Which do you consider to be the more dangerous? Or at what cutoff or ratio would one be more dangerous than the other?
High voltage High available amperage
For example, a certain voltage, like 480 volts or 690 volts has a certain level of danger to it. But at what point of available fault current would you consider the amperage to exceed the voltage as a hazard (such as the arc flash hazard)? When would you raise the system voltage/impedance (at design time) in order to decrease the available fault current to decrease the hazard?
A few years ago a friend told me about a place that had a lot of lighting that was set up on low voltage because someone thought that would be safer. And then because it was believed to be safe (perhaps they used only 12 volts ... I never found out exactly what it was) electricians would regularly work on it energized. That was, until one of them somehow caused a bolted fault that didn't get cleared, and caused the transformer powering it to eventually explode with the ensuing fire destroying a building.
Obviously voltage is unsafe to humans until the system impedance gets high enough that the human is an insignificant part of the circuit. But amperage is certainly unsafe when out of control as it can cause serious arc and heat problems.
Years ago I was considering building a house with low voltage DC wiring throughout for almost all lighting. I'm now reconsidering that, both due to my better understanding of fault conditions, as well as the problem with clearing faults in DC (no zero crossing). Combining DC with high available currents could be serious ... at least that is my thinking and concern right now.
----------- You are looking at two different conditions: High current and the danger from arcs where the source impedance is relatively low as the arc does not limit the current- the system does. As you indicate this can occur at low voltages as well as at high voltages. The other is not unsafe "voltage" but unsafe currents in the range of 100 or so ma (5% probability of fibrillation in an adult males is at about 110ma. Obviously this then becomes a problem of both voltage and contact resistance as in typical 60Hz systems, the system will not limit the current at this level. The body will be the limiting factor. At higher voltages there is often a better chance of living (not that I want to try it) - possibly with burn damage- as the body's reaction can cause a person to be thrown clear - heart stoppage may occur but spontaneous restarting often occurs and has resulted with linemen surviving (the fall might do them in.) - with fibrillation- you need help.
Depends on the scenario...high school physics classes have static electricity demonstations of lighting a fluorescent tube by holding one end in your hand and the other on a high voltage static electricity generator.... its harmless. very little amperage...all voltage.. watts are way down.
Add some amperage (millions of electrons) then you get power..and thats dangerous...the more power the more dangerous... the higher the voltage the farther it will arc.
50 amps at 12 volts will not arc very far..or have the force behind it go through your body producing severe damage at least.
50 amps at 110 volts will kill your... at 240 it will do it faster...at 480 it will cook you well done.
has a
available fault
voltage as a
That was posted on last week a great reference with the charts and numbers well worth a read.
in summary the closer you are to the line transformers the more dangerous... the heavier the service feed wire the more dangerous... the higher the rating of the transformer the more dangerous... the slower the trip time on the fault interrupt device (by tenths of a second) the more dangerous...linearly a half second interupt time way over twice as dangerous as 1/4 second interupt time.
If your theoretical bolted dead short (the power company provides that spec to you based on the transformer, feeds and interrupt they have supplied) can draw say 10,000 amps and the interupt is 1/4 of a second...and you get a path of current though that device and through your body at only 1,000 amps and the device takes 6 seconds to interupt...it will have interrupted long after your body meat has gone crispy.... and is too tough to serve with breakfast.
So its tricky and a compromise...the safer it it the more tripped breakers and this upsets customers... no tripped breakers, heavy feeds, everything oversize...and its a dry location.. no one hosing the place down.. its not real safe but not the danger it would be in a marine environment with salt water around.
< When would you raise the system
available
At the line transformers, interrupt devices and feeders...and you talk this over with the utility engineers... and in sizing the feeders from the transformers to the main distribution panel...and in selecting the main breaker to interupt as fast as possible at its max amp rating, and as instantaeously as possible on a dead short. Devices vary in their performance you need to compare them.
