Constellation uses 120 Vdc likely so it will be compatible with Space Station power. Space Station power is 120 Vdc to save weight over 28 Vdc wiring.

I'm not saying that is the right thing to do, just saying that's how it is. From a human safety aspect, 120 Vdc should be no more dangerous than 120 Vac, which as you note is ubiquitous and people work around it all the time, and there are recognized rules and practices in place.

From a fire hazard aspect, 120 Vdc is a little more dangerous than 120 Vac, because if there is an arc-over just getting started, it will extinguish every time the ac line potential drops below the potential necessary to activate it, whereas with dc power, the potential is always peak, and hence the arc can activate and grow.

There are commercial spacecraft planned that use 28 Vdc power, but when docking with Station, will have a 120 Vdc to 28 Vdc converter so that Station can recharge their batteries while docked.

So if you wanted a 28 Vdc primary power spacecraft, you have to add the weight of that converter to the added wire weight of a 28 Vdc bus relative to that of a 120 Vdc bus; I suppose it starts adding up.

On the other hand, Station could just suck it up and provide 28 Vdc charging capability at the docking interface. It's not like there aren't a lot of these already scattered throughout the Station. They're called SSPCMs - Space Station Power Converter Modules (I guess) - and their entire purpose is to provide 28 Vdc power to loads that were designed to run off 28 Vdc...

It's not a requirement for compatibility with the station, there is plenty of 28VDC power in the ISS, and Constellation is not primarily concerned with the ISS anyway - it was levied entirely in the interest in saving weight on wire. Actually, the community voted against it - due to issues with safety, but primarily the availability of parts, but we were overruled by a certain now-ex-administrator.

If it's about wire weight, then why is the 200+ foot long Ares launch vehicle using 28 Vdc power, while the Orion crew module, which is slightly larger than an Apollo crew capsule, using 120 Vdc power? That doesn't make any sense at all from a wire weight point-of-view, but makes a lot of sense if you are looking at plugging into Station to recharge.

I agree that Station could provide 28 Vdc to recharge a 28 Vdc battery pack. That isn't the issue. The issue is why 120 Vdc, at all?

Well, as long as they are using a nitrogen /oxygen mix instead of 100% oxygen like Apollo 1. that hotter arc from the 120Vdc would definitely ruin the crew's day if it was 100% oxy.

I thought that the answer was given at the start of this blog, but let me repeat it for you:-

Less weight of wire for the same amount of power. In Europe we have mains at 230 volts and our house wiring is generally half the thickness of that used in US homes.....

I would guess that the voltage used in the space units you mentioned may be a function of when the units were first designed.......as I suspect that just because of a voltage change, they do not make retrospective changes to existing designs.......but that is just a guess on my part.......

What part of this is so hard to understand? Constellation is all new. Ares and Orion are paper studies. If a 200 foot long launch vehicle elects to use 28 Vdc power, but the crew capsule, just a little larger than an Apollo crew capsule, elects to use 120 Vdc power, then wire weight CANNOT possibly be the rationale.

Do you get it? The launch vehicle is much, much longer than the payload - this is not rocket science.

If the tiny crew capsule elects to use 120 Vdc, and it is the only part that docks with Station, then maybe, just maybe, they were looking at compatibility with existing Space Station power when they dock.

As other posters correctly pointed out, it's not as if Station couldn't provide 28Vdc at the docking interface; they already provide 28 Vdc to many installed loads; so the Orion compatibility with Station 120 Vdc is only a theory, but what is clear beyond a shadow of a doubt is that whatever the reason for Orion's use of 120 Vdc power, it is not wire weight savings.

If you were signed on with a proper name, we MIGHT believe you a tiny bit, but trolls are really not welcome.....nor desired...

There's a good response: ignore the presented facts and attack the person presenting them.

With which facts do have trouble?

1) That Constellation is an all new paper study - check it out - that's what the web is for.

