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what wire/cable to use for rotax magneto p-leads

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rowlandcarson(at)gmail.co
Guest

Posted: Mon Dec 03, 2012 2:53 am Post subject: what wire/cable to use for rotax magneto p-leads

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.



Is there any recommendation for a type of cable to use for the leads to the Rotax mag switches? Or can anyone tell me what the voltage rating for mag leads ought to be?



in friendship



Rowland



| Rowland Carson          ... that's Rowland with a 'w' ...

| <rowlandcarson(at)gmail.com>            http://www.rowlandcarson.org.uk

| Skype, Twitter: rowland_carson      Facebook: Rowland Carson

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christoph.both(at)acadiau
Guest

Posted: Mon Dec 03, 2012 3:58 am Post subject: what wire/cable to use for rotax magneto p-leads

Rowland:

Best to call your Rotax Dealer to see if they have the two small pins you

need to insert into the Molex Type ignition module connector box. In

Canada it was recommended to use a minimum of 18AWG shielded aircraft type

wire. My Rotax came with two pins you need to insert into the not yet

filled and marked "br" brown section of the Molex Type fastener mounted to

each of the ignition modules. You need a good crimping tool to make the

connection to the wire and I would use shrink wrap to lessen vibration

problems between wire and fastener. Make sure the wire is routed without

too much tension and well supported. Rotax specifies that both ends of the

shielded wire be grounded and that the wire be routed as much as possible

away from other wires, antennas, etc. to minimize interference. I am using

a separate through the firewall conduit  to allow this.

Christoph Both

#223 Classic 912S,finalizing CLASSIC engine installation

Wolfville, Nova Scotia, Canada

 



On 12-12-03 6:52 AM, "Rowland Carson" <rowlandcarson(at)gmail.com> wrote:



 	  Quote: 	
		  

<rowlandcarson(at)gmail.com>



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.



Is there any recommendation for a type of cable to use for the leads to

the Rotax mag switches? Or can anyone tell me what the voltage rating for

mag leads ought to be?



in friendship



Rowland



| Rowland Carson          ... that's Rowland with a 'w' ...

| <rowlandcarson(at)gmail.com>            http://www.rowlandcarson.org.uk

| Skype, Twitter: rowland_carson      Facebook: Rowland Carson

| pictures: http://picasaweb.google.com/rowlandcarson

rowlandcarson(at)gmail.co
Guest

Posted: Mon Dec 03, 2012 5:18 am Post subject: what wire/cable to use for rotax magneto p-leads

On 3 Dec 2012, at 11:57, Christoph Both wrote:



 	  Quote: 	
		   Best to call your Rotax Dealer to see if they have the two small pins you

 need to insert into the Molex Type ignition module connector box


	


Christoph - thanks for your message. I've got the blank pins that came with the engine, and an assortment of crimping tools, one of which I'm sure will be equal to the task.



I'm concerned about whether "ordinary" screened aircraft cable will be OK for the high voltages (thousands of volts) that appear in ignition circuits. In my youth I was involved in the development of colour TV for UK transmission and so did some poking around in boxes that had 30kV lurking inside. I was impressed by the way such voltages could walk across things that I have previously regarded as insulators. Similarly, (before I switched to driving a diesel car) I've been amused by the capability of car ignition voltages to find a route to earth that did NOT involve the appropriate spark plug. So, I am approaching this question with some respect for the physics involved.



in friendship



Rowland



| Rowland Carson          ... that's Rowland with a 'w' ...

| <rowlandcarson(at)gmail.com>            http://www.rowlandcarson.org.uk

| Skype, Twitter: rowland_carson      Facebook: Rowland Carson

| pictures: http://picasaweb.google.com/rowlandcarson

christoph.both(at)acadiau
Guest

Posted: Mon Dec 03, 2012 5:37 am Post subject: what wire/cable to use for rotax magneto p-leads

I am planning to use a small diameter grounding strap conduit to run the

shielded 18AWG wires in, similar to the Rotax shielded AC power coming

from the engine. This should contain any problems.

C

On 12-12-03 9:18 AM, "Rowland Carson" <rowlandcarson(at)gmail.com> wrote:

 	  Quote: 	

<rowlandcarson(at)gmail.com>

On 3 Dec 2012, at 11:57, Christoph Both wrote:

> Best to call your Rotax Dealer to see if they have the two small pins

>you

> need to insert into the Molex Type ignition module connector box

Christoph - thanks for your message. I've got the blank pins that came

with the engine, and an assortment of crimping tools, one of which I'm

sure will be equal to the task.

