Comparative Safety: 12v v 24v v 48

[ramblinrod]

Originally Posted by Dockhead

Just a point of order . . .
Aristotle said -- wisdom begins when you start to understand what you don't know. Unfortunately that is kind of like, you know, a word to the wise

Absolutely.

I absolutely LOVE to learn knew things.

The more one learns about a subject, the more they learn their is to learn.

Understanding electricity is just like sailing, it takes a day to learn, but a lifetime to master.

I ain't dead yet, so, I know I have more to learn (and I don't need a scientific paper to prove this) because I simply know it. ;-)

The only thing I like better than learning is to teach people knew things, especially if it helps them in their career or pastime.

I admit I am wrong when I am.

In this case, I am not.

Despite a little mob of members attempting to contradict my position on this subject, it has withstood every attempt to disprove it (most of which relied on completely incorrect statements that I corrected one after the other).

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.

It has to.

No amount of denial, table pounding, or chest thumping will ever change this.

I promise everyone here this, I will never accept falsehood as correct, just to stop someone else's bullying tactics, or to satisfy their ego, because they themselves cannot admit or accept their own error.

[CatNewBee]

In general you are right, for the same resistance increasing voltage increases current, in the particular case, you are wrong, for a constant power output in watt, a higher voltage with resulting lower current is safer, in magnitudes safer because the losses and the resulting dissipated energy along the pass is in sqr factor lower, a 2 times increase in voltage decreases current to half and power loss along the connection 4 times. Just look at Ohms law and Watts formula, bring them together for the same output in watt and fid it out yourself, it is not so complicated, a 8th grader in basic physics class can do this.

I have an university degree on electronics, I know what I am talking about.

[evm1024]

Originally Posted by ramblinrod

Requesting "proof" of this basic principle, is akin to asking someone to provide scientific evidence that the sun rises in the east and sets in the west.

If one does not already know that a contact or connection resistance can be 10 ohms (or any value between near zero and near infinity), I recommend they take a class, "Basic Fundamentals of Electricity".

If they have already taken such a class, and still don't know this basic fundamental, sorry, I doubt I or anyone can help.

It is quite obvious that your understanding is at a basic level. That is why you do not see the importance of the contact resistance. But for your education I will simply say that If you look beyond what you learned in your "Basic Fundamentals of Electricity" class you would see that the contact resistance is the key to the falseness of your assertion.

Your false belief in the superiority of your knowledge and understanding is amply displayed above. Your understanding lacks depth and you would and do argue without an in depth understanding, as required by the subject.

Case in point - you state

Originally Posted by RR

Requesting "proof" of this basic principle, is akin to asking someone to provide scientific evidence that the sun rises in the east and sets in the west.

A person with a basic understanding would state that the sun rises in the east. And that will get you by in everyday life.

But anyone with more than a basic understanding knows that the Sun is not the body that is moving. They know that we stand on the surface of the earth and that the earth rotates and that it appears that the sun rises in the ease but that that is not the truth.

Your basic understanding fails your again.

[ramblinrod]

Originally Posted by Dockhead

But why is sending 400 amps of power, 30 feet to the bow, at 24v, more dangerous than sending 800 amps (eek) 30 feet to the bow, at 12v?

First of all, it does my soul some good that despite being a reasonably intelligent fellow with a reasonably significant boat, that you realize and acknowledge that you don't understand enough about marine electrical systems, to properly select and install a high load appliance (like a thruster).

A lot of people who think they do, and who may have responded to this thread (perhaps in seemingly convincing ways to those who don't know any better), and even if they claim to be an engineer of some type, really don't.

We don't know what we don't know until we learn so.

Now onto your first question; I have already answered this (or similar) many times in this thread, but I will try again because you have requested respectfully.

First of all, I would generally not run cables 30 ft (60 ft round trip) to carry 800 A to the bow, to power a 12 Vdc thruster.

This is not normally a practical solution.

The electrical cable and component size to stay within the thruster manufacturers minimum terminal voltage recommendation would be too costly.

