Comparative Safety: 12v v 24v v 48

[GrowleyMonster]

Bigger loads, bigger voltage. If all you have is running lights, bilge pump, and a VHF, It makes no sense at all to switch to 48v and not much sense to go 48v for original equipment. Bow thruster, windlass, electric propulsion (main or backup), 3kw+ Inverter, 48v makes a lot of sense, but the owner/operator needs to understand the difference and be very safety aware. 24V? Well, some higher power electronics don't play well with 12v. I am not aware of any turnkey bow thrusters or windlasses for small yachts that require 48VDC but I have little doubt that as electric propulsion (48v is pretty much standard for boats under 40') gains wider acceptance, somebody will jump on the wagon and make bow thrusters and windlasses for this market. Who knows... I might try to adapt a spare propulsion motor to my spare 12v windlass and install it on my 48v boat, just as proof of concept. I really would like to have a windlass on the little boat even though I don't really NEED it like I do on the big boat.


I might add that there are 32v starters. All shrimp boats used to use 32v for house and start banks, back in the days before AC gensets became popular. Fitting a starter from a 353 Cat to a little Yanmar or Beta might be a trick, but no doubt it could be done. Overvolting a 32v starter has most definitely been done, to 36v. 48v is a stretch, yeah, I guess, but probably a stock starter could be rewound for 48v.



48v is all about need. Do you need 48v? Would 48v eliminate dependence on multiparallel 12v? Would it keep currents at acceptable levels that would otherwise be crazy high? It never would have occurred to me to have anything BUT 12v if I had never started messing around with EP. The need was there, for the propulsion bank. The two group 31s were getting old and tired. The big propulsion bank isn't in constant use or anything, takes up space, consumes maintenance time, why not give it other things to do? So I did. Now I don't have a 12v bank on that boat. The big boat has a diesel. No bow thruster. (for now, anyway.) Soon I will install a 12v windlass that can be manually powered if needed. And a 12v watermaker. Converting to 48v might happen some day, but not in the near future. The need for it is not overwhelming, for now.

[CatNewBee]

Originally Posted by GrowleyMonster

Bigger loads, bigger voltage. If all you have is running lights, bilge pump, and a VHF, It makes no sense at all to switch to 48v and not much sense to go 48v for original equipment. Bow thruster, windlass, electric propulsion (main or backup), 3kw+ Inverter, 48v makes a lot of sense, but the owner/operator needs to understand the difference and be very safety aware. 24V? Well, some higher power electronics don't play well with 12v. I am not aware of any turnkey bow thrusters or windlasses for small yachts that require 48VDC but I have little doubt that as electric propulsion (48v is pretty much standard for boats under 40') gains wider acceptance, somebody will jump on the wagon and make bow thrusters and windlasses for this market. Who knows... I might try to adapt a spare propulsion motor to my spare 12v windlass and install it on my 48v boat, just as proof of concept. I really would like to have a windlass on the little boat even though I don't really NEED it like I do on the big boat.


I might add that there are 32v starters. All shrimp boats used to use 32v for house and start banks, back in the days before AC gensets became popular. Fitting a starter from a 353 Cat to a little Yanmar or Beta might be a trick, but no doubt it could be done. Overvolting a 32v starter has most definitely been done, to 36v. 48v is a stretch, yeah, I guess, but probably a stock starter could be rewound for 48v.



48v is all about need. Do you need 48v? Would 48v eliminate dependence on multiparallel 12v? Would it keep currents at acceptable levels that would otherwise be crazy high? It never would have occurred to me to have anything BUT 12v if I had never started messing around with EP. The need was there, for the propulsion bank. The two group 31s were getting old and tired. The big propulsion bank isn't in constant use or anything, takes up space, consumes maintenance time, why not give it other things to do? So I did. Now I don't have a 12v bank on that boat. The big boat has a diesel. No bow thruster. (for now, anyway.) Soon I will install a 12v windlass that can be manually powered if needed. And a 12v watermaker. Converting to 48v might happen some day, but not in the near future. The need for it is not overwhelming, for now.

some truck startes and generators at 24V may fit.

