Master Switch Wiring

[Tickers]

This seems to come up every year, but each time the discussion seems to have a different result.

How do you guys wire your master switches, and do you get any stick from the scrutineers?

Looking at the FSG diagram, the rectifier must run all of its power through the spade terminals on the bottom of the switch. How much current can these terminals take? It's easy to find ratings for the large posts, but not for the smaller ones.

[Tickers]

This seems to come up every year, but each time the discussion seems to have a different result.

How do you guys wire your master switches, and do you get any stick from the scrutineers?

Looking at the FSG diagram, the rectifier must run all of its power through the spade terminals on the bottom of the switch. How much current can these terminals take? It's easy to find ratings for the large posts, but not for the smaller ones.

[PeterK]

I don't know about this 'FSG' diagram, but generally we place the master switch between the battery's ground terminal and the vehicle chassis. Emergency stop button and brake overtravel switch operate through a relay which cuts power to the ECU, fuel pump and coils.

[PabloH]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PeterK:
I don't know about this 'FSG' diagram, but generally we place the master switch between the battery's ground terminal and the vehicle chassis. Emergency stop button and brake overtravel switch operate through a relay which cuts power to the ECU, fuel pump and coils. </div></BLOCKQUOTE>

This is exactly what we do. No trouble at Tech.

[TMichaels]

Hi all,
wrt. the master switch the rules say: "Disable power to ALL electrical circuits, including the battery, alternator, lights, fuel
pump(s), ignition and electrical controls."

Therefore it has to separate alternator, battery and the rest of the car from each other. The often seen solution to only cut between battery and rest of the car is not within the rules, since the alternator/rectifier and the battery are still connected to each other.

Furthermore switching ground is a bad idea and not "good engineering practice", because a ground failure has a way higher probability than a +12V failure.

Short version: You will not pass scrutineering at FSG with a master switch that only cuts between battery and the rest of the car. The alternator/rectifier has also to be cut. This is also why a master switch is shown in the rules which has the ability to also cut the alternator/rectifier and not the standard 2-pole version. The terminals are usually 6,3mm and are able to carry about 30A.

Regards,

Tobias

[PeterK]

Tobias,

I understand where you are going, however, I have to disagree that putting the switch between the battery and the rest of the car on the negative side does not "Disable power to ALL electrical circuits...", nor is it "not "good engineering practice""(sic)

Specifically you mention the alternator/rectifier. If the ground for the rectifier is connected on the battery side of the master switch then power will certainly be disabled to both the alternator and the rectifier as no current can flow out of the rectifier (or stator coils).

If you put the ground for the rectifier on the car side of the switch then you certainly will have a problem and the switch will likely not shut off the car.

Also, in terms of safety, it is far better to remove the ground to the chassis then to switch the +12V at the battery because if you are working on the car (especially anything to do with the battery it is far easier to short the battery if you switch the +12V. This is one of the reasons the first step with most automotive repairs is to remove the negative terminal of the battery. Of course the battery terminals should always be covered and bad things should never happen to good people but sometimes when you are working on the car at 3am...

Maybe things in FSG are different but over here in North America this setup has never been a problem passing tech and it certainly does disable the car.

[TMichaels]

Peter,
we probably have some misunderstandings here.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Specifically you mention the alternator/rectifier. If the ground for the rectifier is connected on the battery side of the master switch then power will certainly be disabled to both the alternator and the rectifier as no current can flow out of the rectifier (or stator coils). </div></BLOCKQUOTE>
With that setup current is flowing from the alternator/rectifier into the battery as long as the engine is spinning, even if the master switch is shut off. This means that there is still a connection through which power flows, but according to the rules it has to be disabled without mentioning a specific direction.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Also, in terms of safety, it is far better to remove the ground to the chassis then to switch the +12V at the battery because if you are working on the car (especially anything to do with the battery it is far easier to short the battery if you switch the +12V. This is one of the reasons the first step with most automotive repairs is to remove the negative terminal of the battery. Of course the battery terminals should always be covered and bad things should never happen to good people but sometimes when you are working on the car at 3am... </div></BLOCKQUOTE>
I agree that this is true for working on the car, but not for operating the car. The master switch is there to safely shut the car down, not to repair it safely.
If ground is connected to the chassis, it is far more likely that contact is accidentially made between the negative pole of the battery and the chassis, which would render the master switch useless.
If the positive pole of the battery contacs the chassis, the short cut will shut down the car as well which makes this setup fail-safe wrt. the function of the master switch, if you switch +12V. Switching ground would not be fail-safe, therefore I have the opinion that this is not good engineering practice.

BTW: All well developed ECUs in the automotive industry switch to the positive pole.

Regards,

Tobias