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Old 16-02-2019, 14:48   #1
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Comparative Safety: 12v v 24v v 48

Quote:
Originally Posted by noelex 77 View Post
I am not a fan of the 48v option. It is going to be difficult to find equipment to run on this voltage, and even small components such as many marine fuse holders are not suitable (often the limit is 32v).

This voltage also starts to present a shock hazzard. Over 50v can be fatal in the wrong circumstances. A tired salt water wet owner who has just come off watch to find the electrical fault in a bouncing healing yacht is the wrong circumstance . A nasty shock is more likely than a fatal result, but both of these possibilities mean the electrical ststem needs to more complex with shrouds, covers etc. A 48v battery system will be over 50v when charging. I am sure it could be made to work, and be safe, but not without significant effort. I think there would have to be a compelling reason, such as the installation of 48v electric main engine, to justify this.

However, I think the suggestion that all higher voltages are inherently dangerous and increase the fire risk is also incorrect. A 24v house system does not decrease safety. I would argue that it is safer than a 12v system.

Letís take a practical example recently reported on Cruisers Forum:

ANL Fuse/holder getting real hot

This involved an ANL fuse holder overheating.

At 12v the fuse holder was carrying around 400A. If the boat had a 24V house bank then the fuse holder would have been carrying half this amount, ie around 200A. Does anyone think the 24v system is less safe?

Remember the power (heat) generated by the resistance in the fuse holder is dependent on the current squared. Reducing the current has a huge impact on the power (heat) that the fuse holder has to dissipate. A cooler fuse holder is a safer fuse holder.
After receiving so many objections to my stated position, I performed a variety of circuit analysis with a variety of loads, contact impedances, and supply voltages.

It turns out that it is much more complicated, than I, or any of the objectors were giving credit.

There is a real balancing act between P = I^2R and P = E^2/R going on at the same time.

One has to perform a proper circuit analysis, considering the reduction in circuit current caused by the increased impedance, and then the resultant voltage drop across, and power dissipated by, the contact resistance.

For example, for a 100W load, here are the actual results...

Supply E 12 Vdc 48 Vdc
R Hi Z P hi Z (W) P hi Z (W)
0.001 0.07 0.004
0.01 0.6 0.04
0.1 6 0.4
1 24 4
10 11 21
100 1.4 15

For a 1000W load

Supply E 12 Vdc 48 Vdc
R Hi Z P hi Z (W) P hi Z (W)
0.001 6.9 0.4
0.01 60.7 4
0.1 242 40
1 110 211
10 14 152
100 1.4 22

For a 10000W load

E supply 12 Vdc 48 Vdc
R Hi Z P hi Z (W) P hi Z (W)
0.001 607 43
0.01 2418 399
0.1 1100 2110
1 139 1521
10 14 220
100 1.4 23

So while the relationship between contact resistance, supply voltage, and power dissipation is not as clear as I had posted, neither is it as clear as some detractors posted.

All this aside, when considering a higher supply voltage:

1. We still have the greater risk of shock.

2. We still have the greater risk of dangerous secondary reaction to shock. (e.g. pulling back quickly into rotating machinery).

3. We still have the greater risk of arcing and contact damage.

4. We still have the more expensive components.

So my position still is unless absolutely necessary, stay with a lower DC system voltage.
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Old 17-02-2019, 08:50   #2
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Re: 48V as main DC voltage on boat - anyone?

Quote:
Originally Posted by noelex 77 View Post
At 12v the fuse holder was carrying around 400A. If the boat had a 24V house bank then the fuse holder would have been carrying half this amount, ie around 200A. Does anyone think the 24v system is less safe?

Remember the power (heat) generated by the resistance in the fuse holder is dependent on the current squared. Reducing the current has a huge impact on the power (heat) that the fuse holder has to dissipate. A cooler fuse holder is a safer fuse holder.
This is exactly the fundamental flaw in (fairly common) thinking I have been attempting to address.

P = E x I

If one doubles voltage and halves current, the power is exactly the same.

For example, a 12 Vdc, 1000W heater element, will draw a current of 83.3R A.

That heater will dissipate 1000W, mostly in heat.