Then you would leave the main distribution panel from a breaker sized for the expected peak load of the panel it supplies (with diversity figured in) .
for instance if you were feeding a 500 amp sub panel .... which with diversity would never draw more than 200 amps... you would supply that panel with a 250 amp main breaker.. with as fast a trip time at 250 amps as possible.... and as fast as possible dead short interrupt. In reality at that level you are going to use whats available from the primary vendors etc. GE, Seimens, Square D etc. they will all be close but you still have choices.
all this adds impedance to the system that minimizes the arc flash hazard or explosion and starts to get anally retentive as you get way from from the main distribution panel to small and smaller panels... the primary hazard is at the main, and these line transformer and main distribution breaker and feeder sizes etc
If I were in a salt water marine environment I would have many smaller panels, low amperage not oversized on the wire...or breakers... rather than several very large ones...this cuts the feeder size and main breaker size to each sub panel and makes the entire arrangement safer.
I designed and built a shipboard system with two Cat gen sets,
250 and 137 kw and a 150 ton refrigeration system a few years ago. (blast freezing lobsters in the south pacific).. I sized the gen sets so that only one refer package could be started at a time.. a staged start... this cut the size of all the wire and panels and reduced the hazard potentials... I also stayed with 230 v 3 ph. running my cost of starter and breakers back up, but reducing the hazards dramatically.
With motor panels you have to take into account start surges as you know (locked rotor amps) and that will affect your breaker size and interupt delay on overload characteristics serving a particular motor, but should not be cumulative for all the motors ...just full load amps but not locked rotor amps... so you end up with an oversize main breaker for that sub panel since all motors will not be starting at once.. these are factors to consider...not set in stone though...the NEC addresses these issues extensively.
for instance if you have 8ea 50 amp 3ph 460v breakers in a sub panel...and these serve a mix of motors, air conditioners and say a trash compactor.. you know they will not all be starting at once so if you want maximum safe operation you would not size that panels main at 400 amps...though its very common. you would size it closer to the diveristy load.. say 200 amps max...and specify maybe 250 amps on the main breaker...with wire feeding that panel sized the same..again the NEC defines this pretty well...but the guess on diversity is largely up to the electrical engineer...most size not for maximum safely but to minimize main breaker trips from what Ive seen, that might start changing as we get litigation on the issue.
Ive never done it but I might well start using several smaller sub panels instead of one larger panel for a single group of large motors.. just to get the wire size down to each one and smaller main breakers at the sub panels to reduce flash hazards...that would be a stretch in most dry locations...it would not be in wet locations.
In the past with a more stable grid, and with smaller service drops and a single small transformer.... hazards were still somewhat contained... but as the utilities start paralleling transfomers and beefing up the service feeders to satisfy a customer ... the arc flash danger goes up by 10 to 1 or so...massively...if its ratty old equipment no originally arranged to handle that much potential you can have big problems... if its a cannery with some guy from Peru hosing the place down at night the risks are high...and way out the top if its a 480v main and subs and thats often the case.
I also see grounding that meets code but not any more than that and in many cases 5 times the bonding is required to be actually safe... bonding goes ignored by many it seems...because its not hot its almost considered peripheral by some people..
Heavy transformer lighting for stages etc presents all sorts of problems with neutrals and bonding.. I run 2 or 3x the neutral wire for those applications. and heavy grounds at multiple locations. (stage lighting people know this, most other electricians cant fathom the range of issues that can develop with these arrangements)
Location is important...if you are in moist area, or a marine salt water environment all of these issues become a lot more important as the path to ground becomes a lot more conductive and available. Personnel proximity to the equipment is a big issue...on ships for instance it can be tight... and its salt water... fault interupts and wire sizing for impedance is even more of an issue.
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voltage DC wiring
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as the
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that is my
stay with AC use the commercially available components..you dont have arc flash issues with home type systems, panels and feeder sizes to any great extent...and those are very slight on the branches...
