Comparative Safety: 12v v 24v v 48

[ramblinrod]

Originally Posted by CatNewBee

...

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?

See?!?!?!

Strawman Alert: I never claimed "480 W as threshold for danger." Not once.

With all respect, any size of 12V battery in a boat delivers more than 480W when shorted.

Strawman Alert: I never claimed they didn't.

The potential energy stored in a battery is Ah * Voltage and is in Wh, your calculations need some review.

Incredibly incorrect:

The potential energy stored in a battery is defined as it's A-hr rating * the time interval the A-hr rating pertains to, * voltage.

Big difference kids, big difference.

AND, this only applied for that very specific current. At much higher currents, the capacity is usually considerably less.

AND, that rating is only until the battery is considered fully discharged at 10.5 Vdc, BUT the battery still has potential energy left in it.

AND, often the fire will have started long, long, long before the battery is completely dead, as it's full capacity was not nearly needed to be dangerous.

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.

Correct, this is the related to the Maximum Power Transfer Theorem discussed earlier in this thread.

Since the internal resistance of a good battery is such that I know (do I really ;-) the battery can unleash 1000's of Amps to a low impedance short circuit, I didn't concern myself with the internal resistance of the battery, even though I know (I really do) that it will come into play. I just hope that my assumption that it would be negligible, is.

I do know (I think) that since the bank in question has undefined capacity (and could be extremely high) that we can disregard it for the purpose of this discussion.

Cheers! [/QUOTE]

See!

[CatNewBee]

See, how easy it can be, the answer is not 42, it's a beer!

[ramblinrod]

Originally Posted by Dockhead

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).

They may not accept it, but I do believe that is exactly what happened here.

I don't mean this disrespectfully at all.

Everyone, along with me, who agreed and would argue that the sun rises in the east, but knows the sun doesn't really rise, in a way, is delusional, I suppose.

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.

I do know the commitment of time, effort, and money, that every engineer (in Canada) have put into their learning.

This commands a level or respect from the get go.

I have a very high respect for every engineer (until they give me cause to lower it, based on their behaviour or deliverables).

I have worked with engineers that I have held in very high regard, and I have worked with some that I have absolutely despised.

Similarly, I have worked with engineers who have despised me, for my audacity to stick my nose into their area of business, whereas I have worked with others who were astounded by my knowledge of complex concepts they barely "knew" themselves.

When a new engineer walks in the door, I have a certain degree of respect.

First of all, the general respect I have for all people, and then the respect I have for this individual for pursuing this career and achieving a level of success to date.

Now they have to show me what they can do and how they treat people along the way.

I will adjust my level of respect for them on this basis, mostly the latter.

If they are struggling with the technical complexity of their assignment, I can assign them to do something else, or assign another more knowledgeable or experienced in that particular issue to assist them.

But if they treat people poorly, they are soon an island, and out the door.

[ramblinrod]

Originally Posted by CatNewBee

See, how easy it can be, the answer is not 42, it's a beer!

I would share one with you any day!

[ramblinrod]

Originally Posted by Dockhead

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.

You may do whatever you wish.

You are correct that my statements regarding shock and fire hazard are simplifications. But IMHO they are not really over simplifications, and certainly not to the extent they are not valid.

It is that last sentence that you quoted but then disregarded, that makes the statements truly valid....

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

..specifically, without any other mitigating factors...

You are correct, if current is somehow limited (a mitigating factor) it could affect the equation for either supply voltage.

But if the current is for all intents and purposes "unlimited" higher voltage is always more dangerous with very few exceptions.

Example, there is a "clamp on" threshold voltage, where if one receives a shock with this voltage, with sufficient current, their muscles can involuntarily contact so the individual cannot let go, so their body absorbs more energy, which can do a lot of damage.

Whereas for a slightly higher voltage, the muscle spasm will throw them away, and they will therefore consume less energy.

However with ever increasing voltage, the muscle spasm will become more violent and the burn more severe, until it is once again more dangerous than that odd "clamp on" voltage.

But we can't really predict where the persons "human resistance" and the corresponding clamp on voltage will be, so for all intents and purposes, we can just claim that danger increases with voltage (even though we know there may be a few odd exceptions to the rule.

The general rule of thumb is that no amount of electrical energy in the human body is safe. The greater the energy, the greater the risk of injury. For a given skin resistance, the greater the voltage the greater the potential energy, the greater the risk of injury should one contact it.

