DC power versus AC power

Barton F Bruce barton at gnaps.com
Mon Dec 30 09:59:12 UTC 2002

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.

More information about the NANOG mailing list