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Old 25-02-2019, 03:03   #181
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Re: Comparative Safety: 12v v 24v v 48

Quote:
Originally Posted by ramblinrod View Post
. . .

In conclusion, everything else equal, consistent with Ohm's law, the risk of electrical shock increases proportionally with voltage.

Consistent with Watt's law, the risk of electrical fire increases proportional with voltage.

Therefore I know that without any other mitigating factors, danger increases proportionally with an increase in system voltage.

. . .

But neither Ohm's law nor Watt's law says that -- even a layman like me can understand it. These are oversimplifications of complicated questions, oversimplified to the point where they lose any relationship to reality and become misleading. EVEN if everything else were equal -- but it never is, and crucially in this case.


The risk of electrical shock certainly does NOT increase proportionately with voltage. Increases of voltage below the point where a dangerous or even unpleasant current can flow through a human body do not increase the risk of shock at all. Furthermore, even very high voltages are not dangerous, if there is very little current. So the statement "the risk of electrical shock increases proportionately with voltage" is obviously wrong -- did the risk of shock double, when you changed your flashlight from 3v to 6v? Zero is not double zero. Likewise, there is no material risk of shock from a 24v electrical system. So telling people to avoid 24v if possible because it increases the risk of electrical shock is just wrong.


The statement "according to Watt's Law, the risk of electrical fire increases proportional with voltage" is also obviously just false, obvious even to layman capable of rational thought. If this were true, then we should all reduce our system voltage to 6v or even 3.2v, to get double and then 4x the safety, respectively, but this would obviously INCREASE the risk of fire because of the great practical difficulty of handling the large currents which would result, which would increase the risk of electrical faults.



The risk of electrical fire doesn't even increase proportionate to HEAT -- for any given situation, over a certain range, increases in heat have no effect on risk, then over a certain other range, you get a fire no matter what, so increases in voltage, power, current are immaterial. Risk doesn't work that way, that it is just dumbly proportional to something, almost never.





So, in summary -- I am really completely unconvinced by any of this. To me it, it looks like wrong conclusions based on gross oversimplifications of rather more complex phenomena. I think I will choose to believe the actual electrical engineers in the room.
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Old 25-02-2019, 04:12   #182
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Re: Comparative Safety: 12v v 24v v 48



Dockhead, take it with a grain of salt, you are completely right. Voltage alone is nothing, rubbing your synthetic clothes will charge you to several thousend volts and nothing will happen.

You need the right tool for the right task, a tiny hammer for goldsmith work and a sledge hammer for demolitions.

In practical installations higher currents cause more trouble, and there are safety level that require additional per-cautions. 50V is one limit, where touch protection becomes mandatory, up to 380V are somewhat manageable by normal people and electricians with per-caution and sticking to the norms. We deal daily with gear operating in this range on our home appliances.

Above 1000V it gets more tricky and even more dangerous, especially when higher currents are involved. But even amateurs handle and repair gear in this range - for instance changing a spark plug in your gas engine, fixing the coil etc. If you check the spark on your ignition and are not cautious, you can get some very unpleasant e-shock with this voltages.

So for your project and your demand, a 48V may be the right thing. There are not many marine devices, that run on 48V, so you may need some step-down converter or a separate voltage installation for this gear - either in 12V or 24V - this are common voltages on board. 48V stuff is simply rare.
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Old 25-02-2019, 04:46   #183
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Re: Comparative Safety: 12v v 24v v 48



... Even AA batteries can be lethal.
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Old 25-02-2019, 06:01   #184
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Re: Comparative Safety: 12v v 24v v 48

That flashy thing is a voltage multiplier. The output voltage is a lot higher than 12V.
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Old 25-02-2019, 06:24   #185
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Re: Comparative Safety: 12v v 24v v 48

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Originally Posted by transmitterdan View Post
That flashy thing is a voltage multiplier. The output voltage is a lot higher than 12V.
You noticed that big bright sparky thing too did you? ;-)
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Old 25-02-2019, 06:32   #186
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Re: Comparative Safety: 12v v 24v v 48

Sure, this is a insect zapper, what many have at home and children often play with, you can buy them in penny stores everywhere.

This are several thousands volts charged in a relatively small capacitor. It is not the total energy stored, it is the current you can release for a sufficient long time, voltage helps of course to overcome the resistance of the mouse.

