heavy85 Posted December 30, 2007 Share Posted December 30, 2007 It seems everyone puts the master disconnect in the + cable. I'm used to off-highway equipment that puts the disconnect in the ground cable. Unless I am missing something the negative side would be the best place to put the disconnect. For example (and this has happened to me) a common thing on Z's is for the battery + to short out on the frame. Having a disconnect in the batt + cable would do nothing to stop the short. Breaking the ground would stop the short. Having the disconnect in the ground requires a double failure to still have a problem if you tried to use it. Are there rules or something driving everyone to put them in the + cable? I checked the NASA rules but it did not specify. Thanks Cameron Quote Link to comment Share on other sites More sharing options...
280Zone Posted December 30, 2007 Share Posted December 30, 2007 I have installed both of mine (2 different cars) on the negative side. I did this becuse the disconnect terminals I use, seemed to fit the negative side better. Quote Link to comment Share on other sites More sharing options...
e_racer1999 Posted December 30, 2007 Share Posted December 30, 2007 from what i've heard sanctioned racing bodies require the disconnect on the + side. that's all WOM, tho. anyway, i run it + disconnect; but i also have it in a box in the back. best option for preventing sheetmetal shorting is by using the rubber clamp surrounds. Quote Link to comment Share on other sites More sharing options...
heavy85 Posted January 1, 2008 Author Share Posted January 1, 2008 Anyone else? Quote Link to comment Share on other sites More sharing options...
Tony D Posted January 1, 2008 Share Posted January 1, 2008 I'm in the "Negative Terminal" camp as well, I just posted above to BJ Hines request about similar issue. I don't see why the sanctioning bodies insist on a positive disconnect, without a ground circuit nothing will function. If the positive battery cable starts grounding to chassis and arc welding, it only does it because you have a negative cable going to the chassis, removing the ground path will stop it as positively as cutting off positive power. And doing it on the negative post kills alternators if you run your chassis ground through the same switch. The alternator still requires a complete positive-to-negative path to make power. If you pull the positive cable, the alternator will still run the engine. You pull the chassis ground/negative post off, and it kills it. Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted January 1, 2008 Share Posted January 1, 2008 . If you pull the positive cable, the alternator will still run the engine. You pull the chassis ground/negative post off, and it kills it. I can see where that is true if the battery ground is the only chassis ground. But is is true if there is also an engine to chassis ground? I know I always pull my negative battery cable first. Maybe they are just stuck in the mindset of electricians where the ground wire is never switched. Doesn't make a lot of sense when it comes to a vehicle with it's floating ground. Quote Link to comment Share on other sites More sharing options...
bjhines Posted January 2, 2008 Share Posted January 2, 2008 There are LOTS of ground points in a vehicle. More than you might think. Also consider that each sender and sensor uses chassis ground as well. You could end up FRYING something that was not meant to take the loads when you disconnect BATT. GROUND with the engine running. I am posting this diagram just for kicks. It is detailed in my thread. ... Quote Link to comment Share on other sites More sharing options...
Tony D Posted January 2, 2008 Share Posted January 2, 2008 I'm no good with drawing diagrams online.... But if the engine to chassis ground exists, it ONLY allows a loop if it's also concurrently linked to the negative post of the battery. That is what I mean, you loose voltage sensing if you disconnect the negative cable, and you simultaneously loose the return path for the field excitation, collapsing the field in the alternator killing everything. All my grounds which normally would come from the battery negative post (To Chassis @ Firewall, Starter Bolt) are all on one side of the switch, and the other side goes to the negative post. If 'chassis' is not connected to the negative post on the battery, there is no 'loop' that is completed, and the electrons can't flow. On the Lemons Car, the "Star Ground Point" on the switch went to the dash ground bar, the head, the intake manifold, the block, the starter mounting bolt, and the chassis ground point on the firewall. Many of my sensors are of the two-wire type (GM) and therefore go back to a common ground bar that is grounded at the same place as the ECU (the dash ground bar)---from experience keeping the grounds on one point when possible keeps sensor error at a minimum. Especially with a Megasquirt. Some sensors for the dash gauges are grounded through the engine head/block, but they are brought back to the same 'star point' on the disconnect switch to the battery negative. The other side of the switch went to the negative post on the battery. "Electricians" wouldn't like how fuel injector circuitry works: every control scenario I know of switches ground on them... I was under the impression this was some sort of 'Emergency Stop' scenario Disconnect and not a routine shutdown circuit. Emergency Shutdown is a particular situation, you shouldn't be killing the car switching off your 'positive disconnect' either. I think that is a moot point. Quote Link to comment Share on other sites More sharing options...
heavy85 Posted January 29, 2008 Author Share Posted January 29, 2008 Per the 2008 SCCA Solo rulebook for Modified class: "2. Master Switch - All cars shall be equipped with a master switch easily accessible from outside the car. Spec Racer Fords shall be wired per RFSRII. The master switch shall be installed directly in either battery cable and shall cut all electrical circuits but not an on-board fire system, if so equipped." So it appears that ground side is acceptable - that's where I put it anyway . Cameron Quote Link to comment Share on other sites More sharing options...
