Comparative Safety: 12v v 24v v 48

[transmitterdan]

Rod, the energy stored in an inductor is entirely defined by the amount of current through said inductor. It is a fundamental law of physics. It matters not how that current came to be in the inductor. Current can be created in an inductor multiple ways. But the energy stored in said inductor does not depend on how the current came to be.

Try to grasp that and a lot of things said in this thread will make a lot more sense to you.

[Dockhead]

Originally Posted by ramblinrod

Energy Efficiency = Power In / Power Out

If one requires 1kW/h of electrical power, it is generally far more efficient to run a 1 kW generator for 24 hours, than a 24 kW generator for one hour.

What makes you think that? Why would that ever be more efficient? In fact, the opposite is true -- the cost of maintenance and amortization of any generator is incurred per hour of operation. A larger generator is more expensive, but the cost to generate one kW/h of power in terms of maintenance and amortization goes down drastically with size. Fuel efficiency of larger generators is also better than that of smaller ones, but fuel is not the biggest cost.


You meant 24kW/h of power, I guess.


But let's use real numbers. I use about 8 kW/h a day of power on my boat when there are a lot of people on board and a lot of electrical demand.



I have a 6.5kW heavy duty generator which cost about £15 000 with installation and which has an expected useful life of about 15 000 hours. So amortization cost is £1 an hour and maintenance and repairs maybe another £0.50 for a total of about £1.50. Fuel costs about £2 per hour when running at about 75% load, the most efficient regime, putting out a bit less than 5kW. So at 75% load, I'm paying about £0.90 per kW/h for power. But if I use the generator at 25% load, the cost per kW/h more than doubles, because I am using up the useful life of the generator at the same rate for half the power, and fuel efficiency is less.



If I went to a smaller generator, and ran it longer, cost would go up drastically. If I went to a larger generator, and ran it for the same period of time (limited by acceptance rate or size of chargers), cost would go up drastically.


So from a systems design point of view, you very much want the generator to be the right size, and used for the minimum hours every day, and if you are forced to size it for outsized peak loads, then you'll never get this right.

Originally Posted by ramblinrod

I think you may be misusing the term efficient. In an of itself one voltage is no more efficient (Power in vs Power out) than another.

Sorry, but higher voltage electric motors of the same type are more efficient than lower voltage ones.


However, AC motors are less efficient than DC motors, so a 230v AC motor might not be as efficient as a 48v DC one.

Originally Posted by ramblinrod

Sorry, but you can't have it both ways.

If you have a daily consumption from a battery bank of 20 kWh, and you want to replenish that with 1 hour of generator run time, you must have a 20 kW generator.

Just to be clear, a 15kW load at 48 Vdc is 312 A. As you mentioned before, "Yikes!"

You totally missed the point. The point is that if you power large loads like bow thrusters with generators, rather than batteries, then you have to size the generator to cover the maximum short term load, which leaves it poorly sized for the average loads. So a 10kW bow thruster, like mine, would require a 12kW generator at least, weighing about half a tonne. This is the worst possible choice for a generator running constantly because you will not be able to keep a healthy % of its maximum load on it (at least about 25% or 3kW) constantly, and even during times of larger loads, the oversized generator will be running in a poor fuel efficiency regime.


So this is inefficient in all possible respects and very poor systems architecture.


Far better, with total cost of producing each kW/h of power literally a fraction of what you recommend, is to run a smaller generator for shorter periods at an efficient load (70% -- 90%), and handle the peak short term loads with battery power, and the long term small loads also with battery power. The batteries allow you to even out the loads on the generator and unlike a generator, they are not bothered by your taking small amounts out at a time.


Thus on a boat like mine you can easily run a 10kW thruster or even a 20kW thruster off batteries, and use a 5kW (or 6.5kW like what I have) generator to put power into the batteries. With lithium, you can capture 8 or 10kW/h in two hours or less of generator running, enough for an electrically intense day on a big boat. Or even without the generator if you've motored for a couple of hours with a large alternator. Even a very large lead bank, say double the size of what I have now, can only store about 8kW/h of power, and it will take hours to get that much power into it.

Originally Posted by ramblinrod

You would need a 350 A fuse or breaker. Any short circuit that draws less than 350 A, will not trip the over current protection. That is a whole lot of power (up to 16.8 kW) than can be smoldering away (or vapourizing in an instant) in your "backbone" somewhere.


This is why I don't recommend another high voltage system in the vessel. Most already have enough trouble not starting fires with the higher voltage AC system; the last thing they need is a higher voltage DC system.