Anyway, there are more dangers around us than 48V in very harmless looking "toys".
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Old 25-02-2019, 06:33   #187
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Re: Comparative Safety: 12v v 24v v 48

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Originally Posted by ramblinrod View Post
You noticed that big bright sparky thing too did you? ;-)

Yes, the voltage in the trap is way more than 100V and it is AC not DC.
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Old 25-02-2019, 06:39   #188
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Re: Comparative Safety: 12v v 24v v 48

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Originally Posted by ramblinrod View Post
You noticed that big bright sparky thing too did you? ;-)
You mean like the bright sparky thing when kids shuffle their feet on the carpet and sneak up behind their buddy and touch them on their ear lobe.
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Old 25-02-2019, 08:44   #189
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Re: Comparative Safety: 12v v 24v v 48

WOW!

Perhaps this thread should have been more aptly titled:

Truth vs Perception vs Mindset vs Pride. ;-)

I hope any psychiatrists or sociologists on the forum can comment...

...or do I actually know that is what I wish? ;-)

Despite all of the light saber rattling, name calling, intelligence questioning, (which is really a shame that some have to resort to this, but that too is human nature unfortunately, I guess, but think I know), I hope everyone has found this thread as interesting and entertaining as I have.

It didn't really strike me until the sun rising issue was discussed.

I declared how I "knew" the sun would rise in the east.

I do know for sure that an event has happened every dawn and sunset, and throughout the day, and technically I knew that the earth revolves around the sun, and spins on it's axis (or do I) ;-), and yet knowing full well that the sun is not actually rising and falling, I was fully prepared to defend my statement that it does.

Then it dawned on me. (har).

How can I possibly be correct and all of these knowledgeable gentlemen be so wrong.

And it struck me, I was trying to present a truth, that contradicted perception, that was firmly entrenched in a mindset, that pride was preventing acknowledement.

Instead, many were prepared to argue ad infinitum their misperceived mindset was correct, as a matter of pride; pride in being correct.

So lets look at the pride issue. From early childhood we are taught that we should strive to succeed. We go to school and are taught that we need to learn things. If we learn well, we score an A+++, and the teacher is proud they did a good job, our parents are proud that they raised a bright child, and we develop a sense of pride in what we know, especially what we know that others don't.

Well that is a generalization that is not always true.

Some people could care less to let others do the thinking and be correct. "Meh", may be a term they use a fair bit.

I admit my own academic pride diminished somewhat about the time I learned about girls, cars, and parties.

Others could spend all of their time with noses in books and worry about their A+++, whereas I would be perfectly satisfied to party hardy and receive a solid B.

The A+++ earners developed a perception that they had greater intelligence.

Though I learned later on in my life that this was not necessarily so.

In fact in many cases, I could learn knew things, and change gears much faster when presented with new information that contradicted my prior thinking.

This came up time and time again when we would have a product development team meeting that I was leading, and when a problem was encountered, all of the table pounding and chest beating took place between all of the "technical" people from Phd's in electrical science, through all the different disciplines and levels of engineers and technologists, down to well, me, a lowly electronic technician.

Yet I led the group.

I was perfectly happy to let the thinkers "know" what they knew, so they could get on with their job, making calculations, researching various technologies and methodologies, and whatever else they did to fill up their day.

But every once in a while, maybe 3 times per project, we would run into a problem that the team really struggled with.

After one such meeting a young, recently hired engineer walked up to me and asked, "How do you do that?"

"Do what?", I asked.

He replied, "We've all been going around in circles and in-fighting for days, you call a meeting, ask 5 questions, and we have an answer and direction forward, yet you didn't come up with it, we did?"

I smiled and replied, "Experience".

I had enough experience in situations where "technical" people tend to stumble, that I could walk in cold, see the fallacy(s) in play, draw them out into the open, so the team could think straight.

I could say that "I know" that is what happened here, but in fact, "I think (with a high degree of confidence"), that is what happened here. ;-)

Truth.

I think everyone here, probably knows that with greater amounts of stored energy, comes greater danger should it be accidentally unleashed.

Take for instance, a jerry can of gasoline and lets say this has X units of stored energy.

While filling the lawnmower, if a drop of gasoline is spilled on the floor, it is no great concern.

But if we knock over that jerry can and empty it's contents onto the floor, we get very concerned.

