Comparative Safety: 12v v 24v v 48

[evm1024]

... gross simplification and strawman intended to defend an untenable position at any cost.

The first google hit gives us:


The NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 ohms. Wet or broken skin may drop the body's resistance to 1,000 ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 ohms".

The argument that 48 volts is too dangerous (as in you are gambling with your and your loved ones life) for use on a boat is not sustained by ABYC standards as well as other industry standards.


Scare tactics


Suffering Succotash

[s/v Jedi]

Here's a white paper from "the industry" showing that 48V is the new 12V and that the train has left the station... no rambling will be able to stop it :-)
https://www.mouser.com/pdfdocs/Why-a...ing-to-48V.pdf

[evm1024]

Having a 48 volt alternator feeding a 48 volt (nominal) house bank makes sense to me.

Running 48 volts forward to feed a thruster and windless makes sense. Many run heavy gauge 12 volt cables forward for this. With the respective I^2R losses.

With 48 volts forward the next step is to have 48 volt thruster and windless there. Even for small thrusters and windless.

The same goes for other loads. Start with a 48 volt feed and include dc to dc converters to get 12 volts. Then the next crop of loads (oh, say VHF) will be designed to accept (oh say) 12 to 60 volts for their power input .

And so the world changes.

[ramblinrod]

Originally Posted by transmitterdan

Danger also increases with higher current.

You're just playing the ohm's law game.

If the resistance is unchanged the current is increased, BY INCREASING VOLTAGE.

So all we can conclude is danger increases with power.

No, this is not true. See my post above.

I am uncomfortable with a blanket assumption that increasing current while holding voltage the same has no additional risk.

Neither would I, and I have never stated this. But everything else equal, voltage is not "held" (by somehow reducing resistance) causing current to increase.

An increase in voltage, increases danger.

In my experience increasing current is the greater danger up to about 48V. At that point we have to do more things to make higher voltage safe enough.

I can't speak to your experience. Mine differs in this regard.

But there is virtually no change in risk between 12V and 48V for currents under about 10A. Above 10A the danger of fire and arcing increase with current. Increasing voltage above 12V comes with much less relative increase in fire or shock risk.

Incorrect.

About 80 mA is sufficient to stop the human heart. It is much easier (about 4 times to be exact) to stop the heart with 48 Vdc as it is with 12 Vdc.

So let us say I need to power a 240W load which is more than 120W. So i could choose 12V and 20A or 48V and 5 A. Which is less safe?

The circuit with the 48 Vdc power supply.

It could feasibly kill if someone laid cut hands across it, whereas a 12 Vdc supply really can't.

It’s obvious that the 12V system has greater risk of fire. Not much greater but certainly greater.

Incorrect.

EEE, the risk of fire increases with heat.

Heat is measured in BTU/h.

1 W = 3.4 BTU/h.

12 Vdc x 20 A = 240 W = 816 BTU/h

48 Vdc x 5 A = 240 W = 816 BTU/h

The danger from fire is equal.

So then why do you claim danger increases with voltage Ramblin Rod?

There is a greater risk of shock from the higher voltage supply.

Now lets take the danger mitigating regulators out of the power supplies in question, so we make everything equal, after all, batteries don't have them...

For a 1 ohm resistive load,

With the 12 Vdc battery,

BTU/h = E^2/R / 3.4 = 144 Vdc / 1 ohm / 3.4 = 42

With the 48 Vdc battery,

BTU/h = E^2/R / 3.4 = 2304 Vdc / 1 ohm / 3.4 = 677

In my experience, 677 BTU/h is much more likely to start a fire than 42 BTU/h.

Shock risk is virtually zero in either case.

Incorrect.

While it may be low under certain circumstances, it is much, much, much higher in the 48 Vdc case, because danger increases with voltage.

Don't believe me, get a small cut on each hand, (maybe while twisting the sharp wires you are working with) dip your hands in the water in the bilge (maybe to pick up that battery terminal nut you just dropped) and lay your hands across that 12 Vdc starter battery, and then the 48 Vdc house bank.

No don't. The last one could kill you, and I really wouldn't want that on my conscience.

If you increase the load from 240W to 1200W the fire risk jumps exponentially in a 12 V system. But it jumps much less if we talk about a 48V system.

Incorrect.

Completely false.

I have not idea where you are getting this from.

As shown above.

If the systems are both limited to 1200 W, the danger from fire is equal, but the danger from electric shock is higher in the 48 Vdc systems.

If the systems are not limited, the danger from fire for the 48 Vdc system is exponentially greater than for the 12 Vdc system.

Why?