There is very little sense in arguing against the rule of thumb on the semantics that there may be a remote scenario where the rule doesn't apply perfectly. for the purpose of the discussion we are having here.

It is true that 1.5 Vdc is generally safer than 12 Vdc.

Most electrical agencies have very little in the form of safety requirements for the use of a 1.5 Vdc AAA sized battery, that has an inherent energy delivery limit.

But as was shown by another poster, a small battery can be used to power a circuit that can kill rodents, and possibly humans.

This doesn't change the fact that the risk of danger increases proportionally with voltage.

(And it is this relationship that the rodent killing device was based on; it raised the voltage, so that even the small amount of current it could deliver could deliver enough power, to kill the rodent(s).)

[GrowleyMonster]

There is a lot of very passionate debate on this topic in this thread. I wonder how many here, besides myself, have 48v systems? I do, and I will weigh in with possibly the least technical but most relevant comments.


Actual electric shock from handling a single conductor in a 48VDC circuit isn't really a thing, unless the negative side (or the positive, actually, but I have never seen a positive grounded system on a yacht) is grounded and you are touching or sitting or kneeling on a grounded metal surface. Most electric propulsion systems are not grounded. Touching both neg and pos with dry hands and just brushing one side or the other can give you a bit of a bite, in my real world actual experience, and I have not had this occur with wet hands so can't comment authoritatively on that. What I CAN comment on is that any direct short through a wrench, screwdriver, metal watchband, etc produces incredible amounts of heat in milliseconds.



Watchband burn. The watchband was destroyed and at the first sparkle I naturally yanked my hand away instantly and still got these cool blisters to show off at cocktail hour that day. A wrench will melt. A quarter lodged across the phase wires of a motor will actually explode when it is energized. Skin contact with 48v is not the demon here. It is contact with metal that contacts the circuit. Eliminate this risk by using proper terminal boots and the battle is half won. The other half is learning not to do stupid stuff. The good thing is that the learning process punishes you just enough to remember your mistakes and not kill or hospitalize you.


So am I saying that 48v is as safe as 12v, all things equal? No, not really. Kind of like asking if looking left and then right before crossing a street is more dangerous than looking right and then left. The risk to life posed by a properly installed 48v system is probably about the same as the risk of playing golf too near a bayou or pond where alligators lurk, or driving to the store every day instead of just once a month, or going sailing where powerboats are known to go, or a number of things that we do without thinking about them. I will go so far as to say that the safety risk inherent with a good 48v setup is trivial. Facts and figures and formulae and theorems and laws of nature aside, how many people have died from 48vdc electrical shock on boats?



I dont think anybody can throw doubt on the idea that 48v for high power loads or long cable runs is more efficient than 12v.



48v bits and pieces are getting easier to source, both in the marine world and the golf cart world. No, you won't find 48v gear at Worst Marine, but it is out there. It didn't take an act of congress to get a 5kw and a 300w inverter, or 30a dc/dc converter, (I still have some 12v gear) or 48v charging systems. On my e-boat I don't have a 12v bank anymore.



If you have ever wanted a "wing motor" to get yourself into your slip when the infernal combustion engine dies unexpectedly, then you probably understand how handy an electric motor driving a pulley on the shaft could be. And for small boats, for propulsion, 48v is king, for a number of reasons. I think bow thrusters and windlasses are also excellent 48v projects. Last but not least, should it be necessary, you can weld straight from a 48v bank. You can't, from a 12v bank. You could, from a 36v bank. 24v? Probably not. I think it has been done with 32v (8v batteries) but not sure.


Higher voltage lends itself well to straight series wiring, too. I have a strong distaste for parallel connections and I won't argue about it, but if you don't like batteries connected in parallel and you like golf cart batteries, then it is pretty obvious what to do to have lots of watt/hours.



On Brute Force, I considered going with a higher voltage as it had one tired old 8D and one dead one, for engine starting and house loads so basically starting from scratch. I even considered going as high as 120v partly because at that voltage I could run an ME0913 or similar motor and at least move the boat along in the event of an engine failure. In the end I decided that the Westerbeast is running dependably enough that I don't need the wing motor, and I can get a pretty fat bank out of Rolls 1000ah cells in series for 12v. But the higher voltages, in light of my own experiences, were very attractive in a way.