We "know" that we just unleashed a significant amount of potential energy onto the floor, and a single fault (flame or spark) could turn that from potential energy to active energy, and very bad things could happen, very quickly.

Now ask yourself this question, (forgetting about evapouration for a second) what real difference does it make how quickly the gasoline poured out of the jerry can?

I think most will agree, that it doesn't really make any difference how quickly the gasoline got there, the real issue is the amount.

Well, it isn't the amount. The same amount of water would not be nearly as dangerous with respect to a fire.

It is the amount of potential energy that is now not under control (on the floor instead of in the jerry can) that has us concerned.

So lets go back to our 12 Vdc vs 48 Vdc discussion.

If the total potential energy available, and which could be released uncontrolled in a dangerous manner is, 480 Watts, it doesn't matter whether that energy is unleashed by 12 Vdc @ 40 A, or 48 Vdc at 10 A, the potential energy available to be unleashed in a dangerous way is exactly the same, 480 W.

Just like the speed of the gasoline spill, the speed of the current flow, has no real bearing, once it is all unleashed.

Now, this is where a whole bunch of intelligent folks stumbled.

Because we normally work with a fixed voltage supply, it doesn't matter what it is, but lets say 12 Vdc, some associated the potential energy (and potential danger) with the amount of current.

Obviously the more current, the more energy and the greater the risk.

200A oh my, 400 A, geesus, 800 A, yikes!!!!

Agreed! 1000%

Why?

Well at 12 Vdc, 200 A = 2400 W of energy, 400 A = 4800W and 800 A = a whopping 9600 W of energy.

So RamblinRod, higher current is more dangerous, so you are incorrect that higher voltage is more dangerous! (A lot of people jumped on this.)

Incorrect.

As I correctly stated, if we have a 12 Vdc circuit of very low resistance, designed to handle 800 A, it isn't dangerous at all.

It sits there happily running day in and day out, doesn't even break a sweat.

If a fault occurs, and the potential energy of that circuit is unleashed in an uncontrolled and dangerous way, the total potential energy is 9600 W.

So if instead we had a circuit capable of delivering only 200 A at 48 Vdc, that would be much safer, because it is only 200 A, right?

Wrong.

Interestingly, every person with any electrical background "knows" this.

Do they?

Yup. They have just established a mindset, from constantly thinking about a higher current with a fixed voltage, that they have incorrectly associated the danger with the current, when the potential danger is really associated with the total energy, which is the product of the voltage and current.

While they were working in their fixed voltage realm, this worked perfectly fine for them.

BUT, as soon as we increase the voltage by the same amount we reduce current, their established mindset causes them to make a mistake, even though they know that P = E*I; total energy = voltage * current, and if we increase one by the same amount we decrease the other, nothing changes as far as risk of danger.

Lowering current only makes the circuit less potentially dangerous if we do not increase voltage "DIRECTLY PROPORTIONALLY". (Dockhead may be pleased to know that I used this term correctly this time.)

If we lower current by 400% and increase voltage by 400% we have the same total potential energy - 9600W.

If that amount of energy can be converted into heat, to start a fire, it makes absolutely no difference whatsoever that one circuit had a 12 Vdc supply and the other 48 Vdc.

OK so we were wrong that risk of danger is proportional with current, but thankfully Ramblin' Rod was also wrong, because he said risk of danger is proportional with voltage and he just proved that voltage doesn't matter it is the total potential energy that is proportional to danger.

Well one might think so, if the amount of current was always reduced by the same amount that the voltage was increased. But it isn't.

As shown in one of the prior examples, when we increase voltage, across a fixed resistance, the current also increases. So if we have a short circuit of X ohms in a 48 Vdc circuit, the potential energy released is exponentially greater than that in the 12 Vdc circuit.

Of course there is no guarantee that a release of potential energy is going to do something bad, just like the gasoline on the garage floor that gets mopped up and safely put away, electrical energy can certainly be released without starting a fire.

It happens every time we use an electrical appliance.

However, there is always the potential that electrical energy can be released in an uncontrolled way, that will be dangerous, and the degree of that potential is proportional to the electrical potential (voltage).

Now, we do have certain measures in most electrical systems to prevent unleashing unlimited electrical potential. Power supplies usually have a current limiter, so that only so much energy can be released in Watts. Batteries often have fuses so that only so much energy can be released before the circuit is opened and current flow stopped. Note that the potential, based on the circuit voltage, is still there. If someone puts a jumper across that fuse. Look Out!!!.

