal lift against the power lines. That isn’t bothersome to the people on the truck because their potential changes to match the potential of the power lines. Weird, but okay. No difference in potential between them on the truck and the power lines, so they are not at immediate risk of harm. Still, there is room for caution given the abrupt rise in potential.

Situation red:
The alarm sounds, not because the truck is grounded (it’s not grounded) but because they have a device that measures the (or their) change in potential. I’m a dummy too, and I figured since their potential changed to match the potential of the power line when the truck’s metal lift brushed up against the power line, that my potential would likewise change leaving me unscathed. Boy was I wrong!!

Next time, i’ll be sure to heed the warning alarm—the alarm that is set to go off—when there is a substantive change (rise?) in potential. Is that something that can be measured without resorting to creating a ground? The alarm needs to exclude the need to ground the truck.

 

[skepticalbip]

Next time, i’ll be sure to heed the warning alarm—the alarm that is set to go off—when there is a substantive change (rise?) in potential. Is that something that can be measured without resorting to creating a ground? The alarm needs to exclude the need to ground the truck.

The alarm sensor needs to monitor the truck's potential with respect to the ground. So one input for the sensor needs to be connected to the truck and another input for the sensor needs to be grounded.

 

[fast]

Next time, i’ll be sure to heed the warning alarm—the alarm that is set to go off—when there is a substantive change (rise?) in potential. Is that something that can be measured without resorting to creating a ground? The alarm needs to exclude the need to ground the truck.

The alarm sensor needs to monitor the truck's potential with respect to the ground. So one input for the sensor needs to be connected to the truck and another input for the sensor needs to be grounded.

Well, that just sucks all to hell. Is there nothing that excludes grounding that can be detected to alert us that live electrical wires have made contact with conductive material? Incredibly cost prohibitive, but we could fit the entire lift with touch sensitive iPad touchscreen technology. Of course, that exceeds the aim to a fault. We don’t want alarms buzzing when a limb, stick, bush, or tree creates pressure upon the sensors.

I envision a bird sitting upon a high voltage powerline. The bird is not touching the ground. I dare say no energy transference has taken place. But, it’s clearly a yellow situation, far more potential danger than should it be sitting upon a tree branch in a green situation. Even a flag pole far from the ground sitting atop a box that sits upon a car placed atop a building that is on the ground is a green situation since there’s no substantive change in potential energy that would be present had there been a high voltage live wire.

I’m no electrician, but what if instead it was a monkey with a test light hanging from a powerline? Could he not manage to test for the dangers without making contact with the ground? Now that I think about it, I recall a test light with a pointy thing that needs to be put in the ground.

But dang, I find it hard to believe that current must pass to the ground before being able to determine that a wire is hot.

 

[steve_bank]

You want to avoid contact. You can sense either the magnetic or electric fields around the wire before contact is made.

 

[bilby]

Next time, i’ll be sure to heed the warning alarm—the alarm that is set to go off—when there is a substantive change (rise?) in potential. Is that something that can be measured without resorting to creating a ground? The alarm needs to exclude the need to ground the truck.

The alarm sensor needs to monitor the truck's potential with respect to the ground. So one input for the sensor needs to be connected to the truck and another input for the sensor needs to be grounded.

Well, that just sucks all to hell. Is there nothing that excludes grounding that can be detected to alert us that live electrical wires have made contact with conductive material? Incredibly cost prohibitive, but we could fit the entire lift with touch sensitive iPad touchscreen technology. Of course, that exceeds the aim to a fault. We don’t want alarms buzzing when a limb, stick, bush, or tree creates pressure upon the sensors.

I envision a bird sitting upon a high voltage powerline. The bird is not touching the ground. I dare say no energy transference has taken place. But, it’s clearly a yellow situation, far more potential danger than should it be sitting upon a tree branch in a green situation. Even a flag pole far from the ground sitting atop a box that sits upon a car placed atop a building that is on the ground is a green situation since there’s no substantive change in potential energy that would be present had there been a high voltage live wire.

I’m no electrician, but what if instead it was a monkey with a test light hanging from a powerline? Could he not manage to test for the dangers without making contact with the ground? Now that I think about it, I recall a test light with a pointy thing that needs to be put in the ground.

But dang, I find it hard to believe that current must pass to the ground before being able to determine that a wire is hot.