Because EEE danger always increases with system voltage.

Please stop before you hurt someone.

I can't afford to spend any more time trying to protect others from this degree of misinformation.

[ramblinrod]

Originally Posted by s/v Jedi

Here's a white paper from "the industry" showing that 48V is the new 12V and that the train has left the station... no rambling will be able to stop it :-)
https://www.mouser.com/pdfdocs/Why-a...ing-to-48V.pdf

I can certainly imagine some marketing manager in a cubicle of a manufacturer of DC-DC converters writing this in an attempt to increase sales at the expense of user safety.

[transmitterdan]

Where do we find these unlimited current sources for 48V DC? I could sell as lot of them.

Which system has a higher self limiting fault current? A 12V 400AH bank or a 48V 100AH battery bank. It’s obvious the 48V system will have a much lower fault current capability. The battery fuses will be 1/4 the current rating. And the batteries will have a much higher internal resistance of about 4 times. So no, it does not follow that higher voltage results in higher current.

[ramblinrod]

Originally Posted by s/v Jedi

Really?! You specifically started this thread...

I didn't expect there would be so many claiming to understand what they are talking about, that would be willing to risk other people's safety.

I have a life and a job and cannot continue to correct the plethora of ridiculous mistakes (and even more disgusting, the intentionally misleading, strawman arguments) being made by others in this thread.

[ramblinrod]

Originally Posted by transmitterdan

Which system has a higher self limiting fault current? A 12V 400AH bank or a 48V 100AH battery bank. It’s obvious the 48V system will have a much lower fault current capability.

Do you understand Ohm's Law and the difference between Amp-hr capacity, MCA, and max Amps deliverable?

Don't answer that.

Go back and read post # 94.

There is nothing incorrect in any of the corrections I posted there.

[evm1024]

Originally Posted by RR

Go back and read post # 94.

There is nothing incorrect in any of the corrections I posted there.

Originally Posted by post94

About 80 mA is sufficient to stop the human heart. It is much easier (about 4 times to be exact) to stop the heart with 48 Vdc as it is with 12 Vdc.

Originally Posted by google

Death can occur from any shock that carries enough sustained current to stop the heart. Low currents (70–700 mA) usually trigger fibrillation in the heart, which is reversible via defibrillator but is nearly always fatal without help. Currents as low as 30 mA AC or 300-500 mA DC applied to the body surface can cause fibrillation. Large currents (> 1 A) cause permanent damage via burns and cellular damage.

Perhaps we can all respond with articulate deliberation and avoid histrionic, invective posts.

[ramblinrod]

Originally Posted by evm1024

Having a 48 volt alternator feeding a 48 volt (nominal) house bank makes sense to me.

Running 48 volts forward to feed a thruster and windless makes sense. Many run heavy gauge 12 volt cables forward for this. With the respective I^2R losses.

With 48 volts forward the next step is to have 48 volt thruster and windless there. Even for small thrusters and windless.

The same goes for other loads. Start with a 48 volt feed and include dc to dc converters to get 12 volts. Then the next crop of loads (oh, say VHF) will be designed to accept (oh say) 12 to 60 volts for their power input .

And so the world changes.

For your size of boat I would not recommend proceeding with your proposal, due to the unnecessary safety hazards and failure risks and costs this could incur.

I would need to perform a cost / benefit / risk assessment for the particular circumstance.

In general, for the benefit of others following this thread who may be confused by the mistakes and misinformation posted by others...

For a typical 50ft cruising sailboat with a 12 Vdc house bank...

If the house bank is amidship, a 12 Vdc bow thruster connected to it, may be our top recommendation, based on cost, safety, and performance considerations.

If the house bank is aft, it may be necessary to place a dedicated 12 Vdc battery(s) near the thruster, with some form of charging that would be included in the assessment.

After the assessment if it is determined that a 24 Vdc thruster is warranted, I would recommend leaving the 12 Vdc electrical system as is, but mounting a dedicated 24 Vdc bank near the thruster, and charging them either with a 120 Vac charger, echo charger, or larger DC-DC battery charger. (The assessment would determine my top recommendations of these 3 options.)

For this size of vessel a 48 Vdc thruster is not generally required. If 24 Vdc wouldn't do it, I would likely recommend putting in an AC thruster.

Increasing the main DC voltage for this purpose would be incredibly cost prohibitive.

Additionally, using DC-DC converters to power 12 Vdc safety devices, eliminates reserve power capacity in the even of a system failure and introduces a potential failure node(s) by the DC-DC converter for no real benefit.

Of course, everyone can do whatever they wish, and increase system voltage needlessly while increasing danger to themselves, loved ones, and others, if they desire.

