Comparative Safety: 12v v 24v v 48

[ramblinrod]

Originally Posted by Jim Cate

This statement is the crux of the problem, Rod. YOU don't require proof because you KNOW you are right.

This makes for kinda one sided discussions and thus the endless arguments with folks who DO require proof.

Jim

I would not present a position and defend it, if I didn't know it to be correct.

If one does present a position and defend it, when they know they are incorrect, that would be stupid and self-defeating.

I suspect I am neither.

(Not actually sure of this, and would not attempt to debate a contradiction, as I am still here taking this ridiculous abuse from y'all.) ;-)

[ramblinrod]

Originally Posted by CatNewBee

This is a false thought. it is not necessary the 240W that start the fire.

Incorrect.

Strawman Alert: No where did I suggest that the 240 W would definitely start a fire.

This is hugely different from the suggestion that something has fire starting potential.

I know that under certain circumstances, for example, 10 W can start a fire, whereas in others, 1000's of Watts can't start a fire.

But in general, 1000's of Watts has much greater fire starting potential than 10 Watts.

Assume you have a faulty contact with a resistance of 0.1 Ohm, same contact in both cases.

When at 12V 20 Amps are going through, it would have a voltage drop of 2V and over the contact a power transferred in heat of 40W (2V * 20A).

In 48V, there is a current of 5A, over the same connection Voltage drop is 5A * 0.1 Ohm = 0.5V, and the power burned over the connection is 0.5V * 5A = 2.5W, it would not get even warm.

So running on 48V (4 times higher) is 16 times safer (4^2) and more efficient.

Incorrect (again).

Please refer to post # 1 and 5 and the posts about the Maximum Power Transfer Theorem.

[ramblinrod]

Originally Posted by transmitterdan

Undesired power in a faulty connection is proportional the current squared times resistance which inevitably increases the hotter it gets. So heating rises until you get thermal runaway and something burns.

If you lower current a factor of 4 by raising supply voltage from 12 to 48 you have the same power to your device. Bolted connections of a given size have a certain contact resistance. So if you use a 1/4-20 or 6mm bolt to connect a device then the heat generated in that connection is 16 times more for the 12V case.

So current matters way more than voltage when we talk about heat. In fact voltage contributes nothing to heat. You can raise the voltage to 1,000 volts and the heat in the connection is the same for any given current. And the current in a faulty connection will always be less for a higher system voltage.

It has been argued that higher voltage somehow increases fire hazard. This hypothesis is not supported by any math in electrical engineering. Fire danger increases with decreasing voltage. Fault current decreases with increasing voltage. Why? Two reasons: First the circuit interrupters are set at lower current for higher voltage. A 100A fuse in a 12V system becomes a 25A fuse in a 48V system. Second, the system impedance will be proportional to the voltage. So higher voltage systems have correspondingly higher internal resistance. The fault current available from 4 series batteries is exactly the same as what is available from 4 12V batteries in parallel. So we have the same fault current available with 1/4 the the fuse/breaker trip current.

It has been argued that for some unspecified or wholly incorrect reasons, exposed 48V is a death trap but 12V is not. This is an unwarranted fear held by people with little or no experience with high voltage. It’s not the voltage that kills you it’s the current. Utility line repair technicians routinely touch 250 thousand volt power lines and they live. Birds to it all the time. Why! Because the current is too low to cause injury.

Exposed DC circuits are a terrible idea no matter the voltage. ABYC wants 12V exposed metal covered. The reason is not shock hazard. It is because the high current available will melt things like rings and wrenches which can start a fire.48V means less current available.

Fire is the danger. Heat and flammable material are the ingredients of fire. Four times less current means heat is more than an order of magnitude less. Less heat means less chance of fire. It’s not that complex or mysterious. So 48V is much safer any way to look at it.

Incorrect.

The heat generated by a resistance is directly proportional to the power (energy) dissipated.

Power (P) = E (voltage) * I (current).

I (current) = E (voltage) / R (resistance)

The lower the resistance the greater the current for a specific voltage.

So the power dissipated with a specific voltage applied will increase with a decrease in resistance.