As I have stated many times, every installation requires a thorough vessel design and construction review and a customer interview.

Usually, after such, for a thruster up to about 8 HP (6 kW) I recommend a 12 Vdc model, and install a 12 Vdc battery bank of adequate capacity (MCA) very close to it.

The proper battery capacity minimizes voltage sag, and the close proximity minimizes voltage drop. Together, this assures the minimum thruster terminal voltage is realized.

If a 10 HP (7.5 kW) thruster is needed, I would normally recommend installing a 24 Vdc model, and a 24 Vdc battery bank of adequate capacity in close proximity.

Again, while 24 Vdc is inherently less safe that 12 Vdc, using 12 Vdc would be impractical due to the size of the load and resulting electrical cables and components.

I would normally recommend against running 24 Vdc cables 30' (60' round trip), as this would usually be too expensive (compared to a close proximity battery) greater safety risk, and poorer performing due to excessive voltage drop.

Because everything else equal, danger increases proportionally with voltage.

Another term for voltage is "potential".

EEE, a circuit having a higher voltage, has higher potential to cause an electric shock or fire.

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

For a thruster of 7.5 kW, the properly installed over-current protection device will not likely trip (else the thruster could never run).

Therefore, there is much greater risk of fire from a short circuit in a 24 Vdc system than there is in a 12 Vdc system. Unfortunately, for this size load, the 12 Vdc system voltage is impractical, so the greater risk has to be mitigate to the extent practical and just accepted. But if a 12 Vdc will suffice, this is generally my recommendation.

EEE, danger increases proportionally with voltage.

Always.

Has to.

As dictated by Ohm's Law and Watt's Law, and presented by RamblinRod to the shocking disbelief of many on this forum who should know better. ;-)

[ramblinrod]

Originally Posted by CatNewBee

In general you are right, for the same resistance increasing voltage increases current, in the particular case, you are wrong, for a constant power output in watt, a higher voltage with resulting lower current is safer, in magnitudes safer because the losses and the resulting dissipated energy along the pass is in sqr factor lower, a 2 times increase in voltage decreases current to half and power loss along the connection 4 times. Just look at Ohms law and Watts formula, bring them together for the same output in watt and fid it out yourself, it is not so complicated, a 8th grader in basic physics class can do this.

I have an university degree on electronics, I know what I am talking about.

Despite your university degree, you are making a fundamental error.

Yes a lower voltage will demand a higher current to power the same load.

So what. Current is not directly proportional to danger; power is.

And both circuits powering the same load will dissipate the same power.

Please see the post I just made to Dockhead, regarding a 1 ohm short circuit occurring in either a 12 Vdc or 24 Vdc circuit.

The 24 Vdc short circuit, dissipates 4 times more power and produces 4 times more heat. Thus, much more dangerous.

[evm1024]

Ah I see that we are no longer trying to defend the 10 ohm series resistance strawman. Kirchhoff's Laws intrude.

Now we are onto a 1 ohm short strawman.

Lets see, 1000' of Awg 10 wire has a resistance of about 1 ohm. We put 12 volts across it. It gets hot. we do the same with 24 volts, it gets hot.

Sounds like an electric blanket to me.

OK, so 1000' of 10 awg does not support your assertion. OK lets make that a 1 ohm 10 watt power resistor. At 12 volts we get a resistor that takes some time to burn up. But it does burn up. OK, so how about at 24 volts you ask. Oh, the resistor burns up. It burns up in less time and thus has less time to start things on fire.

So what is the disconnect? Well it is the difference between a basic understanding that does not know or ignores the details.

It is not quite so simple. We can start with some basic thermodynamics.

Fires are not started only by heat in this case. Simply stated you can hold a match to a tree for hours and not start a fire.

Moving on to the thruster example. So for thrusters above some given size 24 volts makes more sense. Even someone with a basic understanding sees that because of P = I^2 R a 800 amp 12 volt circuit is quite a bit different than a 400 amp 24 volt circuit.

Of course that is moot because faults in either circuit will likely create major problems.

So the 12 volt thruster even with batteries right next to the thruster has a greater risk simply due to the currents involved.

