Comparative Safety: 12v v 24v v 48

[KTP]

Originally Posted by Jammer

The load dissipates 144 watts. 12v x 12a.





Each dissipates 36 watts, a quarter of the intended load. 6 volts x 6 amps. (This is the most that can be dissipated at the "contacts" since any other resistance chosen will dissipate less. You can try a bunch of values for yourself and see, or look at the math I linked upthread)


The 1/4 of the load is the maximum regardless of the voltage used.

Yeah I was typing while eating pizza and forgot to square my voltage before multiplying the resistance.

[ramblinrod]

Originally Posted by Kelkara

I'm sure you understand the "law" ... these graphs show how the power in the load and the resistive contact will vary with the resistance of the contact for 12V and 48V. As you can see the graphs are almost exactly the same, with the peak occurring where the resistance of the bad contact equals the resistance of the load ... The only difference is that the peak occurs at a 16x higher resistance ... but the shape of the graph is exactly the same, with the same power dissipated.

Since the discussion is about safety ... I'm going to ask the question from a different angle ... assuming that the equipment was properly installed and so working correctly originally (OK, I admit that this is a big ask) ... that means that our bad contact is deteriorating.

When are we going to notice that we have a problem? Either when we notice the contact getting hot, or when we notice the load is starved of power ... in both the 12V and 48V system it is the same ... the power dissipated by the bad contact will follow the same curve for the loss of power in the load. The only difference is that by this time the bad contact will have deteriorated 16x as much in the 48V system than the 12V system.

So which is safer? the 12V system where the problem becomes apparent earlier, or the 48V system where the early stages go unnoticed? Either way we have exactly the same problem to fix once we do notice it. Personally I think that the difference is in the original installation, which for such a high current load would be easier to get right with 48V.

Gooooooood Post!

OK, one more response and that's it.

To answer your questions, now we have to consider what is causing the high impedance connection.

a) Is it a gradually deteriorating situation, like a contact whose impedance is gradually increasing which each closure, or...

b) Is it a dramatic change, like a contact that just arced and it's impedance changed drastically.

c) Is it a dramatic change, like a wire or connector that has become corroded and with a single stressor it opens but arcs across.

We don't know.

It could be a long drawn out change or instant.

Then there is the variable as to whether the issue will be detected sooner or later in the various systems. We won't know that either.

One could have a 48 Vdc major fault and not realize it at all because the DC to DC converter is still feeding 12 Vdc to the TV, until they went to start the propulsion system and POOF!.

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.

Hope you are close to shore so your cell phone works.

Oh yeah, and I hope you remembered to charge it.

So in my opinion, this has been a great discussion but the KISS principle is usually pretty safe.

When possible, stay with the lowest voltage practical.

If you need to power loads in excess of 3 kW for any duration, consider installing a generator.

If you are trying to power this off a DC bank, you are going to have carry one mother of a battery bank, and recharge it with something fairly significant anyway.

Why not just fire up the generator to power the high sustained loads with that? Forget the high voltage DC system. You've already got a generator. (Or if you don't sounds like you're going to need one, and when you do put that in, all of the effort that went into the 48 Vdc system was for not.)

If you do have to go with a higher system voltage, ensure you have some double, preferably triple or quadruple redundant means to power your 12 Vdc safety equipment beyond a "made in China" DC-DC converter.

So again, right back to my first response to the original thread, the first step is to perform an energy needs analysis, the next step is determine charging system preferences, and the next step is to determine energy production and storage capacity.

If y'all got a big ass boat and yer gonna run the genny 24/7 to keep the A/C on, why are we even discussing this?

[transmitterdan]

The 1/4 power rule is just a restatement of the maximum power transfer theorem.

It is a fundamental truth that a flaky contact can never dissipate more than 1/4 the design max power of a fixed resistance DC load. Voltage does not affect this law of physics. A million volt system obeys the MPT theorem. Even if the flaky contact “resistance” does not obey Ohm’s law the MPT theorem cannot be violated.

