Unnamed Administration sources reported that Charles Sprickman said:
Additionally I wonder why non-conductive tools wouldn't be the norm in an environment where there's an "open" power grid? :)
"Fools rush in where....." It was in years past. I recall how excited the power room guys were over TI's first all-plastic watches. They simply did not ever wear same until then. -- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433
A few points: Below 50 volts, anyone can do the wiring. No licensed electrician is needed im most jurisdictions. Total fault current available determines the damage that will be done when something like a wrench falls across bus bars. Time, too, counts. If you can't vaporize the whole wrench before the upstream fuse or breaker opens, then you just have scarred metal. For any given load, the AC wiring, running higher voltage than 48VDC CO batteries, will be smaller due to the lower amperage requirements and the larger typically allowed voltage drop. The AC network's source impedance is going to be a lot higher than a local battery string. too. A 600 amp capacity BDFB panel in a COLO room even just a 100' of cable loop length (50' end to end) from the main fuse panel at the batteries may well be wired with excess ampacity just to limit the IR drop. Instead of just a single 535MCM cable (that has adequate ampacity) that might mean up to 3x535MCM or 2x777MCM in parallel for both battery and return for both A and B battery feeds! That is a lot of copper. Look at the current available from a typical 9,000 ampere hour battery through just a single 100' length of 750MCM cable that has a resistance of .00150 ohms, assuming, for simplicity no resistance for the battery. That is 48 / .00150 = 32,000 amps, and that is very comfortable short term for a battery that can deliver 9,000 amps for an hour! The cable between the battery and the main fuse panel is unprotected other than by the ability to vaporise the straps between cells that are often substantially smaller in cross section than the cable. After the main fuse panel, typically there would be a 600 amp fuse protecting the feed to a BDFB Before that fuse goes, there is still plenty of energy to vaporize common hand tools. Typical 120/208V small branch circuit breakers in small buildings and homes have an interrupting capacity rated at 10,000 amps, and should not be deployed where that can be exceeded. It will be on the label. Some carriers have reservations about the small plug in (bullet pins on rear) BDFB panel breakers that in one case size range from a few amps to 100. Apparently there have been fires, and fuses are considered safer. Once you get past dry skin resistance, I have several times seen human body resistance given as 1 ohm. I have no idea how/where that is being measured.
"Barton F Bruce" <barton@gnaps.com> writes:
Typical 120/208V small branch circuit breakers in small buildings and homes have an interrupting capacity rated at 10,000 amps, and should not be deployed where that can be exceeded. It will be on the label.
It's worth noting that the interrupting capacity of the aforementioned breakers is 10,000 amps *AC*, and that said circuit breakers should not be used in *DC* applications despite the fact that the voltage is less than half as much and the fact that they're downstream from a 600A fuse (and have smaller wire in the circuit that will naturally limit how many amps can go into a short anyway). I'm hazy on the theory (perhaps someone more knowledgeable can post it), but my understanding is that with AC the arc has a chance to quench 120 times per second (ie, every time there's a zero crossing), and with DC that opportunity (obviously) does not exist. Bottom line is that one should buy breakers and fuses that are designed for use in DC powerplants, rather than trying to cheap out with something you picked up at Home Depot or Pep Boys. I'm sure I'm wasting my breath since _nobody_ who reads NANOG would ever try to cut corners to save a few bucks... :) ---Rob
Unnamed Administration sources reported that Robert E. Seastrom said:
Bottom line is that one should buy breakers and fuses that are designed for use in DC powerplants, rather than trying to cheap out with something you picked up at Home Depot or Pep Boys. I'm sure I'm wasting my breath since _nobody_ who reads NANOG would ever try to cut corners to save a few bucks... :)
Gawd yes. We all know those little 3AG glass fuses, right? They also come in ceramic. A regular PITA -- you can not look and see they are blown. But they are there for reason. They are typ. full of power to quench the resulting arc. A glass 3AG can and will open, yet the arc just keeps going....slagging the fuseholder, and whatever was errr... protected. There is as much diversity and engineering in fusing as router design. Voltage to be broken, AC vs DC, time curve to open, other factors all enter into it. I see two fuses in series on "pole pigs" primaries around here. Finally had a chance to ask the foreman. One is {say} 10A and that is sized for overloads. The second is 15A, but its sole function is to open FAST if the primary takes a lightning hit and the spark-gap on the transformer conducts. Seems they can't get both qualities right in one fuse so they use two. And RES is correct; DC circuit breakers are different animals than AC by a long shot. Fuses have different ratings as well. RTFM. Please be careful; while you might [now] be easily replaced, the suffering VC's will not appreciate losing all to a dropped tool. ps: I've not even mentioned the acid and H2 gas [kaaBOOM] issues... -- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433
I deny saying:
But they are there for reason. They are typ. full of power to
powDer -- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433
Also, some AC circuit breakers are of a design that counts on the magnetic properties of AC, and, therefore, won't trip due to ANY DC load. I think these are mostly not available any more, but I remember encountering them some time ago and realizing that it would be _REALLY_ bad if someone put them in a DC plant accidentally. Owen --On Monday, December 30, 2002 9:18 -0500 "Robert E. Seastrom" <rs@seastrom.com> wrote:
"Barton F Bruce" <barton@gnaps.com> writes:
Typical 120/208V small branch circuit breakers in small buildings and homes have an interrupting capacity rated at 10,000 amps, and should not be deployed where that can be exceeded. It will be on the label.
It's worth noting that the interrupting capacity of the aforementioned breakers is 10,000 amps *AC*, and that said circuit breakers should not be used in *DC* applications despite the fact that the voltage is less than half as much and the fact that they're downstream from a 600A fuse (and have smaller wire in the circuit that will naturally limit how many amps can go into a short anyway).
I'm hazy on the theory (perhaps someone more knowledgeable can post it), but my understanding is that with AC the arc has a chance to quench 120 times per second (ie, every time there's a zero crossing), and with DC that opportunity (obviously) does not exist.
Bottom line is that one should buy breakers and fuses that are designed for use in DC powerplants, rather than trying to cheap out with something you picked up at Home Depot or Pep Boys. I'm sure I'm wasting my breath since _nobody_ who reads NANOG would ever try to cut corners to save a few bucks... :)
---Rob
participants (4)
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Barton F Bruce -
David Lesher -
Owen DeLong -
rs＠seastrom.com