Honestly, what are you talking about? Powering an equally sized large load with whatever voltage is going to require the same size breaker in terms of power, and will result in exactly the same potential of smoldering or vaporizing, in terms of power. But this is double nonsense, because what kind of short circuit fault, is going to have just the right impedence to smolder or vaporize something, but not enough to trip a breaker?

Originally Posted by ramblinrod

An ELCI, pretty much eliminates the risk of electric shock or electrocution from the protected AC circuit (whatever the voltage).

So how is it that hundreds of people are killed every year from electrocution working on circuits protected by such devices?


See: Is It Possible to Be Electrocuted From a Properly Functioning GFCI Circuit https://diy.stackexchange.com/questi...-circuit#10667

Originally Posted by ramblinrod

This is why I suggest, if considering 48 Vdc, consider moving the higher loads to the AC system and powering with a generator instead, and staying with a lower voltage DC system for the safety equipment, nav lights, cabin lighting, instruments, etc.

And this is why I will hire a qualified electrical engineer and experienced designer, and not an electrician, to design the electrical systems on my next boat!

[ramblinrod]

Originally Posted by transmitterdan

Rod, the energy stored in an inductor is entirely defined by the amount of current through said inductor. It is a fundamental law of physics. It matters not how that current came to be in the inductor. Current can be created in an inductor multiple ways. But the energy stored in said inductor does not depend on how the current came to be.

I am reasonably sure you believe what you claim.

I have repeatedly referred you to Ohm's Law, (a fundamental law of physics), that dictates for a circuit having fixed resistance, current is proportional to voltage.

You have claimed that a Law of Physics exists, that will support your notion that current through an inductor can be independent of voltage.

Please refer me to this law.

Until then, I must respectfully disagree with your belief, and stand by my position that Ohm's Law applies, and in any closed DC circuit having fixed resistance, current is proportional voltage, and without voltage, we can have no current.

[fivecapes]

Originally Posted by ramblinrod

Many technical papers may discuss I^2R generated heat.

Some may be considering this to mean current generated heat.

It doesn't.

It means electrical energy generated heat.

Electrical energy is measured in Watts, and may be referred to a P.

P (energy in Watts) = E (electro-motive force in Volts) x I (current in Amps).

This is Watts Law.

For clarity
The SI unit for energy is Joules, not Watts
The SI unit for power is Watt, or Joules/s

Power=/=Energy



Current does generate heat
Energy (E) =Voltage (V) x Charge (Q)
where Q is in A.s

[CatNewBee]

I don't like electrons either, they are always so negative.

Energy is power * time

[evm1024]

Originally Posted by CatNewBee

I don't like electrons either, they are always so negative.

Energy is power * time

I hear that antimatter batteries (either anti lead acid or anti LiFePO4) have a more positive disposition.

[evm1024]

Originally Posted by ramblinrod

SNIP %<

An ELCI, pretty much eliminates the risk of electric shock or electrocution from the protected AC circuit (whatever the voltage).

SNIP %<

Once again... Misinformation.



WHY ELCI BREAKERS?
ELCI Breakers are safety devices to prevent Electric Shock Drownings. While more dangerous in fresh water, drownings happen in both fresh and saltwater when people become better electrical conductors of leakage A/C current than the surrounding water. Faulty dock or boat wiring is a common cause of dangerous electrical power in waters around docks and marinas. The danger only becomes known when someone enters the water and is immobilized by the electrical shock. These tragedies grow when rescuers enter the same dangerous waters in an attempt to help, and become victims as well.

ELCI Breakers detect amperage difference (imbalance) between the hot (black) wire and the neutral (white) wire and trip if there is a significant difference (leakage) between power going to the boat (hot wire) and that coming back (neutral wire) from the boat to shore power.

ELCI, RCD, GFCI, GFI DIFFERENT OR SAME?
All of these devices perform the same function of detecting an amperage imbalance. The difference is in the level of imbalance, or leaking amperage, that trips the breaker. In-home GFI and GFCI breakers trip at very low imbalance levels while ELCI breakers trip at much higher imbalance levels. RCD’s trip at imbalance levels between GFI and ELCI.


30 mA vs 5 mA

[brownr377]

Faradays law
Gauss law
Ampere's law
Maxwells equations

There is more to it than Ohms law. [edit] Ohms law, yeah, it's good and all, but the other laws are needed to deal with anything other than a pure resistance.

[I'm sure this is already known but why not put it out there for other folks that might be interested in learning things.]

[brownr377]

Hell in for a penny in for a pound.