12vac wire has to be ten times the size per watt than 120vac wire its not economical and the components are not available commonsly.. if you want transformer lighting in spots use whats available, plug it into a 110 source... those small transformers are dry and impedance protected they dont blow up.
I would rather work 480v @ 4000a than 120v @ 20a. When I check out I want my boots smoking.
High voltage High available amperage
For example, a certain voltage, like 480 volts or 690 volts has a certain level of danger to it. But at what point of available fault current would you consider the amperage to exceed the voltage as a hazard (such as the arc flash hazard)? When would you raise the system voltage/impedance (at design time) in order to decrease the available fault current to decrease the hazard?
A few years ago a friend told me about a place that had a lot of lighting that was set up on low voltage because someone thought that would be safer. And then because it was believed to be safe (perhaps they used only 12 volts ... I never found out exactly what it was) electricians would regularly work on it energized. That was, until one of them somehow caused a bolted fault that didn't get cleared, and caused the transformer powering it to eventually explode with the ensuing fire destroying a building.
Obviously voltage is unsafe to humans until the system impedance gets high enough that the human is an insignificant part of the circuit. But amperage is certainly unsafe when out of control as it can cause serious arc and heat problems.
Years ago I was considering building a house with low voltage DC wiring throughout for almost all lighting. I'm now reconsidering that, both due to my better understanding of fault conditions, as well as the problem with clearing faults in DC (no zero crossing). Combining DC with high available currents could be serious ... at least that is my thinking and concern right now.
--
----------------------------------------------------------------------------- | Phil Howard KA9WGN |
i would like to propose the theory that voltage has never killed anyone as it is only a theoretical concept. current however is the movement of electrons and does supply energy to a load (i squared r).
electronic parameters are set to accomplish a given task. then insulation/shielding/grounding is employed to minimize hazard
seems to me that this could happen at any voltage level when a breaker fails to function (or when the system is improperly designed)
you are trying to separate 2 factors that need to be considered together.
for the same amount of power loads the cost of larger copper wire and heavy duty switches may make LVDC lighting unattractive. might be nice for emergency backup lighting... personally i just use a flashlight :)
Does arc hazard depend on available fault current or available fault VA? (Pls let me know). If VA, then changing secondary voltage won't help much. Available secondary VA would be a function of txf rated VA and txf %Z (per unit impedance). So you might have to go with the increased impedance route you mentioned. Or, split loads up onto smaller txfs if that is an option. Sounds messy. I bet a lot of designers wouldn't like selecting voltage based on arc hazard ... when they want 120, they want 120 - and when they want 480, they want 480.
Re dc fault clearing. I've seen breaker catalogs / data sheets where they list the dc duty a breaker can handle. Maybe you can get what you need off the shelf & cheap. Are you going to be able to find appliances / devices that will take dc at the voltage you have chosen? Would you expect average field strengths to be (a) higher, (b) lower, or (c) don't care, in a dc-wired home?
| Does arc hazard depend on available fault current or available fault | VA? (Pls let me know). If VA, then changing secondary voltage won't | help much. Available secondary VA would be a function of txf rated VA | and txf %Z (per unit impedance). So you might have to go with the | increased impedance route you mentioned. Or, split loads up onto | smaller txfs if that is an option. Sounds messy. I bet a lot of | designers wouldn't like selecting voltage based on arc hazard ... when | they want 120, they want 120 - and when they want 480, they want 480.
All the data centers I have designed were all based on 120/240 or 208Y/120 for power source. Usually various kinds of UPS systems are involved. I was talking with a friend who has a friend who runs a very large data center. They have a pair of 500 kVA UPS systems running the place, and behind that a single transfer switch between a fault tolerant utility feed (unknown primary voltage) and a 1500 kW generator.
Technicians NOT trained in power engineering would often be working around inside the cabinets of the computer and network systems. They would be dealing with the secondary LV power. While 120 volts is not much, that 500 kVA UPS is delivering 4166 amps and potentially as much as 80 kA of fault current. That means the distribution panels will have to be rated to interrupt perhaps at least 100 kA (depending on lots of things such as actual UPS fault availability, series ratings of the whole system, etc).