[Q Xopa]

Originally Posted by GrowleyMonster

Bigger loads, bigger voltage. If all you have is running lights, bilge pump, and a VHF, It makes no sense at all to switch to 48v and not much sense to go 48v for original equipment. Bow thruster, windlass, electric propulsion (main or backup), 3kw+ Inverter, 48v makes a lot of sense, but the owner/operator needs to understand the difference and be very safety aware. 24V? Well, some higher power electronics don't play well with 12v. I am not aware of any turnkey bow thrusters or windlasses for small yachts that require 48VDC but I have little doubt that as electric propulsion (48v is pretty much standard for boats under 40') gains wider acceptance, somebody will jump on the wagon and make bow thrusters and windlasses for this market. Who knows... I might try to adapt a spare propulsion motor to my spare 12v windlass and install it on my 48v boat, just as proof of concept. I really would like to have a windlass on the little boat even though I don't really NEED it like I do on the big boat.


I might add that there are 32v starters. All shrimp boats used to use 32v for house and start banks, back in the days before AC gensets became popular. Fitting a starter from a 353 Cat to a little Yanmar or Beta might be a trick, but no doubt it could be done. Overvolting a 32v starter has most definitely been done, to 36v. 48v is a stretch, yeah, I guess, but probably a stock starter could be rewound for 48v.



48v is all about need. Do you need 48v? Would 48v eliminate dependence on multiparallel 12v? Would it keep currents at acceptable levels that would otherwise be crazy high? It never would have occurred to me to have anything BUT 12v if I had never started messing around with EP. The need was there, for the propulsion bank. The two group 31s were getting old and tired. The big propulsion bank isn't in constant use or anything, takes up space, consumes maintenance time, why not give it other things to do? So I did. Now I don't have a 12v bank on that boat. The big boat has a diesel. No bow thruster. (for now, anyway.) Soon I will install a 12v windlass that can be manually powered if needed. And a 12v watermaker. Converting to 48v might happen some day, but not in the near future. The need for it is not overwhelming, for now.

I'm thinking an MPPT should be able to charge your 12 or 24v house bank from a 48v propulsion bank? I could be wrong.

This should effectively mean you have a larger combined batt storage bank.
It should be doable the other way too through an inverter and AC charger.
Losses and efficiencies yes of course through extra conversions.
But if you can use say solar after your house batts are topped off, that would usually be not used you are ahead.
Just some crazy thoughts.

I'm taking the middle ground, doing a 24v from 12 upgrade on my CT49. My windlass, invertors and most electronics and fridges run happily off 24v. No thruster.

It would be much less compex if I can get or rewind, or just run my 4JH4 starter on 24v, I wont need a 12v batt system. At $139 for a starter Im thinking I will give it a go. Ive got a few spares.

Then I will just have two smaller parallel 24v banks, with seperate switches. Lets call them Main and Reserve, in liue of House and Start.

The few devices that do need 12v I can easily just use a converter. I couldnt run the Starter off a converter though obviously.

[ramblinrod]

Originally Posted by Q Xopa

Strawman alert-
Disrespectful and Cyber bulling. Arent you precious, accusing others of this.
Incorrect-
I get it, that this is what you believe.

Correct!

The following post is directly related to this thread, but not to the subject title.

OK, I think I may understand what has happened here.

Everything everyone posts on a forum, is their opinion. Even if they claim a statement is a fact, it is still their opinion. Perhaps the opinion is supported by facts, or perhaps not. Perhaps they have mistakenly considered a fact applicable, or applied it incorrectly. Or perhaps the "fact" is merely the opinion of a respected authority on the subject, and supported by many. It is still an opinion. To err is human, whether singular, or a large group, whether highly educated, or not.

So if someone has a differing opinion, they may precede their reasoning or justification for their opinion by stating...

Thank you for taking valuable time from your busy day to share this information with the forum, however, based on my humble opinion, based on X amount of relevant education, y amount of relevant training, and z amount of relevant practical experience, much of it directly related to this subject matter, I have a differing opinion, and believe the statements referenced are "Correct! or "Incorrect"!

Or, to save time, space, and bandwidth, and avoid distracting attention from the main point, or making the post way too boring for anyone to suffer through all of the fluff to find out what the real issue is, they may just post...

"Correct!" or "Incorrect!"

(and then just get to the point).

Note that where I agreed with a statement by other posters and replied with simply "Correct!", (followed by justification) the author of the post I was referring to was not offended, but any who disagreed with the author or my opinion, may have been.

Some may believe that anyone posting the clear and concise statement "Correct!" or "Incorrect!" (followed by supposed "facts", reasoning or logical justification), considers themselves the ultimate authority on what is correct and incorrect.