As I said before a basic understanding does not take into account the details (i.e. the sun does not actually rise).

As has been stated a 24 volt thruster makes sense in some circumstances. Which means that you need a 24 volt charging source and all that that implies. The 24 volt thruster is not in isolation.


Lastly, I'm sure that a fire caused by a 490 BTU fault is much safer than a fire caused by a 1958 BTU fault.

[ramblinrod]

Originally Posted by evm1024

That is why you do not see the importance of the contact resistance.

False Statement Alert:

I posted the importance of contact resistance in post # 1, and have maintained its importance throughout this thread (and all of my professional life for that matter).

But for your education...

Misleading Statement Alert:

No greater than a fundamental understanding of electrical principles are required to understand Ohm's Law and Watts Law as it applies to this subject, and as I have presented.

ALL of my education, training, and experience, (quite substantial and relevant to the subject) supports these fundamental principles.

Whether one has the most basic understanding of electrical principles or a PhD in electrical science, they should "know" that the facts as I have presented them in this thread, are all true.

But anyone...

More Rhetoric Alert:

I "know" that from my vantage point on earth, the sun has risen in the eastern sky every morning, and fallen in the western sky every evening.

I expect, barring any celestial catastrophe, it will tomorrow too, and for each day of the remainder of my life.

I expect most astrophysicists would agree; but I do not require their level of knowledge, nor a scientific paper from them to "know" this.

[ramblinrod]

Originally Posted by evm1024

Ah I see tha….

Far better things to do than try to explain to this poster why the risk of fire generally increases proportionally with the temperature of a heat source accidentally exposed to a combustible (such as cable insulation).

Apparently they believe that because holding a match to a tree can't light it on fire (don't suggest this to anyone who used to own a home in California) that a much hotter short circuit poses no greater risk of fire than a much cooler one.

For the record, I do not plant trees in boats I install thrusters in, so my customers need not fear forest fires in their V-berth.

Additionally, they can count on the fact that I will install the safest system I can, and that does mean recommending the lowest system voltage possible and practical, capable of meeting the energy requirements of the loads they desire.

[transmitterdan]

Originally Posted by CatNewBee

nope, 1 and one is 1 in binary.

Ok, a 1 beside a 1 is 3. It was a joke not a computer quiz.

[Jdege]

1 + 1 = 3, for particularly small values of 3

[evm1024]

I think that I will come about this in a different way. It is time to revisit the basic comparison between 12 volts, 24 volts and 48 volts in terms of safety.

I'll have to go back through the thread to see if I can list all of the reasons why 12 volts is safer than 48 volts. Three come to mind off the bat.

1) Shock hazards

2) risk of fire in a short circuit

3) risk of fire in a HRC (high resistance connection) that develops in the circuit

It will take a little bit.

---------------------------

Speaking of what we think we know.... (and please feel free to stop reading here I just ramble a bit below)

Just for fun you remember my 18 watt soldering iron from post #4. That Iron has a 735 degree F tip temp (390 C). I plugged it in and let it heat up fully.

If we just do the math and don't worry about any losses 18 watts is 61 BTU per hour. (it is key that BTU is per hour). Piddly you say and I agree. Until you get the tip on your skin...

I took some newspaper and put about an inch of the tip on it. Now you would expect that 735 degrees would be more than enough to start the paper on fire. No such luck. I tried and tried and tried. I held the tip on the paper for more than 2 minutes and did not get a fire. Oh, yes of course I got smoke and charred paper. I had to keep moving more paper onto the tip to replace the charred paper. But no fire.

I got some glowing embers and Ah I thought now we will get a fire. No alas no fire.

Lastly I blew on the embers and I got fire.

I should note that all through this test the silver solder remained molten on the tip. This means that the tip temp was still quite hot.

Then I took some more paper and put some 200 proof Absolute, anhydrous ethanol on it. 100 % alcohol, that should burn right? No such luck. The ethanol evaporated when touched with the soldering iron tip. And once the paper was "dry" it started to char. No fire.