My tendency is to go with smaller UPS systems on the order of 10 kVA to 30 kVA at most. Enough for 1 to 3 cabinets. But these can be had with 120 volt output and either 208Y/120 in, or 480Y/277 in. I have been thinking of starting to go with the latter. But at least with the smaller UPSes, fault currents within a data cabinet will be small.
Going from 208Y/120 to 480Y/277 raises the system impedance to 5.333 times. Fault currents would be 43.3% as much on the higher voltage feeds. I find myself wishing 600Y/346 was more common in the US as it is in Canada. The thing is, I have heard of (and under my watch happened once, but no one was hurt ... just one guy's pride lost) computer technicians shorting something out much more often than getting themselves shocked or electrocuted. So it seems to me that voltage isn't as much of an issue as amperage, as long as the voltage is within what can be safely handled through insulation (e.g. up to
600 volts perhaps).
| Re dc fault clearing. I've seen breaker catalogs / data sheets where | they list the dc duty a breaker can handle. Maybe you can get what | you need off the shelf & cheap. Are you going to be able to find | appliances / devices that will take dc at the voltage you have chosen? | Would you expect average field strengths to be (a) higher, (b) lower, | or (c) don't care, in a dc-wired home?
Incandescent lighting. The decision is 12 volts vs. 120 volts.
Ultra hard self forced caughing can stop fibrillation in many but of couse not all cases....its worth a search on google of you can remember the drill in such a situation. I found your points on fibrilation thresholds useful.
4166? Would this not be a 3phase unit @ 1400A per phase?
small.
Agreed.
Fault currents would be 43.3% as much on the higher voltage
Agreed, but, available fault VA is the same either way. Can you tell me whether arc hazard depends on fault VA or fault current?
I find myself wishing 600Y/346 was more common in the US as
Yay Canada.
The thing is, I have heard of (and under my watch
voltage
Do you think you WOULD hear about the 120V shocks that technicians might accidentally get? I wonder if they'd go unmentioned for whatever reasons. I have not gotten into the arc hazard stuff at all. So, of course I still instinctively think of lower voltages as safer, and I still think of shock hazard being way more of a threat than arcs. Maybe I'll learn otherwise.
|> High voltage |> High available amperage | | i would like to propose the theory that voltage has never killed anyone as | it is only a theoretical concept. current however is the movement of | electrons and does supply energy to a load (i squared r).
There is a real voltage at the source. For example batteries will have a specific voltage based on their type of construction. And I presume a given voltage will be present on generator windings under certain conditions. But beyond there, it's all about system impedance and what proportion of that impedance your load is (or what proportion of that impedance your body is in instances where that matters) which comes down to how the current dissipation is distributed (as r goes up, i goes down, so it's not like r can't have significant influence).
|> For example, a certain voltage, like 480 volts or 690 volts has a |> certain level of danger to it. But at what point of available fault |> current would you consider the amperage to exceed the voltage as a |> hazard (such as the arc flash hazard)? When would you raise the system |> voltage/impedance (at design time) in order to decrease the available |> fault current to decrease the hazard? |>
| electronic parameters are set to accomplish a given task. then | insulation/shielding/grounding is employed to minimize hazard
There's really a lot of technical flexibility in choosing a voltage. What influences that choice so heavily, however, is what you cannot readily change, such as the standard voltage that loads want to have. I'd rather go with lights on 277 volts at home instead of 120 volts, but the standard is 120 and that extensively influences the choices. Incandescent bulbs for 277 volts are hard to find, have less variety of wattage, and are more expensive. HID lights are what get used at that voltage for the most part. But I don't consider HID to be viable lighting for the home (I favor incandescent even over CF and LED).