I think most irrational fear of the 48v is simply not being familiar with it. At one time, 6v car electrical systems were common. I wonder if there were naysayers and doom predictors giving the newfangled 12v cars the badmouth? And yet now, 6v cars are rare.



Not saying everybody needs to get with the new and improved 48v standard, but it is viable and will become more so in the future, I think, especially as electric propulsion becomes more widely accepted.

[evm1024]

Originally Posted by Dockhead

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.

I like your optimism!

Today the sun rose in the eastern sky. And it is certainly valid to plan a rocket launch with the time of sunrise in mind.

The launch planners however do take into account that it is the earth that rotates. That the earth rotates is very important to their calculations.

Anyone who missed the point should take a second look.

Simply stated saying that the sun rises in the east is fine for day to day life but is fatally flawed if you are planning an orbital launch.

The task requires a greater understanding.

Rules of thumb are just that. Sometimes real thinking is required.

Originally Posted by Q_Xopa

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).

I like it, how apropos.

[transmitterdan]

With all due respect, respect is something we earn from our interaction with others. No one is owed respect just because they themselves think it should be so or they attended some university or attained some certification.

Respect is like beauty, it only has meaning for someone other than ourselves. We don’t walk around with “i must be respected” tee shirts or hats. Well, you can I guess, but it would have no meaning.

[CatNewBee]

Originally Posted by transmitterdan

With all due respect, respect is something we earn from our interaction with others. No one is owed respect just because they themselves think it should be so or they attended some university or attained some certification.

Respect is like beauty, it only has meaning for someone other than ourselves. We don’t walk around with “i must be respected” tee shirts or hats. Well, you can I guess, but it would have no meaning.

... but you can sing it!

R.E.S.P.E.C.T!

https://www.youtube.com/watch?v=6FOUqQt3Kg0

[Steve_C]

Semantics!!!!

"48 Volts CAN be MORE Dangerous than 12 Volts"

"12 Volts CAN be MORE Dangerous than 48 Volts"

Relative terms!!!!!

"All things being equal" = Virtually impossible for these scenarios.

There are very clear advantages to engineering things to different voltages and each has its strengths and weaknesses. The VAST majority of everyone's opinions in this thread are absolutely based in truth and fact.

You are simply arguing about the "Relative" meaning of Rods one phrase.

"risk of DANGER increases proportionally with voltage."

Some aspects may be more dangerous and others not. Every situation is not the same and needs to be considered.

I personally hate blanket statements and find them to FAR more likely to be wrong or at least misinterpreted that not. The relative nature of many statements can be impossible to compare when two parties are looking at it from vastly differing veiwpoints.

I personally have spent my entire adult life in the electrical utility industry. I have dealt with live line maintenance up to 39KV and worked on circuits up to 500 KV.

I have immense respect for ALL voltages including 12 volts. For the appropriate load on my vessel I would not hesitate to use 48 volts, But only if the loads or equipment justified it. Even with my 12 volt systems I have always taken great strides to protect all open points of the positive circuit as well as pay extreme attention to fusing, circuit protection and proper connections. ALL electrical systems are dangerous!!!! (I know, its blanket statment)

I am planning of a setup very similar to CatNewBee's only without a generator. I plan on 1 or 2 high output alternators, or possibly the Integrel system now being marketed. I want the capacity just for emergencies, and with a really large LFP bank I would like to be able to charge them up really quickly if needed. The Integrel system puts out 48 Volts to mitigate problems with excessive currents.

https://triskelmarine.co.uk/integrel/
https://www.youtube.com/watch?time_c...&v=p6jMN2HTlmc

I definitely struggle with the option for 48 volt vs 24 volt vs 12 volt. I REALLY like the 48 volt approach to charging, but would prefer to keep things simple (KISS) I will consider a DC to DC stepdown, but still undecided if its worth it. I feel there are no exact right or wrong answers. just degrees of advantages vs risk.

I recently helped my nephew install a US 12 volt stereo into his vintage Japanese land cruiser with a 24 volt electrical system.

I think this entire thread has been immensely educational and entertaining. I applaud the amount of professionalism and restraint that has mostly been observed. Perhaps its time to quit focusing on 1 small RELATIVE statement that is causing such concern.

Thanks for the entertainment

[ramblinrod]

Originally Posted by evm1024

Rules of thumb are just that. Sometimes real thinking is required.