Lastly, I took a little 30 gauge wire and hooked it up to a 12 volt battery. The wire got hot and melted. With it in contact with the paper I got no fire.

Oh but now the fun begins. Paper, ethanol and wire. The wire gets hot, melts and no fire. Try again. Ah, when the wire melted this time there was a little spark. And that ignited the ethanol.

One last test. This time I used a 1.5 volts AA battery. 3" of 30 gauge is too long for the current available from the AA. It got warm but did not glow. I shorten up the wire and finely with only 1/2" of wire I get a red hot wire. As you might guess no fire. But, once again with a spark there was ignition.

OK, so what does this prove? Well nothing really.

I suppose that it illustrates that what we think we know is not always true. That we need a deeper understanding to, er, understand what is going on.

That 61 BTU per hour is enough to start a fire under the right conditions.

Of note is the auto-ignition temperature. This is the is the lowest temperature at which something spontaneously ignites. Below that temp you need a source of ignition such as a flame or spark.

See: https://en.wikipedia.org/wiki/Autoignition_temperature

For the most part we get by in the world thinking that putting out soldering iron into some paper is going to start a fire. But I had to work at it.

My basic assumption was a good one (heat = fire) but does not cover the technical aspects of it. And after all this thread is a technical one regardless of the excursions it has taken.

[s/v Jedi]

Originally Posted by ramblinrod

Incorrect.

Heat is proportional to Watts dissipated. 1 W = 3.4 BTU/h.

Therefore BTU/h = V * I * 3.4

48 V at 5 A = 240 W * 3.4 = 816 BTU/h
12 V at 20A = 240 W * 3.4 = 816 BTU/h

Exact same fire danger potential.

(People incorrectly associate current with heat. Not true. Power is associated with heat. This is why the heat output of many heaters is rated in Watts.

Correct.

This shows either an utter lack of understanding or just trolling the forum. I would hope for the latter but fear it's the former.

What you show here is the heat produced in case the 240W load is a resistive heater, which has nothing to do with the statement you are calling to be incorrect. Total BS answer. The statement was talking about a 240W load in a circuit that somehow (corrosion, wear) has developed a resistance. I have shown the math for that before in this thread but that was ignored because it was probably too complex.

Also. what is completely lost as a factor of safety is the actual operational status of the device powered by the circuit. The higher the power consumption, the higher the reliability becomes with a higher voltage power source as I demonstrated earlier as well. I even showed that, although with reduced effect, this is even true for small loads where even good old 12V would still do okay'ish.

[evm1024]

As promised here are "few" thoughts on electric shock that I threw together. I've not proof read it or checked it for omissions or errors. I am sure there are a few.

Rather than trying to pick it apart I suggest that you take the part that you disagree with and write something that presents your point of view.

I wonder how long it will take me to write up a few thoughts on shorts and HRC....

-----------------------------------------------------------------------------------

Electric shock can be loosely defined as the passage of an electric current through a human body as the result of contact with energized conductors. For this posting I am not considering static electric shocks (which range from the shock you get when rubbing your cat on a dry day to lightning bolts).
Now for some ground work. We all know Ohms law (E=I*R), Watts law (P=E*R) and their various permutations (e.g. P=I^2*R etc). Those with a basic electrical understanding also know Kirchhoff’s circuit laws although we may not remember them by name. Kirchhoff’s first law basically says that in any given series circuit the current flowing through one part of a circuit is the same as the current flowing through any other part in the circuit. Kirchhoff’s second law basically says that in a series circuit the voltage drops of each resistance in the circuit will add up to equal the supply voltage. See Wikipedia for a better definition if needed. In addition to these laws we should keep parallel and series circuit laws in mind. Resistors in series add (Rt = R1 + R2) and resistors in parallel follow the reciprocal rule. 1/Rt = 1/R1 + 1/R2 + 1/Rn. When we toss all this together we end up with the basis of Nodal Analysis of electric circuits. There are computer implementations of nodal analysis of course. My own first exposure to computer based nodal analysis was back when I worked for Tektronix in the mid 70’s. I was able to help develop in a very minor way (very, very minor) some of the code in SPICE (google if you care) to model some Tek IC’s. I digress.