An example from my world, which is more dangerous - a 16 kV ESD pulse (defined by IEC 61000-4-2) or a 2kV Surge pulse (defined by IEC 61000-4-5). If the be all and end all parameter is the voltage, the ESD pulse should be more dangerous, however, it's not. Clearly there are other things going on that need to be understood.

[ramblinrod]

Originally Posted by Dockhead

What makes you think that?

As I have said now, soooooooo many times, before making any modification one needs to perform a vessel design and configuration and user needs and wants assessment.

You did refer to "efficiency", which generally implies energy efficiency when referring to electrical rotating machinery.

But then you referred to other matters indicating you may have meant "cost effectiveness".

Depending on the circumstances it could be (never said it most definitely would be) more efficient (and cost effective), to run a smaller generator 24 hours/day vs a large generator for 1 hour/day, to produce the same amount of energy.

I expect in most cases it would not be desirable to the customer, to run either of these scenarios, all relevant things considered. The design, configuration, wants, and needs review would determine this.

In reality, it would be fairly unlikely I would recommend a system based on either scenario.

However it is highly likely that after such an analysis, I would recommend moving some heavy loads to the AC electrical system, to optimize the battery bank size and renewable energy charging sources to something that would realistically fit, in order to be reasonably energy efficient, cost effective,and safe.

It is unlikely, except in the case of electric propulsion, that I would recommend going above 24 Vdc, and then it could be higher, even a lot higher.

There are many reasons for this, but I have already worn out my keyboard on this thread. ;-)

I saw an excellent T-shirt the other day, that is somewhat relevant to this thread, so I thought I would post it.

"ELECTRICIAN"

Standard Rate: $100/h
If You Watch: $150/h
If I Train You: $175/h
If You Help: $200/h
If You Tell Me How To Do My Job: $250/h

(It made me chuckle.)

I saw another cute T-shirt...

"I know everyone thinks education is really important,
but I think getting the job done right is importanter."

This is interesting, as those who value education over actual ability will say, "See", and those who value actual ability over education with say, "See".

[

So how is it that hundreds of people are killed every year from electrocution working on circuits protected by such devices?

A GFCI or ELCI is to protect one from electrical shock from an accidental ground fault, not any incredibly stupid act.

What can I say?

Some people do some pretty dumb things, (like stick two forks into a receptacle).

Comparatively, on average each year 700 die in an aircraft crash, and 1.5 million will die in a car accident.

If only 100s are dying from electrocution by GFCI protected circuit every year, that is incredibly low, and a testament to just how effective they really are.

RE: Final disrespectful remark in your post (others intentionally ignored).

You can hire whoever you wish.

Similarly, a business can take on any new customer they wish, based on evaluation of the prospects honesty, integrity, and general good nature.

[ramblinrod]

Originally Posted by evm1024

Once again... Misinformation.

Nope, you just don't understand.

(Watch your sources when you Google something.)

Cutting and pasting something off the WEB is not indicative of understanding.

GFCIs are not only to protect from electric shock drowning.

They are to protect from an electric shock caused by any ground fault.

It is designed to trip if the differential current threshold is reached between the ungrounded and grounded conductor, based on the assumption that some current may be flowing through the grounding conductor, which should not normally be carrying current, if nobody is in the process of being given and electrical shock.

For a better understanding of how GFCIs work and what they do, you should have just asked. I used to manage a large portfolio of them, among other commercial and premise wiring devices. I don't have to Google this stuff and hope the source is knowledgeable and correct. I know it. Have for years.

[Dockhead]

Originally Posted by ramblinrod

. . . Depending on the circumstances it could be (never said it most definitely would be) more efficient (and cost effective), to run a smaller generator 24 hours/day vs a large generator for 1 hour/day, to produce the same amount of energy.. . .

For this to have even equal cost even in case the fuel efficiency is the same (which it won't be), the amortization and operating costs of the smaller generator would have to 24 times less per hour, than that of the larger generator. I guess there wouldn't be even one single example of that in real life. A case of your favorite beer if you can provide even one real life example, of any marine diesel generator running 24 hours a day, which produces power for less per kW/h than a properly sized diesel generator producing the same amount of power in one hour, considering fuel, maintenance, amortization, and all other costs.




For the record -- "efficiency" means getting the most desired effect for the least effort or cost. Mechanical efficiency or electrical efficiency is only one aspect of this. The end user will care very much about cost, in this equation, and will not care about efficiency considered in a vacuum without regard to cost.

[evm1024]

Originally Posted by ramblinrod

SNIP %<

Ummm, yes I do, and I have understood this since grade 9, "Fundamentals of Electricity" class.