2) That the launch vehicle, at over 200 feet, is much larger than the payload vehicle? If you have a problem with this concept, it is your credibility which is in question.

3) That Station primary power is 120 Vdc? See (1) above for your way forward.

If you had bothered to fully read and understand what I wrote, you would not have posted.

But trolls will be trolls I suppose.......they never learn!!

I could tell you that it was 65 degrees yesterday, after a long cold spell, so I hopped on my bicycle, went for a ride, which carried me by my neighbor's home, and he was out in the yard, and we stopped to talk. My neighbor works for NASA, on the Constellation and Space Station programs, while I am a support contractor. We talked about 120 vs 28 Vdc power, and other matters.

Now I could sign in and give you the above information, but it is totally unverifiable, signed in or anonymous. Instead, I presented the verifiable facts from that conversation, with my conclusion attached.

And you can only attack the messenger, totally ignoring the message, while stating that I haven't read your fallacious statements about wire weight savings, at the same time conjecturing absolutely and verifiably false statements about the Constellation program.

The part of this thread about wire weight is just opinion with no factual backup whatsoever.

I thought this was an engineering forum.

Your argument is based on false information.

1) Ares is no longer 28VDC - so much for getting info from the Internet.

2) While much of Constellation is in the "paper study" stage, the Ares I-X rocket is now being assembled at KSC. And my project - the docking system - delivers hardware at the end of this year.

So, can anyone help me with my original question?

I have picked out just a few lines to answer for you as generally I completely ignore guests as not being worth while trying to educate them. In this case I did offer some possible plausible reasons as to why a particular unit was given a particular voltage. I did not present them as known facts...:-

The part of this thread about wire weight is just opinion with no factual backup whatsoever.

Just learn simple Ohm's/Joule's Law, this will bring you to correctly understand that power distributed can be done with a much thinner cable when using a higher voltage.....thinner cables can actually weigh less!!!!

Both of these laws are absolute facts, there is no argument against their validity.

For example, if a circuit needs 1 amp at 28 volts, it will need only (after redesign for the higher voltage of course!) 0.5 amps at 56 volts, or 0.25 amps at 112 volts. The power used is still the same with each change in voltage!......smaller currents thus allow a thinner conductor to be used.

In space, every gram of weight costs real money to transport from the earth up to orbit or further... Weight is extremely important.......and REALLY expensive!

I thought this was an engineering forum.

It is, part of that is the fact that most of the people here a) sign on with a proper name and let themselves be identified in some way and b) most of them remember their simple Ohm's Law and Joule's Law from their first year Physics at High School or the equivalent......you obviously don't. For me they are both burnt into my brain and still get used from time to time.......seldom a week goes by without having used them.....

Please click on either of these links to learn more/educate yourself in simple matters electric:-

Ohm's Law

Joule's Law

I likely learned those laws before you were born. The issue isn't and never has been the validity of basic physical laws. The issue is practical. You have two subsystems that work together: the launch vehicle (Ares) and the payload (Orion). The launch vehicle is over 200 feet long, and the payload is slight larger than an Apollo space capsule: the launch vehicle is orders of magnitude larger than the payload, as dictated by the physics of climbing out of a gravitational well.

Under those circumstances, if the much larger subsystem, which uses longer runs of wire, chooses to use 28 Vdc power, while the much smaller subsystem chooses to use 120 Vdc power, then some other explanation must be sought for these decisions besides conserving mass of electrical wire. Especially if you consider the possible loads in the two subsystems: the launch vehicle will likely have to use electrical energy to direct thrust; moving the nozzles that direct that thrust is going to be a heavier electrical load than anything done in the payload capsule.

Now bhankii has recently (today) posted that Ares is changing over to 120 Vdc power, and if that is indeed the case, then it is clear that wire weight is the driving factor.

I have sent a message to the NASA colleague I mentioned in an earlier post asking to verify that, because he hadn't mentioned it when we talked on Saturday - I did not get my information off the internet as bhankii snidely implied.