So if you use 12V wire and 12V terminations for a 48V system I posit the 48V system is just as safe and arguably safer. But if you skimp on the connection/wire sizing then you are back to about the same set of problems. But you save a little money with a 48V system in terms of hardware cost. And the skill of the electrician can be a little bit relaxed too. Not that anyone would ever do that.

[s/v Jedi]

12W lamp on 12V gives 1A and on 48V it’s 0.25A. So let’s introduce a 0.5 Ohm bad contact which is about as bad as it gets:

12V: lamp is 12 Ohm so total circuit becomes 12.5 Ohm; new current 0.96A so the lamp gets 11.5V and burns at 11W
48V: lamp is 192 Ohm so total circuit becomes 192.5 Ohm; new current 0.24935A so lamp gets 47.88V and burns at 11.92W

It doesn’t really matter… the 12V lamp will shine a little less bright so technically 48V wins

Now a 5 Ohm bad contact which I have never yet seen:

12V: lamp is 12 Ohm so total circuit becomes 17 Ohm; new current 0.7A so the lamp gets 8.4V and burns at 5.88W
48V: lamp is 192 Ohm so total circuit becomes 197 Ohm; new current 0.24365A so lamp gets 46.787V and burns at 11.4W

The 12V lamp is rendered useless but the 48V lamp keeps shining bright so clearly wins.

Now we make its ridiculous with a 10 Ohm bad contact:

12V: lamp is 12 Ohm so total circuit becomes 22 Ohm; new current 0.55A so the lamp gets 6.55V and burns at 3.6W
48V: lamp is 192 Ohm so total circuit becomes 202 Ohm; new current 0.23762A so lamp gets 45.624V and burns at 10.84W

Now even in ridiculous mode the 48V lamp still works while the 12V is the ridiculous one so 48V wins.

Now for the risk of fire:

at 0.5 Ohm resistance we have 0.46W heat dissipation for 12V and 0.03W for 48V so 48V wins
at 5 Ohm we get 2.45W heat for 12V and 0.3W for 48V so 48V wins
at 10 Ohm we get 3.03W heat for 12V and 0.56W for 48V so 48V wins again and again.

Result: at 48V the lamp keeps working while the 12V lamp is rendered useless in 2 out of 3 cases. To be fair, in real life only the first case is probable so for light loads like lamps 12V works okay even though 48V does better. Not so for big loads as this is where 48V really wins.

p.s. I know 24V boats that just run their 12V starters on the 24V without trouble

[Jammer]

Originally Posted by evm1024

At the moment we are fixated on the heating and fire potential in a faulty connection that introduces a series resistance (HRC).

The two most likely causes of fire on a boat are:


1) The boat in the slip next to it catching fire; and
2) DC wiring faults.


I think it's a legitimate concern. I note that series resistance faults are recognized as a common cause of structure (house) fires. The recent introduction of arc fault breakers, which are now mandated for at least some circuits in most jurisdictions, may change that.


I am unaware of any arc fault breakers that are suitable for DC.


Other faults -- besides series resistance -- are responsible for a good deal of boat fires. I don't have any data that breaks down the "DC wiring faults" statistic into more detail but anecdotal evidence would suggest that shorts due to poor wiring practices, overfusing, and internal failures of high-current equipment (inverters, alternators, and chargers) are a big part of it.

[Jammer]

Originally Posted by s/v Jedi

p.s. I know 24V boats that just run their 12V starters on the 24V without trouble

I keep reading stuff like this and maybe it works for a while -- but -- I ran a 6 volt starter on a 12 volt system for some years and it failed early because it operated at excessive speed, damaging the pinion gear. I in fact went through several of these until I found a shop that could convert the field coils to 12v.


Starters are series wound, and it is not necessary to rewind the armature to change the voltage, just change out the field coils.

[evm1024]

Originally Posted by Jammer

The two most likely causes of fire on a boat are:


1) The boat in the slip next to it catching fire; and
2) DC wiring faults.


I think it's a legitimate concern. I note that series resistance faults are recognized as a common cause of structure (house) fires. The recent introduction of arc fault breakers, which are now mandated for at least some circuits in most jurisdictions, may change that.


I am unaware of any arc fault breakers that are suitable for DC.


