Comparative Safety: 12v v 24v v 48

[ramblinrod]

Originally Posted by evm1024

I think that the ocean is large enough and that the desires of owners broad enough such that it is fruitless to attempt to force any specific opinion onto others. Dogma often just annoys people.

Thank you for your contribution, that a highly respected electrical safety agency understands as I do, that danger increases proportionally with system voltage.

This confirms my opinion of the validity of post # 72

[evm1024]

Originally Posted by ramblinrod

Thank you for your contribution, that a highly respected electrical safety agency understands as I do, that danger increases proportionally with system voltage.

You are welcome. I am glad you like my post.

I presume that you are speaking of the ABYC.

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

When I read through the ABYC AC AND DC ELECTRICAL SYSTEMS ON
BOATS standards it is quite obvious that ABYC considers DC systems of 50 volts nominal or less as on par with 12 volt DC systems. You would think that if 48 volt DC systems were so dangerous then ABYC would include standards to reflect that risk.

Basically, ABYC does not include any additional safety measures for voltages under 50 volts. Thus your argument that because additional safety measures have to be employed is invalid. No additional safety measures need to be employed according to ABYC.

Simply put if 48 volts were so dangerous so as to risk life and limb (and pocketbook) as you stated -

Originally Posted by RR

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. ;-)

- 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?

[transmitterdan]

Danger also increases with higher current. So all we can conclude is danger increases with power. I am uncomfortable with a blanket assumption that increasing current while holding voltage the same has no additional risk.

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.

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.

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? It’s obvious that the 12V system has greater risk of fire. Not much greater but certainly greater. Shock risk is virtually zero in either case.

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.

[Dockhead]

Originally Posted by ramblinrod

Thank you for your contribution, that a highly respected electrical safety agency understands as I do, that danger increases proportionally with system voltage.

This confirms my opinion of the validity of post # 72

Originally Posted by transmitterdan

Danger also increases with higher current. So all we can conclude is danger increases with power. I am uncomfortable with a blanket assumption that increasing current while holding voltage the same has no additional risk.

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.

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.

So let us say I need to power a 240W loaf which is more than 120W. So i could choose 12V and 20A or 48V and 5 A. Which is less safe? It’s obvious that the 12V system has greater risk of fire. Not much greater but certainly greater. Shock risk is virtually zero in either case.

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.

So here we see that although we seem to have more or less agreed about all the technical details, conclusions reached are diametrically opposite. One says that 48v is "proportionately more dangerous" (400% as dangerous?!) than 12v, and another says that 48v is either just as safe, or if the loads are big, 48v is safer than 12v.


I don't really see what in the world in the technical details we disussed supports the idea that 48v is FOUR TIMES more dangerous, than 12v -- but conclusions are a bit subjective, I guess. Everyone will have to make up his own mind.

[CatNewBee]

Originally Posted by Dockhead

So here we see that although we seem to have more or less agreed about all the technical details, conclusions reached are diametrically opposite. One says that 48v is "proportionately more dangerous" (400% as dangerous?!) than 12v, and another says that 48v is either just as safe, or if the loads are big, 48v is safer than 12v.


I don't really see what in the world in the technical details we disussed supports the idea that 48v is FOUR TIMES more dangerous, than 12v -- but conclusions are a bit subjective, I guess. Everyone will have to make up his own mind.

we can make it easier for the people that fear 48V, lets reduce the risk of the 12V systems and use 1.5V instead.

Even 3.4V are dangerous if you look at exploding Android and iPhones.

[transmitterdan]

I would like to see a scientific paper documenting the increased risk of relay contact “burning” between 12V and 48V. I don’t believe it is significant.

[evm1024]

Originally Posted by transmitterdan

I would like to see a scientific paper documenting the increased risk of relay contact “burning” between 12V and 48V. I don’t believe it is significant.

I have been searching for any paper that documents the actual resistance of High Resistance Contacts for any cause. I've not really found one.

I have found a paper that describes an increase in contact resistance due to fretting. That paper indicates that the contact resistance increases to 50 milli ohms after 32000 cycles.

It appears from the graphs shown prior in the thread that for contact resistances greater than the load resistance lower voltages are favored.

And for contact resistances lower than the load resistance higher voltages are favored (in terms of lower heat generated)

The example given of a 10 ohm faulty contact does not appear to be anything close the the actual resistances of faulty contacts. Every where I am looking points to faulty contacts having resistances of less than 200 milli ohms.

If I find a paper I will share it.

[evm1024]

Originally Posted by CatNewBee

we can make it easier for the people that fear 48V, lets reduce the risk of the 12V systems and use 1.5V instead.

Even 3.4V are dangerous if you look at exploding Android and iPhones.

I used to work on some equipment that had -3.2 volt power supplies at 500 amps each.

Get a watch or ring across the bus bar and you will lose a hand or finger.

Perhaps the scariest piece of hardware that I worked on. The ground and -3.2 volt buses were only about 1/2" apart. Respect the current!

[transmitterdan]

When a relay contact opens and there is a spark why is that so? And does the base voltage of the system (12V or 48V) have any impact on the “size” or duration of the spark. If so, how much and why?