[evm1024]

Originally Posted by RR

For your size of boat I would not recommend proceeding with your proposal, due to the unnecessary safety hazards and failure risks and costs this could incur.

I would need to perform a cost / benefit / risk assessment for the particular circumstance.

In general, for the benefit of others following this thread who may be confused by the mistakes and misinformation posted by others...

For a typical 50ft cruising sailboat with a 12 Vdc house bank...

SNIP

Rod,

Your recommendations with regards to my boats electrical system are unasked for, unwarranted and unwanted.

You are listed as a Marine Service Provider and thus I must conclude that your posting is commercial in nature. You appear to be promoting your business either directly to me or to others on the list.

Further I would note that your recommendations are made in a vacuum. They do not take into account the needs or desires of the boat owner. They appear to dictate to the boat owner what is good for the boat owner. And more specifically they do not take into account my stated desires.

I would never hire a Marine Service Provider that believes that they have the only answer, does not look to the future and treats their customers desires with the contempt that you have shown.

Lastly, you may recall that I asked a question -

Originally Posted by preamble

As we all know ABYC "standards" are all about mitigating risks. ABYC develops new standards as they are needed. Oh, of note ABYC appears to consider 48 volts on par with 12 volts:

11.2.2 to direct current (DC) electrical systems on boats that operate at potentials of 50 volts or less

Originally Posted by question

then you would think that ABYC would not use 50 volts as its upper limit for DC voltages.

Can we conclude that you as an ABYC Certified marine tech consider the ABYC standards to be in error?

Were I to hire an ABYC certified Marine Service Provider I would never hire one who cannot answer that basic question.

With all due respect

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

Mods: Please feel free to delete any and all of my post(s) that have crossed the line.

[ramblinrod]

Originally Posted by evm1024

Rod,

Your recommendations with regards to my boats electrical system are unasked for, unwarranted and unwanted.

…

With all due respect.

Incorrect.

My post was not about any specific boat, nor to solicit business.

It was a general post about how we address the installation of a bow thruster on any customer boat.

I feel it was very relevant to the discussion of 12 vs 24 vs 48 Vdc electrical systems.

If you do not agree with the information I posted, feel free to disregard.

This post is to address a post that I believe was intended to challenge my professional integrity.

Yes, I make my living performing this type of work.

This gives me a great deal of education, training, and experience that I can share here to benefit others.

The post in question was an explanation of how a professional marine service provider should address the selection and installation of a thruster.

It is how we do it every single time.

We never perform work on a customer boat in a manner that has not been specifically authorized. It's their boat.

We make recommendations (normally based on vessel inspection and customer interview).

They make choices, based on our recommendations, and authorize the work to be performed, if it meets their approval.

If a customer demands work be performed in a manner that will not meet our business standards of quality, profitability, risk tolerance, and brand integrity, we decline.

Our recommendation to approval rate (conversion ratio) is very high by any business standard, especially the marine industry.

I answer the forum questions I choose to.

I am not obligated to answer any specific one.

If I believe my answer may help a forum member, I do my best.

If I believe a question to me (directly or implied) is misleading, deceiptful, antagonistic, or disrespectful, I rarely answer, other than maybe to draw attention to these shameful tactics.

This may be the reason for any unanswered questions.

If I believe a forum member has posted something incorrect and/or that could cause a forum member harm, I generally respond.

We do have a very thorough customer screening process.

We choose to help those boaters who need our services and treat us "with all due respect" for real.

Posting "with all due respect" does not make a disrespectful post respectful.

[evm1024]

Oh, so sorry it just happens that you divide the need for higher voltages at 50'. And that 48 volts is safe on boats over 50'.

That I have a 50' boat and your recommendation was in response to my post is totally coincidental. What was I thinking!

It appears that you are speaking for all ABYC techs.

I think that we all know that no specific ABYC tech speaks for the others and that they each have different ways of going about making recommendations.

In addition you state:

Originally Posted by RR

This gives me a great deal of education, training, and experience that I can share here to benefit others.

As a general rule we do not know the qualifications of others on the list. I do not know yours and you certainly do not know mine. We do know that ABYC certification is not that hard to get and that good sense has nothing to do with training.

The requirements for an ABYC Marine Electrical Certification can be met after passing a 200 question test (advisor, tech requires work experience).

Usually the test is taken after completing a 3 day course. But the course is not a requirement. The test can be challenged.

https://abycinc.org/page/Education

https://abycinc.org/page/cert_reqs_faq


With all due respect means exactly that. If you are due respect then it is given.