If indeed the entire Constellation program has opted for 120 Vdc over 28 Vdc, then one can surmise the issue is wire weight.

However if it remains that the larger platform which uses more and heavier gauge wire in its electrical power distribution system elects to use 28 Vdc power, then the weight savings argument is out the window.

In fact, somewhat in agreement with bhankii, if wire weight and safety were both a concern, then it would seem more logical to use 120 Vdc power in the launch vehicle, and reserve 28 Vdc power distribution in the Orion spacecraft. You have a minimum weight impact that way, and get all the safety benefits where the astronauts are.

I wasn't being snide - just responding to this:

1) That Constellation is an all new paper study - check it out - that's what the web is for.

That stuff looks like it would burn.

It does if you get it directly into the eyes!!!

I imagine it would...

The use of 28V for space suits is a safety issue. No one wants to plug an astronaut into 120VDC. Actually, the spacesuit support system is the only Ares system that will get 28VDC, from a 120VDC converter.

Ares is switching to 120VDC. CEV is switching to 120VDC. Altair is already 120VDC. Hence the need for this topic.

I beg to differ on whether 120VDC is just as dangerous as 120VAC. Due to the fact that it is DC, it will cause the muscles in the current path to contract and stay contracted, while 120VAC is cyclic and will cause the muscles to clench and unclench. It will be harder to remove yourself or your co-worker from a DC circuit than an AC circuit. (I have seen someone's arm touch the positive and negative terminal of one cell of a submarine's battery and his co-worker had a hell of a time trying to get him off.) Most likely if a 120VAC circuit is touched, the body's reaction is to let go.

In actual fact, AC tries to move the muscles in both directions at 50 or 60 hz, which of course is impossible. The net result is you stay locked in one position......

DC drives the muscles involuntarily in one direction, this could of course also be the wrong direction!!! I was lucky, in the literally 100s of DC 220volt shocks that I had, none of them drove me in the wrong direction. It does not even hurt like AC does....of which I have had a few and I try not to add to the list ever again.....

Been there done that and NOT got the T-Shirt!!

If you are not writing from extensive personal experience, you are making a really serious "Faux pas"!

By the way, I am surprised that anyone even felt the voltage on a submarine battery as they are only 2.1 to 2.2 volts, but can weigh a ton each sometimes......he must have been exceedingly sensitive to voltage.....perhaps he got stuck on something else other than a battery.....?

To show just how forgiving DC at 220 volts is, I was present on board when a messmate had a very serious accident. He was searching for an earth on a main generator breaker (25,000 amps at 220VDC, a box probably 10 feet high x 6 feet wide x 4 foot deep, a bloody big bit of equipment!) on HMS Triumph (in the far east at the time), in about 1965 or so, with an uninsulated "Long red" screwdriver. This in itself was contrary to all the rules as you were supposed to insulate the whole length and just leave the blade uninsulated....

He accidentally dropped the screwdriver and it shorted across the main busbars of the generator, bending the ammeter needle in the main switchboard if I remember correctly on the min stop, but approaching from the over current direction!

The screwdriver was almost instantly glowing like a light bulb filament!!

The correct procedure would have been to close the doors and let it burn out! But he tried to pick it off with his bare hands. The ship rolled or something and he fell, putting one hand on "+" and one hand on "-"! Covering the burning screwdriver with his bare hands, getting some severe burns to both palms and getting a bad dose of "arc-eye" at the same time. He was no lying on the deck plates...

A colleague helped him get off the busbars, but he would have achieved it himself if he had been alone, so he said to me, but the colleague was simply quicker.

He had to be in the sickbay for a week or so till he recovered his sight and new skin had grown new skin on his hands. Other than that he was fine.....if it had been AC, he would have been dead as he got the shock passing through one hand, over his chest to the other hand. The single most dangerous shock path!!!

The nice thing about 28VDC is that it's really hard to kill yourself with it, whereas 120VDC is potentially very dangerous.