'Ground' is an arbitrary potential. It's no different from any other potential, until a comparison is made.

A bird sitting on a 240V line will be fried if the 11kV distributor gets too close to him - as far as he's concerned, there's exactly zero difference between that situation and a 10,760V line while he is on a ground wire.

Voltage is all about difference. That's why birds can land on 40kV lines with impunity. In any system with only one electrical potential, everything is at 'ground' potential. But ground can mean different things in different systems. We use it to refer to the mean potential of the entire local electrical environment.

If you brige the gap between two phases of a three phase power supply, you will fry even though both are nominally the same voltage above 'ground' - because in alternating current, it's phase angle that matters. Neutral can be the same as ground (and for safety it usually is tied to ground somewhere); But it needn't be, and it's very unwise to assume that it will be.

Ground is one of those local concepts that is only meaningful because we view our special situation of being on a planetary surface as being 'normal'. But ground doesn't really occupy a special place in the physics.

Detecting a 'hot' wire is not only impossible without a reference to 'ground'; it's actually meaningless.

It's also a needlessly complex solution to a simple problem. Around here, the electricity supply isn't switched off when people are working on the lines; They use a cherry-picker with an insulated bucket and arm. They can work in direct contact with 'live' wires, and anyone on the ground who touches the truck is in no danger at all.

 

[Treedbear]

... But dang, I find it hard to believe that current must pass to the ground before being able to determine that a wire is hot.

Well in order to measure anything there needs to be a power exchange somewhere. That means I*I*R or V*I or V*V/R. But you need to pass some current so you need a connection. Take any cheap digital votlmeter. Typical input resistance is 10 megohms, so it draws very little current even up to 1000 volts. Also most of the power created by the current flow will be across the DVM's input resistance, rather than across any other resistance in the circuit. That includes imperfections in the ground connection. Therefore a direct connection with a spike hammered in the ground might not be necessary, and maybe something like a chain dragging on the ground would suffice. The other type of connection is a capacitive one in which there is no direct connection at all. Just a capacitive plate which is in relatively close proximity to the ground (relative to how large it is) on which electric charge can accumulate and which is connected in series with the DVM in the current measuring (very low resistance) mode. When the power line first makes contact a current is induced momentarily as charge rushes onto the plate, which can also be measured by a sensitive DVM. One that doesn't need to be connected to ground.

 

[fast]

... But dang, I find it hard to believe that current must pass to the ground before being able to determine that a wire is hot.

Well in order to measure anything there needs to be a power exchange somewhere. That means I*I*R or V*I or V*V/R. But you need to pass some current so you need a connection.

Well, I suppose there needs to be current to measure current, but it’s current that changes yellow to red. I want to be alerted when the situation changes from green to yellow. Something is changing. It’s the potential with respect to ground. The change in potential doesn’t require current, right?

An observer can visually detect that a dangerous powerline has made contact with a metal lift, and we don’t need current for it to be true that the variance of potential has changed. Is there no artificial device capable of replacing the observational and deductive tasks of a person?

 

[skepticalbip]

... But dang, I find it hard to believe that current must pass to the ground before being able to determine that a wire is hot.

Well in order to measure anything there needs to be a power exchange somewhere. That means I*I*R or V*I or V*V/R. But you need to pass some current so you need a connection.

Well, I suppose there needs to be current to measure current, but it’s current that changes yellow to red. I want to be alerted when the situation changes from green to yellow. Something is changing. It’s the potential with respect to ground. Current doesn’t require that, right?

An observer can visually detect that a dangerous powerline has made contact with a metal lift, and we don’t need current for it to be true that the variance of potential has changed. Is there no artificial device capable of replacing the observational and deductive tasks of a person?

What changed between yellow and red is that you provided a different potential (and current path) from the potential that the truck and those in it were at. There needs to be two different potentials (and a path for current flow) for there to be a shock hazard. It is the difference in potentials of you and the truck and your body providing the current path between those potentials that is the shock hazard.

ETA:
You could take a leap onto the truck without harm because in that case your body wouldn't be providing a current path from the truck's potential to the ground's potential.

 

[Treedbear]

... But dang, I find it hard to believe that current must pass to the ground before being able to determine that a wire is hot.

Well in order to measure anything there needs to be a power exchange somewhere. That means I*I*R or V*I or V*V/R. But you need to pass some current so you need a connection.