I'm concerned about whether "ordinary" screened aircraft cable will be OK

for the high voltages (thousands of volts) that appear in ignition

circuits. In my youth I was involved in the development of colour TV for

UK transmission and so did some poking around in boxes that had 30kV

lurking inside. I was impressed by the way such voltages could walk

across things that I have previously regarded as insulators. Similarly,

(before I switched to driving a diesel car) I've been amused by the

capability of car ignition voltages to find a route to earth that did NOT

involve the appropriate spark plug. So, I am approaching this question

with some respect for the physics involved.

in friendship

Rowland

| Rowland Carson          ... that's Rowland with a 'w' ...

| <rowlandcarson(at)gmail.com>            http://www.rowlandcarson.org.uk

| Skype, Twitter: rowland_carson      Facebook: Rowland Carson

| pictures: http://picasaweb.google.com/rowlandcarson

frans(at)privatepilots.nl
Guest

Posted: Mon Dec 03, 2012 7:34 am Post subject: what wire/cable to use for rotax magneto p-leads

On 12/03/2012 11:52 AM, Rowland Carson wrote:



 	  Quote: 	
		   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

frans(at)privatepilots.nl
Guest

Posted: Mon Dec 03, 2012 7:40 am Post subject: what wire/cable to use for rotax magneto p-leads

On 12/03/2012 02:18 PM, Rowland Carson wrote:



 	  Quote: 	
		   I'm concerned about whether "ordinary" screened aircraft cable will

 be OK for the high voltages (thousands of volts) that appear in

 ignition circuits.


	


We are not talking here about the HT-leads to the spark plugs, but about

the ignition kill wire.



 	  Quote: 	
		   In my youth I was involved in the development of

 colour TV for UK transmission and so did some poking around in boxes

 that had 30kV lurking inside. I was impressed by the way such

 voltages could walk across things that I have previously regarded as

 insulators.


	


I'm sure you must have been impressed , but the Rotax ignition kill

circuit is far from comparable to the 30kV of your colour TV's. It is

just low voltage.



Frans

brian.davies(at)clara.co.
Guest

Posted: Mon Dec 03, 2012 8:39 am Post subject: what wire/cable to use for rotax magneto p-leads

Hi Rowland,



I used standard aircraft grade screened cable and have not had any problems.

As the ignition modules are not really magnetos I suspect we are simply

controlling a transistor.



Regards



Brian



--

rowlandcarson(at)gmail.co
Guest

Posted: Mon Dec 03, 2012 10:53 am Post subject: what wire/cable to use for rotax magneto p-leads

On 3 Dec 2012, at 15:34, Frans Veldman wrote:



 	  Quote: 	
		   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



| Rowland Carson          ... that's Rowland with a 'w' ...

| <rowlandcarson(at)gmail.com>            http://www.rowlandcarson.org.uk

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frans(at)privatepilots.nl
Guest

Posted: Mon Dec 03, 2012 12:34 pm Post subject: what wire/cable to use for rotax magneto p-leads

On 12/03/2012 07:53 PM, Rowland Carson wrote:



 	  Quote: 	
		   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!


	


The official Rotax electrical installation diagram assumes that you

don't have a ground connection at the switch end of your setup.

Make sure you meet that assumption, or very unpleasant things can

happen. This means that you have to double check that no ground wire,

metal bracket, or whatever is attached to the ignition switch in any way.



Also, I still feel that the risk of accidentially shorting the ignition

wire to ground (with engine stoppage as a result) is higher in a

screened wire than in an unscreened wire. Sometimes aircraft

manufacturers follow a standard approach without considering other

options, options that might be better in that specific situation.

The screened ignition wire really is a Lycosaurus thing.



Anyway, a sensible compromise might be to use the screened wire, but to

keep one of the ends of the screen disconnected, and to run a separate

ground cable to the ignition switch. This to avoid any possibility of

running ground currents through the screen, which might melt the core

and screen together, leading to engine stoppage.



If I would ever get corrosion on my battery or alternator ground

connection, I would prefer a failure of my electrical system but with

the engine still running, than the ignition screen taking over the

ground connection for a short while before melting everything together,

and giving me an electrical failure PLUS an engine stoppage.



Frans

rowlandcarson(at)gmail.co
Guest

Posted: Mon Dec 03, 2012 2:07 pm Post subject: what wire/cable to use for rotax magneto p-leads

On 3 Dec 2012, at 20:33, Frans Veldman wrote:



 	  Quote: 	
		   The official Rotax electrical installation diagram assumes that you

 don't have a ground connection at the switch end of your setup.

 Make sure you meet that assumption, or very unpleasant things can

 happen. This means that you have to double check that no ground wire,

 metal bracket, or whatever is attached to the ignition switch in any way.


	


Frans - thanks for your reply.



I don't want to keep flogging a dead horse as I think I have a satisfactory answer to my original question.



But it seems the diagram I attached to my earlier posting did not make it to the list (for some unknown reason I didn't get an echo of the message myself so can't be sure), and it clearly showed earth connexions to the 2 wire screens and the 2 switch cases as well as to the 2 normally-open poles of the A & B switches. So, I have some difficulty understanding why you say that Rotax assumes no earth at the switch end of the "p-leads".