|> A few years ago a friend told me about a place that had a lot of lighting |> that was set up on low voltage because someone thought that would be |> safer. And then because it was believed to be safe (perhaps they used |> only 12 volts ... I never found out exactly what it was) electricians |> would regularly work on it energized. That was, until one of them |> somehow caused a bolted fault that didn't get cleared, and caused the |> transformer powering it to eventually explode with the ensuing fire |> destroying a building. |>
| seems to me that this could happen at any voltage level when a breaker fails | to function (or when the system is improperly designed)
What came across to me was that the people working on it apparently presumed it was "safe" due to the low voltage. I think that assumption just simply cannot be allowed to be made. Still, I'd use voltages like that for special situations like swimming pool lighting. But there you can at least put each light on its own transformer to be extreme.
|> Obviously voltage is unsafe to humans until the system impedance gets |> high enough that the human is an insignificant part of the circuit. |> But amperage is certainly unsafe when out of control as it can cause |> serious arc and heat problems. |>
| you are trying to separate 2 factors that need to be considered together.
Both need to be considered dangerous in their own way. Circumstances do vary, and in some places high voltage works because the voltage hazard is less likely to be realized (where people cannot touch live wires). I think far too few people realize that high currents are just as dangerous as high voltage. It's just a matter of what is the best balance in some particular situation, and how close to that you can get to given limits like budgets and available equipment.
How often do you see signs saying "Danger High Amperage".
|> Years ago I was considering building a house with low voltage DC wiring |> throughout for almost all lighting. I'm now reconsidering that, both |> due to my better understanding of fault conditions, as well as the |> problem with clearing faults in DC (no zero crossing). Combining DC |> with high available currents could be serious ... at least that is my |> thinking and concern right now. |>
| | for the same amount of power loads the cost of larger copper wire and heavy | duty switches may make LVDC lighting unattractive. might be nice for | emergency backup lighting... personally i just use a flashlight :)
And Edison was distributing DC around the streets at utilization voltage. At least there weren't so many loads in that day that he would have needed the enormous amperage that would have to be present to supply the same few city blocks today at 120/240 utilization.
I am looking at using small low power LED lights for emergency lighting. But the cost of a power supply for each one becomes the big issue.
On Thu, 14 Oct 2004 00:48:51 GMT Phil Scott wrote:
|> For example, a certain voltage, like 480 volts or 600 volts | has a |> certain level of danger to it. But at what point of | available fault |> current would you consider the amperage to exceed the | voltage as a |> hazard (such as the arc flash hazard)? | | That was posted on last week a great reference with the charts | and numbers well worth a read.
I read it. It set me to thinking about this again, and with even more insight than I had before.
| in summary the closer you are to the line transformers the | more dangerous... the heavier the service feed wire the more | dangerous... the higher the rating of the transformer the more | dangerous... the slower the trip time on the fault interrupt | device (by tenths of a second) the more dangerous...linearly | a half second interupt time way over twice as dangerous as 1/4 | second interupt time. | | If your theoretical bolted dead short (the power company | provides that spec to you based on the transformer, feeds and | interrupt they have supplied) can draw say 10,000 amps and | the interupt is 1/4 of a second...and you get a path of | current though that device and through your body at only 1,000 | amps and the device takes 6 seconds to interupt...it will have | interrupted long after your body meat has gone crispy.... and | is too tough to serve with breakfast.
Aha ... a way to cook dinner even faster than a microwave :-)
| So its tricky and a compromise...the safer it it the more | tripped breakers and this upsets customers... no tripped | breakers, heavy feeds, everything oversize...and its a dry | location.. no one hosing the place down.. its not real safe | but not the danger it would be in a marine environment with | salt water around.
