Master Cutoff Switch, Alternators, and the rules?

[Chris Boyden]

When we went through scrutineering at comp,
we had a disagreement with one of the judges over the alternator connection to the battery.
I wired the hot lead on the battery side of the single spole single throw master switch. So, when you kill the master switch, the alternator lead is isolated from the rest of the car which keeps the charging system from energizing the ECU etc... On our '03 car, I wired the hot lead on the switched side of the master, and when one of us reached out to kill the car, it DIDN'T die, because the alternator kept things going.

This is how the judges wanted it wired and asked me to change it. Their argument was that all circuits need to be de-energized in case of a fuel leak. And that the alternator would still put out current, just for a little while, as the engine winds down, which posed a spark hazard.

I argued that it was more dangerous in the other scenario because, if you needed to kill the car in an emergency, by flipping the master off, the car would possibly stay on if the alternator/rec/reg was still cranking out current. Eventually, I was able to keep the wiring with the hot lead of the alternator on the battery after a lengthy discussion.

The solution to the problem is to use a 2pole double throw cutoff switch that grounds the alternator hot lead through a resistor when the master is shut off. But, these are relatively expensive ($60) compared to the $14 cutoff switches (SPST)from Delcity.

Did anybody else have problems with this?? It seems like there is a fair amount of confusion over this topic. Maybe the rules should be clarified?

Chris

[Chris Boyden]

When we went through scrutineering at comp,
we had a disagreement with one of the judges over the alternator connection to the battery.
I wired the hot lead on the battery side of the single spole single throw master switch. So, when you kill the master switch, the alternator lead is isolated from the rest of the car which keeps the charging system from energizing the ECU etc... On our '03 car, I wired the hot lead on the switched side of the master, and when one of us reached out to kill the car, it DIDN'T die, because the alternator kept things going.

This is how the judges wanted it wired and asked me to change it. Their argument was that all circuits need to be de-energized in case of a fuel leak. And that the alternator would still put out current, just for a little while, as the engine winds down, which posed a spark hazard.

I argued that it was more dangerous in the other scenario because, if you needed to kill the car in an emergency, by flipping the master off, the car would possibly stay on if the alternator/rec/reg was still cranking out current. Eventually, I was able to keep the wiring with the hot lead of the alternator on the battery after a lengthy discussion.

The solution to the problem is to use a 2pole double throw cutoff switch that grounds the alternator hot lead through a resistor when the master is shut off. But, these are relatively expensive ($60) compared to the $14 cutoff switches (SPST)from Delcity.

Did anybody else have problems with this?? It seems like there is a fair amount of confusion over this topic. Maybe the rules should be clarified?

Chris

[Colin]

i've connected the alternator to the battery for the last two years and have had no trouble at scrutineering at the AUS comp. I agree with you that this is by far the safest way to go if your only using a single pole cut-off switch. On a similar note what's with the change to the brake cut off switch rule, something about it can no longer be an ECU function but has to be an analogue circuit? caus that's all we need on the car's more relays and wiring!

[Chris Boyden]

I just wire the main ignition switch on the dash in series with the brake cutoff switch. Both are single pole single throw switches and are powered through a small fuse. That signal triggers the main relay which powers the ecu and accessories which may be triggered by additional relays. This meets the rules because the brake cutoff and ignition switch can be activated by a relay. It would sure suck if you couldn't because it would take a lot of wire to run all of the power through the brake and ignition switches!

[Mi_Ko]

Can I go a little offtopic?

For how many Amper did you design the cutoff switch?
We calculated we would need arround 45 Ampere, but we couldn't find any switch (in normal dimensions) defined for that much Ampere. The best we coul'd find (and probably going to use) is a 15A 125VAC switch.

[fsae_alum]

Here's one for everybody. What about cutting the negative side (ground from the battery)? I ask because I heard that this is the preferred method in some racing sanctioning bodies.

[Chris Boyden]

Mi_Ko,

your point is well taken about designing for the current requirements. The master switches that most of us use are rated for enourmous currents (200 amps?). be careful about rating your switch off the power rating (volts AC x amps)....the curves are not always linear with respect to power when rating it for 12V operation. In my experience, fuses and switches are still concerned with the current flowing through them not the power. here's an example
SPST-NO-DB 100mA 50 VDC Res
SPST-NO-DB 125mA 125 VAC Res
the current rating actually went down for this switch even at a lower voltage level (DC as well).
There's other examples that the current spec does go up when the voltage goes down, but it's not linear, it seems that it's generally only a couple of amps more, not a factor of 3.

