From: John De Armond
Subject: Re: Where to put a 3rd battery on a 5th wheel
Date: Tue, 04 Nov 2003 12:40:40 -0500
On Tue, 04 Nov 2003 08:52:06 -0600, Eric Roellig <Eric.Roellig@motorola.com>
>John, Arn't the requirements for DC vs AC breakers different? Do you
>have a rule of thumb to give the estimated DC rating for an AC breaker?
>I know that the spec's I read on plain ol' switches give different
>ratings between DC & AC.
At higher voltages and currents, yes there are significant differences. AC
breakers rely, at least in part, on the zero crossing to break the arc during
interrupting. DC doesn't have a zero crossing so other means are necessary.
The most usual is a blowout coil, a solenoid carrying the full load located so
that the magnetic field will blow out the arc. Many AC breakers also use
blowout coils but they're generally not as effective as on DC.
None of this matters much if the open circuit voltage (including inductive
voltage) is below the arc sustaining threshold voltage. It varies by metal.
About 26 volts for copper, significantly higher for the silver alloys used in
breaker contacts. Below the threshold, the arc will not sustain itself
regardless of the current.
This is the feature that lets us get away with using AC breakers on 12vdc.
And since the AC breaker is designed to open against typically 10,000 amps in
a fault, the contacts open very wide, further precluding arcing on DC.
The proof's in the pudding, of course. I did fairly extensive testing before
I relied on an AC breaker. This includes closing the breaker into a direct
short circuit across a pair of Group 29's, the worst possible case situation,
both because of the high current and because the closing in on a short invites
contact welding. The residential AC breaker is an ingenious device. The
mechanism wiggles the contact in the process of opening which breaks any
welding. It's important to remember that these breakers are designed to
interrupt a fault current of 10,000+ amps in normal service. I've never
measured a peak short circuit current of more than a couple thousand amps
using a pair of Group 29's.
Most all breakers have an arc chute that directs the arc vapors safely out the
breaker. This usually discharges directly above the wire connector. There is
a line of sight path to the contacts so you can observe the arc. On AC, a
breaker will occasionally blow some arc flame out the chute. This is normal.
On 12vdc, the arc is barely visible, even breaking a dead short.
>I saw a video of what happens to a breaker that is asked to interrupt
>(way) more current than it was designed for in an EE class way many
>moons ago. Suffice to say the result was spectacular.
Yup. I've had the occasion to peer into the void left when double fault
(breaker closing in a 10MW generator 180 deg out of phase with the line) blew
it out of its cabinet. This was a roll-out breaker that weighed perhaps 1000
Which brings me to another important point. NEMA vs IEC ratings. NEMA is our
traditional standards body. IEC is the new ISO (and thus better.... NOT!)
body. There is an important difference. Under NEMA standards, an electrical
apparatus subjected to design basis fault current must interrupt it, withstand
all forces involved and FUNCTION AFTERWARDS. Damage is permitted but none so
bad as to cause the device to malfunction.
IEC, on the other hand, allows the device to be destroyed as long as it
interrupts the load. In effect, this standard turns a breaker into a very
messy fuse. This is the standard that permits these little tiny motor
starters that look like they couldn't support a light bulb. Of course, since
this allows cheaper devices, industry has jumped all over the standard. At
least until the first big fault.
Moral: Stick with NEMA ratings.
>BTW: Your BBQ is >>WONDERFUL<<! Thanks again.
Thanks much. I was trying to figure out which Eric on the group belonged to
you. Really enjoyed having you stop by.
From: John De Armond
Subject: Re: Modified UPS with Golf Cart Batteries
Date: Sat, 21 Apr 2007 18:55:01 -0400
On Sat, 21 Apr 2007 04:26:18 +0000, dave
>I've got an APC Back-Ups ES500 UPS that I had modified to use a pair
>of 220AH 6V golf cart batteries instead of the 7AH battery that's
>inside the UPS.
>It works, but there are a few things that concern me. It's using a
>240V AC 50 amp circuit breaker between the batteries and the UPS, and
>it makes a buzzing noise when it's under load. Is this safe? I'm not
>using it right now because I don't think it's right.
Yes, that's fine. Unsurprising that the breaker would buzz under
load, as the inverter draws more power at the peak of each half cycle.
I've tested domestic type AC circuit breakers rather extensively for
use on DC relative to my electric car work. This type of breaker
interrupts what golf cart batteries can throw at it up to about 48
volts. Above 48 volts to 72 (the highest I've tested), the
interruption is accompanied with some rather violent arcing that comes
out the arc chute. The breaker still functions just fine but I'd
replace it after a short circuit interruption. It's perfectly fine
for your 12 volt system.
>I'm looking for an appropriate DC fuse and cut-off switch, but the DC
>fuse is not easy to find. Is there generally any sort of
>short-protection for the batteries built into the small APC UPS? Would
>it be ok to not use any fuse?
>The UPS has a 300 watt inverter, but I'm thinking it may place a
>larger load on the batteries. Would a 30 or 40 amp blade-type
>automotive fuse be ok for this?
Don't know. If there is and it's an AG3 type glass fuse (or ceramic
version) then it'll be fine. Those are rated to 32 volts DC.
I like the APC spade type fuses better and the Maxi-fuse even better.
The connection, made with spade lugs, is much more secure and lower in
resistance than glass fuses. At 30 amps many glass fuse holders
overheat. You'll not be pulling that much current, of course, but I'd
still use that type fuse. You don't need to buy a fuse holder - a
pair of female spade lugs will do just fine.
>I also have a larger 12 amp battery charger that I'll use to recharge
>the batteries after a long power outage. Right now I'm cutting off
>the batteries and unplugging/turning off the UPS during charging.
Depending on the type of charger that may not be necessary. If it's a
modern smart charger then no need. If it's a dumb old simple
transformer/rectifier type then it might be a good idea to disconnect,
depending on what the charger's voltage capability is.
>I left the batteries connected and the small internal UPS charger
>going while using the external charger, would it cause problems?
That works fine - I do it that way myself - but the batteries will
need an occasional equalization charge to bring all the cells up to
full charge and stir the electrolyte. The built-in charger won't do
that, for it its voltage were that high, the batteries would quickly
gas and dry out. A couple of times a year, disconnect the batteries
and give 'em a good equalization charge. let the built-in charger do
its thing otherwise, unless you need them charged faster.