Incorrect! (At least as it pertains to when I used this statement.) Despite my qualifications relevant to the subject matter, and what I have learned to date, (perhaps more than some and less than others) I know there is a lot more to learn, which is why I am involved in life-long continuing education, and pursuit of additional experience in the field of marine electrical and electronic systems, theories, and practices.

I understand that on any opinion I post that I believe is "Correct!", it is possible that I am actually "Incorrect!".

When anyone states that something is simply "Correct!" or "Incorrect!", basically the entire sentence in blue above is implied, every time.

(All except for maybe one or two other posters in this thread, who seem to believe they are the ultimate authority and will attempt to shun any contradictor, by bombarding them with a barrage of "incorrect" statements, and intentionally misleading statements to purposefully slight the other poster, and a variety of other tactics, just to be mean (basically), and distract attention from the bully's posts that were in fact "Incorrect!" nad the other poster who was in fact "Correct!".

From this point forward, should I (or pretty much anyone) ever state that something is "Correct" or "Incorrect", please understand that the entire sentence in blue is implied.

So based on the sentence in blue, I believe I am "Correct" in stating that if anyone's ego is so sensitive, that they cannot stand someone else declaring their position "Incorrect!" that they cannot rebut, without becoming extremely offensive, especially when they realize that in fact they were "Incorrect!", and this "forces" them to launch an irrelevant offensive against the "corrector" for the purpose of distracting attention away from their error, instead of just letting it go unsaid (acceptable), or admitting the mistake (the honourable thing to do, (followed by the "blue" statement above), probably shouldn't post stuff on an internet forum.

Additionally, it appears that some consider anyone who posts a large number of lengthy posts is being disrespectful or bullying.

Incorrect!

If someone disagrees in an honourable way, presenting facts or opinions as they know them, this is neither disrespectful nor bullying.

The more complex the nature of the disagreement and the more willing the poster is to devote time and effort to crafting their detailed position, the more lengthy it may become.

Long posts that raise many different but relevant issues and go into detail to explain each one, are actually quite valuable (blue statement).

The more people who disagree and wish to post so, challenging the position of another poster, the more posts the originator may post in direct response to each, if they wish to respond in this manner.

This is not bullying, it is just responding personally and with detail.

So what is bullying?

Well, attempting to disparage a poster, based on education, experience, race, creed, colour, or any other irrelevant factor, in fact any disparaging attempt, is a form of bullying.

It's kind of like stating, "It doesn't matter if you are "Correct!", what is important is that in my opinion, I am more (blank) than you, and therefore you should just keep quiet instead of having the audacity to contradict me", or "Because you have lesser (blank), you MUST be incorrect, regardless what your position is.

Obviously this isn't necessarily true, and I certainly don't believe so, or I would never disagree with anyone having higher (blank).

This would be difficult to even do, as most posters don't include their (blank) in their signature or profile. The others may attempt to present themselves as having more (blank) than they really do.

Also, we do have to be aware that this is the internet and not everyone will be honest. If I post something original or contradict another, please know that it is my honest opinion, and I will not contradict someone based on a flippant notion; I truly believe someone is "Incorrect!" and if others mistake their statement as "Correct!" it could cause them harm.

Is the original poster wrong just because a number of posters disagree?

Not necessarily.

What can often happen on a forum such as this, is that a poster may become known for bullying others who contradict them.

If a member posts something and this poster disagrees, they may not be willing to stand their ground for fear of retaliation from the bully.

If they do stand their ground, a number of bully supporters may launch posts (some of them having nothing to do with topic, but just attempts to disparage the non-bully, as disagreeing with a bully can expose one to their wrath.

I am not such a bully. If others disagree with me, I hope they will post contradiction. This will challenge me to support my position. It will help me to confirm my opinion is "Correct!" or show my why it is "Incorrect!"

I am all for this type of learning opportunity. (And why I am so objectionable to those who constantly bully others, diminishing this opportunity.)

I do my best to stay cool, and not loose my temper when others launch these highly offensive bullying attacks, (intentionally and sometimes disguised in a way the average poster or moderator may not see it), but of course, in a case like this thread, it can become like attempted murder by a 1000 paper cuts, and sometimes my posts, after several disrespectful attempts against me, may start to become less cordial. Sorry, I do have thick skin, but I am human.