Other faults -- besides series resistance -- are responsible for a good deal of boat fires. I don't have any data that breaks down the "DC wiring faults" statistic into more detail but anecdotal evidence would suggest that shorts due to poor wiring practices, overfusing, and internal failures of high-current equipment (inverters, alternators, and chargers) are a big part of it.

Good post.

I worded it that way to point out that we were focused on judging whether 12 volts or 48 volts systems are more, less or equally safe based on a specific wiring fault.

And that chasing a specific fault is fruitless.

As you note there are many more faults which often come down to installation/design errors.

[CatNewBee]

Well, every accident is different, so you cannot blame it on the voltage alone.

Assuming 1000W load, 12V results in 83A trough the faulty connector,

Assuming 1000W load, 48V results in 21A trough the connector.

Scenario 1: the connector can stand the current of 80A somehow, it only heats up, 21A current would pass without heating, so 48V are safer then 12V, at 12V more likely to ignite something.

Scenario 2: the connector cannot stand 80A, but can stand 21A, so in 12V it will blow up and brake the circuit, in 48V it will heat up., potentially 12V more dangerous to start a fire by ignition of the spark when it breaks, but safer afterwards (connection broken, no more heat) while at 48V still some danger to start a fire by overheating.

This is not the way to decide what system to use.

In normal use case less current is better, less power losses, smaller installation, more reliable contacts.

This statement is valid for using the SAME sizing of wires and connections in 12V and 48V. If you design the circuits to a specific voltage, you would use 1/4 of the size for wiring and contacts to the specs, so both systems would be equaly safe or dangerous, but the 48V will be not so heavy, will use smaller and potentially cheaper installations.

[Dockhead]

Originally Posted by ramblinrod

No.

This is covered in post # 1, but go to post # 5, its simpler and clearer.

Draw yourself a picture. On the left a battery, across this battery draw two resistors in series. Call the first resistor R contact, and the second resistor R load.

From post # 5....

For a 1000W load supplied by 12 Vdc, the power dissipated by the contact impedance, varies with the contact impedance, as follows:

For 0.001 ohms, 6.9W
For 0.01 ohms, 60.7 W
For 0.1 ohms, 242 W
For 1 ohm, 110 W
For 10 ohms, 14 W
For 100 ohms, 1.4W

Now if we make that a 1000 W load supplied by 48 Vdc, the following results occur...

For 0.001 ohms, 0.4 W
For 0.01 ohms, 4 W
For 0.1 ohms, 40 W
For 1 ohm, 211 W
For 10 ohms, 152 W
For 100 ohms, 22

<<<<<<<<< >>>>>>>>>>>

For the first example, R Load = 1/ P / E^2 = 1/ 1000W / (12 * 12)V = 1/1000W/144V = 0.14 ohms.

For each case of contact resistance, R total = R load + R contact.

For each case of contact resistance, I = E/R total.

For each case of contact resistance, P contact = I^2*Rcontact.

Or this could be calculated by, P contact = Econtact^2/R.

The same holds true for the 48 Vdc circuit, except we have modified the load to dissipate the same power as in the first circuit despite the higher supply voltage.

For the second example, R Load = 1 / P / E^2 = 1/ 1000 W / (48 * 48) V = 1 / 1000W / 2304 V = 2.304 ohms.

This now has to be used to calculate Rtotal, and all of the other calcs to derive P contact.

OK guys, that is all I can play today, I have to prepare a presentation I am delivering to a boating group tomorrow.

Fortunately it is not about 12 vs 24 vs 48 Vdc electrical systems....this time. ;-)

OK, so I think I understand it now.


And remarkably, it looks like everyone in this conversation has backed away from categorical positions and roughly agree. That's a first for electrical arguments, I think!


So if I now understand it -- no one any longer says that driving the same power load with different system voltages has a single, simple effect on the amount of power which will be dissipated at the bad connection. It turns out to be a complicated relationship. There might be some advantage for low voltage systems in that the same amount of resistance in a bad connection will knock out the device being driven, faster, so give earlier warning of the fault, and lower voltage means less risk of shock, which is why lower voltage is preferred by Rod, who is the tech, the actual electrician in the room. There might be some other advantages of higher voltage systems, which seem to be preferred by ALL of the actual electrical engineers participating in the discussion.