If one replaces that with a 24 Vdc, 1000W heater element, it will draw a current of 41.6R A.

The heater will dissipate 1000 W, mostly in heat.

The power consumption and heat dissipation is identical.

The 24 Vdc system is not "safer" than the 12 Vdc system.

In fact, it may be "less safe", for the other reasons previously stated.
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Old 17-02-2019, 09:39   #3
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Re: 48V as main DC voltage on boat - anyone?

Right but aside from shock potential, the I^2 R losses in a poor connection at 48V are a lot less than those in a 12V connection for a given load.

Say you have a 1000 watt load. At 12V that is 83.3 amps. If you have some flaw in a connection somewhere that has 0.05 ohm resistance, the connection is going to dissipate 83.3^2 x 0.05 = 347 watts. This might be enough to catch surrounding materials on fire.

Now with the same 1000 watt load in a 48V system, you would be drawing just 20.8 amps. The poor 0.05 ohm connection would then dissipate 20.8^2 x 0.05 = 21.6 watts, unlikely enough to set surrounding materials on fire.

edit: Well, not quite so simple as the poor connection in the 12V system would have a 4V drop across it, thus reducing the voltage at the load to 8V. Depending on what type of load this is, the current may drop and the connection would not actually dissipate the full 347 watts. It would still be more than the 48V system though. I think the calculation for a pure resistive 12V 1000 watt load being fed through a flawed connection where the load saw just 8V would mean the connection would dissipate 191 watts.
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Old 17-02-2019, 09:42   #4
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Re: 48V as main DC voltage on boat - anyone?

Quote:
Originally Posted by ramblinrod View Post
This is exactly the fundamental flaw in (fairly common) thinking I have been attempting to address.

P = E x I

If one doubles voltage and halves current, the power is exactly the same.

For example, a 12 Vdc, 1000W heater element, will draw a current of 83.3R A.

That heater will dissipate 1000W, mostly in heat.

If one replaces that with a 24 Vdc, 1000W heater element, it will draw a current of 41.6R A.

The heater will dissipate 1000 W, mostly in heat.

The power consumption and heat dissipation is identical.

The 24 Vdc system is not "safer" than the 12 Vdc system.

In fact, it may be "less safe", for the other reasons previously stated.

Well, at least we are making progress. Rod had come to understand that a 24 volt system may not be less safe.

May not is a long way from anything not 12 volts is too unsafe to have on a boat.

As I say progress.

Back to the example given. It is clear that heater is designed to make heat and is just doing its job as designed.

This example just shows electricity at work in a non faulted circuit.

Go back and read post #154 where you can see a 1000 watt windless (not a heater...) in a faulted system. Oh, heck. I'll quote it for you.

Quote:
Originally Posted by faulted-system
Take the case of a 1000 watt windless.

Let's say that the solenoid contact has a 0.01 ohm resistance (typical for a car type solenoid) and that at 12 volts we get 83.33 amps current and
at 24 volts we get 41.666 amps.

The voltage drop in the 12 volt system at 83.33 amps across the 0.01 ohms is (83.33 * 0.01) 0.833 volts. And the power dissipated in that resistance is (0.833v * 83.33A) 69 watts.

The voltage drop in the 24 volt system at 41.66 amps across the 0.01 ohms is (41.66 * 0.01) 0.4166 volts. And the power dissipated in that resistance is (0.4166v * 41.66A) 17 watts.

It is extremely well known that resistive electrical losses decrease by the square with increasing voltage. Keep in mind P=I^2*R

It should be obvious that a 48 volts system would only need 20.833 amps for that same 1 KW windless and that the power loss in the solenoid would be (20.833 * 20.833 * 0.01) 4.3 watts.

Now tell us again which voltage is more likely to cause a fire EEE?
As you can see it is the current squared through the fault generated resistance that set the power dissipated in the fault. And thus the heat generated in the fault.

P = I^2 * R

I've attached a photo of my small soldering iron. Please note that it has an 18 Watt heating element and generated a 725 degree F tip.

The point here is that a faulty contact that dissipates 18 watts can reach 725 F.

And at 12 volts we need only pass 1.5 amps through the faulty contact. As Rod noted the 1000 watt heater passes 83 amps at 12 volts. Much, much more current than what is needed to start a fire.