The basic electrical circuit diagram in chapter 25 of the Europa builder manual, although slightly different in detail from the one in the Rotax manual, also calls for the screens on the magneto wires to be earthed at both ends.



In Bob Nuckolls' widely-respected AeroElectric Connexion, his wiring diagrams show the leads to the mags shielded and grounded at the switch end.



 	  Quote: 	
		   If I would ever get corrosion on my battery or alternator ground

 connection, I would prefer a failure of my electrical system but with

 the engine still running, than the ignition screen taking over the

 ground connection for a short while before melting everything together,

 and giving me an electrical failure PLUS an engine stoppage.


	


I understand your concern - IF the normal earth should fail. I plan to use a B&C common earth system which has on each side of the firewall a forest of male blade connectors soldered to a brass plate. A brass bolt about 8mm diameter passes through the firewall to connect the 2 plates, and provide an anchorage for a ring termination on a 10AWG wire from the battery. I consider the physical integrity of that arrangement, and my planned maintenance schedule for it, good enough not to cause me any sleepless nights.



in friendship



Rowland



| Rowland Carson          ... that's Rowland with a 'w' ...

| <rowlandcarson(at)gmail.com>            http://www.rowlandcarson.org.uk

| Skype, Twitter: rowland_carson      Facebook: Rowland Carson

| pictures: http://picasaweb.google.com/rowlandcarson

frans(at)privatepilots.nl
Guest

Posted: Mon Dec 03, 2012 3:03 pm Post subject: what wire/cable to use for rotax magneto p-leads

On 12/03/2012 11:06 PM, Rowland Carson wrote:



 	  Quote: 	
		   But it seems the diagram I attached to my earlier posting did not

 make it to the list (for some unknown reason I didn't get an echo of

 the message myself so can't be sure), and it clearly showed earth

 connexions to the 2 wire screens and the 2 switch cases as well as to

 the 2 normally-open poles of the A & B switches.


	


Yes, but it is separate from any other ground connection in the

instrument panel... That means, if you have a ground bus bar in your

instrument panel, you should take care that it doesn't somehow connect

to the screen as well.



This would for instance happen when you have a metal facias on your

panel. Your ignition-switch touches it, and some other switch from,

let's say your landing light, touches it as well. This other switch has

a ground connection to your ground bus. Without realizing it you have

just created a ground loop.



Now suppose something happens to your landing light ground connection.

The electrons now find a path via the switch case to the metal facias,

to your ignition switch, and then via the ignition screen, to ground.

Your landing light will still work, so you won't notice that something

isn't as it should be. Maybe you will only detect a funny smell, when

the 3 amps of your landing light travel via the facias and screen of

your ignition cable, a cable not designed to run the 3 amps via the screen.



Ok, you make good connections you say, your ground cables won't fail. I

take that for granted. But here is another problem, and that is

electrical resistance. Your perfect ground cable has still resistance.

And when your starter cranks it will pull a few hundred amps through the

cable, and due to the resistance some electrons find an easier route via

the screen of your ignition cable. Repeat this a dozen times, and for

some "unknown reason" suddenly your ignition cable fails.



Some day you have a short cut somewhere in your electrical system. No

worries, you have a circuit breaker. But just at the instant between the

short cut occurence and the opening of the circuit breaker, you have an

amp spike on your system, possibly a few hundred amps. It doesn't cause

a fire because it only lasts a microsecond, but at the moment your

ground bus is overloaded some electrons will choose the screen as an

easier route. The amp spike on the screen causes an induction in the

core wire, and that core wire is attached to your precious ignition

coil... Just cross your fingers and hope it doesn't do anything bad.



I can give you many more examples. The bottomline is: Ground Loops Are

Bad (Pun intended). Always make sure there is only ONE way to ground.

This screen of the ignition wire makes your system vulnerable to ground

loops, hard to trouble shoot because you think "ground is ground" and

undetectable for simple measurements because you can't measure which

path the electrons have taken.

A much safer way is to connect the ignition switch to your ground bus in

the instrument panel, and not connect it to the screen. The result is

exactly the same, but the dangerous ground loop can never develop.



 	  Quote: 	
		   I understand your concern - IF the normal earth should fail.


	


Not only when it fails, but also when it has resistance... and it has.



Of course it is your airplane, but think about it for a moment, why you

should prefer an error-prone solution over a safer solution without

sacrificing anything in the process.



 	  Quote: 	
		   I plan

 to use a B&C common earth system which has on each side of the

 firewall a forest of male blade connectors soldered to a brass plate.

 A brass bolt about 8mm diameter passes through the firewall to

 connect the 2 plates, and provide an anchorage for a ring termination

 on a 10AWG wire from the battery.


	


When you are done installing this, switch on all appliances in your

airplane, and take a multi-meter and measure the voltage drop from your

battery to the ground bus in the cockpit. You will be surprised!



Frans

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