|> 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
|> 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
| 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
I'll probably leave the low voltage lights to places like pools
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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