Note that I do my Name, Business and (some) relevant qualifications, abundantly clear.

I do not operate in anonymity, like many others here, who can post whatever nonsense, disparaging attack they like and yet it won't stick to them personally.

I like to be honest about who I am, what my qualifications are, and sign my name to it every time. If I wasn't that sure, I wouldn't present it as that sure.

I hope others will respect this, and not launch disparaging attacks, while they themselves hide behind anonymity.

[weavis]

Let me be clear about this.

It is not for any CF member to outline the rules. We have set the rules.......

Do not think that the body of moderators or Admin is not able to see what is going on. We have to see it from all sides of the issue and decisions made on that basis.

This thread has degraded. For all participants, you need to get back on track collectively or the thread will be no longer useful and we will pull the plug.

[ramblinrod]

Everyone is entitled to their opinion.

1. Existing...

Originally Posted by s/v Jedi

is that a pro? 12V is better because it exists? I would delete this point.

If a boat comes with an existing, fully functional, and safe electrical system, that is a pro. If a 12 Vdc electrical system fits the bill, and it is already installed, there is no need to change it.

2. Product Base...

all the wiring, breakers, terminals, terminal blocks etc. are the same so availability is the same.

I disagree. In contemplating a modification or new design to a higher system voltage, one has to review the design voltage of all components. As discussed earlier in the thread, many components that are rated for 12 Vdc are not rated for 48 Vdc. All of these components would have to be replaced with 48 Vdc rated components (which the associated cost of unnecessary replacement, with the additional cost of many 48 Vdc rated components being more expensive than their 12Vdc counterparts.

In case of motors for starters, winches, windlass etc. then there are more offerings for 12V than there are for 48V.

Correct! and generally lower cost and better value for money too.

But for boats who need more than a car-grade 12V system, all the motors they require are available in 24V so that makes 12V less desirable than 24V.

I believe this is an intentionally misleading statement attempting to associate a negative connotation to 12 Vdc.

There are 12 Vdc "automotive" grade components and 12 Vdc "Marine Grade" components. I believe "Marine Grade" 12 Vdc components should always be use, with the understanding that in some cases, the product sold as Marine Grade, may be similar to the automotive grade.

A perfect example of this is electrical wire and connectors. Anyone in the know, knows that the difference is night and day, "Marine Grade" tends to be vastly superior.

3. Cost...

in case of motors etc. 12V may be cheaper yes.

4. electrocution:

delete, irrelevant for anything below 50V

I disagree. As previously posted, there is no magic number immediately below which constitutes "No Danger" and above which constitutes "Danger"

No one can predict if a definite electrocution hazard exists, with looking at the specific circumstances of each case. But the math and science tends to support that the risk of electrocution is less at 12 Vdc, and more at 48 Vdc, which logically supports the position, "Danger increases with Voltage".

5. electrical shock:

delete, irrelevant for anything below 50V

Disagree. I have received an electrical chock from 12 Vdc (and even less) many times. The shock has been far less severe than the shock received from 48 Vdc under similar conditions.

6. fire:

more risk for 12V so this is a con for 12V and a pro for 24/48V. Reason is simply that with 12V we have a current that is 4 times higher so in any hotspot due to corrosion etc. we get 16x as much heat development ( I^2 = 4^2 = 16 )

The math and science and my experience do not support this claim, but od support the exact opposite.

The heat generated by a resistive load (which may be a high Z connection) is proportional to the electrical energy dissipated as measured in Watts.

If the voltage is increased across a resistance, the heat generated in BTU/h is directly proportional to the energy dissipated in Watts/h, 1 W/h = 3.4 BTU/h.

The energy dissipated in this case is calculated using the formula P(Watts) = E^2/R.

7. arcing:

more arcing for 12V because current is higher and arcing occurs when disconnecting a load. The big error here is that for high voltage arcing we mean that an arc forms through insulation or just through plasma in an otherwise static environment. We talking many kV here so all that is irrelevant for anything below 50V. The arcing described by me and other EE's is contact-arcing inside a switch, breaker, solenoid etc. when a large current is disconnected. Here it is current, not voltage, that is damaging.

I believe this to be incorrect and either misinformed, or intentionally misleading.

For a switch, breaker, or an open circuit connection (such as a loose terminal), there is no current flowing. It is only when the voltage is of sufficient magnitude to force an arc, that the arc is formed, at which time it can pass almost all of the current that is available.