I recall being shocked very badly with 24 volts one cool and damp summer morning in 1966 as I hooked up jumper cables to a bulldozer's 24 volt battery. I grabbed each clamp in each hand to twist them back and forth to make good contact. As soon as I made good contact with condensate enhancing the whole affair, I learned very quickly that 24 volts is high enough to shock you. Never thought so before that.

But, you didn't kill yourself, right? You can get a nasty shock from a 9V battery if you touch it to your tongue, but you wouldn't think twice about carrying one in your pocket.

Well, I won't say that 24V will kill someone, but I can only imagine that it could. It felt pretty much as intense as being shocked by 120 VAC, which I've experienced several times.

http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html

Just a good page on the subject--K T

I'm not particularly concerned for myself, I've been shocked enough to know better. But we have some youngsters in the lab - and it's been 30 years since my last CPR class. We'll be dealing with 2kW worth of 120VDC, and I'm as concerned for our equipment as I am for our people. Each of our boxes is $300K (just for parts) - it's a bad day when we burn one up.

Did I mention our lab is a converted machine shop with a grounded metal floor?

Worked with 120VDC UPS battery systems - much more dangerous than 120VAC (greater shock risk and high A batteries could explode if short became welded into output). Company (Chloride Systems) had their own internal safety instructions. Suggest treating the lab with same safety as High Voltage Systems.www.ehs.uci.edu/programs/safety/hvoltageprog.html

Also suggest having all sources equipped with GFI breakers. Training is OK but applied learning is not infallible. Good Luck

Thanks - that's outstanding!

Bhankii, I would strongly encourage you to put down some of those semi-conductive anti-static mats. They are conductive enough to keep you from generating ESD, but are not so conductive that you are not the only voltage drop in the circuit. They are generally equivalent to about a megaohm or so. It might just save your life. I'm serious, this is a MAJOR H&S problem.

We plan to. We also plan to float our 120VDC relative to the bench, which is connected to line ground. The benches have ESD tops, but there is some metal framing (painted) on the benches as well. I'd like to eliminate that metal as a potential path.

Dear Guest.

In UPS systems [more than 50KVA] Higher DC Bus is recommended; as hi as 500V to have smaller DC currents in system; at least Emerson has this practice in its UPS Systems.

In domestic UPS mostly 12V Battery is a common practice; I suppose for most of the cars have it; but famous manufacturers use more than 48V as Lead-acid-Sealed batteries of higher AH or not suitable to be used in Built-in Battery-Tray; while smaller AH batteries at higher voltage can handle smaller currents for the same back-up time.

Any comments are welcome !

Andy,

You get a GA from me for your precise comments. For those who disagree that DC is safer than AC, do some history research on Edison's safety concerns for Tesla's work within Westinghouse on AC power distribution.

Thanks for your needed and very accurate support and knowledge.

The reasons why many other people err in this area is that there are several text books around, written by people with no personal experience or understanding of the problems....they can only speak "second hand", whereas I speak from firsthand personal experience!!

I too have been shocked by AC and DC (various voltages) and was a US Navy Nuclear Electronics Technician on 3 different class submarines plus 8 years in submarine construction where I have been at the scene of and the aftermath of electrical shocks. Both voltage types will kill you but have seen with experience that AC shocks tend to "throw" the person shocked (unless it is extremely high voltage and current, I got a story about that, but for a later time). As I have said in a previous post to this thread DC causes the muscles to contract therefore if holding on to a conductor one cannot let go.

Regards.

I hope you may have seen the problems of "Switching" devices in DC; @ High-Current or High-Voltage.

If have experience please a summary.