We have all heard the adage that “it is not the voltage that kills, it is the current”. And this is true. However, ohms law intrudes and says that without the voltage to overcome the resistance there will be very little current flow. Current is defined as the number of electrons (or any other carrier of charge) that flow and is measured in amps (1 ampere is 1 coulomb of charge flowing per second, a coulomb is 6.242×10^18 charges , electrons for our use), A volt is much harder to define being a derived unit. The easiest way to look at it is a measure of the potential energy per charge. In other words how powerful is each of those electrons flowing in the current. In Ohm’s law we indicate the voltage with the symbol E for electro motive force. In general conversation we use the term potential but that has a very specific meaning. Just contrast in Wikipedia Electric Potential, electromotive force and voltage to see how quickly it get complex.

Moving back to shock – The governing bodies (NFPA and OSHA in the USA for example) typically break the risk of electric shock into 3 broad categories based on the voltage of the current source. It is presumed that the voltage source has sufficient power to cause injury. For example, Van de Graaff generators have high voltages but because of the low current they are for the most part unregulated toys.

Shock by high voltages – No, no, 48 volts are not considered a high voltage. High voltages are defined as at or above 600 volts. I should note that I am ignoring whether the voltage is AC or DC for this post but please do remember that AC is much more hazardous than DC.

In basic terms shocks delivered by high voltages (600 volts or greater) cause massive damage and are nearly always fatal. Dry skin is basically an insulator with a hand to hand resistance through the body of 100,000 ohms or more. There are many things that change skin resistance and do not really enter here in any case. 600 volts is high enough to cause dielectric breakdown of the skin. It is great enough to exceed the breakdown voltage of the skin thus causes the skin to be conductive. At this point the resistance drops from the 100,000 or so ohms to the body’s internal resistance. The internal resistance of course varies depending on many factors but that really does not matter with high voltages. If we use 500 ohms for the internal resistance in this example we can see that using ohms law we end up with (600/500) 1.2 amps of current flow through the body. Watts law tells us that this is (600 * 1.2) 720 watts. The actual wattage does not really matter. What does matter is that current flows through all paths, each path can be represented as a resistance and that as a set of parallel resistive paths the current will divide in accords with the reciprocal rule with the most current flowing through the path of least resistances. These lower resistance paths see the greatest heating. Blood is full of salts and has a fairly low resistance. Blood rich muscles and blood veins and arteries are some such paths. It really is rather gruesome. Cooked from the inside, then putrefaction. Plus, blood cooked this way blocks further blood flow and areas not cooked end up dying from lack of blood.

No recreational boat that I’ve been on has high voltages. The research vessels that I’ve been on do have high voltages but you really cannot get near it.

Right, what is the next category? Low Voltage of course. Not 48 volts. Voltages between (depending on the regulating body) 50 or 60 volts and 600 volts are considered low voltages. With Low Voltages we do not get dielectric breakdown of the skin typically and thus the major threat to life is not deep cooking of tissues. Low voltage shocks have a whole range of effects. Muscle spasms can cause tears in the various tissues that the current flows through (Muscles, ligaments and tendons). Tissues can be burned if the current flows for enough time. If a current flows through the heart ventricular fibrillation can occur. This appears to be the more common cause of death in a low voltage shock. Additional effects are seen. Nerves have fairly low resistance and thus are subject to damage in electric shock.

Low voltage systems on our boats are for the most part confined to our AC house/mains currents. 120 volts in the USA and 220 to 240 in much of the rest of the world are typical. These voltages are able to produce lethal shocks on a recreational boat just as they are in our homes or factories. And as such should be accorded the respect that the various standards require.

Lastly we come to the voltages that are un-specified under the various codes. Typically these voltages are under 60 volts. In the ABYC standards 50 volts and lower is indicated.