But I digress.

You have stated that 48 volts is dangerous. You have gone to great lengths to force that viewpoint on the list. For example:

Originally Posted by RR

Then again, one could have a 48 Vdc propulsion system where one little spike takes out the motor controller and DC - DC converter so the engine is dead, and so is the sole power source for all the safety equipment aboard.

18. Everyone is absolutely free to disagree with these statements, and do as they wish on their boat. But if you make poor decisions that end up in property damage or personal injury, for you or your loved ones, hold YOURSELF personally accountable, and please don't park your vessel near mine. ;-)

It is not needed and it is less safe.

One more post, just to hopefully help prevent the careless posts by some others from possibly resulting in a forum member getting hurt.

Please stop before you hurt someone.

And so on.

ABYC standards to not indicate that any additional safety measures are needed for 48 volt DC systems.

Your statements as to the Comparative Safety of 12 volts over 48 volts are not supported by ABYC standards.

Rather than evade answering the question please explain why your opinion differs from that of the ABYC.

[evm1024]

I expect the next post to start with:

Incorrect

But regardless of how the post starts the question of the comparative safety of 48 volts in a boat does not change with the length of the boat.

50' is not a magic number that makes 48 volts safe, if it is unsafe in boats under 50' then it is unsafe in boats over 50'.

When one uses electric propulsion or has a large thruster for whatever reason 48 volts becomes more desirable.

That the recreational boating industry standardized on 12 volt systems is the chance of the automotive industry selection of 12 volts for their reasons. Not for any reason of safety.

That 48 volts needs no additional safety measures is clearly reflected in ABYC recommendations where all voltages below 50 volts are treated the same.

[ramblinrod]

Originally Posted by evm1024

Oh, so sorry it just happens that you divide the need for higher voltages at 50'.

Based on my experience (pretty extensive) 42' is about the LOA where it may not be best to run cables from an aft 12 Vdc house bank, and 50' LOA is about where it may not be best to run cables from an existing midship 12 Vdc housebank.

In these cases, it is best to locate a dedicated 12 Vdc battery(s) near the load.

For a larger vessel yet, it may be justifiable (considering cost, performance, and safety) to move up to a 24 Vdc bank at the thruster.

It does depend on the size of windlass and/or thruster, and the boat configuration, but that is where the specific vessel review comes into play.

I have stated this many times in this thread and others.

This has absolutely nothing to do with your boat in particular.

If your boat happens to be at one of the thresholds I have recommended, it is purely coincidence.

These general recommendations, are the result of many vessel reviews for bow thruster and windlass installation.

I have never reviewed your boat, so I cannot even comment on your particular vessel. Most likely, it would fall within my normal recommendations.

"Strawman Alert": I have never claimed 48 Vdc is safe on boats over 50'.

Quite the opposite, the thrust of my position, for anyone paying any attention, is that increased system voltage is more dangerous, and therefore to be avoided where possible / practical.

Frankly, if 24 Vdc won't do it (very, very large boat) I recommend an AC thruster to run off the generator (as previously stated).

Note that the ABYC compliant AC system has all kinds of protection to mitigate danger due to the higher voltage, life GFCIs and ELCIs and breakers rather than fuses, reverse polarity warning, double pole main breaker, etc., etc. etc.

Why?

Because danger increases proportionally with system voltage; ABYC recognizes this, and has developed more stringent standards for higher voltages.

Of course the standard developers have to draw the line somewhere, on the "Danger Is Proportional With System Voltage" curve.

It would not be practical, feasible, or relevant, to have increasing safety requirements for every possible increment of system voltage increase to mitigate the higher danger it invokes.

Only a fool would argue that 48 Vdc is safe and 50 Vdc is not.

For the record, to become ABYC certified one must have at least 1000 hours of documented relevant experience in that field during the prior 2 years, BEFORE they are qualified to write the certification exam.

The member has 3 hours to complete the ~200 questions, and must score greater than 85% to pass.

If they do not write the exam at an ABYC training event (which typically cost around $1000 plus flight, plus accommodation, plus expenses) and challenge the exam on their own, they must do it through a certified proxy organization.

But wait, that's not all.

If they pass the exam, the certified member must commit to a lifetime of continuing education to maintain their certification, which is reviewed every 5 years.

Then, there is the annual membership fee.

And yes, there is an obligation to perform all work to ABYC standards, which my company does, every single job, without exception.

If the customer doesn't like that, they are free to go elsewhere.

Just curious; what is your ABYC certification in?

As to the balance of your post...

Please re-read the last sentence of your post # 99, and the last half of my post # 102.