I think that I understand much better now. Electricity can start fires and is dangerous. Heck even at 3.2 volts we need only 5.6 amps to cause that fault to heat up to 700 degrees.

(start tongue in cheek mode - easily offended please stop reading)

That does it - no more dangerous electricity on my boat. I'm going to use kerosene on my boat so as to avoid the risk of an electrical fire. (remove tongue in cheek)
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Old 17-02-2019, 15:23   #5
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Re: 48V as main DC voltage on boat - anyone?

Well, as I indicated in post # 169 (unfortunately the table format didn't post well), the calculation for the effect of a high Z connection, is not as simple as everyone (including myself before that exercise) was claiming.

One must perform a proper circuit analysis.

This is a simple series circuit having a power supply (12 or 48 Vdc) with a 1000 W load (corresponding to the supply voltage), and calculating R total (RT) which is the sum of RL and Rcontact, to identify circuit current (I = E / RT).

Then one can calculate the power consumed by R contact, using any power formula they desire, P=E*I, P=I^2*R or P=E^2/R.

The results will be the same.

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 W

So the 12 Vdc system has higher power dissipated across a low impedance connection, but lower power dissipated across a high impedance connection.

Conversely, the 48 Vdc system has lower power dissipated across a low impedance connection, but higher power dissipated across a high impedance connection.
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Old 17-02-2019, 16:41   #6
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Re: 48V as main DC voltage on boat - anyone?

Quote:
Originally Posted by ramblinrod View Post
Well, as I indicated in post # 169 (unfortunately the table format didn't post well), the calculation for the effect of a high Z connection, is not as simple as everyone (including myself before that exercise) was claiming.

One must perform a proper circuit analysis.

SNIP.
Rod has correctly described HRC faults (High Resistance Circuit) and also noted that proper circuit analysis needs to be done.

However in his efforts to prove (wait, I'm not sure what he is trying to prove - no matter) he has not taken into account time.

As calculated with a faulty connection with a resistance of 10 ohms he notes that at 12 volts we end up with a power dissipation 14 watts.

And at 48 volts we end up with a power dissipation of 152 watts through that same 10 ohm faulty connection.

As you recall I stated that we have an incorrect circuit analysis because we did not take into account time. How so you might ask.

Well it is simple and yet quite complex.

The answer lies in the difference between power and energy. Quickly (stolen from wikipedia):

The terms power and energy are frequently confused. Power is the rate of delivery of energy. Power is work performed per unit of time. Energy is the work performed (over a period of time).

As an example we might have a 4 KW radar but it does not consume 4 KWH when in operation. This is because the peak power is 4 KW but the duty cycle (i.e. time) is very low.

Back to our 12 volt and 48 volt examples.

In the case of the 48 volt 10 ohm HRC fault the power dissipation is 152 watts which creates a very high temperature in a very short time. This high temp is more likely to arc, spark and burn itself into an open circuit is a very short time.

In the case of the 12 volt 10 ohm HRC fault the power dissipation is 14 watts which creates a high temperature but because of the much lower energy does not "blow" itself to bits in short order and thus has time, in some cases a lot of time to catch your boat on fire.

All this really begs the question - is a faulty 12 volt system safer than a faulty 48 volt system? I answer hardly.
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Old 17-02-2019, 18:26   #7
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Re: 48V as main DC voltage on boat - anyone?

Quote:
Originally Posted by evm1024 View Post
Rod has correctly described HRC faults (High Resistance Circuit) and also noted that proper circuit analysis needs to be done.

However in his efforts to prove (wait, I'm not sure what he is trying to prove - no matter) he has not taken into account time.

As calculated with a faulty connection with a resistance of 10 ohms he notes that at 12 volts we end up with a power dissipation 14 watts.

And at 48 volts we end up with a power dissipation of 152 watts through that same 10 ohm faulty connection.

As you recall I stated that we have an incorrect circuit analysis because we did not take into account time. How so you might ask.

Well it is simple and yet quite complex.

The answer lies in the difference between power and energy. Quickly (stolen from wikipedia):

The terms power and energy are frequently confused. Power is the rate of delivery of energy. Power is work performed per unit of time. Energy is the work performed (over a period of time).

