Comparative Safety: 12v v 24v v 48

[Dockhead]

Originally Posted by transmitterdan

DH, if you want to discuss this send me a PM. I am an experienced high power engineer that happens to own a boat. I don't charge anything for sharing my experience.

Thank you -- that's very kind. I will definitely take you up on that.

[Dockhead]

Originally Posted by CatNewBee

You are right for a typical low-power marine set-up, the 12V are sufficient, like 6V installations on older cars with weak headlights . When the headlights go to 55W/65W bulbs, 2x21W blinkers, 2x21W or 4x21W brake lights, stereo radios, electrical heating of seats and rear windows, electrical mirrors etc, 6V installation hit a barrier and 12V was introduced to make the installation more stable and more safe.

Trucks have 24V board systems, also busses and other commercial vehicles, commercial boats and larger yachts have also often 24V board systems for this reason.

If you go to a super yacht or a larger vessel, you will find 10kVA inverters, even 3-phase systems with 380V to power all the A/C units , refrigeration and galleys. Some run generators 24/7 some use lithium batteries for a quiet sleep. When it comes to inverter in this power range you are safer and better off with high voltage batteries, no question at all. If you look at hybrid / electrical propulsion the engines are also not at 12V, they use higher voltages to keep the currents in the installation under control.

So it depends on what you want to have on board. A dinghy even do not need an electric system, you can run your navigation lights on AAA batteries and do visual shore navigation.


In my opinion, it is a mistake to only look at the voltage, you have to consider both, voltage and current when deciding about a set up and safety. Higher currents make things complicated, so sometimes it is better to increase the voltage in an installation to make it more reliable.

I agree of course, for a bare boat with only navigation lights, some illumination inside and a chart plotter and a camping fridge where your power draw is around 10..20A, it makes no sense to go for 48V or even 24V.

If you regularly go over 400/500A, things are different. It starts with the wire gauge, the fuse holders and fuses, the solenoids, the switches and so on. This is a completely different playground.

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.

[ramblinrod]

Originally Posted by CatNewBee

The higher voltage increases the safety in high demand applications, simply by reducing currents. To handle currents above 400A requires very serious installations in 12V, because a weak contact can produce a lot of heat. For the same load in 48V you need 1/4 of the current, easier manageable and therefore safer in operation.

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.

A proper installation is in any case a necessity too.

Correct.

[CatNewBee]

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

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.

[ramblinrod]

Originally Posted by Jim Cate

Rod, I wonder if you have any documented cases of folks being seriously injured, either by shock or by consequent injury, on boats with systems of greater than 24 volts and less than 50 volts dc?

I do not.

The math and current threshold to stop a human heart is all of the evidence I need. To be satisfied that danger of shock increases proportionally with voltage. I have heard references to electrocution having happened as low as 42 Vdc. If interested in digging deeper into this subject, anyone can google it. Beware of poor and incorrect information sources on the internet. There can be as many mistakes made as have been in this thread.

If I follow your arguments correctly, your main objection to such voltages is in personal hazard to the operator, not in electrical failures which have been shown to not be greatly more hazardous if at all.

Incorrect.

One of my objections to using higher than necessary voltage is the higher risk of electrical shock.

The other one, is that another term commonly used for voltage is "potential".

Higher voltage has a higher potential for danger of fire.

When a resistance of any specific value is placed across a voltage (potential), the current that flows is based on the voltage. The higher the voltage, the higher the current.

Since P = E*I, and BTU/h = P * 3.4, the higher the voltage, the more heat generated, and the greater the risk of fire.

Lets take for example, a resistance of 10 ohms accidentally placed across a voltage source.

For 12 Vdc, I = E/R = 12V/10 ohm = 1.2 A
P = E* I = 12V x 1.2A = 14.4W
Heat = P(W) * 3.4 = 49 BTU/h

This is a considerable amount of heat that could be a danger.

However,

For 48 Vdc, I = E/R = 48/10 ohm = 4.8 A
P = E* I = 48V x 4.8A = 230.4W
Heat = P(W) * 3.4 = 783 BTU/h

Note the exponential relationship.

For a 400% increase in voltage, the heat generated increased 1600%.

1. Danger from shock and electrocution increases proportionally with voltage.

2. Danger from accidental fire increases exponentially with voltage.

Put simply, EEE danger increase proportionally with voltage.

Therefore, I recommend using the lowest voltage possible and practical to power loads.

Based on my experience analysing this on many, many boats for high load additions such as thrusters and windlasses, it is often best to stay with a 12 Vdc electrical system on vessels around 50 ft and less (actually based on a review of each specific vessel).

Where the load just gets too high for 12 Vdc, then 24 Vdc should be considered.

Based on my evaluations, this is around 3 kW for durations up to about 1 hour (e.g. inverter) and 6 kW for durations up to about 5 minutes (e.g. thruster)

When a more dangerous, higher voltage electrical system is introduced, it is safest to keep it small and localized to the load.

If this worry is valid, there should be a rich history of injury to fall back on for proof.

For me, there is enough evidence just in the more stringent safety standards for increasing voltages.

All electrical safety standards I have reviewed do this. The arbitrary voltage levels they choose is by no means a threshold above which is not safe and below which is. They are just a level where they drew the line, below which is incrementally more safe with decreasing voltage, and above which is incrementally less safe with increasing voltage, until one reaches the arbitrary limit where even more safety measures must be implemented.

Electricity of any voltage has the "potential" to be dangerous.

Electricity of higher voltage has a higher "potential" for danger.