One thing we probably need to address is why we have so many tripped breakers. And that comes down to, at least in the cases I can think of, the startup current surges. That happens even on an incandescent light bulb. Power companies do want customers with big motors to use slow start controls to keep the surge of current down so it doesn't blink lights elsewhere. But this can also allow the use of overcurrent protection that can respond faster and at lower currents. It can make things safer that way.
| < When would you raise the system |> voltage/impedance (at design time) in order to decrease the | available |> fault current to decrease the hazard? | | At the line transformers, interrupt devices and feeders...and | you talk this over with the utility engineers... and in sizing | the feeders from the transformers to the main distribution | panel...and in selecting the main breaker to interupt as fast | as possible at its max amp rating, and as instantaeously as | possible on a dead short. Devices vary in their performance | you need to compare them. | | Then you would leave the main distribution panel from a | breaker sized for the expected peak load of the panel it | supplies (with diversity figured in) . | | for instance if you were feeding a 500 amp sub panel .... | which with diversity would never draw more than 200 amps... | you would supply that panel with a 250 amp main breaker.. with | as fast a trip time at 250 amps as possible.... and as fast | as possible dead short interrupt. In reality at that level you | are going to use whats available from the primary vendors etc. | GE, Seimens, Square D etc. they will all be close but you | still have choices. | | | | all this adds impedance to the system that minimizes the arc | flash hazard or explosion and starts to get anally retentive | as you get way from from the main distribution panel to small | and smaller panels... the primary hazard is at the main, and | these line transformer and main distribution breaker and | feeder sizes etc
In the case of computer rooms, the distance from the main and the transformer may not be very much. So even a 20 amp branch circuit could pull some serious amperage for an instant if the building is served by a big transformer (say 750 kVA). If the breakers fail, the branch circuit itself could well be vaporized while the guy who touched a screwdriver to the wrong spot gets his face cooked. I'd want fast trip breakers, but also current limiting that won't prevent that fast trip. The article previously posted shows how this kind of choice is not easy to make because time is perhaps an even larger factor tha current.
| I designed and built a shipboard system with two Cat gen sets, | 250 and 137 kw and a 150 ton refrigeration system a few years | ago. (blast freezing lobsters in the south pacific).. I sized | the gen sets so that only one refer package could be started | at a time.. a staged start... this cut the size of all the | wire and panels and reduced the hazard potentials... I also | stayed with 230 v 3 ph. running my cost of starter and | breakers back up, but reducing the hazards dramatically.
You'd have 400 volts between phases in that, right?
| for instance if you have 8ea 50 amp 3ph 460v breakers in a sub | panel...and these serve a mix of motors, air conditioners and | say a trash compactor.. you know they will not all be starting | at once so if you want maximum safe operation you would not | size that panels main at 400 amps...though its very common. | you would size it closer to the diveristy load.. say 200 amps | max...and specify maybe 250 amps on the main breaker...with | wire feeding that panel sized the same..again the NEC defines | this pretty well...but the guess on diversity is largely up to | the electrical engineer...most size not for maximum safely but | to minimize main breaker trips from what Ive seen, that might | start changing as we get litigation on the issue.
And someone might also get upset if they do happen to try to start too many loads at once and trip the breaker. You're damned if you do and you're damned if you don't.
| In the past with a more stable grid, and with smaller service | drops and a single small transformer.... hazards were still | somewhat contained... but as the utilities start paralleling | transfomers and beefing up the service feeders to satisfy a | customer ... the arc flash danger goes up by 10 to 1 or | so...massively...if its ratty old equipment no originally | arranged to handle that much potential you can have big | problems... if its a cannery with some guy from Peru hosing | the place down at night the risks are high...and way out the | top if its a 480v main and subs and thats often the case.
Or like network services in many downtown areas where they have added more parallel transformers, while many buildings still have lower rated breakers or fuses. Imagine the utility beefing up their network to 6000 amps, while many customers have 10000 AIC rated 1950's era breakers.
| I also see grounding that meets code but not any more than | that and in many cases 5 times the bonding is required to be | actually safe... bonding goes ignored by many it | seems...because its not hot its almost considered peripheral | by some people..
And it can fail without being noticed. Redundancy would be good, too.
| stay with AC use the commercially available components..you | dont have arc flash issues with home type systems, panels and | feeder sizes to any great extent...and those are very slight | on the branches...