As an example we might have a 4 KW radar but it does not consume 4 KWH when in operation. This is because the peak power is 4 KW but the duty cycle (i.e. time) is very low.

Back to our 12 volt and 48 volt examples.

In the case of the 48 volt 10 ohm HRC fault the power dissipation is 152 watts which creates a very high temperature in a very short time. This high temp is more likely to arc, spark and burn itself into an open circuit is a very short time.

In the case of the 12 volt 10 ohm HRC fault the power dissipation is 14 watts which creates a high temperature but because of the much lower energy does not "blow" itself to bits in short order and thus has time, in some cases a lot of time to catch your boat on fire.

All this really begs the question - is a faulty 12 volt system safer than a faulty 48 volt system? I answer hardly.
Or, EVM1024 could just admit he made a mistake and get over it. ;-)
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Old 17-02-2019, 18:48   #8
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Re: 48V as main DC voltage on boat - anyone?

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Originally Posted by ramblinrod View Post
Or, EVM1024 could just admit he made a mistake and get over it. ;-)
I make a lot of mistakes - to which mistake are you referring to?
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Old 17-02-2019, 23:56   #9
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Comparative Safety: 12v v 24v v 48v

A side discussion from 48V as main DC voltage on boat - anyone? concerning the safety of different DC voltage system turned into a hijack of the original thread. This is the new place for that discussion.


Please be polite and respectful to each other, or there will be consequences.
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Old 18-02-2019, 05:45   #10
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Re: Comparative Safety: 12v v 24v v 48

Most Telcos use 48V for standby power but they don't need inverters to power the gear. Almost all their gear runs off 48V directly. So their use case is different than a boat. Their loads are steady and they have automatic monitoring of the system that communicates back to a central automated attendant. But 48V battery systems are not more dangerous than 12V. If they were the telcos could never get a permit to install hundreds of thousands of them in buildings all over the world.

Also, telco equipment is designed to draw DC current from the batteries. The biggest load on a boat is often the AC inverter. Those do not draw steady current from the battery. Each 1/2 cycle of 50 or 60Hz (8-10 milliseconds) draws a half cycle of AC from the battery. So the peak current is much higher than the "DC" current. This is a good exercise for a first year engineering student. Calculate the RMS current in a DC cable feeding a high efficiency inverter. The RMS is higher than the DC current. It is the RMS current that should be used to estimate heating in wires and connections. A 48V inverter can be much better than a 12V in both efficiency and RMS to DC ratio. Also, don't forget abut wiring inductance. Since 48V systems draw less current (75% less) they should radiate 75% less noise from switching currents. In dB language a 48V DC system should produce 12dB less interference noise which is about an order of magnitude.

So overall the engineering favors 48V. But the economics and market place don't. As I know about engineering and not so much about markets I leave that to someone else.

But all in all 48V is "better" from the engineering side in every way you analyze it. There is a slight increase in shock hazard but not enough to change the equation.
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Old 18-02-2019, 06:28   #11
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Re: Comparative Safety: 12v v 24v v 48

Automotive engineers began discussing higher-voltage systems in the 1990's, when they first noticed the difficulties associated with thick wiring bundles. Those problems grew as they added audio, video, cellular, and other components. Predictions were that the first 42-volt systems would be available as early as 2002 or 2003. So much for that prediction.
The industry wide standard of 42-volts was selected by an automaker consortium led by the Massachusetts Institute of Technology (MIT, Cambridge, MA). The Consortium on Advanced Automotive Electrical/Electronic Systems and Components, which operated under the auspices of MIT’s Laboratory for Electromagnetic and Electronic Systems, was working to resolve challenges surrounding 42-volt systems. 42 volts was the threshold, because anything higher presents safety concerns. Because 50 volts can stop a human heart, anything higher than that requires special safety systems to prevent contact with wiring. In addition, any voltage above 60 needs more heavily insulated wires and connectors that would add weight.