We interrupt this program for the following public service announcement...Many boats have been built or modified with no fuse on the house bank.

For ABYC standards compliance , a fuse is not required on the starter battery (if there is no way it can be combined with the house bank, or anything else, and current distributed out of the starter circuit).

If your boat does not have a fuse (or breaker) very near the house bank positive terminal, please put one there, of the proper size and ratings.

If your boat does not have a fuse (or breaker) very near the starter battery positive terminal, please put one there, of the proper size and ratings. (While this is not required for ABYC standards compliance, these standards represent the minimum safety level. The starter battery fuse, greatly improves safety for very little money. (Just ensure the fuse or breaker is rated high enough, that it will not interrupt starter motor current under normal and even extreme (cold weather, fully loaded engine) conditions.


Now back to our regularly scheduled program.

In the example illustrated in post # 169....

Quote:
Lets consider the example that a short circuit of 1 ohm develops somewhere across the 30' power cables.

P (Watts) = E (Volts) ^ 2 / R (ohms)

Heat (BTU/h) = P (Watts) X 3.4

For 12 Vdc, P = E^2/R = 144V/1 ohm = 144 W * 3.4 = 490 BTU/h

For 24 Vdc, P = E^2/R = 576V /1 ohm = 576W * 3.4 = 1958 BTU/h
This illustrates that the risk of danger (unleashing energy in an uncontrolled way that could be bad) increase significantly, proportional with voltage.

The risk of danger, is as simple as using Ohm's and Watt's laws.

Whether the danger is unleashed, (potential energy in the electrical circuit converted to active energy that can be harmful) depends on all of the circumstances, which is most definitely more complicated, but pretty much completely unnecessary to consider for these purposes.

More unleashed potentially dangerous energy is bad as compared to less unleashed potentially dangerous energy.

Every thing else equal, as shown in the short circuit example above, (which is quite plausible) in a condition where current is not otherwise limited (which can be many cases) the risk of danger increases proportionally with voltage.

The really surprising thing is; everyone responding in this thread with an electrical background knows this, and likely has for a long, long time; it was just their continued experience always worrying about higher current with a fixed voltage, that was preventing them from realizing the truth.

Then pride and inherent desire to be correct crept in, and prevented them from seeing the truth, that they actually knew, even when I spelled it out directly in front of them.

No offense meant to anyone in this thread by this post whatsoever.

I just hope others find this phenomenon as fascinating as I do, and can use this example to help them understand it, should they encounter it again in the future.

Let this be proof, that the "opinion" of the majority, even when it is filled with subject matter experts, that are basing their arguments on "facts", is not necessarily always the "truth" and in fact, they may even "know" differently but due to apparently conflicting information, can't access that knowledge.

FWIW, when considering adding a high power load to a vessel, it is always safer to choose the lowest voltage supply practical.

If the vessel has a 12 Vdc supply, and that can be safely used (there reasonably affordable cables and components necessary to supply the recommend current and voltage to the terminal) it is usually best to go with that.

When designing a new electrical system form scratch, it is still safest, to choose the lowest voltage supply possible and practical, to supply the electrical appliances in the vessel.

For this reason, I generally recommend, for all rec boats less than about 50', that a primary 12 Vdc electrical system be used. If there is a very high load, like a thruster, it may be wise to mount a 12 Vdc battery very close to it. To reduce the cabling in a high load circuit that could potentially short circuit anywhere along it's length, and unleash a significant amount of potentially dangerous energy, because due to the nature of the load, the over-current protection devices in the circuit MUST allow a high degree of energy to be released, WHICH IS PROPORTIONAL TO CIRCUIT VOLTAGE (up to the limit of the over-current protection device).

So if there are any psychologists or sociologists in the house, that can help me understand / define the phenomenon described herein, please either post (if your OK with declaring in public), or PM me (for complete confidentiality).

Absolutely fascinating.
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Old 25-02-2019, 09:09   #190
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Re: Comparative Safety: 12v v 24v v 48

Anyway, this leads me to have a new concern.

I am always concerned about those who "think" they know enough (who really don't) about matters to muck about with the electrical systems on their boats, increasing the risk of unleashing potential energy that could harm themselves or others.

Now I have to be concerned about those who definitely do know enough about matters to design or modify a marine electrical system, but can't use that long held knowledge, because of a flawed mindset getting in the way.

