QO Breakers and Panels [RE-wrenches]

[Bill Brooks]

Wrenches,

Can't let this one go. First of all--I love QO breakers. They make the best
low voltage switches money can buy. BUT, I have personally witnessed a QO
fry at about 70 volts with a DC motor load (kind of worst case scenario).
The dc-rating in the Square D digest is 48 volts. Pulse got special ratings
on QOU breakers to use in their 48-Volt DC systems, but that does not mean
that we are allowed to do the same.

First of all, standard QO breakers, as Bob-O and others have pointed out are
not allowed on 48-Volt systems since the code considers the voltage on the
battery system as the highest equalization voltage (~64V). The PV array is
even worse with voltage reaching 95 Volts in Central California and upwards
of 108 Volts in cold climates like the high Sierras.

When QO breakers are used as combiners for PV arrays, we have two problems.
24-Volt systems are just under the wire for Central California, but too high
of a voltage for cold climates. This one is right on the bubble. This is
after great care was taken to remove the blanket 1.25 multiplier in the 1996
code and replace it with a table that gave the designer much more
flexibility in the 1999 code (count your blessings). The infamous 1.56
number has nothing to do with Voltage, it is a current multiplier
(1.25x1.25) required on short circuit current of PV modules for very good
reasons.

The second problem with using a QO or any dc-rated snap-in breaker as an
array combiner is that it is clearly a "back-fed" breaker, and therefore is
required to have a secondary locking mechanism by the code. This is to
prevent the breaker from becoming dislodged (as these breakers can become in
a messy installation)and arching all over the place. Some inspectors allow
for the protective cover to be that secondary locking mechanism, but I have
met way more that do not see it that way.

John Wiles points out the issue of AIR rating in his manual. There is always
a different AIR rating for DC and AC so you can never use the AC AIR rating
on the DC side. QO breakers, as is required for all branch service breakers
for residences, has a 10,000 Amp AC AIR. The 48-Volt DC AIR is 5,000
volts--probably too low for golf cart batteries (although I would certainly
use them oversees on grass huts without any concern). As we parallel
batteries, the maximum short circuit current goes up clearly making it
unacceptable to use QO breakers without some circuit limiting fuse between
the breakers and the battery (Class T or similar).

I still maintain that wrenches that have the majority of their experience
with 12 and 24 volt systems do not appreciate the hazards, and need for
ratings, that become very evident even at 48-Volts. DC is a totally
different animal than AC and needs to be treated with the respect it
deserves (just as batteries deserve a lot of respect). It's easy to assume
that the world is out to ruin your life with excessive restrictions until
the day you see the results of the problems.

There is a very real misconception that there is no voltage at short
circuit. There may be only a small amount of voltage at first, but it very
quickly rises to whatever voltage it needs to sustain the arch. I have seen
500 Volt fuses completely burned in a short circuit because a 1200 volts got
applied to it by accident. How long did the voltage stay below 10 Volts?
maybe a few milliseconds. One of the real hazards we have is
counter-intuitive--since we have a limited current source (PV array) it does
not tend to fuse wires and do things as are cited below. It just burns and
burns until there is not longer any fuel. It can take a long time to use up
all the fuel in many systems were we have plastics and EVA and wire
insulation, etc....

Enough of this. We have listed equipment at 125 Volts for 12, 24, and
48-Volts systems that is readily available, inexpensive, and works. I'm
wondering what the problem is.

Bill.


-----Original Message-----
From: Bob-O Schultze, Electron Connection
[mailto:econnect at snowcrest.net]
Sent: Tuesday, March 06, 2001 5:23 PM
To: RE-wrenches at topica.com
Subject: Re: QO Breakers and Panels [RE-wrenches]


>
>I agree. I have always thought that a little ridiculous but I doubt have a
>good grasp of the big picture.  Yes, by the time you apply all the de-rate
>factors you can get the OCV of a 24v array near 48 volts but there is no
>current which = no energy. In my way of thinking.  If there is a fault then
>the array is instantly pulled to near zero volts at short circuit current
>which = just a little above no energy.

Nothing wrong with your grasp of the picture, Travis.

>
>I wonder if a lot of the overkill on electrical code as it relates to solar
>is thinking of the array as a infinite energy source just like you have to
>consider the service coming into the house.  For example if there is a
>problem with the grid wires in between your main breaker and the utility
>transformer then the problem either burns the wire in two or trips the
>transformer.

In my kinder moments, I've thought about that as well. Unfortunately,
the folks charged with giving input to 690 of the Code are SUPPOSED
to know about PVs and should know better. I believe the motivations,
and hence the regulations, are far more political in nature.
Bob-O

Bob-O Schultze, Electron Connection
PO Box 203, Hornbrook, CA 96044
800.945.7587 or 530.475.3402
fax 530-475-3401
www.electronconnection.com

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 www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine: 
 www.homepower.com

For info contact list moderator by email:
 michael.welch at homepower.com

____________________________________________________________
T O P I C A  -- Learn More. Surf Less. 
Newsletters, Tips and Discussions on Topics You Choose.
http://www.topica.com/partner/tag01