Simply stated it is highly unlikely that you will go into ventricular fibrillation should you receive a shock from voltages less than 60 volts. There has been at least one case of ventricular fibrillation when shocked with 42 volts. You may recall that I stated that AC is more dangerous than DC. In this case it was 42 volts AC and the current path led directly through the heart.

Data indicates that it takes about 300 to 500 mA DC or as low as 60 to 100 mA AC to cause ventricular fibrillation. See https://hypertextbook.com/facts/2000/JackHsu.shtml for something on this.

If we use the higher AC limit of 100 mA and 42 volts we would need a (42 / 0.1) 420 ohm resistance path through the heart to induce ventricular fibrillation. 420 ohms is close to the basic internal body resistance so a person who has salt water soaked hands with open sores who gets 42 volts AC flowing from one hand to another could easily receive a lethal current flow through their heart.

When using the lower limit for DC current of 300 mA and upping the voltage to 48 volts we end up needing a resistance of (48 / 0.3) 160 ohms in the current path. Short of taking some knitting needles and impaling the chest cavity it is nearly impossible to get a 160 ohm resistance through the human body. Thus the risk of receiving a 48 volt DC shock that leads to ventricular fibrillation is clearly vanishingly small. Because the risk approaches zero we can say in effect that the risk is zero. Clearly lowering the voltage to 12 volts from 48 volts would require an even lower resistance in the path through the heart in order to induce ventricular fibrillation. On one hand you might say that 12 volts is safer than 48 volts. This is academic, a risk nearly approaching zero and a risk even more closely approaching zero are indistinguishable.

So what happens if you do get shocked with voltages under 0 volts? In general it appears that the lower threshold of perception is a current flow of 1 mA to 5 mA. The pain threshold is around 10 mA and the severe pain threshold is upwards of that.

Posters have indicated that they have been “shocked” by 48 volts and felt only a tingling. From that we can conclude that the current path had a resistance 50,000 ohms or so. This is a commonly cited number for the resistance for dry skin. To get a similar current flow at 12 volts would require a skin resistance of around 12,000 ohms. Not an uncommon resistance. I myself have felt a tingle from 12 volts.

To receive a painful shock from 48 volts you would need a current path of about 5000 ohms. At 12 volts that same path would need to be around 1800 ohms. With the exception of some sweaty palmed individuals nearly everyone has a hand to hand resistance greater than 5000 ohms. It is not a question of degree but of thresholds and limits. With a resistance path of 10,000 ohms which is much more common the current flow would be felt but not painful in both voltages (12 or 48).

Lastly, in order to get a painful shock from either voltage one would need to break a few rules. One would need to be sitting in a pool of salt water (grounded), while using your salty wet hands to touch an energized conductor. I guess we can conclude that some people are safer than others

[CatNewBee]

Love you guys, nobody would expect in a sailing forum a lecture in electrical engineering.

So we have Ohms laws, Watts laws, Kirchhoff's laws...
So here to complete it, for those who are curious how it it wold look like in mathematical formulas for the nodes and mashes:





Some mention all the time the energy, and resulting induction laws + energy persistence:



and here we go for AC with changing magnetic fields over time:



.. just to give you some brain food...

Disclaimer:
Electricity is dangerous, you may experience some headache dealing with the formulae above


Have fun!

[Q Xopa]

Originally Posted by s/v Jedi

This shows either an utter lack of understanding or just trolling the forum. I would hope for the latter but fear it's the former.

What you show here is the heat produced in case the 240W load is a resistive heater, which has nothing to do with the statement you are calling to be incorrect. Total BS answer. The statement was talking about a 240W load in a circuit that somehow (corrosion, wear) has developed a resistance. I have shown the math for that before in this thread but that was ignored because it was probably too complex.

Also. what is completely lost as a factor of safety is the actual operational status of the device powered by the circuit. The higher the power consumption, the higher the reliability becomes with a higher voltage power source as I demonstrated earlier as well. I even showed that, although with reduced effect, this is even true for small loads where even good old 12V would still do okay'ish.

Correct- Trolling obviously.

Incorrect- He cant be that out step with commonly accepted wisdom not pretending.

He's having a laugh.