Scary.
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Old 25-02-2019, 09:15   #191
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Re: Comparative Safety: 12v v 24v v 48

...

The answer on all Questions is 42 !

...
The Hitchhiker's Guide to the Galaxy by Douglas Adams, "Answer to the Ultimate Question of Life, the Universe, and Everything", calculated by an enormous supercomputer named Deep Thought over a period of 7.5 million years., Unfortunately, no one knows what the question was..."


What a long post, mate!

How in the world you end up with 480W as threshold for danger?

With all respect, any size of 12V battery in a boat delivers more than 480W when shorted. The potential energy stored in a battery is Ah * Voltage and is in Wh, your calculations need some review. If you refer of the Wattage a battery can emit on a shortcut, you must take the internal resistance (based on chemistry and size) into account to calculate the Ic (maximal shortcut current) and the resistance of the material that causes the short cut.

Cheers!

https://www.sbsbattery.com/PDFs/VRLA...erySystems.pdf
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Old 25-02-2019, 09:17   #192
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Re: Comparative Safety: 12v v 24v v 48

Hi Brandon. I'm a regulatory compliance engineer. I work with high power programmable power supplies. The regulatory standards I work with most are EN61010, EN60950 and EN60601. I will try to make this short, but all the standards consider a voltage safe to touch if it is below 60Vdc, or 42.4Vrms. The caviot her is that this must be isolated from primary power sources by reinforced insulation. Reinforced is defined by 3 layers two of which are capable of withstanding a hipot test appropriate for the working voltage.

Risk of shock is not greater with 48V, nor is burn hazard, nor is arcing. It typically takes more than 1000V / mm to arc. The difference between 48V and 12V is insignificant when dealing with arc hazard. Since the energy level is presumed to be the same between a 48V and a 12V system the burn hazard is the same. The advantage with the 48V system is you can reduce the gauge of wire thus reducing copper losses. In short..48V is more efficient and the voltage drops will be less due to the reduced current for the same power.
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Old 25-02-2019, 09:31   #193
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Re: Comparative Safety: 12v v 24v v 48

Quote:
Originally Posted by SV MAKAI View Post
Hi Brandon. I'm a regulatory compliance engineer. I work with high power programmable power supplies. The regulatory standards I work with most are EN61010, EN60950 and EN60601. I will try to make this short, but all the standards consider a voltage safe to touch if it is below 60Vdc, or 42.4Vrms. The caviot her is that this must be isolated from primary power sources by reinforced insulation. Reinforced is defined by 3 layers two of which are capable of withstanding a hipot test appropriate for the working voltage.

Risk of shock is not greater with 48V, nor is burn hazard, nor is arcing. It typically takes more than 1000V / mm to arc. The difference between 48V and 12V is insignificant when dealing with arc hazard. Since the energy level is presumed to be the same between a 48V and a 12V system the burn hazard is the same. The advantage with the 48V system is you can reduce the gauge of wire thus reducing copper losses. In short..48V is more efficient and the voltage drops will be less due to the reduced current for the same power.
See! ;-)
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Old 25-02-2019, 09:33   #194
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Re: Comparative Safety: 12v v 24v v 48

Quote:
Originally Posted by ramblinrod View Post
WOW!

Perhaps this thread should have been more aptly titled:

Truth vs Perception vs Mindset vs Pride. ;-)

I hope any psychiatrists or sociologists on the forum can comment...

...or do I actually know that is what I wish? ;-)

Despite all of the light saber rattling, name calling, intelligence questioning, (which is really a shame that some have to resort to this, but that is human nature unfortunately), I hope everyone has found this thread as interesting and entertaining as I have.

It didn't really strike me until the sun rising issue was discussed.

I declared how I knew the sun would rise in the east.

I do know for sure that an event has happened every dawn and sunset, and throughout the day, and technically I knew that the earth revolves around the sun, and spins on it's axis (or do I) ;-), and yet knowing full well that the sun is not actually rising and falling, I was fully prepared to defend my statement that it does.

Then it dawned on me.

How can I possibly be correct and all of these knowledgeable gentlemen be so wrong.

And it struck me, I was trying to present a truth, that contradicted perception, that was firmly entrenched in a mindset, that pride was preventing acknowledement.

Instead, many were prepared to argue ad infinitum their misperceived mindset was correct, as a matter of pride; pride in being correct.