The damage potentially caused by the arc is directly proportional to power (not just current).

So if the voltage is lower, it may not have sufficient electromotive-force (voltage AKA potential) to jump the gap, form an arc, and allow a high amount of current to pass. No damage.

If the voltage is higher, it may have sufficient electro-motive force to jump the same gap, allowing current to flow, allowing power (Watts) to be dissipated.

In either case, if an arc is formed, the energy dissipate will be P (Watts) = E (Volts) * I (Amps).

When we talk about high current capable sources such as batteries, the current available is mostly limited by the resistance of the cabling.

In most if not all cases, the probability of an arc forming increases with voltage, and with increased voltage, the damaging energy dissipated by an arc is also proportional to voltage which cause the resultant current to flow.

Danger Increases with Voltage. The Amount of Voltage is what determines the amount of current can flow, and hence the energy dissipated.

8. Familiarity:

irrelevant because everything is the same. No additional knowledge required.

I disagree. While all basic principles of electrical theory apply to any voltage, like P = E^2/R, general familiarity with relatively uncommon electrical products of a different voltage will be less. For example, if a person works on marine electrical electrical systems regularly, they will know off the top of their head which components are suitable. If they then work on a system having a different voltage than they are familiar, they will have to verify products are compatible or source different products. All of this takes time, and time is money.

9. Availability:

yes, 12V pumps etc. are easier to find than 24V let alone 48V

Agreed. To me, initial and emergence replacement component availability is highly important. In the case of working on a customer boat, I don't want to be held up for a special 48 Vdc part to come in, when I have lots of 12 Vdc parts in stock, because of the "familiarity" of 12 Vdc systems. On my own boat, I don't want to be stuck in a remote location, or living without the convenience or safety and security of electrical systems, because a "unique" or "unusual" part (such as 48 Vdc rated) is not readily available.

10. Necessity:

compared with 48V correct. Compared with 24V (which was part of it) incorrect. Every winch, windless, starter etc. is available for 24V.

I think you missed the point. For many, an electrical system voltage higher than 12 Vdc is not necessary, and really would not benefit them in anyway.

On my own boat for example, I have a 12 Vdc system and a 120 Vac system. I have a refrigeration system, nice assortment of electronics, 1000W inverter, 1000W windlass, lighting, audio and video appliances, typical (actually a little more than average) for a boat the size of mine.

A system voltage level higher than 12 Vdc is not a necessity in any way, shape, or form. To change the electrical system to a higher voltage would be a huge cost for no benefit. The current may be lower to some loads. So what. The current to present loads, is not a problem, and because the system voltage is lower, the power (hence heat and risk of fire) available to short circuit faults is lower.

So, I posted a summary/review, that I believed was "Correct".

You have contradicted that post (indicating you believe it "Incorrect, and stating the reason why.

I am not offended at all.

This gave me the opportunity to review my review for veracity (correctness) based on the contradictory opinion of another.

Despite your comments (and thank you for taking the time and effort to do so) that were posted in a reasonably respectful manner, I agree with some, disagree with others, for the reasons, that I took my time and effort to craft and post in response. Even if we disagree, I hope you can appreciate that I responded with my disagreement in a respectful manner, even if I used the words, "Correct!" or "Incorrect!" (which themselves are neither respectful nor disrespectful, they are just words to indicate "agreement" or "disagreement".)


Therefore my position is unchanged.

[weavis]

Rod... Better to say agree or disagree. Correct or incorrect makes you the arbiter of what is right. And that is your opinion.

[Jdege]

So, I've been reading My Electric Boats.

Mathys makes a good argument on the superiority of the three-phase AC induction motor over any type of DC motor.

AC is generally considered far more dangerous than DC, it seems to me that this might be manageable, given that the AC would run only from the inverter to the motor, and that's likely to be a single, short run.

[ramblinrod]

Originally Posted by Jdege

So, I've been reading My Electric Boats.

Mathys makes a good argument on the superiority of the three-phase AC induction motor over any type of DC motor.

AC is generally considered far more dangerous than DC, it seems to me that this might be manageable, given that the AC would run only from the inverter to the motor, and that's likely to be a single, short run.

Good point.

I have spec'd a lot of motors and 3 phase AC are vastly superior to DC or single phase AC.