But did not learn anything new since then? Some may wonder.

Moving on.

Originally Posted by TD

Thus the battery voltage does not determine the amount of damage caused by arcing as you believe.

to which RR replied:

Originally Posted by RR

Yes it does. The energy stored in the inductor is proportional to the current through it. The current through it is proportional to the system voltage.

Therefore the energy stored in the inductor is proportional to system voltage.

A failure to understand again. You forgot Ohm's law. You left out a term. Let me correct that for you"

The current through it is proportional to the system voltage and inversely proportional to the resistance.

This is a big difference and the reason for your lack of understanding.

Originally Posted by RR

I disagree.

What a surprise.


SNIP %< errors addresses by other posters.

Originally Posted by RR

Now, when the inductor circuit is opened, all kinds of interesting things happen, the inductor magnetic field collapses which generates an EMF, which can be higher than the circuit voltage. I referred to this earlier as back EMF (some may prefer a different term). There can be other things going on with respect to circuit capacitance and resistance, that will affect the magnitude and duration of the transient voltage spike.

Note that we call it a transient voltage spike.

Hard to have a productive discussion when the basic definitions are ignored or not known.

Here is a pdf that talks about the breaking arc of contacts in switches and relays.
https://www.pickercomponents.com/pdf...Phenomenon.pdf

Originally Posted by PDF

An arc ignites in similar manner upon contact break. As the contacts begin to separate, less and less
contact area carries load current. Load current begins to funnel into this constricted area and I²R heat
begins to increase. The very last point of contact melts and, as the contacts continue to separate, a thin
bridge of molten metal is stretched between the contacts. The air in the gap begins to ionize. The I²R
energy in the bridge generates so much energy that the bridge literally explodes, showering the gap with
metallic ions. Again, if contact voltage is sufficient, an arc will ignite.
Different contact materials have different arc voltage ratings. For fine silver, the arc voltage is 12 volts.
For cadmium, it is 10 volts; and for gold and palladium it is 15 volts. Let’s assume the contacts are fine
silver. Within nanoseconds after the molten bridge explodes, if the material is silver and if circuit voltage
is 12 volts or more, voltage break over occurs. If circuit voltage is less than 12 volts, break over cannot
occur and there will be no arc.

I thought you knew these things.

Peak voltage in a transient voltage spike is reached in 1 to 2 uS. That is a really fast rise time. You discription (below) is not consistent with reality. For voltages over about 12 volts (and the voltage spike is way over 12 volts) an arc ignites as the molten metallic bridge becomes an ion bridge. More or less in the nano second range.

In short you have it backwards. The opening of the contact gap is what extinguishes the breaking arc. You confuse a breaking arc with a dielectric breakdown arc.

Originally Posted by RR

If the contact is open wide enough, the transient voltage is not high enough to cause an arc, and there is no transient current.

But, the higher the transient voltage the greater the risk of an arc, and the greater the energy dissipated in the arc if it ignites.

As stated you keep leaving out resistance and reality. I know it is hard for you to see this. But try.

When you EEE and increase the voltage you need to increase the resistance to keep the power the same. When you say that EEE means that the resistance stays the same you are misusing EEE. Is this a mental block or a deliberate subterfuge?

Originally Posted by RR

EEE the higher the circuit voltage, the higher the inductor current, the stronger the magnetic field, the higher the transient voltage spike, the greater the risk of arc and arc damage.

[ramblinrod]

Originally Posted by brownr377

Hell in for a penny in for a pound.

An example from my world, which is more dangerous - a 16 kV ESD pulse (defined by IEC 61000-4-2) or a 2kV Surge pulse (defined by IEC 61000-4-5). If the be all and end all parameter is the voltage, the ESD pulse should be more dangerous, however, it's not. Clearly there are other things going on that need to be understood.

Can you illustrate a correlation to the discussion?

Unless you can show that a 16 Kv ESD pulse is more dangerous than a 64 kV ESD pulse, or a 2 kV surge pulse is more dangerous than a 8 kV surge pulse, your statement seems to support my claim that EEE and BMF, danger increases with voltage.

[evm1024]

Originally Posted by ramblinrod

As I have said now, soooooooo many times, before making any modification one needs to perform a vessel design and configuration and user needs and wants assessment.

SNIP %< %< %<

__________________
Rod Brandon
Sheen Marine - President
ABYC Certified

Yes we know your business model - One man shop, working out of your van or boat, has an ABYC Marine Systems Certification for 1 year.

No need to sell to us.