[aprssig] Snow, Back-feed, Shocking things
ko6th_greg at hotmail.com
Mon Feb 8 22:06:32 CST 2010
Not to keep the discussion going, but I haven't seen this morsel of input...
I am told that a Transfer Switch also is constructed to prevent an arc-welded contact from keeping the circuit live. With standard circuit breakers (like the 100A "mains" breakers in a typical home panel), there is a small but non-zero chance that opening the breakers will not actually open the circuit due to a failed contact. There is pretty much no way to detect that this has occurred, until you've killed someone upstream.
> From: k7ftp at k7ftp.net
> To: aprssig at tapr.org
> Date: Mon, 8 Feb 2010 15:14:28 -0700
> Subject: Re: [aprssig] Snow, Back-feed, Shocking things
> From: "Tom Mandera" <tsm1 at tmcom.com>
> > How is a transfer switch significantly different from disconnecting the
> > main breaker (which could have failed and be passing electricity even when
> > off, but uncommon) before plugging on the backup power?
> A real transfer switch employs a BREAK BEFORE MAKE architecture. Either the
> generator is connected to the circuit, nothing is connected to the circuit,
> or the utility is connected to the circuit. Really a DPTT with a center off
> A real transfer switch connects between the house circuit breaker panel and
> the circuits that can be generator powered. When they are properly
> installed, they guarantee that you can't backfeed the utility and
> potentially cause problems (or death).
> Here is a link to an installation of a real transfer switch.
> Personally, I believe that we have way too many laws and restrictions in
> this country, but that there are things that are SMART to do. We shouldn't
> legislate them, but our elected officials (and the appointed ones they
> select) love to try to legislate away all stupidity and carelessness, and
> they usually legislate away all of the freedom in the process.
> But like I said - there are things that are SMART to do. Electricians don't
> use MALE plugs as power SOURCES because of the exposed contacts. FEMALE
> receptacles are used for SOURCES because the contacts are protected -
> reducing the likelihood of someone getting shocked. The setup that Bob has
> created is acceptable as long as BOB is the one working with it and he
> ALWAYS does it correctly. If he doesn't, or if someone else is working with
> it, the likelihood of a problem is GREATLY increased.
> Let's compare this with the rules of firearm safety.
> TREAT EVERY FIREARM AS IF IT IS LOADED
> NEVER ALLOW THE MUZZLE TO POINT AT ANYTHING YOU ARE NOT WILLING TO DESTROY
> KEEP YOUR FINGER OFF OF THE TRIGGER UNTIL THE FIREARM IS POINTED AT A TARGET
> YOU ARE PREPARED TO SHOOT
> If these rules are always followed, then the likelihood of injury or death
> in the event of a malfunction is little or none. If any one of the rules is
> ignored, the likelihood goes up. If two of the rules are ignored, they go
> up more. It's the same with this situation.
> There is a more practical standpoint on this. When I lived in Oregon, the
> local power utility would not start to work on the lines in an area if there
> was any sign of a generator in operation until someone had verified that the
> generator was incapable of being connected to a home - or that a transfer
> switch of the type I linked to above was installed and being used. If they
> found a setup like what Bob is using, they would not work on the lines in
> that area until it was disconnected and the cable used was "safed". Bob
> might have some very angry neighbors that are waiting for their power to be
> restored when they find out that he is the one holding it up while he argues
> with the utility company. While his setup might be safe as long as he is
> the one that sets it up and he does everything right, there is enough
> potential for issues that they would be within their abilities under local
> laws and codes to demand that he disconnect it and keep it that way.
> Just as a telco can disconnect any phone line which has problem-causing
> equipment attached, the power utility can do the same thing.
> In this case, there is a legal and affordable way to avoid the issue. I'd
> recommend it.
> Jim - K7FTP
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