But for most rec boats 50 ft and under, 3 phase AC is overkill, and standard 120 Vac (NA) or 220 Vac (ROW) is a perfectly adequate electrical system to run high load appliances, that are available in this voltage everywhere and inexpensively.

This is why I suggest if one is considering a higher voltage DC system, just determine if the heavy loads (refrigeration, Air Conditioning, and Water Making) can be run off a common AC generator.

If so, this means a smaller inverter can be used, a smaller house bank maintained, etc, etc, etc.

Is a generator heavy? Yup. But so is a big 48 Vdc house bank, and quite frankly, a generator is likely going to be necessary to recharge it anyway.

[ramblinrod]

Originally Posted by weavis

Rod... Better to say agree or disagree. Correct or incorrect makes you the arbiter of what is right. And that is your opinion.

Hi Weavis,

Well now I don't know what to do.

If this post is as a member, I would like to rebut with my opinion.

If this post is as a moderator, forum rules would seem to prohibit rebuttal.

[Jdege]

Originally Posted by ramblinrod

Good point.

I have spec'd a lot of motors and 3 phase AC are vastly superior to DC or single phase AC.

But for most rec boats 50 ft and under, 3 phase AC is overkill, and standard 120 Vac (NA) or 220 Vac (ROW) is a perfectly adequate electrical system to run high load appliances, that are available in this voltage everywhere and inexpensively.

The author is a retired electrical engineer, and he:

1. Rewired the stator coils of an off-the-shelf 220v 3-phase to be appropriate for 48v (fewer coils of a heavier-gauge wire), and
2. Built a 48v DC to 3-phase AC inverter.

I'm not sure either is within the purview of the average DIY boat owner.

[transmitterdan]

Rod,

With respect to your opinion about arcing let me try to explain how arcs form in low voltage DC systems.

The dominant method of arc formation in DC systems is when a current interrupting device (fuse, breaker, relay, contactor or manual switch) is opened with current flowing. The inductance in the load and wiring creates a back EMF that attempts to keep the same current flowing. The voltage across the contacts will rise high enough to arc across the air gap.

The amount of energy in such an arc is completely unrelated to the system voltage which is contrary to what you believe. The voltage of the arc will be about 50V regardless of the system voltage. That is the voltage across a good strong plasma arc in air. That blue flash you see is the plasma (ionized air).

The amount of energy in the arc is proportional to the energy stored in the magnetic field. The formula for the energy is 1/2 inductance times current squared (E = 1/2 X L X I^2). Note that system voltage is no where in this equation.

12V devices have higher current than 24 or 48V devices of the same power. So the energy available can only be determined from the inductance of the load and wiring.

Now, 48V motors have higher inductance than 12V motors for a similar power rating. But how much more inductance? For small DC motors in the range of 100W to 2kW the inductance ratio of 12V to 48V is slightly less than the voltage ratio squared. A 48V motor will have about 14-15 times the inductance of a 12V motor. So the energy stored in the motor windings is slightly less for a 48V motor compared to a 12V. Not only that but the resistance of a 48V motor is 4 times that of a 12V motor which dissipates the energy in the arc much faster.

For non-inductive loads such as resistance heaters and inverters the situation is even better for 48V. The inductance is the same but current is 1/4 so arc energy is 16 times less for a 48V system compared to a 12V system.

Thus your insistence that arcing damage increases with system voltage is not supported by any engineering mathematics. The exact opposite is true.

Likewise, if there is a failure of insulation then the available energy to a resulting arc is about the same for a 12V and 48V system because the system impedance for 48V is going to be 4 times higher than for a 12V one of the same kWh rating. But the rate of insulation failure in sub 100V systems is about the same regardless of system voltage. 48V suffers no higher insulation failure rate than 12V systems.

No need to tell me I am incorrect. I do this stuff for a living and have used the aforementioned mathematics many times in solving arc energy calculations.

I mean no disrespect when I say that you are not helping your cause by always trying to be right. You can get a lot of satisfaction from learning if you try just a little.

[weavis]

Originally Posted by ramblinrod

Hi Weavis,

Well now I don't know what to do.

If this post is as a member, I would like to rebut with my opinion.

If this post is as a moderator, forum rules would seem to prohibit rebuttal.

Rod.

If you want to have a discussion, Then agree or disagree would be the way to go.

If you want to fight, then correct or incorrect is the way to go. It will result in a lot of wasted time and energy and bad feeling if others feel different and eventually be a contributing factor to the closure of the thread.