So lets look at the pride issue. From early childhood we are taught that we should strive to succeed. We go to school and are taught that we need to learn things. If we learn well, we score an A+++, and the teacher is proud they did a good job, our parents are proud that they raised a bright child, and we develop a sense of pride in what we know, especially what we know that others don't.

Well that is generalization that is not always true. Some people could care less to let others do the thinking and be correct. "Meh", may be a term they use a fair bit.

I admit my own academic pride diminished somewhat about the time I learned about girls, cars, and parties. Others could spend all of their time with noses in books and worry about their A+++, whereas I would be perfectly satisfied to party hardy and receive a solid B.

The A+++ earners developed a perception that they had greater intelligence. Though I learned later on my life that this was not necessarily so. In fact in many cases, I could learn knew things, and change gears much faster when presented with new information that contradicted my prior thinking.

This came up time and time again when we would have a product development team meeting that I was leading, and when a problem was encountered, all of the table pounding and chest beating took place between all of the "technical" people from Phd's in electrical science, through all the different disciplines and levels of engineers and technologists, down to well, me, a lowly electronic technician.

Yet I led the group.

I was perfectly happy to let the thinkers "know" what they knew, so they could get on with their job, making calculations, researching various technologies and methodologies, and whatever else they did to fill up their day.

But every once a while, at least 3 times per project, we would run into a problem that the team really struggled with.

After one such meeting a young, recently hired engineer walked up to me and asked, "How do you do that?"

"Do what?", I asked.

He replied, "We've all been going around in circles and in-fighting for days, you call a meeting, ask 5 questions, and we have an answer and direction forward, yet you didn't come up with it, we did?

I smiled and replied, "Experience".

I had enough experience in situations where "technical" people tend to stumble, that I could walk in cold, see the fallacy(s) in play, draw them out into the open, so the team could think straight.

I could say that "I know" that is what here, but in fact, "I think (with a high degree of confidence", that is what happened here. ;-)

Truth.

I think everyone here, probably knows that with greater amounts of stored energy, comes greater danger should it be accidentally unleashed.

Take for instance, a jerry can of gasoline and lets say this has X units of stored energy.

While filling the lawnmower, if a drop of gasoline is spilled on the for, it is no great concern.

But if we knock over that jerry can and empty it's contents onto the floor, we get very concerned. We "know" that we just unleashed a significant amount of potential energy onto the floor, and a single fault (flame or spark) could turn that from potential energy to active energy, and very bad things could happen, very quickly.

Now ask yourself this question, (forgetting about evapouration for a second) what real difference does it make how quickly the gasoline poured out of the jerry can? I think most will agree, that it doesn't really make any difference how quickly the gasoline got there, the real issue is the amount.

Well, it isn't the amount. The same amount of water would not be nearly as dangerous with respect to a fire. It is the amount of potential energy that is now not under control (on the floor instead of in the jerry can) that has us concerned.

So lets go back to our 12 Vdc vs 48 Vdc discussion.

If the total potential energy available, and which could be released uncontrolled in a dangerous manner is, 480 Watts, it doesn't matter whether that energy is unleashed by 12 Vdc @ 40 A, or 48 Vdc at 10 A, the potential energy available to be unleashed in a dangerous way is exactly the same 480 W. Just like the speed of the gasoline spill, the speed of the current flow, has no real bearing, once it is all unleashed.

Now, this is where a whole bunch of intelligent folks stumbled.

Because we normally work with a fixed voltage supply, it doesn't matter what it is, but lets say 12 Vdc, some associated the potential energy (and potential danger) with the amount of current. Obviously the more current the more energy and the greater the risk. 200A oh my, 400 A, geesus, 800 A, yikes!!!!

Agreed! 1000%

Why?

Well at 12 Vdc, 200 A = 2400 W of energy, 400 A = 4800W and 800 A = a whopping 9600 W of energy.

So RamblinRod, higher current is more dangerous, so you are incorrect that higher voltage is more dangerous! (A lot of people jumped on this.)

Incorrect.

As I correctly stated, if we have a 12 Vdc circuit of very low resistance, designed to handle 800 A, it isn't dangerous at all. It sits there happily running day in and day out, doesn't even break a sweat.

If a fault occurs, and the potential energy of that circuit is unleashed in an uncontrolled and dangerous way, the total potential energy is 9600 W.