Data centers are, however, a different issue. And some are run on DC (telco ones tend to be). That 48 VDC stuff can deliver a load of amps.
| 12vac wire has to be ten times the size per watt than 120vac | wire its not economical and the components are not available | commonsly.. if you want transformer lighting in spots use | whats available, plug it into a 110 source... those small | transformers are dry and impedance protected they dont blow | up.
I'll probably leave the low voltage lights to places like pools and such.
What your remark here tells me is that with heavy potentials involved we may need to think about standing 120 volt panels off alone and at a distance from the main service and on lighter wire... and even think about running some 14 gage wire on circuits that serve single devices or controllers in close proximity to the primary service..
Maybe the NEC will require signs as transformers get paralleled onto antiquated existing building services. Floor in the area painted red,,,with conductive paint so the victim won't suffer.
"You are at the buildings primary electrical service location... ground fault amperage is 10,000 times higher here, on the same voltage than more distant from these feeders on lighter conductors.... Utility company ground fault potential here is EXPLOSIVE... rated _________ killowatts" Utility company fills in the blank to match thier latest grid and transformer upgrades in the area
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correct. Lawsuits will determine the eventual choices. Starting them manually isnt typically a problem since LRA is only for a second or two... it takes that long to reach the next breaker. But in the event of a power failure, and power coming back on...you would trip a lot of breakers... so... the people just have to go reset them one at a time...that works. I will going a little lighter in the future...more but smaller sub panels... lighter wire to max divirsity load especially in high risk locations.
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Nothing will be done until Underwriters Lab gets on it...along with the NEC...and that might happen in a few years if there are a few more bad accidents or one spectacular one .... and an attorney wins big on this combination of oversights.
It seems the utility could be held reponsiible as most of thier customers are not qualified and registered EE's. .. the reasonable man priniciple dictates that the utility has the responsibliity to advise its customers and see to it that the system they are feeding is suitable... EE's will in order to protect thier interests, liability etc will have to start paying attention to these issues. So will building inspectors.
be
peripheral
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and
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Im not a telco guy... what size wire are we talking at the vdc source? typical range for what you do.
but their capacity is rated in ampere-hours not volt-hours
And
this does not prove that voltage exists as anything other then a convenient theoretical concept.
how about phosphorescent strips? no heat, no moving parts
high voltage works because it is the most efficient way to accomplish a given task. safety precautions are then effected to minimize the hazard. some guy didnt just come along one day and say "wow, lookie at those wires up on those poles. i think it would be a great way to run high voltage lines"
I
any combination of factors that manipulates energy can be "dangerous". without voltage there is no current... just a bunch of unemployed electrons hanging around waitihg for the government check to come.
It's just a matter of what is the best balance in some
i have one that says "danger 1,000,000 ohms"
i guess i could make one that says "danger: red herrings" :)
the install cost of LED lighting is still outrageous when compared with incandescent. the power savings can be 90% using LEDs.
the estimated (predicted) life of a LED FAA tower light flasher (obstruction becon) is 7 years (if operated only in the nightime). unless it gets hit by lightning.
i think the true answer to the question posed by the origional poster is: both or neither equally.
Another interesting issue is that the last time I was setting up a computer center, it was less money to buy a 700 VA individual UPS for each system than to buy the big system and all the distro to go with it. This removes a single point of failure. Since the particular application was a linux server farm, any one machine dropping out was not a big deal. Got a 1400 VA UPS to operate the KVM switch, hubs and the common monitor... and a small fluorescent desk lamp.
|> would be dealing with the secondary LV power. While 120 volts is | not |> much, that 500 kVA UPS is delivering 4166 amps and potentially as | much | | 4166? Would this not be a 3phase unit @ 1400A per phase?
You're right. I was using a new program to get the numbers and forgot to put the /3 in there. OK, so it's not quite as dangerous as previously thought.
| Fault currents would be 43.3% as much on the higher voltage |> feeds. | | Agreed, but, available fault VA is the same either way. Can you tell | me whether arc hazard depends on fault VA or fault current?