Well, I suppose there needs to be current to measure current, but it’s current that changes yellow to red. I want to be alerted when the situation changes from green to yellow. Something is changing. It’s the potential with respect to ground. The change in potential doesn’t require current, right?

An observer can visually detect that a dangerous powerline has made contact with a metal lift, and we don’t need current for it to be true that the variance of potential has changed. Is there no artificial device capable of replacing the observational and deductive tasks of a person?

I think you're over-complicating this. Changing potential causes current flow. When the truck touches the power line it acquires electrical charge. That means a momentary current flow. And a very tiny current flow can be used to detect that charge.

 

[Derec]

The alarm sensor needs to monitor the truck's potential with respect to the ground. So one input for the sensor needs to be connected to the truck and another input for the sensor needs to be grounded.

Potential difference between truck and ground also means an electric field. You could fit an electric field meter to measure the electric field between the chassis and the ground without it being actually connected to the ground. It would have to be capable of detecting alternating electric fields to be useful of course.

 

[Derec]

I think you're over-complicating this. Changing potential causes current flow. When the truck touches the power line it acquires electrical charge. That means a momentary current flow. And a very tiny current flow can be used to detect that charge.

It would be momentary in DC, but this is AC, so the truck would be changing from positively to negatively charged 60 times each second, so a little current would be flowing in and out all the time.

 

[fast]

Maybe it’s the terminology, but there is something about the word, “potential” that bugs me. It reminds me of risk. I can drive 90mph in a 30mph zone all day long and not cause an accident or run someone over. Doing so increases the risk of causing harm, but no amount of risk equates to any actual harm. In some convoluted sense, maybe, but not in straight forward talk.

What I’m left thinking is that potential energy is not in fact a kind of energy at all—or at least not a type of energy. If one says x has energy, then if the only energy one can point to that x has is potential energy, then one was incorrect to begin with to say x has energy. That would be like saying risk of harm is a type of harm.

In fact, maybe that’s why the word, “potential” is used, not to suggest that something has energy but rather that something can produce it. There is no energy UNTIL the circuit is completed. When I grab ahold of the truck, energy then exists, but prior to touching it, although there are things with divergent potential energies, it’s not to say there are two things each with its own present energy. No, its not pent up energy that begins flowing—even if energy begins flowing.

I’m just speculating, and if I have it wrong, then common usage may just be the way it is despite the difference I make.

 

[bilby]

Maybe it’s the terminology, but there is something about the word, “potential” that bugs me. It reminds me of risk. I can drive 90mph in a 30mph zone all day long and not cause an accident or run someone over. Doing so increases the risk of causing harm, but no amount of risk equates to any actual harm. In some convoluted sense, maybe, but not in straight forward talk.

What I’m left thinking is that potential energy is not in fact a kind of energy at all—or at least not a type of energy. If one says x has energy, then if the only energy one can point to that x has is potential energy, then one was incorrect to begin with to say x has energy. That would be like saying risk of harm is a type of harm.

In fact, maybe that’s why the word, “potential” is used, not to suggest that something has energy but rather that something can produce it. There is no energy UNTIL the circuit is completed. When I grab ahold of the truck, energy then exists, but prior to touching it, although there are things with divergent potential energies, it’s not to say there are two things each with its own present energy. No, its not pent up energy that begins flowing—even if energy begins flowing.

I’m just speculating, and if I have it wrong, then common usage may just be the way it is despite the difference I make.

Potential energy certainly is a form of energy. It's energy that's being resisted (or that hasn't yet finished being expended) If you are in an airplane at 40,000 feet, you have a gravitational potential energy, that is being resisted by the lift generated by the plane's wings and engines. Step out of the plane, and that potential energy is translated into kinetic energy, until something (eg the surface of the Earth) intervenes to stop that from happening.

On the roof of a tall building, your gravitational potential is resisted by the structure of the building itself.

Electrical potential is similar - while there's a sufficient resistance to stop a current from flowing, the potential energy is sustained. But if that resistance isn't present, the energy will rapidly emerge in another form - as the kinetic energy of electrons pouring 'down' the potential well from the high energy state in the cable, to the lower energy state of the ground, until the potential difference is eliminated. Of course, if you keep pumping electrons back up as fast as they fall down, (eg using a generator); or keep releasing more electrons with high potential from a reservoir (eg a battery), the current can keep flowing.