nuckolls.bob(at)aeroelect
Guest
|
 Posted: Wed Sep 02, 2009 4:32 am Post subject: Fried Master switch wires |
 |
|
At 03:26 PM 9/1/2009, you wrote:
Here is a real puzzle.
I used Bob's Z-11 wiring plan to structure my wiring system in my RV.
All was well (3 years) until I inadvertently jumped a dead battery
and reversed polarity. The master switch wire that runs from the
switch to the master relay, that powers up the starter relay burned
(smoked actually). Nothing else burned, just ancillary damage to wire
insulation from the melted insulation on the master to ground and
master to relay and 7 burned fuses.
There was a diode on both relays. The yellow diode on the MR showed
discoloration (a small brown spot) but appeared intact. I replaced
all the master wiring and the Odyssey battery and tried to lite it.
Again it smoked the master wires to the MR and to ground.
"Appearing intact" doesn't mean it was not damaged.
In fact, the reversed polarity on this device caused
the coil of your contactor to appear shorted which is
why the control wire burned. The same current fused
the diode's junction causing it to become shorted
which caused your contactor to appear shorted after
the wire was replaced.
Then I replaced the MR, the Starter Relay, both diodes AND the master
switch along with the burned up wire. Seems fine now and I flew it
home. The problem is this. Except for the Yellow MR diode frying
during the second attempt, nothing in that circuit appears damaged.
We cut open the MR and it is as new inside. Same with the Master
Switch....contact are normal appearing. So what happened the second
time??? Why does everything appear OK on inspection?
The contactor itself would not have been damaged.
It's not a polarity sensitive device. Replacing
the diodes and damaged wire was all that was
necessary.
One side note....except for a loss of a battery, one microphone jack
and the Alternator now putting out only 13.9 volts, the rest of the
avionics and electrical system appear unharmed. Burned out 7 fuses on
the first incident, but no apparent damage to the expensive
equipment. That alone makes fuses superior to circuit breakers in my mind.
I DO favor them for their much faster response to
an overload condition. The first condition (initial
application of reverse polarity) was the only
one that put your system at risk. The second event
was because the diode was also shorted causing a
large fault current to flow in the replacement
wire. The contactors would have been okay to leave
in.
I appreciate any electrical Guru's thoughts on this event and what I
might still need to do to insure safety in flight. Not knowing what
caused the second event makes me a bit nervous that It might still be
lurking. Smoke in the cockpit is not good!
You're experience is not uncommon. I reversed a
set of jumper cables on a couple of cars last
winter when I was working in the dark and it
was snowing on me! In most automotive applications,
the diode array in the alternators is what
keeps the system from going negative by more
than a couple of volts. This may have figured
strongly in your experience too.
This is another example of how adding a ground
power jack (with ov and reverse polarity protection)
would have proved useful. I'm pleased that the
outcome of your experience was so benign . . . it
could have been much worse.
I'm mystified about the microphone jack. What
kind of voltage regulator does your airplane have?
If it's a built in regulator and it shifted in
regulation set point as a consequence of this
event, then it HAS been affected and is now suspect.
Bob . . .
---------------------------------------
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
Get Email Distribution Too!
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