But no one any longer says simply -- lower voltage systems are safer because less voltage means less power at the bad connection, or that higher voltage systems are safer because lower current means less power at the bad connection. Correct?


And in any case, without regard to system voltage, the upper limit of power which can be dissipated at a bad connection is 25% of the power of the load.


Fair summary? In the end, a good discussion! I certainly learned a lot.

[GordMay]

Originally Posted by Jammer

The two most likely causes of fire on a boat are:
1) The boat in the slip next to it catching fire; and
2) DC wiring faults...

"Causes Of Boat Fires" ~ By Beth A. Leonard
☞ https://www.boatus.com/magazine/2015...boat-fires.asp

What the claim files teach us about preventing boat fires.
Fire ranked number five among the causes of loss for BoatUS Marine Insurance between 2008 and 2012.

Off-Boat Sources (26%)
More than a quarter of the time, our insured's boat burns when something else goes up in flames — the marina, the storage facility, the house, the garage, the barn, the neighbor's house. In more than 70 percent of those cases, it's the marina that burns. A high percentage of those fires start on someone else's boat.

Engine Electrical (20%)
Wiring harnesses and starters cause a disproportionate number of fires on boats more than 25 years old.

Other DC Electrical (15%)
While loose battery connections, chafed battery cables, and aged battery switches can all cause fires aboard, the most common cause of battery-related fires is operator error: reversing the battery cables or connecting them in series when they should have been in parallel, or vice versa.

AC Electrical (12%)
Bringing air conditioning, microwaves, electric heaters, and other AC appliances aboard makes life on the dock more comfortable and convenient but also greatly increases the risk of fire. Most AC electrical fires start somewhere between the marina pedestal and the shorepower inlet on the boat. BoatUS has long recommended using only marine-grade power cords with proper adapters and replacing them at the first sign of wear on the cord or pitting on the blades of the plug. But the analysis of our fire claims has identified another high-risk area on boats more than 10 years old: the back of the shorepower inlet where the ship's wiring connects to the terminals.

Other Engine (9%)
Any interruption of cooling water can lead to overheating and then to a fire. In this case, a blockage of the raw-water intake caused the overheating. Other exhaust fires are caused by impeller failures due to age or to sediment in the water.

Batteries (8%)
On older outboards, the voltage regulator is by far the most common cause of fires. The failure rate increases with age after 10 years.

[transmitterdan]

The first cause is a bit misleading because there are many boats in proximity to a boat on fire. So just probability demands that external causes are the number 1 fire claim reason. But in terms of risk the electrical problems dwarf all others by a margin much higher than it should be IMO. So I would say these stats are a good basis to posit that 12V DC is not very safe.

Any time you have a risk that has a probability of occurrence way higher than all other risks then that particular risk has not been sufficiently reduced. In other words it is not random therefore it is an un-retired risk. You can’t do very much about failures that are statistically random. But electrical fires on boats are clearly not random and in any other endeavor/industry would demand action.

[ramblinrod]

Originally Posted by Dockhead

OK, so I think I understand it now.


And remarkably, it looks like everyone in this conversation has backed away from categorical positions and roughly agree. That's a first for electrical arguments, I think!


So if I now understand it -- no one any longer says that driving the same power load with different system voltages has a single, simple effect on the amount of power which will be dissipated at the bad connection. It turns out to be a complicated relationship. There might be some advantage for low voltage systems in that the same amount of resistance in a bad connection will knock out the device being driven, faster, so give earlier warning of the fault, and lower voltage means less risk of shock, which is why lower voltage is preferred by Rod, who is the tech, the actual electrician in the room. There might be some other advantages of higher voltage systems, which seem to be preferred by ALL of the actual electrical engineers participating in the discussion.

But no one any longer says simply -- lower voltage systems are safer because less voltage means less power at the bad connection, or that higher voltage systems are safer because lower current means less power at the bad connection. Correct?


