On 3 Dec 2012, at 15:34, Frans Veldman wrote:
> 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
Frans - thanks for your message.
Yes, I appreciate that the purpose of the "P-lead" is to allow the
ignition to be short-circuited to earth and so prevent sparks at the
plugs - much the same as that bit of bent metal you sometimes see on
lawnmower engines (which also tend to have magneto ignition) that shorts
the top of the spark plug to the case. Of course that is working on the
HT side of the circuit and I understand now that "P-leads" are usually
connected to the LT side.
I understand that a screened wire is used to prevent radiation of
electromagnetic interference from the ignition circuit; as the LT
circuit is opened and closed each time a spark is required at a plug,
the resulting transients can cause radiation even though the voltage may
not be as high as on the HT (spark plug) side.
Brian Davies' reply to my posting, and other offline communications,
indicate that the voltages appearing on the inner of the Rotax "P-lead"
are quite low and so may not have much potential (pun intended) to
generate interference as a traditional magneto installation. But the
official Rotax electrical installation diagram (see attached) calls for
screened wires (and even for the switch cases to be screened) so I see
no reason to invite possible interference by ignoring that advice!
Thanks for all the responses on this topic - I'm now happy to use
standard aircraft-grade screened cable for the ignition kill switch
connexions as there is no EHT voltage involved.
in friendship
Rowland
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