The power dissipated will also increase with an increase in voltage for a specific resistance.

The thermal runaway argument is a "red herring".

It is reasonable to consider a fixed resistance.

Not all occurrences of heat generation result in thermal run-away, or the universe would not exist.

Likewise, not all occurrences of heat generation result in fire.

However, in general, with the presence of oxygen and a combustible, (of which both usually exist on a boat) the risk of fire increases with heat generated.

The fire may be started long before there is any significant change in resistance.

One can have 1000's of amps flowing through a very low resistance with no appreciable heat generated.

Heat is not proportional to current.

EEE the risk of fire increases proportionally with voltage.

EEE the risk of electrical shock increases proportionally with voltage.

Fire and electrical shock are dangerous.

Danger increases proportionally with voltage.

[evm1024]

Originally Posted by RR

I would not present a position and defend it, if I didn't know it to be correct.

If one does present a position and defend it, when they know they are incorrect, that would be stupid and self-defeating.

I suspect I am neither.
SNIP

Trying to defend a 10 ohm faulty connection fills the bill.

Please cite any study of the resistance of series faults or parallel faults where they state that the faulty resistance was 10 ohms.

I have been looking to find such a study and have not. I would welcome such a study.

As I have noted many time in this thread I have been able to calculate what the typical faulty connection resistance is. I pretty much all cases the HRC (high resistance contact) resistance is on the close order of 0.2 ohms.

One more popped up where the connection was glowing. This connection had a resistance of 0.160 ohms.

Without any evidence for you to base your opinion on a 10 ohm HRC is as you stated above.

Simply put you have"cooked" the data to support your (incorrect) position.

[ramblinrod]

Originally Posted by Dockhead

I think these are wise comments.

My boat has a 10 horsepower bow thruster which draws something like 400 amps at 24v. The installation is well designed and works well, but I wonder how much the voltage sag is and how much better it would work, with how much lighter cables, on 48v. Surely no one would say that moving that kind of power, you wouldn't want the highest voltage you could get.

Not only are the cables massive, but the circuit protection devices are massive and no doubt very expensive.

You make a good point about 6v on cars. I had a 6v car once (old VW van), and I hated how the 6v gear worked on it. Everything was so laggy and weak. If safety were proportional to voltage, wouldn't we want to go back to 6v? I wouldn't. Going from 6v to 12v was like going to 12v to 24v on this boat. Much more realistic voltage for moving even moderate amounts of power around, and the gear runs so much better. Of course you're right -- a lake sailer with nothing but a bit of lighting and a bit of nav gear, hardly cares. Even 6v would probably be ok for that.

Not surprisingly, yours is the size of vessel and your thruster is the size of load, where I prescribed that an increase from 12 Vdc to 24 Vdc (for those loads) may be warranted for practical reasons.

Make no mistake, 24 Vdc is more dangerous, but it's also more practical for a short duration 7.5 kW load.

PS, an unacceptable voltage sag in a DC system powered by FLA batteries can be resolved by increasing capacity. It can also be resolved by a change in battery technology less prone to sag, but that kettle of fish is unrelated to the subject of this thread, so lets keep it separate please.

[ramblinrod]

Originally Posted by TeddyDiver

You seem to have a confusion which part of the current is wasted (and converted to heat) and what's not..

No confusion.

[ramblinrod]

Originally Posted by evm1024

Agreed, Sigh (I mean Correct)

It appears that we are back to debating Rod's opinion. It is all about what Rod dictates is correct. That and what Rod recommends, nothing else cuts it.

Oh well, there was 7 good posts in the discussion.

Another option would be for you to refer to the last sentence of Post # 99, and follow it.

Let's see.

[evm1024]

Originally Posted by RR

Another option would be for you to refer to the last sentence of Post # 99, and follow it.

Let's see.

Of course we could have a constructive conversation. Unfortunately that is not in everyone's nature.

What is it you said once? Something about beating people down until they stay down.

Facts do not matter it appears where opinion rules.

And by the way, I am still waiting for you to provide a reference that indicates the 10 ohm fault resistance that you go on about.

