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When should a switch be in the Live or Earth line?


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On 9/11/2020 at 11:12 AM, mmgemini said:

So you are quite happy to be able to run the engine with the neg wiring switched off to blow the alternator.

An alternator disconnect is a different subject.  Disconnecting the positive wire from the battery has no different affect on the alternator's circuit than does disconnecting the negative wire.

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17 minutes ago, Bowie69 said:

Point is, you CAN'T disrupt the negative of the alternator, it is the casing.

First off, this whole thing has nothing to do with the subject of this thread....

I understand how the alternator is wired but it is these details that are not included in the discussion and then cause people to do things wrong. mmgemini is trying to tell people to put the disconnect on the positive wire and that will prevent the engine from continuing to run and not damage the alternator.  That is completely wrong.  You would also need to run the alternator output wire through the disconnect which is not standard wiring. Killing the engine with a disconnect is a different subject to this entire thread.

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Railing against you?

I'm trying to close the off topic discussion on master kill switches which is confusing the whole issue of the decision of which side of a load to switch. It is a separate and different topic.  A master kill switch contains three separate switches and really should be a topic for different thread is that is of interest.

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No, I said railing against my reply, not me.

Why are you shutting down discussion? Is that your prerogative? Who gave you that right? 

There have been ~11 separate posts about the kill switch, and is certainly related to the original topic. It's not like we are talking about concreting floors when the thread is supposed to be about installing a Lexus V8 in a Sandringham Six, is it?

 

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28 minutes ago, Gluv said:

Jeez fellows , everybody chill out ! . This mini heat wave is boiling everybody's brain 🙂

Gluv🇬🇧

I know Gluv, I'm chilled, very. Just been out fertilising the lawn. ✌️

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16 hours ago, Red90 said:

First off, this whole thing has nothing to do with the subject of this thread....

I understand how the alternator is wired but it is these details that are not included in the discussion and then cause people to do things wrong. mmgemini is trying to tell people to put the disconnect on the positive wire and that will prevent the engine from continuing to run and not damage the alternator.  That is completely wrong.  You would also need to run the alternator output wire through the disconnect which is not standard wiring. Killing the engine with a disconnect is a different subject to this entire thread.

You set your vehicle on fire then. I wondr if you knoe how an FIA switch works, aparently you do not. So yo,Red 90< fit the FIA switch in the Neg side of the circuit. Now tell me how the live ign side will work. Yes I will fit a battery cut off switch in Neg line but never fit an FIA switch in there. A cutof switch will not kill the enging. It can easily over come with a p encil. Something an FIA switch can not be done

 

Red 90 now please explakn to me and everybody else. You fit an FIA switch inti the neg line. HOW DO YOU WIRE THE  LIVE IGNITION WIRES IN ?

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On 9/11/2020 at 6:12 PM, mmgemini said:

So you are quite happy to be able to run rthe engine with the neg wiring switched off to blow the alternator.

It makes no difference which side you switch, negative or live. The FIA switch has 3 independent switches, the main one for the power and two additional ones that are used to stop the engine and put a load on the alternator. That's the difference between an "FIA Switch" and a "battery cutoff switch". I personally prefer to switch the live but it's not an issue switching the earth as long as you remember to make sure that the alternator load resistor is connected to the unswitched earth (the same earth the alternator is connected to). There is a regulation (not sure if it's MSA or FIA) for all UK motorsport that the negative lead of the battery be easy to identify and/or have yellow tape on it, in the event of an incident this allows marshals to quickly identify the earth lead on the battery and cut it. I assume this is because most vehicles these days are negative earth and getting a pair of bolt croppers between the positive and the earth wouldn't be a good thing. It does, however, show that disconnecting the earth is an accepted way of isolating a battery and electrical system.

On the more general query, switching earths is so much easier than switching positives for electronic controllers as they are then voltage independent. Decent powered darlington transistors are cheaper and more common for switching the earth side too. I'm in the process of converting my camper controller from a centralised power controller to a distributed CAN bus system with a controller for each item to be switched and most of the work has been in converting from positive to negative switching. 

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Yes Dave the Neg lead needs to be easily identifying or painted yellow. crtainly an RAC MSA rule but I don't know about FISA as it's quite a long time since I tried to understand the yellow book

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