Six_Shooter Posted January 29, 2008 Share Posted January 29, 2008 Tony D, you are missing a lot of the basics of how an automotive electrical system works. You can completly remove the battery from a running vehicle disconnecting the negitive first if you like, and it will continue to run on, I have done this several times, out of need to move cars around with an improperly sized battery or none at all and boosted it from another vehcile, I do not however recommend doing this as it can ruin electrical components, due to there being no filter to remove the AC ripple that is present in the charge lead from an automotive alternator. Many sanctioned racing bodies specify the master kill switch must be on the positive side, NHRA, IHRA being a couple, although I haven't looked myself, but I believe NDRA, NMRA are also like this. It's MUCH easier to have a car shut off, by switching the positive side of the elctrical system, since it's much easier to cut the charge lead from the alternator than it is to interrupt the ground path of the alternator. It can be done as simply as running the charge lead from the alternator to the battery side of the master kill switch. Better option is to use a 4 pole master disconnect to also shut off the charge lead and have no positive potential on that charge lead back to the alternator. If you were to run the ground from the engine block to the battery through the switch, you still have the potential that a gorund path may be present, through sensors, the tranny, to the drive shaft, to the Diff, etc. In a negitive ground car it's much easier and more finite to switch the positive, there's just far too much potential for the ground path to not be interupted. Quote Link to comment Share on other sites More sharing options...
heavy85 Posted January 30, 2008 Author Share Posted January 30, 2008 Tony D, you are missing a lot of the basics of how an automotive electrical system works. You can completly remove the battery from a running vehicle disconnecting the negitive first if you like, and it will continue to run on, I have done this several times, out of need to move cars around with an improperly sized battery or none at all and boosted it from another vehcile, I do not however recommend doing this as it can ruin electrical components, due to there being no filter to remove the AC ripple that is present in the charge lead from an automotive alternator. Many sanctioned racing bodies specify the master kill switch must be on the positive side, NHRA, IHRA being a couple, although I haven't looked myself, but I believe NDRA, NMRA are also like this. It's MUCH easier to have a car shut off, by switching the positive side of the elctrical system, since it's much easier to cut the charge lead from the alternator than it is to interrupt the ground path of the alternator. It can be done as simply as running the charge lead from the alternator to the battery side of the master kill switch. Better option is to use a 4 pole master disconnect to also shut off the charge lead and have no positive potential on that charge lead back to the alternator. If you were to run the ground from the engine block to the battery through the switch, you still have the potential that a gorund path may be present, through sensors, the tranny, to the drive shaft, to the Diff, etc. In a negitive ground car it's much easier and more finite to switch the positive, there's just far too much potential for the ground path to not be interupted. I think your missing the point. Sure you may need to kill the alternator somehow but otherwise the ground is the more robust way to isolate the battery. If you look at all the failure modes where you might need to kill the battery the negative side will take care of many more than the positive side would. All it takes is a short from positive to ground ahead of the switch to make the switch worthless. Those of us who have experienced this on the Z with it's close proximity to the hood will attest to this being a fairly common. Cameron Quote Link to comment Share on other sites More sharing options...
Six_Shooter Posted January 30, 2008 Share Posted January 30, 2008 I think your missing the point. Sure you may need to kill the alternator somehow but otherwise the ground is the more robust way to isolate the battery. If you look at all the failure modes where you might need to kill the battery the negative side will take care of many more than the positive side would. All it takes is a short from positive to ground ahead of the switch to make the switch worthless. Those of us who have experienced this on the Z with it's close proximity to the hood will attest to this being a fairly common. Cameron I'm not missing the point. I've been around racing for many years, and seen people try to use the negitive side for this purpose and cause problems. Putting a switch on the negitive side of the battery is a poor "repair" for a battery that is not bolted down well, and/or has inadequate terminal protection. I work with automotive electrical daily, and know this is not a difficult thing to fix properly. Most racing bodies I have seen require the vehcile to shut off, when the switch is also turned off, and as I stated, this can be possible to do by switching the negitive side of the battery, it is much more difficult than using the positive side. The cable between the positive terminal on the battery and the master cut off switch should be as short as possible. Also the same "problem" that suggest applies to even the negitive cable, it can make a curcuit in teh very same way as a shorted positive cable, whihc can actually be looked at as "shorting" as well, though all it will be doing in the case of a negitive ground vehcile is completling the circuit and making the master cut off switch pointless. Quote Link to comment Share on other sites More sharing options...
heavy85 Posted January 30, 2008 Author Share Posted January 30, 2008 The cable between the positive terminal on the battery and the master cut off switch should be as short as possible. Also the same "problem" that suggest applies to even the negitive cable, it can make a curcuit in teh very same way as a shorted positive cable, whihc can actually be looked at as "shorting" as well, though all it will be doing in the case of a negitive ground vehcile is completling the circuit and making the master cut off switch pointless. Depending on where the battery is at you cannot make the positive terminal short (no pun intended) and still have access from the outside or near the window (SCCA type rules). Also, the 'problem' would require a double failure on the ground where only a single failure on the positive side would cause major issues. Hence why I say it's more robust. For example if there was an accident and the positive battery cable before the switch got damaged and shorted to ground. This has nothing to do with the quality of installation but has bad side effects if on the positive side. If the same thing happened except the ground shorted to frame then (one failure) then the switch still does not work but you are also not shorting the battery so there is really no issue unless the positive cable simultaneously shorts (two failures). True you cannot 'cut-off' power but at least you can use the ignition switch to turn everything off. Understand the alternator concerns but still overall see the ground and more reliable again taking into account possible failure modes and their effects. Cameron Quote Link to comment Share on other sites More sharing options...