And in any case, without regard to system voltage, the upper limit of power which can be dissipated at a bad connection is 25% of the power of the load.


Fair summary? In the end, a good discussion! I certainly learned a lot.

Pretty good summary.

So here is my position, based on 40+ years of experience in various industries, designing, developing, installing, troubleshooting, repairing and marketing electrical products and systems, for the average ICE propelled rec cruising boat from 20 to 50 ft:

1. 48 Vdc is more prone to arcing and damaging contacts than 12 Vdc.

2. Damaged, arced contacts may cause a high Z connection.

3. High Z connections can heat up and cause fires.

4. 12 Vdc high Z connections, though less likely to occur, are more likely to manifest themselves easily (by improper load operation), prompting the boater to investigate and repair the problem before a fire ignites.

5. Receiving an electric shock is likely when contacting 48 Vdc, and unlikely at 12 Vdc.

6. While the 48 Vdc shock itself is not likely to be lethal, the involuntary instinctive response to electric shock can very well be more dangerous than the shock itself (e.g. jerk hand back into rotating machinery).

7. Most marine electrical products rated for 48 Vdc are more expensive.

8. Using 48 Vdc to reduce cable and connector size, may save some money, but will increase impedance, making the installation less safe.

9. From a builder standpoint, one has to calculate the cost / weight savings in cable gauge reduction against the cost / weight increase of the 48 Vdc breakers, switches, electrical products, etc.

10. From a modification standpoint, for up to a 3 kW moderate duration load (e.g. inverter) and a 6 kW temporary load (e.g. thruster) changing system voltage (even locally) is rarely worth the cable size reduction advantage. (I have evaluated many.)

11. For higher loads of longer duration, regardless of DC system voltage, the same power will be required, the battery bank will have to be very large, and it will be discharged rapidly, requiring ICE generator re-charging in most cases, that could have simply been used to power the appliance(s) in the first place, reducing the high stress on the batteries and electrical circuits.

12. Developing a high load, high voltage DC system, and using a DC-DC converter to power safety equipment, introduces a potential single point of failure node, that could render all 12 Vdc safety and nav equipment and lighting inoperable.

13. Below 50 ft LOA, a 12 Vdc electrical system in conjunction with a proper corresponding AC system, will provide excellent service, providing the best compromise between cost, weight, safety, reliability, ease of use, maintenance.

14. In the case of an electric propulsion system, being a > 3 kW and long duration load, a higher voltage DC electrical system is justified.

This system should be completely isolated from the vessel 12 vdc system, the latter having it's own charging systems, to ensure multiple charging source redundancy assuring power availability to all safety equipment in the event the propulsion electrical system (or DC-DC converter) fails.

14. All of these same principles hold true for a 12 Vdc vs 24 Vdc decision. (The electrical product costs will be closer but so will the cable and connector cost.)

15. Above about 50 ft, a 24 Vdc primary electrical system may be justified for a new build.

16. Above about 50 ft, a 24 Vdc local electrical system may be justified for a modification such as a windlass or thruster.

17. Some will propose a design solution they feel is more exciting for them to develop, rather than follow more traditional and proven solutions that are better justified by safety, reliability, life expectancy, and cost.

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. ;-)

What I did gain from this thread, is that the electrical and acoustic impedance matching I have been employing for years to achieve Maximum Power Transfer, also applies to the situation where a contact resistance is in series with a load, (aha moment, explaining the reason for the phenom posted in # 1 and 5). In this case, we are not really trying to achieve Maximum Power Transfer in a fault condition, but it absolutely explains the effect of the fault.

Special thanks go out to "Jammer", "Transmitter Dan", and "Kelkara" and others, for your valid contributions.

Thanks to Dockhead for keeping the discussion alive to this conclusion.

We may all disagree on some or all points, but that's OK, as long as we treat one another with respect, and keep the best interest of forum members (whatever their knowledge and skill level) in mind.

[transmitterdan]

From more than 40 years designing high voltage systems I can say that 48V systems are not more prone to arcing compared to 12V. If so it’s only because people did not design the equipment right. There are DC power systems at 500kV that don’t burn up contactors.