[ramblinrod]

Sorry forum members,

I just can't tolerate the silly games some posters are playing any longer.

I simply have better things to do.

I stand by post #72 and all others I have made in this thread, in their entirety.

I believe they are 100% true and accurate, and I have made no attempt to mislead anyone.

I do take electrical safety very seriously and as a result of this thread, I accept that others may not.

Everyone is free to believe whatever they wish.

[CatNewBee]

Originally Posted by transmitterdan

When a relay contact opens and there is a spark why is that so? And does the base voltage of the system (12V or 48V) have any impact on the “size” or duration of the spark. If so, how much and why?

It happens especially if you have inductive loads, when the magnetic field breaks down on switching, it induces a voltage with reverse polarity into the coil that may cause a spark, like the ignition coil in a gas engine.

[mako]

Originally Posted by transmitterdan

Danger also increases with higher current. So all we can conclude is danger increases with power. I am uncomfortable with a blanket assumption that increasing current while holding voltage the same has no additional risk.

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 think your statement is accurate. Two examples would be the danger of an arc welder (low voltage - high current) and the safety of my daughter rubbing her feet on a carpet and zapping me with 20,000 volts of static electricity.

[ramblinrod]

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

Under normal conditions (skin resistance ~ 5 Kohms)...

A) If one lays their bare hands across a 3.2 Vdc, 500 A max, (1600 W rated) power supply, they won't even feel it. If their hands were sweaty, salty, or nicked, they may still not even feel it.

B) If one lays their bare hands across a 48 Vdc, 33.3 A max, (1600 W rated) power supply, they will most definitely feel it. If their hands were sweaty and salty, it could be a good jolt (which could cause a dangerous involuntary reaction). If hands were nicked, it could be lethal.

C) If one lays their bare hands across a 600 Vdc, 2.7 A max, (1600 W rated) power supply, they will most definitely be seriously injured if not killed.

These voltages selected, (similar to the ones in the electrical safety standards) are not threshold limits, immediately below which means no danger and immediately above which means definite danger.

They are simply selections on the curve of danger increasing proportionally with voltage, because the safety agency, whether it be IEC or ABYC or any body, has to draw the line(s) somewhere.

Make no mistake, EEE, a system design voltage change from 12 Vdc to 48 Vdc is more dangerous. Has to be. The laws of physics don't change just because we are on a boat.

[evm1024]

Originally Posted by transmitterdan

When a relay contact opens and there is a spark why is that so? And does the base voltage of the system (12V or 48V) have any impact on the “size” or duration of the spark. If so, how much and why?

I'm a research engineer at some university and we research things that no-one knows about.

I've been trying to find the resistance of a bad contact without any luck but I am very impressed by the information that I have found.

The description of contact resistance is really very complex and many of the papers I was able to find show the the majority of the research on basic parameters was done nearly 100 years ago.

Circuit breaker and switch designers have used this research to solve many of the problems that come with making and breaking a contact.

For example, contacts are designed to "wipe" across each other slightly so as to overcome much of the problems with oxides and contaminants on the contact.

In terms of arcing the contacts on break may form an arc (more current based rather than voltage based it appears - voltage defines the length at which the arc extinguishes).

This publication gives some insight into arcing and how it is dealt with. See section 3.1

http://copperalliance.org.uk/uploads...ntacts-pdf.pdf

Lastly, I was off into the field of Ductors which are milli and micro ohm meters used for measuring resistance in contacts.

What I was able to come up with there is that good contacts have a resistance in 10 milli ohm or less range with bad contacts typically ranging up to 100 milli ohms (0.1 ohm). No where near the 10 ohms that was used to attempt to prove a point.

One of the reasons that I could not find a "single" good/bad contact resistance is because each manufacture has their own standards on a contact type by contact type basis.

Additionally, in the case of connecting to bus bars (e.g. a 2/0 lug to the positive bus on a boat) the typical contact resistance is very likely less than the resistance of the actual run of 2/0 cable. Surface area of the contact....

It appears that the only way we end up with heating in a contact sufficient to start a fire is in one where the lug has come loose. This is in turn caused by improper torque in installation, no or damaged lock washer. no strain relief which allows the cable to vibrate and the like. These are mechanical failures caused by installer or designer error. And are not dependent on the voltage used in the circuit.

We have seen in this thread that the power dissipated in a high resistance contact is limited to 1/4 of the power dissipated in the load without a resistive contact in the circuit.

So for that 1000 watt load we were talking about we end up with a maximum of 250 watts dissipated in the resistive contact regardless of the voltage. I.E. they have the same risk of fire.

[s/v Jedi]

Originally Posted by ramblinrod

Sorry forum members,

I just can't tolerate the silly games some posters are playing any longer.

I simply have better things to do.

Everyone is free to believe whatever they wish.

Really?! You specifically started this thread to argue about this in a way where you take on the whole electric engineering community by taking position that goes against all accepted principles and conclusions :-) 48V DC is the best option not only as accepted by engineering consensus but also as demonstrated by multiple posters in this thread.

You are indeed free to believe whatever you wish, but do not try to confuse the non-engineer sailors with wrong data, nor try to scare them away from the best options, even if those aren't mainstream