Six_Shooter Posted January 31, 2008 Share Posted January 31, 2008 Depending on where the battery is at you cannot make the positive terminal short (no pun intended) and still have access from the outside or near the window (SCCA type rules). Also, the 'problem' would require a double failure on the ground where only a single failure on the positive side would cause major issues. Hence why I say it's more robust. For example if there was an accident and the positive battery cable before the switch got damaged and shorted to ground. This has nothing to do with the quality of installation but has bad side effects if on the positive side. If the same thing happened except the ground shorted to frame then (one failure) then the switch still does not work but you are also not shorting the battery so there is really no issue unless the positive cable simultaneously shorts (two failures). True you cannot 'cut-off' power but at least you can use the ignition switch to turn everything off. Understand the alternator concerns but still overall see the ground and more reliable again taking into account possible failure modes and their effects. Cameron Do the SCCA rules dictate that the car needs to shut off (while running) via the master disconnect? The need for this in other santioning bodies is in case the driver (possibly you ) is for some reason incapacitated, hit your head, knocked out, fainted, etc and are otherwise unconcious and can not shut off the engine. This situation could really go from mildly uncomfortable to a real night mare quickly. Broken fuel lines fed by electric pumps, and similar things. Although, just thinking about it now, with a simple relay set-up this could be done, possibly even on the negitive side, without running the alternator charge lead to the other side of the switch. I'll have to draw it up. Hmmmmmm Quote Link to comment Share on other sites More sharing options...
Nostalgic_hero Posted February 3, 2008 Share Posted February 3, 2008 Im on the positive terminal cut camp.. but it does have its limitations. Mainly the wire running from the Alternator to the Positive terminal of the battery. Even when the main disconnect switch has been deactivated, there is a serious short potential when servicing the alternator if the alternator power output wire were to contact ground. There are some other options out there for people that NEED that extra step to keep sleeplessness at bay ... Kissling sells remote high amperage breakers that can be controlled electronically. so the alternator can be physically disconnected from the battery when ignition is cut. http://www.kissling.de/english/start_e/start_e.html cheers! Quote Link to comment Share on other sites More sharing options...
Six_Shooter Posted February 3, 2008 Share Posted February 3, 2008 Im on the positive terminal cut camp.. but it does have its limitations. Mainly the wire running from the Alternator to the Positive terminal of the battery. Even when the main disconnect switch has been deactivated, there is a serious short potential when servicing the alternator if the alternator power output wire were to contact ground. There are some other options out there for people that NEED that extra step to keep sleeplessness at bay ... Kissling sells remote high amperage breakers that can be controlled electronically. so the alternator can be physically disconnected from the battery when ignition is cut. http://www.kissling.de/english/start_e/start_e.html cheers! You can also get a 4 terminal master cut off switch. Taylor has them. This would provide the main battery disconnect along with an alternator specific cut off. This same switch would also need to be used on the negative cut off, to control relays for ignition cut off. When disconnect the negative, the car would still need to shut off, before the ground potential would be lost from the alternator, so a 4 pole switch would need to be used to cut off the negative on the coil (or control) side of the relay(s), to use a negative only cut system. P/N 1033 http://www.taylorvertex.com/Products/pdfs/taylorcatalog37.pdf Quote Link to comment Share on other sites More sharing options...
Nostalgic_hero Posted February 5, 2008 Share Posted February 5, 2008 absolutely six shooter... i just posted the relay because it allows you to greatly shorten that alternator hot side (say by putting the relay right by the alternator. (this helps in rear battery mount, rear kill switch, front engine arrangements) cheers! Quote Link to comment Share on other sites More sharing options...
Six_Shooter Posted February 5, 2008 Share Posted February 5, 2008 absolutely six shooter... i just posted the relay because it allows you to greatly shorten that alternator hot side (say by putting the relay right by the alternator. (this helps in rear battery mount, rear kill switch, front engine arrangements) cheers! The live lead would be shorter going to the switch (4 post) than using the relay at the alternator, with a rear mount battery. The 4 post switch has two isolated circuits, one for main battery cut off and the other to kill the alternator lead without running the charge side directly to the battery. Now if you were keeping the battery upfront, then I could maybe see a use for the relay, but then you could just disconnect the battery, since it will be right beside you while working on the alternator. Quote Link to comment Share on other sites More sharing options...
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