I disagree.

For this (and any) electrical installation, real thinking is always required.

This is why we do a vessel design and configuration review and customer interview for every job.

And part of that review is, "What should I chose for a product terminal and supply voltage?"

"Is the increased danger risk associated with a higher system voltage, worth the benefit of reduced cable cost?"

Often, the answer is "no", until we get to a current level where there simply is no cable or components easily available that are adequate, or they add significantly to the total job cost.

Remember that for an 8 HP (decent brand) thruster, we would normally be talking in the realm of a 40 - 56 ft boat, likely having a value of $150 K or more.

Everything else being equal, unless there are other mitigating factors, due to the greater risks of the higher DC voltage in a fault condition, I recommend going with 12 Vdc.

To go with the safer 12 Vdc thruster, costs $40 more, or ~ 0.3% of the job, or < 0.02% of the value of the boat.

Cheapest insurance that owner will ever buy for that vessel.

[ramblinrod]

Originally Posted by evm1024

I like it, how apropos.

Careful, or that could be your new moniker....

[Dockhead]

Originally Posted by Steve_C

. . . I am planning of a setup very similar to CatNewBee's only without a generator. I plan on 1 or 2 high output alternators, or possibly the Integrel system now being marketed. I want the capacity just for emergencies, and with a really large LFP bank I would like to be able to charge them up really quickly if needed. The Integrel system puts out 48 Volts to mitigate problems with excessive currents.

https://triskelmarine.co.uk/integrel/
https://www.youtube.com/watch?time_c...&v=p6jMN2HTlmc

I definitely struggle with the option for 48 volt vs 24 volt vs 12 volt. I REALLY like the 48 volt approach to charging, but would prefer to keep things simple (KISS) I will consider a DC to DC stepdown, but still undecided if its worth it. I feel there are no exact right or wrong answers. just degrees of advantages vs risk.. .

What kind of high power gear do you have on board? How large a vessel?


Large 48 volt alternator charging a large 48v lithium bank powering a 48v inverter bank is very straightforward and deals with the large power involved, knocking down the large currents. From what the engineers are saying, and you be one yourself I guess, that would seem like a superior and generally safer solution. But what kind of high power gear do you have? 48v thrusters exist, but I'm not sure less then 15hp. 48v windlasses exist. I don't know about winches or furlers, but those use fairly generic motors, so you could probably change or rewind the motors if you had to.


If you can get your high power gear in native 48v, then it would be pretty straightforward to use a small 12v or 24v bank for all the non-high power gear, with separate switch panels and busses. Charge via battery-to-battery charger. This is the very architecture I'm considering for my next boat. Small increase in complexity but then you gain backup power.



We separate high power systems from low power systems in buildings (I deal with this in my professional life), and even at three different levels ("high power" for HVAC, elevators, escalators, other really high power systems, at 380v 3 phase; "low power" for lighting and office equipment at 230v 1 phase; and "weak current systems" for data, video, control systems); it seems to me that on a largish yacht it also makes sense not to try to make one voltage fit all. Using different voltages is maybe not so crazy.


My present boat has 2x completely separate 12v engine control and starting systems, each with its own alternator, plus 1x 24v system with own large frame heavy duty alternator for house and technical loads. Seems like an absolutely reasonable approach, and I don't see why you couldn't do something similar with 48v and 12v. No need for droppers; just separate the systems with own small battery bank for the 12v (and/or 24v) system(s).




I think your idea of eliminating the generator and using a very large 48v alternator on the main engine is a good one. Many systems design arguments for that. Getting rid of the generator saves huge weight, volume and cost. My next boat will be designed for extreme latitudes and extremely remote places, and so needs extreme reliability and redundancy, so I won't get rid of the generator, but I might have just a small emergency generator (NexGen or something like that), rather than a half-tonne heavy duty low-speed prime power rated one like what I have now. With primary generation just like what you're considering.

[leftbrainstuff]

Originally Posted by transmitterdan

Most Telcos use 48V for standby power but they don't need inverters to power the gear. Almost all their gear runs off 48V directly. So their use case is different than a boat. Their loads are steady and they have automatic monitoring of the system that communicates back to a central automated attendant. But 48V battery systems are not more dangerous than 12V. If they were the telcos could never get a permit to install hundreds of thousands of them in buildings all over the world.