I don't require any additional proof. I have known this for over 40 years, as I was taught in my first "principles of electricity" class, each subsequent class on electrical theory, and as proven in practice, time and time again since.

[evm1024]

Originally Posted by transmitterdan

Sigh....

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.

[CatNewBee]

at ramblingrod.

I will try to follow your quite queer calculations.

A human body produces heat of about 60W, when you lay in your bed and are covered too good you may self ignite.
There are evidences of self igniton of human bodies on the Internet, and also most people die in beds and not on electrocution, despite the fact that electricity has more regulations than building sleeping furniture.

60W * 3.4 results in 204 BTU, it gets even worse if your spouse joins you sleeping to 408btu/h, a serious danger and a 200% risk of self inflation.

[Dockhead]

Originally Posted by CatNewBee

. . . 60W * 3.4 results in 204 BTU, it gets even worse if your spouse joins you sleeping to 408btu/h, a serious danger and a 200% risk of self inflation.

But that also depends on how hot the spouse or, er, companion is, too, doesn't it?

[Steve_C]

Originally Posted by Dockhead

But that also depends on how hot the spouse or, er, companion is, too, doesn't it?

That is not always correct! My wife is crazy hot, but believe me she generates no "Heat" quite the opposite really [emoji16]

[TeddyDiver]

Originally Posted by ramblinrod

I do not.

The math and current threshold to stop a human heart is all of the evidence I need. To be satisfied that danger of shock increases proportionally with voltage. I have heard references to electrocution having happened as low as 42 Vdc. If interested in digging deeper into this subject, anyone can google it. Beware of poor and incorrect information sources on the internet. There can be as many mistakes made as have been in this thread.



Incorrect.

One of my objections to using higher than necessary voltage is the higher risk of electrical shock.

The other one, is that another term commonly used for voltage is "potential".

Higher voltage has a higher potential for danger of fire.

When a resistance of any specific value is placed across a voltage (potential), the current that flows is based on the voltage. The higher the voltage, the higher the current.

Since P = E*I, and BTU/h = P * 3.4, the higher the voltage, the more heat generated, and the greater the risk of fire.

Lets take for example, a resistance of 10 ohms accidentally placed across a voltage source.

For 12 Vdc, I = E/R = 12V/10 ohm = 1.2 A
P = E* I = 12V x 1.2A = 14.4W
Heat = P(W) * 3.4 = 49 BTU/h

This is a considerable amount of heat that could be a danger.

However,

For 48 Vdc, I = E/R = 48/10 ohm = 4.8 A
P = E* I = 48V x 4.8A = 230.4W
Heat = P(W) * 3.4 = 783 BTU/h

Note the exponential relationship.

For a 400% increase in voltage, the heat generated increased 1600%.

1. Danger from shock and electrocution increases proportionally with voltage.

2. Danger from accidental fire increases exponentially with voltage.

Put simply, EEE danger increase proportionally with voltage.

Therefore, I recommend using the lowest voltage possible and practical to power loads.

Based on my experience analysing this on many, many boats for high load additions such as thrusters and windlasses, it is often best to stay with a 12 Vdc electrical system on vessels around 50 ft and less (actually based on a review of each specific vessel).

Where the load just gets too high for 12 Vdc, then 24 Vdc should be considered.

Based on my evaluations, this is around 3 kW for durations up to about 1 hour (e.g. inverter) and 6 kW for durations up to about 5 minutes (e.g. thruster)

When a more dangerous, higher voltage electrical system is introduced, it is safest to keep it small and localized to the load.



For me, there is enough evidence just in the more stringent safety standards for increasing voltages.

All electrical safety standards I have reviewed do this. The arbitrary voltage levels they choose is by no means a threshold above which is not safe and below which is. They are just a level where they drew the line, below which is incrementally more safe with decreasing voltage, and above which is incrementally less safe with increasing voltage, until one reaches the arbitrary limit where even more safety measures must be implemented.

Electricity of any voltage has the "potential" to be dangerous.

Electricity of higher voltage has a higher "potential" for danger.

I don't require any additional proof. I have known this for over 40 years, as I was taught in my first "principles of electricity" class, each subsequent class on electrical theory, and as proven in practice, time and time again since.

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

[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.

[CatNewBee]

[emoji106] [emoji112]

[Jim Cate]

I don't require any additional proof. I have known this for over 40 years,

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

[Jdege]

I was watching a YouTube video a couple was making as their boat sank.
(They'd had the water speed sensor fail, and hadn't remembered to check it along with their thru hulls, when they first found water riding in the bilge.)

They were, of course, bailing like crazy and talking to the Coast Guard on the radio. The water was gaining on them, and they told the CG that they expected to lose communications soon, when the water level reached the battery terminals. Fortunately, they remembered about the sensor, checked it, drove in the bung, and saved the boat.

But it raised in my mind a question - how safe is the water in a flooded boat when the water reaches the battery terminals?

Is it more dangerous at 48v than 12?

[Dockhead]

Someone wrote me offline, why one wouldn't just go to 230v/50hz for really heavy power users like bow thrusters.


It's a good question, and probably that's the right answer for a big power boat that has generators running all the time. 230v ac is of course a really efficient way to transport larger amounts of power around a larger boat.


On my boat, however, both this one and the one I hope to start building, I don't run the generator all the time, and the main engine mounted alternators produce DC. So I would have to have an inverter between alternators or batteries and the consumer. My present bow thruster needs about 10kW. That's a lot of AC power for an inverter. So I think higher voltage DC would be better -- can be directly connected to batteries and alternator.