There are a number of highly qualified members in this discussion. On that basis, it does not single out one of you to the authoritative voice regarding matters electrical. Given the history of these exchanges, it would benefit the thread if a disagreement over protocols is acknowledged as a disagreement and NOT as correct or incorrect, even though sometimes it may be a correct understanding, or as has been seen, not as correct as a member would like it to be.

I have offered you considered advice from dealing with thousands of threads and personalities. Take it or leave it.

Its a Discussion Board with professionals and amateurs alike. The admin's job is to keep the forum running smoothly and harmoniously, NOT to judge who is right or wrong in a technical subject. Where we see issues in communication, we offer advice to help because the threads are helpful to a lot of people. Your fellow professionals will decide who is correct or incorrect collectively, and better to be a dissenting voice with disagreement than an inflammatory statement that detracts from the subject under discussion.

[ramblinrod]

Originally Posted by transmitterdan

Rod,

With respect to your opinion about arcing let me try to explain how arcs form in low voltage DC systems.

The dominant method of arc formation in DC systems is when a current interrupting device (fuse, breaker, relay, contactor or manual switch) is opened with current flowing. The inductance in the load and wiring creates a back EMF that attempts to keep the same current flowing. The voltage across the contacts will rise high enough to arc across the air gap.

The amount of energy in such an arc is completely unrelated to the system voltage which is contrary to what you believe. The voltage of the arc will be about 50V regardless of the system voltage. That is the voltage across a good strong plasma arc in air. That blue flash you see is the plasma (ionized air).

The amount of energy in the arc is proportional to the energy stored in the magnetic field. The formula for the energy is 1/2 inductance times current squared (E = 1/2 X L X I^2). Note that system voltage is no where in this equation.

12V devices have higher current than 24 or 48V devices of the same power. So the energy available can only be determined from the inductance of the load and wiring.

Now, 48V motors have higher inductance than 12V motors for a similar power rating. But how much more inductance? For small DC motors in the range of 100W to 2kW the inductance ratio of 12V to 48V is slightly less than the voltage ratio squared. A 48V motor will have about 14-15 times the inductance of a 12V motor. So the energy stored in the motor windings is slightly less for a 48V motor compared to a 12V. Not only that but the resistance of a 48V motor is 4 times that of a 12V motor which dissipates the energy in the arc much faster.

For non-inductive loads such as resistance heaters and inverters the situation is even better for 48V. The inductance is the same but current is 1/4 so arc energy is 16 times less for a 48V system compared to a 12V system.

Thus your insistence that arcing damage increases with system voltage is not supported by any engineering mathematics. The exact opposite is true.

Likewise, if there is a failure of insulation then the available energy to a resulting arc is about the same for a 12V and 48V system because the system impedance for 48V is going to be 4 times higher than for a 12V one of the same kWh rating. But the rate of insulation failure in sub 100V systems is about the same regardless of system voltage. 48V suffers no higher insulation failure rate than 12V systems.

No need to tell me I am incorrect. I do this stuff for a living and have used the aforementioned mathematics many times in solving arc energy calculations.

I mean no disrespect when I say that you are not helping your cause by always trying to be right. You can get a lot of satisfaction from learning if you try just a little.

I mean no disrespect, but the main cause of arcing in a 12 Vdc contact, is due to the back EMF caused by an inductive load.

In other words increased voltage.

Resistive loads, like pretty much everything that does not have a motor, do not produce any significant back EMF.

So for a 12 Vdc system, there is about 15 Vdc max across the contacts, and very little chance of arcing problems.

When one switches a 12 Vdc inductive load like a fridge compressor, there can be an instantaneous back EMF of higher voltage, but this generally will not be high enough or sustained long enough, to damage a marine rated switch or breaker. I have seen many over 40 years old and still going strong.

Step the system voltage up to 48 Vdc and we now have a problem.

That 48 Vdc switching a resistive load is sufficient to cause an arc.

Now change that to an inductive load and ZAP!, that contact is getting damaged with every opening.

This is why standard marine switches and breakers are not rated for 48 Vdc.

For 48 Vdc, a better mechanism or arc suppression is required to avoid contact damage by arcing. Without it, there is little chance a 12 Vdc rated switch is going to last 40+ years on a 48 vdc electrical system.

[CatNewBee]

Hey, rod,

Induction is caused by current, not by voltage. Just saying...