Please cite where you got that contact resistance or have the honesty to state that you pulled that resistance out of the air to reinforce you opinion.

Changing ones mind is a virtue.

(mods feel free to change my post as you see fit)

[ramblinrod]

Originally Posted by evm1024

Of course we could have a constructive conversation.

I didn't think so.

The vitriol after this first sentence proved my suspicion.

[transmitterdan]

Rod,

FWIW, please stop quoting my posts as “Incorrect” without stating what fact is incorrect. The facts i posted are exactly correct and can easily be verified by any first year engineering student.

If you think my opinions are “incorrect” that is fine but you should know that opinions cannot be incorrect by definition. That’s why they are called opinions and not facts.

[ramblinrod]

Originally Posted by transmitterdan

Rod,

FWIW, please stop quoting my posts as “Incorrect” without stating what fact is incorrect.

I would never claim a "fact" is incorrect.

By definition facts are correct.

When an "incorrect statement" is posted, to help forum members separate facts from incorrect statements, I sometimes post that it is "incorrect" and explain why with "facts", including examples that support them.

I hope this and every explanation I post, is clear enough that it is obvious which "incorrect statement(s)" it is referring to.

I think if one reads the original post (or quote) I have referenced, and then reads the correction I post carefully, it should be obvious which incorrect statement the facts I post correspond to, even for those without any engineering background at all.

[ramblinrod]

Originally Posted by evm1024

Please cite any study of the resistance of series faults or parallel faults where they state that the faulty resistance was 10 ohms.

After all the time I have spent trying to correct the incorrect statements posted in this thread by others, that could cause forum members harm...

I am not about to waste time searching for studies to prove something that I have known to be correct for over 40 years.

A good contact or connection (switch, breaker, wire connector, etc.) can be near zero resistance.

A bad contact or connection can be any resistance value above design, up to and including "open" circuit.

This is pretty fundamental.

[transmitterdan]

Meesa give up...

[ramblinrod]

Originally Posted by transmitterdan

If you think my opinions are “incorrect” that is fine but you should know that opinions cannot be incorrect by definition. That’s why they are called opinions and not facts.

Incorrect.

If one states their opinion is that 1+1=3, that may very well be their opinion, but their opinion on that subject, is factually incorrect, because 1+1=2.

OK mathematicians reading this, cut me a little slack here, you know what I mean.

;-)

Edit: Posted before I saw the tap out.

[evm1024]

Originally Posted by ramblinrod

After all the time I have spent trying to correct the incorrect statements posted in this thread by others, that could cause forum members harm...

I am not about to waste time searching for studies to prove something that I have known to be correct for over 40 years.

A good contact or connection (switch, breaker, wire connector, etc.) can be near zero resistance.

A bad contact or connection can be any resistance value above design, up to and including "open" circuit.

This is pretty fundamental.

In other words you have no data to back up your opinions. And that rather than man-up and admit that you have no factual basis for selecting 10 ohms you bluster and say you have no time to waste. When in fact most of your posts in this thread and many others are a waste of everyone's time.

What is fundamental is that if indeed you have a fact that you have known for 40 years you would be able to prove it with little or no effort. There is not proof and what you know is wrong.

That you willfully misrepresent and in general being "right" is more important than being honest. Misleading, misdirection and sidestepping are your primary tools. Case in point one needs only look at your post quoted above.

It is sad because you do have knowledge and information that has been helpful to other but that you are so invested in being right that you are a liability rather than an asset to the CF community.

You did learn things 40 years ago but some of what you learned has been shown to be false. Most of us call that scientific progress. You do not know the limits of your knowledge and you mistake your opinions for fact.

You claim to correct incorrect statements with the goal of keeping forum members from harm. And yet in the past you have not correct yourself when you suggested specific courses that lead to danger.

Lastly I want to highlight your post again

Originally Posted by Rod

After all the time I have spent trying to correct the incorrect statements posted in this thread by others, that could cause forum members harm...

You make that sound like you post to help people. That is about as far from the truth as can be.

Your posts - oh well I'm sure that the casual observer understands.