So if instead we had a circuit capable of delivering only 200 A at 48 Vdc, that would be much safer, because it is only 200 A, right?

Wrong.

Interestingly, every person with any electrical background "knows" this.

Do they?

Yup. They have just established a mindset, from constantly thinking about a higher current with a fixed voltage, that they have incorrectly associated the danger with the current, when the potential danger is really associated with the total energy, which is the product of the voltage and current.

While they were working in their fixed voltage realm, this worked perfectly fine for them.

BUT, as soon as we increase the voltage by the same amount we reduce current, their established mindset causes them to make a mistake, even though they know that P = E*I; total energy = voltage * current, and if we increase one by the same amount we decrease the other, nothing changes as far as risk of danger.

Lowering current only makes the circuit less potentially dangerous if we do not increase voltage "DIRECTLY PROPORTIONALLY". (Dockhead may be pleased to know that I used this term correctly this time.)

If we lower current by 400% and increase voltage by 400% we have the same total potential energy - 9600W.

If that amount of energy can be converted into heat, to start a fire, it makes absolutely no difference whatsoever that one circuit had a 12 Vdc supply and the other 48 Vdc.

OK so we were wrong that risk of danger is proportional with current, but thankfully Ramblin' Rod was also wrong, because he said risk of danger is proportional with voltage and he just proved that voltage doesn't matter it is the total potential energy that is proportional to danger.

Well one might think so, if the amount of current was always reduced by the same amount that the voltage was increased. But it isn't.

As shown in one of the prior examples, when we increase voltage, across a fixed resistance, the current also increases. So if we have a short circuit of X ohms in a 48 Vdc circuit, the potential energy releases is exponentially greater than that in the 12 Vdc circuit.

Of course there is no guarantee that a release of potential energy is going to do something bad, just like the gasoline on the garage floor that gets mopped up and safely put away, electrical energy can certainly be released without starting a fire. It happens every time we use an electrical appliance.

However, there is always the potential that electrical energy can be released in an uncontrolled way, that will be dangerous, and the degree of the that potential is proportional to the electrical potential (voltage).

Now, we do have certain measures in most electrical systems to prevent unleashing unlimited electrical potential. Power supplies usually have a current limiter, so that only so much energy can be released in Watts. Batteries often have fuses so that only so much energy can be released before the circuit is opened and current flow stopped. Note that the potential, based on the circuit voltage, is still there. If someone puts a jumper across that fuse. Look Out!!!.

We interrupt this program for the following public service announcement...Many boats have been built or modified with no fuse on the house bank.

For ABYC standards compliance , a fuse is not required on the starter battery (if there is no way it can be combined with the house bank, or anything else, and current distributed out of the starter circuit).

If your boat does not have a fuse (or breaker) very near the house bank positive terminal, please put one there, of the proper size and ratings.

If your boat does not have a fuse (or breaker) very near the starter battery positive terminal, please put one there, of the proper size and ratings. (While this is not required for ABYC standards compliance, these standards represent the minimum safety level. The starter battery fuse, greatly improves safety for very little money. (Just ensure the fuse or breaker is rated high enough, that it will not interrupt starter motor current under normal and even extreme (cold weather, fully loaded engine) conditions.


Now back to our regularly scheduled program.

In the example illustrated in post # 169....



This illustrates that the risk of danger (unleashing energy in an uncontrolled way that could be bad) increase significantly, proportional with voltage.

The risk of danger, is as simple as using Ohm's and Watt's laws.

Whether the danger is unleashed, (potential energy in the electrical circuit converted to active energy that can be harmful) depends on all of the circumstances, which is most definitely more complicated, but pretty much completely unnecessary to consider for these purposes.

More unleashed potentially dangerous energy is bad as compared to less unleashed potentially dangerous energy.

Every thing else equal, as shown in the short circuit example above, (which is quite plausible) in a condition where current is not otherwise limited (which can be many cases) the risk of danger increases proportionally with voltage.

The really surprising thing is; everyone responding in this thread with an electrical background knows this, and likely has for a long, long time; it was just their continued experience always worrying about higher current with a fixed voltage, that was preventing them from realizing the truth.

Then pride and inherent desire to be correct crept in, and prevented them from seeing the truth, that they actually knew, even when I spelled it out directly in front of them.

No offense meant to anyone in this thread by this post whatsoever.