The voltage spike caused by interrupting high current in an inductive load (windlass for example) is about the same for 48V and 12V. That’s because the current-inductance ratio will be the same for a given power motor. So it’s a fallacy to say 48V is harder on contacts than 12V. All inductive switching is hard on contacts. They have to be designed for the application.

At 48V it is much easier to completely eliminate contact arcing using a diode to stop the arc. It’s too expensive to use diodes on 12V systems because the diodes have to be 4 times bigger and 10 times more expensive than for 48V. But even the good 12 volt systems use diodes to eliminate contact arcing.

Relays and fuses designed just for 12V will not be ok for 48V. It’s a simple case of using the right part within its design rating.

[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.

With that said I for one will try to limit my contributions to this thread to those that work toward best practices in the design of a 48 volt boat system.

The topic is Comparative Safety... and I think there are plenty of examples that show that a 48 volt DC system is not intrinsically more un safe than a 12 volt or 24 volt system.

We have seen a number of examples - 48 volt telco systems are common for example.

Along those lines I came across this:

IEC 62368-1:2014 defines "safe" as 60VDC or 2mA DC, whichever is less. For AC the limits are 30VAC and 0.5mA. These are considered safe for an ordinary person but you still cannot leave exposed connectors/wires lying around as there's the whole chapter of "electrical fires" to consider.

There are higher limits for an "instructed person" e.g. service techies. These are 120VDC and 50VAC, with a special exception given for 90VAC telephone network ringing. These are defined as being noticeable or even unpleasant but usually would not produce harmful electrical physiological effects.. Current limits are 25mA DC and 5mA AC.

It should be obvious that taking a 12 volt system and just running it at 48 volts would not work. Sure you would change out the 12 volt loads for 48 volt loads but if you did not replace the circuit breaker rated for a max of 30 volts DC with one rated for 48 volts DC you would be exceeding rated limits.

To stay apples to apples you would need to replace those breakers and any other component not rated for 48 VDC operation with one that was rated for 48 volts.

There are efficiencies in 48 volt power generation (as noted in the quote by Ocean Planet), and in power distribution (4 time the power for the same current over 12 volts) that make it a desirable choice to become mainstream.

[ramblinrod]

Originally Posted by transmitterdan

From more than 40 years designing high voltage systems I can say that 48V systems are not more prone to arcing compared to 12V. If so it’s only because people did not design the equipment right. There are DC power systems at 500kV that don’t burn up contactors.

The voltage spike caused by interrupting high current in an inductive load (windlass for example) is about the same for 48V and 12V. That’s because the current-inductance ratio will be the same for a given power motor. So it’s a fallacy to say 48V is harder on contacts than 12V. All inductive switching is hard on contacts. They have to be designed for the application.

At 48V it is much easier to completely eliminate contact arcing using a diode to stop the arc. It’s too expensive to use diodes on 12V systems because the diodes have to be 4 times bigger and 10 times more expensive than for 48V. But even the good 12 volt systems use diodes to eliminate contact arcing.

Relays and fuses designed just for 12V will not be ok for 48V. It’s a simple case of using the right part within its design rating.

Everyone is entitled to their opinion.

I too have 40 years experience with products and systems using various DC voltages.

In my opinion, declaring one thing is no more dangerous than another, because additional safety measures have to be employed, is an invalid argument, and actually proves the alternative argument.

Apples to apples, up to a load limit, 48 Vdc is more dangerous than 12 Vdc.

As a result, I do recommend higher voltage when required to handle higher loads (> 3 kW for ~ 1 hour, or > 6 kW for ~ 5 minutes), but I do not recommend higher voltage for lower loads for one simple reason.

It is not needed and it is less safe.

The only real benefit of higher voltage than needed, is where the cable cost reduction exceeds the higher cost of other components like switches, breakers, and loads.

In other words, the designer / builder is willing to accept the higher danger to the user, to reduce product cost and increase profit.

Again, everyone is entitled to their opinion.

Don't confuse increasing danger and employing mitigation (to maximize profit), with no change in danger; it simply isn't true.