In AC "Zero-Crossing" avoid Hi-Flsh on Switch-Off; which damage the switches

Thanks

Bhankiii,

A number of months ago there was a forum thread on relative safety of AC vs. DC shocks. Either type of voltage causes the same muscles to contract, if applied to the same points on the body. However, when applied across the heart muscle, AC will cause ventricular fibrillation while DC will not. So, in shocks which pass across the thorax (such as left arm to right leg), there is a much greater potential for harm from AC shocks. Also, AC voltage is measured as the RMS (root mean square) of the waveform; so the peak voltage is 1.4 times larger than the stated RMS voltage. This means that AC shocks are able to penetrate deeper into the body's tissues than DC shocks of the same "stated" voltage.

Back to the question of 120VDC vs. 28VDC, I believe it all has to do with reducing the current for the same power output. This has been well-answered by earlier posts.

--JMM

GA from me for a well thought out statement of the <REAL> facts, thanks.

Bhankiii,

Regarding your question of lab safety training for the techs and engineers, may I suggest:

--Requiring all metal tools to have truly insulated handles and shanks, not just a cushion grip.

--A demonstration of what happens when you short the terminals of a battery or large power supply (complete with explosion of battery and spraying acid or fireball from vaporizing "tool").

--Perhaps an after view of some equipment that was shorted, along with the cost of this damage.

--Stories found on safety sites, regarding what has happened in real life in other facilities.

--Requiring all personnel to be current in their CPR training.

--Mandatory use of appropriate personal protective equipment, and having it readily available for use.

--Leading by example.

Regards--JMM

I'm surprised that no one has hit on an obvious reason for going with a higher voltage. The I²R losses of a 120VDC are 95% less than for identical power requirements @ 28VDC.

Example: 100W @ 28VDC takes 3.57A; traveling through 1Ω of wire produces 12.75W of lost power, manifested as heat. 100W @ 120VDC takes 0.83A; using the same 1Ω wire produces only 0.69W of loss.

That's not how you design a power distribution system: wire resistance is not the independent variable. Voltage drop between power source and load is the controlled variable, wire resistance (gauge) is chosen to provide the required voltage drop.

Andy Germany is correct that if minimizing wire weight is the goal, higher potential is the solution.

The issue is why conserve wire mass in maybe 10% of the system when you aren't trying to in the other 90%. Especially when said 10% is inhabited, and the 90% isn't.

That is the question, and if bhankii is correct that the entire system has gone to 120 Vdc, then wire mass is the reason for doing so.

I am sure though that power loss is also a concern when power is limited.....just a guess on my part though.

What effects it might have on air conditioning requirements is a further point, possibly... probably a bit of a long shot though......

No, actually the question is, "Where can I find a 120VDC safety class?"

You will probably need to plan and write one yourself.....shouldn't be too difficult....

If I find one, I will let you know.....

Yeah - it's looking that way, which is fine, until some numbnut comes along and shuts down my lab because we didn't get the "properly certified" training.

I'll try to answer your original question. There's really no difference between AC and DC in electrical safety. Various people have different opinions as to severity of injury and the effects on muscle tissue, but anything over 30 volts, AC or DC, should be considered hazardous. You should be able to use a standard occupational electrical safety training program. Any decent course will be tailored to the specific hazards of the workplace in question.

I've attached a number of resources to help determine what requirements you must meet, as well as some examples of training guides.

OSHA

http://www.osha.gov/Publications/osha3075.pdf

http://www.osha.gov/SLTC/electrical/index.html

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9868

NC State R&D Equipment Electrical Safety http://www.ncsu.edu/ehs/electrical_safety/training.htm

MIT Dept of Electrical Engineering & Computer Science

http://web.mit.edu/6.111/www/s2008/handouts/SAFETY.pdf

Electric Safety Foundation International

http://www.esfi.org/cms/workplace

There are also many companies which specialize in safety training. You might also check with nearby universities, especially the Electrical Engineering Dept. They may have such a course already designed for their own staff & students. 120VDC is not unusual in R&D settings, and is quite common in stationary batteries for electric and telecommunication substations.

Thanks!

What a brilliant point! Totally forgotten by myself and many others!! Many thanks for pointing that out!!!