The following article lays out some of the engineering challenges posed by higher voltages:

“The Challenges and Opportunities of 42 Volt Systems” (2004)
https://www.eetimes.com/document.asp?doc_id=1272660#
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Old 18-02-2019, 10:56   #12
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Comparative Safety: 12v v 24v v 48

It seems logical to use 42V so that max charging voltage for lead acid does not exceed 50V.

Death due to heart stoppage from a 50V DC source must be exceedingly rare.

The greater danger of DC systems is burns from jewelry getting hot. Metal or carbon ings, necklaces and watch bands are verboten in DC repair work.
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Old 18-02-2019, 11:43   #13
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Re: Comparative Safety: 12v v 24v v 48

Like others I have worked in telecom where various DC supplies are used, usually 48 volt positive ground for switching systems and 24 volt negative ground for cellular sites. I have worked with 6v and 12v automotive and marine systems and 32 volt wind power systems.


It is my view that there is no "safe" voltage below which DC is harmless. The widely used 50 volt threshold is a useful stake in the ground but it does not mean that all voltages below 50 volts are equally harmless, nor that all voltages above 50 volts are equally harmful.


Shocks and burns are a hazard in any power system. I got a nasty burn once from a short on a 4.5 volt system. Whether an electric shock is painful, incapacitating, or deadly depends on circumstances as much as on voltage. I would not consider electric shock risk to be a serious hazard in a 48 VDC system although that's no excuse for sloppy procedures.


In DC power systems the main risk is fire from either a short circuit or series resistance at a poor connection. Fuses and breakers are effective protection for short circuits. Series resistance is more problematic. In general, the recreational marine market doesn't offer products that are well designed for higher currents (over 100 amps). The gold standard for these connections is a two-hole terminal lug crimped onto the wire with a hydraulic crimper, and then attached to the bus or utilization equipment or whatever using two bolts with lockwashers.



https://www.tessco.com/product/42198


With such connections, the wire does not tighten or loosen the bolt with movement or vibration.


Using lugs like that it's not unusual to run 750 mcm cable at 400 amps continuous, and they work reliably. An infra-red scan of the terminals is part of preventative maintenance, and in some cases the bolts are re-torqued annually.


The problem in the marine environment is that equipment that operates at these sort of power levels -- like large inverter/chargers, high capacity alternators, bow thrusters, and the fusing and switching equipment associated with them -- don't have provisions for two-bolt lugs, and so the best you can do is be careful about torque and try to restrain the cable so it doesn't move. In some cases, the design of the equipment and installation make periodic inspection impossible without disturbing the connection and running the risk of creating the problem you're trying to prevent. Inverter-chargers with back-mounted battery connections are especially bad in this area.


In practice that is difficult to do on a 400 amp circuit on a boat. The space isn't quite there. So if you're going to run a big bow thruster, or a big inverter/charger, you're better off IMO with the higher voltage. The balance of harms works in your favor because the fire risk from loose (high-resistance) connection is reduced far more than the slight increase in shock risk.


Equipment for recreational marine use comes either in 12v only, or in a choice of 12 or 24v, or in some cases the equipment adapts to any voltage within a 10-30 volt range. All that means that there are really only two plausible choices, 12v and 24v, for marine use on recreational vessels. Of these, I believe 24v is safer.


As an aside, there were 120 volt DC systems in reasonably widespread use in the early part of the 20th century. The advantage being that they can be battery powered without an inverter (well, a rotary converter back then), and in many cases equipment designed for AC, particularly incandescent lights, cooking and heating equipment, and household motors, would work without modification or with minimal modification. Contact erosion on switches was the main practical problem with these systems.
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Old 18-02-2019, 11:53   #14
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Re: 48V as main DC voltage on boat - anyone?

I have made no such statement as you claimed...

Quote:
Originally Posted by evm1024 View Post

…"anything not 12 volts is too unsafe to have on a boat".
My basic general premise is that complexity and danger increases with electrical system voltage, so it is wise to use the lowest electrical system voltage practical.

For most boats rec boats in the 30 to 50 ft range (except those with large electrical propulsion systems) that is 12 Vdc.
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Old 18-02-2019, 12:00   #15
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Re: Comparative Safety: 12v v 24v v 48

I remind everyone that this thread is a Hot Topic. We've already had to start editing posts. Keep it polite and respectful, please.
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