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<  Technical  ~  External Circuit Breaker

PostPosted: Mon Apr 09, 2018 4:33 pm
Posts: 22Joined: Fri Jul 08, 2016 2:12 am
In the blue book (K 8.1 p168) it says that this must isolate all electrical circuits etc.

But it doesnt have to shut down the engine?


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PostPosted: Mon Apr 09, 2018 8:47 pm
Posts: 11Joined: Mon Dec 23, 2013 10:59 pm
As far as I am aware the cut off switch has to kill the engine. In a single seater running without an alternator isolating all the electrical circuits through the cut off switch will kill the engine.


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PostPosted: Tue Apr 10, 2018 6:20 pm
Posts: 22Joined: Fri Jul 08, 2016 2:12 am
Thanks for the reply Martin.

It has an alternator but I would have thought electrical circuits would include the fuel pump!
There is the external circuit breaker and also a red kill/ignition switch inside the cockpit (next to the start button).
Both the kill/ignition switch and external CB are clearly identified.

The car has competed in circuit racing years ago and has done a few sprints including boyndie before I bought it so would be surprised if it wasn’t ok.

Did yours kill the engine with the external CB?

Graeme


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PostPosted: Wed Apr 11, 2018 3:22 pm
User avatarPosts: 458Location: DunfermlineJoined: Fri Dec 28, 2007 6:35 pm
It is normal to use an FIA Master type switch which has auxiliary contacts (one set NO one set NC).
The NC (closed when running) contacts are connected to the ignition circuit and the NO contacts dump the power to earth via a large 3.3 Ohm resistor to stop the alternator running the engine or blowing up the diode packs, when the master switch is switched off with the engine still running.

We have had scrutineers ask to prove that the master switch will kill the engine.



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Hamblin Cadet Austin Seven Special
Jeffrey Mk111b Historic 750
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PostPosted: Thu Apr 12, 2018 12:33 am
Posts: 22Joined: Fri Jul 08, 2016 2:12 am
Hi Doug

I had a quick look tonight to see if there were aux contacts.
Saw what looked like a main contact and two aux contacts. Didn’t check contact logic, I had just assumed N/O.
Sounds like I will need to sort it, but there are no spare contacts and I couldn’t see anything obvious that might have been previously disconnected.

Will have a better look at weekend and check with multi meter.
But have a feeling it is not a quick fix....

Graeme


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PostPosted: Thu Apr 12, 2018 2:57 pm
User avatarPosts: 927Location: AberdeenJoined: Sat Dec 29, 2007 10:22 pm
CTB if all electrical circuits are isolated the engine will not run(unless diesel). Doug will help you with the wiring but to emphasise the point that if not earthed through the resistor you will total the regulator . Ask me how I know. For that reason I do not switch off at the red switch but if challenged prove that the system will not energise when switched off. This is to obviate the risk of a bad earth/broken wire causing the expensive damage above. Quite prepared to sacrifice the diode pack in an emergency but not risk it needlessly.


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PostPosted: Fri Apr 13, 2018 5:29 pm
Posts: 22Joined: Fri Jul 08, 2016 2:12 am
I do follow you guys about earthing via resistor, ie to prevent a short circuit.
So is it the alternator output being earthed, ie to prevent any potential being generated?

Wouldn’t opening up the supply to the fuel pump be sufficient? That would also kill the engine.
Or must it be a shutdown action?

Graeme


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PostPosted: Sat Apr 14, 2018 6:11 pm
User avatarPosts: 927Location: AberdeenJoined: Sat Dec 29, 2007 10:22 pm
There are generally two switches in play. First is the master switch which kills power to all circuits. This must be accessible to the marshals in the event of a shunt. They must be able to isolate the battery from pumping fuel and also from generating sparks from a short circuit. Switch this on first, them the secondary switch which is the conventional ignition switch and can be anywhere.

If, with the engine running, the master is switched off the alternator, which is producing charge, suddenly doesn't "see" the battery and hikes up the output frying the regulator diodes(usually in a finned metal box). The kill switch when switched off should offer a pathway to earth for this spike of voltage. Hence the earth through a resistor.

As you will have realised I am not an electrician, and others will explain it more correctly. The sense of my comments is however correct. There are quite a few explanations online, and Douglas at Westgarage can fit it if maxed out by this.

Hope this of some help.


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PostPosted: Mon Apr 16, 2018 8:03 am
Posts: 22Joined: Fri Jul 08, 2016 2:12 am
It happens that I’m an Electrical Engineer!
So I understand what you mean. It’s a surge arrestor.

I had a look again, have a main switch that is connected to ground. So this isolates the battery.
There are also 2 aux contacts but the connections to each one are looped together so it looks like one circuit being switched.

I might understand it but it doesn’t mean it will be easy to fix!


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PostPosted: Tue Apr 17, 2018 5:17 pm
User avatarPosts: 927Location: AberdeenJoined: Sat Dec 29, 2007 10:22 pm
so I should be asking you!! I will try and look out a diagram to send.


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PostPosted: Tue Apr 17, 2018 5:22 pm
User avatarPosts: 927Location: AberdeenJoined: Sat Dec 29, 2007 10:22 pm
I think there is a circuit in a posting on TenTenths website.


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PostPosted: Tue Apr 17, 2018 7:22 pm
User avatarPosts: 458Location: DunfermlineJoined: Fri Dec 28, 2007 6:35 pm
asklepios wrote:
....... T*nT*nths website.


Please don't use that sort of language on this forum - you will get banned :lol:



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Hamblin Cadet Austin Seven Special
Jeffrey Mk111b Historic 750
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PostPosted: Tue Apr 17, 2018 9:38 pm
User avatarPosts: 927Location: AberdeenJoined: Sat Dec 29, 2007 10:22 pm
Or go to .ell....oh I forgot it apparently doesn't exist according to a "senior source". Good news for the MSA then!


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PostPosted: Fri Apr 20, 2018 5:29 pm
Posts: 22Joined: Fri Jul 08, 2016 2:12 am
Cheers, I will go and have a look for it.


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PostPosted: Fri May 04, 2018 10:10 pm
Posts: 22Joined: Fri Jul 08, 2016 2:12 am
Had a quick look last night with the multimeter and tried to trace stuff.

Had another look again tonight and I’ve sussed it. It’s quite an easy fix and this diagram helped a lot (from ten tenths):
http://www.zetecinside.com/xr2/pictures ... ircuit.gif

The only difference is mine is negatively switched at main terminal.
I think the aux circuit is the switches loop to the ignition module. It’s +12V when the ignition switch is on and I traced the connection back to a nearby relay. The wire colours don’t match but I’m fairly confident they are the same electrically.

The weird thing is that the aux circuit was connected to both aux contacts which are defo n/o and n/c so it was always closed no matter the switch position!
So I will sort that and wire across the n/o aux contacts.
At the alternator there are three cables, haven’t bothered to check them all but the biggest one appears to be connection to battery, have checked continuity back to the permanent +12V at the relay cluster and all good.
So pretty confident I just jump off this and connect to n/c aux contact with the other side connected to chassis via a recently procured resistor.

Job done! (Well almost, I think the solution is found)

Cheers for the help and steer, was lost. So glad I did not have to rip things to bits.
Also gained a bit of a better understanding of the cars wiring system too. So all good.

Graeme


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