I think the answer would be, it depends. The higher voltage can draw a longer arc for sure.
| I find myself wishing 600Y/346 was more common in the US as |> it is in Canada. | | Yay Canada.
I've seen 600Y/346 in one place when I was in Texas. I've seen it nowhere else in the US. My grandfather used to work on 1000Y/577 but that was at 25 Hz.
| The thing is, I have heard of (and under my watch |> happened once, but no one was hurt ... just one guy's pride lost) |> computer technicians shorting something out much more often than |> getting themselves shocked or electrocuted. So it seems to me that |> voltage isn't as much of an issue as amperage, as long as the | voltage |> is within what can be safely handled through insulation (e.g. up to |> 600 volts perhaps). | | Do you think you WOULD hear about the 120V shocks that technicians | might accidentally get? I wonder if they'd go unmentioned for | whatever reasons. I have not gotten into the arc hazard stuff at all. | So, of course I still instinctively think of lower voltages as safer, | and I still think of shock hazard being way more of a threat than | arcs. Maybe I'll learn otherwise.
You may have a point there. Such things could go unreported since no one dies or goes to the hospital, and no equipment is damaged.
| Another interesting issue is that the last time I was setting up a computer | center, it was less money to buy a 700 VA individual UPS for each system | than to buy the big system and all the distro to go with it. This removes a | single point of failure. Since the particular application was a linux server | farm, any one machine dropping out was not a big deal. Got a 1400 VA | UPS to operate the KVM switch, hubs and the common monitor... and a | small fluorescent desk lamp.
There are some advantages to smaller. Even the servers cost less these days when you have many smaller ones to do the work of a few big ones. But putting several small UPSes in a rack cabinet is not a good fit. Maybe 2 or 3 small 2U versions designed for the rack. But you can't get these small ones in 277 volt or three phase input. And they often have as much harmonic problems as the PC itself. At least the big ones try to remove harmonics some.
|> You'd have 400 volts between phases in that, right? | | 230 the way the gen set was wired.
But if that was 230 volts 3 phase, and each phase was on a common neutral, you have 400 volts between phases. Was it wires wye/star or delta? Or maybe 230Y/133?
| It seems the utility could be held reponsiible as most of | thier customers are not qualified and registered EE's. .. the | reasonable man priniciple dictates that the utility has the | responsibliity to advise its customers and see to it that the | system they are feeding is suitable... EE's will in order to | protect thier interests, liability etc will have to start | paying attention to these issues. So will building inspectors.
If the utility hooks me up and says available fault current is 12 kA and I have breakers rated 25 kA, I'm OK and they are OK. But if years down the road they upgrade the network and there is then 36 kA of available fault current, and an accident happens, and the 25 kA breaker blows up or melts a contact closed, it's sure not the breaker manufacturer at fault.
|> Data centers are, however, a different issue. And some are |> run on DC (telco ones tend to be). That 48 VDC stuff can |> deliver a load of amps. | | Im not a telco guy... what size wire are we talking at the | vdc source? | typical range for what you do.
The one I saw had some 5 inch wide bus bars that looked to be maybe 3/8 inch thick. I don't know if they were aluminum or plated copper. The guy that showed me around said there were 960 single battery cells that weighed in at 300 pounds each. I saw a few of them and volume wise each was about the size of 4 to 6 car batteries. I presume 2 volts each so that should be 24 in series, so there would be 40 in parallel. I don't know if everything was all parallel or if there were separate isolated circuits. But I'd imagine all of them in parallel could do some serious vaporizing.
I've never had to work on DC systems, and I don't think I want to.
| this does not prove that voltage exists as anything other then a convenient | theoretical concept.
Semiconductors and insulation (including air) have specific breakdown voltage levels. I don't think electrical pressure is just pure theory.
| how about phosphorescent strips? no heat, no moving parts
Powered how? The previously powered lighting? I might need to run down a hallway that had been dark all night, and would have lighted up automatically from motion detectors.
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