I just hope others find this phenomenon as fascinating as I do, and can use this example to help them understand it, should they encounter it again in the future.

Let this be proof, that the "opinion" of the majority, even when it is filled with subject matter experts, that are basing their arguments on "facts", is not necessarily always the "truth" and in fact, they may even "know" differently but due to apparently conflicting information, can't access that knowledge.

FWIW, when considering adding a high power load to a vessel, it is always safer to choose the lowest voltage supply practical.

If the vessel has a 12 Vdc supply, and that can be safely used (there reasonably affordable cables and components necessary to supply the recommend current and voltage to the terminal) it is usually best to go with that.

When designing a new electrical system form scratch, it is still safest, to choose the lowest voltage supply possible and practical, to supply the electrical appliances in the vessel.

For this reason, I generally recommend, for all rec boats less than about 50', that a primary 12 Vdc electrical system be used. If there is a very high load, like a thruster, it may be wise to mount a 12 Vdc battery very close to it. To reduce the cabling in a high load circuit that could potentially short circuit anywhere along it's length, and unleash a significant amount of potentially dangerous energy, because due to the nature of the load, the over-current protection devices in the circuit MUST allow a high degree of energy to be released, WHICH IS PROPORTIONAL TO CIRCUIT VOLTAGE (up to the limit of the over-current protection device).

So if there are any psychologists or sociologists in the house, that can help me understand / define the phenomenon described herein, please either post (if your OK with declaring in public), or PM me (for complete confidentiality).

Absolutely fascinating.

We don't need a psychologist for this at all -- it's not a psychological question (unless you think that your opinion is the only truth, and that the entire house full of electrical engineers are suffering from mass delusion, which is not a framework which anyone is going to accept).


What is needed is rather an epistemologist -- so, an expert in the nature of knowledge. I don't claim to be that, although one of my three university degrees is in Philosophy (magna cum laude), so I do know at least something about it, more than I know about electricity anyway


And what I think I'm seeing here in this thread is exactly a clash of epistemology. It's a clash between the point of view and system of knowledge of the practical technician with a modest amount of electrical engineering knowledge but a massive amount of practical experience. Versus the point of view and system of knowledge of actual electrical engineers with vastly more knowledge of engineering, but who don't use that knowledge for simplified practical day to day advice to practical non-knowledgeable clients.



So for the technician, the key thing to know is how practically to advise his clients in simple practical solutions for the best practical results, without access to extensive engineering work around every practical question. So what the technician needs to know comes down to mostly rules of thumb based on engineering principles which may or may not be deeply understood, but that doesn't matter as long as they practically work.



These rules of thumb ("shock danger goes up proportionately with voltage") drive the engineers up the wall, because such statements are based on a different construction of knowledge, which is unsuitable for the real engineering mind, even though it might be perfectly fine for the technician.



All of us are working on different constructions of knowledge, which are all based on rules of thumb of one kind or another, just because our minds simply don't have access to ultimate truths about anything. For example, Newtonian physics aren't really true -- as we know since Max Planck and Einstein -- but they approximate truth closely enough, that we still use them in engineering.



So to continue the metaphor -- a physicist might get irritated with an engineer, in a discussion like this, just like the engineers have gotten irritated with the technician, in this thread. Would be a similar clash of epistemology.


Neither system of knowledge is inherently wrong -- they are constructed for different purposes. However, I would think that the technician might treat the engineers' point of view with somewhat greater respect -- he could gain some valuable corrections to his rules of thumb. Just my personal opinion.
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Old 25-02-2019, 09:33   #195
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Re: Comparative Safety: 12v v 24v v 48

Quote:
Originally Posted by ramblinrod View Post
Anyway, this leads me to have a new concern.

I am always concerned about those who "think" they know enough (who really don't) about matters to muck about with the electrical systems on their boats, increasing the risk of unleashing potential energy that could harm themselves or others.

Now I have to be concerned about those who definitely do know enough about matters to design or modify a marine electrical system, but can't use that long held knowledge, because of a flawed mindset getting in the way.

Scary.
Thats funny I was just thinking something along similar lines. Except I was thinking that I feel so much more reassured that we have some people who know everything. Thankfully we can easily identify them as they are happy to tell us idiots how much they know and we dont.
Pheeeew.
Australian's have a term of enderment for these particulrly knowledgable individuals- 'FIG JAM's (F... Im Good, Just Ask Me).
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