On 12/03/2012 11:52 AM, Rowland Carson wrote:
> My Rotax 912S did not come with leads for killing the mags (what are
> normally called p-leads in standard aircraft practice). I almost
> bought some screened cable that was on special offer at the LAA
> Rally, but couldn't get any assurance from the vendor that it would
> be suitable for leads to the mag switches - ie sufficiently high
> voltage rating.
I recently wrote a private message to someone else about this subject.
You may be interested in it:
I have not earthed it at all, I even didn't use shielded cable. I think
the diagram only shows it that way because it makes a less confusing
drawing. The whole point of the diagram is that you need to connect the
ignition wire to ground in order to kill the ignition. If you have a
ground bus in the cockpit anyway (I'm sure you have), you should use
that to connect to the switch, and just run a standard non shielded
cable from the ignition to the switch.
(Shielding is necessary for cables which carry radio signals or are
sensitive for picking up noise, your ignition wire doesn't fit into
either category).
If your starter switch is already connected somehow to ground, you
should NOT connect both ends of the shield! Otherwise the shield of the
ignition wire will act as a shortcut for the other ground connection.
This might route heavy currents through the shield, it may get hot and
melt the insulation between the two layers, stopping the engine! In
other words, from the starter switch there may only be ONE path to ground.
So, it is either that you connect the starter switch to ground via the
ignition shield, or that you connect the starter switch via another path
to ground, but not both at the same time. Explaining this goes a long
way, trust me on this.
I would recommend option 2, to connect the starter switch to some ground
bus bar somewhere in the cockpit, and forget the whole shield, or
connect only one end of the shield in case you really insist to have the
shield.
Use plastic ty wraps or some other
"soft" way to keep the cable in place. Preferably use some sleeve at the
places where you clamp it with the ty wraps, to avoid any stress points,
chafing, etc. This is a very important wire, if it connects somehow to
ground your engine will stop. Securing it with a metal screw is
dangerous, the metal screw may work its way through the cable connecting
the shield to the core, and this effectively stops the engine.
Just give the starter switch its own ground connection, and run a normal
teflon wire in one piece from the engine terminal to the switch,
securing that cable at strategic places with ty wraps, making sure it
can never chafe even if the engine starts vibrating madly. Remember,
earthing it kills the ignition, so if the insulation chafes through and
the metal wire inside touches any metal object connected to ground, your
engine will stop.
If you feel unsure about soldering or routing the cable, or lack the
correct tools, seek professional help. This is not a cable to fool with.
Frans
|