But you could always test it and see!

FSAE_alum,

That's a different idea, but I'll have to think about that one some more before I decide if I like or not. Any idea why it is prefered??

-Chris

[Igor]

Well, if you disconnect the ground lead, then you can use a spanner on both the positive and the negative battery terminal without worrying about it touching the frame with the other end. (resulting in fireworks)
DC ratings on a switch are almost always lower than AC ratings as AC tends to quench it's own spark where DC keeps it going for a while. This leads to serious spark erosion on the switch contacts when you get to the number of switch throws the switch is rated for. If you do not plan to switch thousands of times, you could go somewhat over the rating and end up at the point where the contact resistance will make your switch heat up and melt.
The main problem for your battery switch will be standing up to the starter current, that will be much larger than the rest of the loads combined.

Of course all the caveat emptors apply, I don't want any teams coming after me at competition with torches and pitch forks when their contact switch dies during the endurance. :-)

And for everyone to enjoy; don't use your fingers as a switch when installing live 230V equipment like I did yesterday. It doesn't hurt that much but it leaves two small red craters in your fingers :-)

Enjoy,
Igor

[Chris Boyden]

thanks Igor, that makes sense...Now it does seem like a pretty good idea to disconnect the ground. I wonder if it's still better to ground the alternator wire through a resistor rather than suddenly remove the alternator load and damage the regulator rectifier? I've tried contacting Shindengen, but they're not much help. Maybe I should try again.

Any body ever measure the starter current on an F4i?

[Chris Boyden]

We also had our little incident with the 230V line on our bridgeport mill. The switch was vibrating off of its mounting studs. So, one day, I took the cover off to fix it, after I flipped the breaker off and unplugged it. http://fsae.com/groupee_common/emoticons/icon_wink.gif
I noticed that the ground wire was disconnected, and the other three phases were connected like they should be. So put a terminal on it and connected it to the chassis of the mill. I buttoned everything up and flipped the switch and everything seemed o.k.
It wasn't long after that, somebody was going through some metal that was touching the mill and the drill press at the same time and got shocked. I thought it was the drill press and checked it out and there wasn't any problems with grounds. Well, it shock the shit out of somebody again so I started checking out the mill. Somebody wired up the plug wrong and the ground was actually hot and the other 3 phases connected to the motor were 1,2, and ground. So, we were "single" phasing the motor while the whole damn mill was hot! I'm suprised it didn't burn up the motor, because it was like that for quite a while. I should have checked the switch and everything better the first time instead of assuming it was good. http://fsae.com/groupee_common/emoticons/icon_mad.gif But I learned something, I didn't know a three phase would run on two and be able to reverse as well.

[Igor]

I think it would be best to always keep the alternator connected to the battery and have a cut-off switch behind these two items. This would prevent the dreaded load-dump which automotive electronics should be rated for, but which is scary none the less. (at least to the EE guys) I'm not sure whether the rules allow this though.
You could catch most of the load dump with a big resistor across the switch like they do in the more expensive switches. I wouldn't know what resistor value they use though.

Igor

[fsae_alum]

I heard about cutting the negative side from a very good vintage racecar mechanic. I was helping him put a Lotus Formula Junior back together and he told me that on all the cars he does (which see races regularly) he has to wire the cutoff on the negative side of the battery. He said that the sanctioning bodies for the vintage races prefer it if not all but require it.

[Big Daddy]

I have a question for you what if your battery is only for starting??? You know a magneto type where the coil is charged and fired by a passing magnet. So cutting off the battery and alt will not kill the engine but cutting the coil will. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[Igor]

Then you have to use a multi pole switch, so you can cut both circuits. Grab one of the six-pole versions; they have an extra make and an extra break switch. I bought one the other day and it also had an extra resistor which is connected to your alternator output by the make switch to catch the transient. It was a 3ohm 11Watt one.

Igor

[El Mug]

The current measured at the battery pole in an f4i is around 8 amp dc whit the starter on, also the peak of current of the coils are around 8 amps. In the alternator the current is up to 40 amps ac, that was the top of our measured lead. I dont know how up from that is the value. We use a cutoff switch of 10 amps, disconecting the battery from the ergulator. In the 4fi thats enough to stop your engine.

I hope it helps you
Andres