Also, telco equipment is designed to draw DC current from the batteries. The biggest load on a boat is often the AC inverter. Those do not draw steady current from the battery. Each 1/2 cycle of 50 or 60Hz (8-10 milliseconds) draws a half cycle of AC from the battery. So the peak current is much higher than the "DC" current. This is a good exercise for a first year engineering student. Calculate the RMS current in a DC cable feeding a high efficiency inverter. The RMS is higher than the DC current. It is the RMS current that should be used to estimate heating in wires and connections. A 48V inverter can be much better than a 12V in both efficiency and RMS to DC ratio. Also, don't forget abut wiring inductance. Since 48V systems draw less current (75% less) they should radiate 75% less noise from switching currents. In dB language a 48V DC system should produce 12dB less interference noise which is about an order of magnitude.

So overall the engineering favors 48V. But the economics and market place don't. As I know about engineering and not so much about markets I leave that to someone else.

But all in all 48V is "better" from the engineering side in every way you analyze it. There is a slight increase in shock hazard but not enough to change the equation.

48V in commercial use is probably a legacy choice rather than for pure engineering goodness.

All the old military comms systems (pre 1980s) were 48V (or similar) mostly due to less weight for wiring and because battery technology was a real constraint.

Also real old generator and remote power systems (pre war) were often based on 32V as many tractors ran 8V batteries.

Today 12V is a no brainer IMHO. Even for our winches which have a very low duty cycle. There are so many choices that the increase in wiring is usually not an issue. We also have more redundancy and less complexity avoiding 24 or 48V.

But if you're considering electric power, more load (induction cooking, AC off battery, etc) then 48V is a good choice.

Even my cnc machine uses 48V. I could have chosen 24V but the stepper motors run cooler on higher voltages.

[evm1024]

Originally Posted by Dockhead

What kind of high power gear do you have on board? How large a vessel?


Large 48 volt alternator charging a large 48v lithium bank powering a 48v inverter bank is very straightforward and deals with the large power involved, knocking down the large currents. From what the engineers are saying, and you be one yourself I guess, that would seem like a superior and generally safer solution. But what kind of high power gear do you have? 48v thrusters exist, but I'm not sure less then 15hp. 48v windlasses exist. I don't know about winches or furlers, but those use fairly generic motors, so you could probably change or rewind the motors if you had to.


If you can get your high power gear in native 48v, then it would be pretty straightforward to use a small 12v or 24v bank for all the non-high power gear, with separate switch panels and busses. Charge via battery-to-battery charger. This is the very architecture I'm considering for my next boat. Small increase in complexity but then you gain backup power.



We separate high power systems from low power systems in buildings (I deal with this in my professional life), and even at three different levels ("high power" for HVAC, elevators, escalators, other really high power systems, at 380v 3 phase; "low power" for lighting and office equipment at 230v 1 phase; and "weak current systems" for data, video, control systems); it seems to me that on a largish yacht it also makes sense not to try to make one voltage fit all. Using different voltages is maybe not so crazy.


My present boat has 2x completely separate 12v engine control and starting systems, each with its own alternator, plus 1x 24v system with own large frame heavy duty alternator for house and technical loads. Seems like an absolutely reasonable approach, and I don't see why you couldn't do something similar with 48v and 12v. No need for droppers; just separate the systems with own small battery bank for the 12v (and/or 24v) system(s).




I think your idea of eliminating the generator and using a very large 48v alternator on the main engine is a good one. Many systems design arguments for that. Getting rid of the generator saves huge weight, volume and cost. My next boat will be designed for extreme latitudes and extremely remote places, and so needs extreme reliability and redundancy, so I won't get rid of the generator, but I might have just a small emergency generator (NexGen or something like that), rather than a half-tonne heavy duty low-speed prime power rated one like what I have now. With primary generation just like what you're considering.

You have made some really great points here that are really worthwhile. This is especially true for anyone who is contemplating new design.

One of the impediments to progress is thinking that the way we have done it in the past is the best and that there is not need to use other ways. Of course those ways are comfortable and comforting and low risk. This is especially true if you make your living based on those ways. This resistance to change, mental inertia takes many forms. Lithium - not needed, Lead (and ICE) does it all. 48 volts - danger, danger Will Robinson.

But with those old ways you never get the leaps of performance.