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Discussion Starter #1
I asked the NHRA this yesterday, and I'm waiting for a reply.

<some unimportant intro snipped>

I understand the reason for NHRA rules concerning a kill switch that
disables electronics and the engine in very fast cars. To me, the rule for
faster cars make sense. That clarity disappears with slower vehicles that
have relocated batteries. I understand the increased hazard of locating a
battery in the trunk, but I think the NHRA is actually increasing the
likelihood of fire with the rule wording for simple battery relocations. I'm
not sure what the NHRA intended. The problem stems from the alternator
system.

First, there is an apparent conflict. Without a relocated battery, the rules
have no provision for a engine kill switch on slower cars. This leads me to
believe killing the engine is not generally a big worry, and that makes
sense. Most cars have fuel pump shutoff on impact, or other safety features,
and a runaway passenger car is much less likely to be a missile that just
sits there trying to re-launch after an incident.

The problem surrounds relocated batteries. Let me explain the conflict and
the hazards. Following the letter of NHRA rules, the alternator wire would
have to be disabled, not just the battery, or the vehicle would remain
running with an open battery line. To get around this, a second high current
wire has to be routed to the battery. This doubles chances of pinched wire
or insulation failure, increases expense, and complicates the wiring. Two
large conductors have to be routed, instead of one.

If two wires are routed, and if the positive voltage feed lead to the engine
is opened, the alternator wire would remain hot from battery supply voltage.
This would comply with the apparent NHRA rule about killing the engine,
something that is NOT a concern with slower passenger cars that do not have relocated batteries. It would, however, violate the NHRA rule of making all electrical circuits or systems dead. The alternator and the high current
cable from the alternator would be back-fed from the battery, regardless of
positive kill switch position.

If wiring is pinched or the engine leaks fuel, you will still have a hot
wire running the length of the vehicle.

To fully comply with NHRA rules, a person with a street car or slower car
having a relocated battery would have to use a DPST disconnect. As far as I
know, DPST or DPDT disconnects fitting size and type requirements are rare
(if they even exist), and the wiring would just further confuse people that
already can barely understand safe wiring.

If NHRA's intention is just to open the battery supply in the event of a
fire or wreck, then the section about killing the engine should be deleted
from slower car requirements. The result would be a safer, more reliable
electrical system, with the NHRA being no part in confusing people or
forcing them to add long high-current alternator feeds that are permanently
energized.

If the NHRA's intention is to kill the engine, then why is it not included
in factory battery positions for all slower speeds and ET's?

If the rule is to just be able to kill battery power if someone's relocated
wiring has an insulation failure, or in an accident, then the engine shut off
requirement should be deleted for slower cars.

Also, if the NHRA is going to force use of true kill switches, the NHRA
should mandate proper fuse links on the alternator lead. It would also be a
large improvement in safety if the NHRA mandated a fuse link from the
battery to chassis or "ground". In a passenger car, the safest way to
disconnect a battery is opening the ground terminal, and a fuse link is
mandatory for improved fire protection if a high current cable faults.

As it is right now, NHRA rules apparently disagree with themselves. The
rules actually force or steer people toward use of less safe wiring.

Thanks,
Tom
 

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The last thing you said there is actually how I hooked mine up. It's not NHRA legal if they were to inspect the wiring, but the car shuts off when you flip the cutoff switch. I wired up the ground wire from the battery to the kill switch. How is a car going to get or become a ground when the battery isn't grounding it? Would it get ground from the "ground or concrete" if you have a flat tire or something?
 

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Discussion Starter #3 (Edited)
The last thing you said there is actually how I hooked mine up. It's not NHRA legal if they were to inspect the wiring, but the car shuts off when you flip the cutoff switch. I wired up the ground wire from the battery to the kill switch. How is a car going to get or become a ground when the battery isn't grounding it? Would it get ground from the "ground or concrete" if you have a flat tire or something?
Concrete does not "wear out" batteries, and it does not conduct enough to be a problem in low voltage systems. It is not like metal at all. Concrete is more like slightly moist dirt for conductivity. It is a post of the battery we need to isolate, because the battery is a source of power for things connected between the positive terminal post and negative terminal post, not to anything else in the world. You disconnect and isolate at least one of those posts, and nothing can happen no matter what.

I am not sure where they got the "positive terminal only" kill switch rule, unless it is from dual batteries. With dual batteries there is some chance the second battery could get loose in a wreck and have the negative post grounded, defeating the switch. That is the ONLY case where a kill switch is probably just as safe in the positive as negative path.

As I read the NHRA rules, it appears they just casually threw in the switch requirement for relocated batteries in slower cars. I'm only guessing (from reverse engineering), but it appears they wanted to have some protection with a cable short since the owner could do about any sloppy thing with high current wiring.

I had a certified chassis Mustang that would run in the 8's that I ran occasionally in national events. I raced that car 5 or 10 years, and my cut out switch killed a small number 6 that powered the electronics. That number 6 was also fused with a fuse link at the battery feed end. The starter cable was 0 copper, and it ran from the starter to a start relay near the rear batteries. That relay only put power on the big cable when the relay was closed. This system passed their tech several times.

With a single battery, ignoring alternator run on (that may not be an issue), it is SAFER to place the cutoff in the negative. If the battery gets loose, the only thing that can happen is the switch might be bypassed. With the switch in the positive lead, you can have a battery fire with a loose battery. All that has to happen is the positive battery post contact the chassis or contact something grounded. This is the same reason everyone in the world tells people to disconnect the negative FIRST when servicing a battery or 12V system, or when killing battery power. Yet for some reason, the NHRA wants disconnects in the positive, which is clearly less safe. Service procedures drum in our heads to pull the negative, and that is correct. It is safer.

It will be interesting to see if and how they respond.

Tom
 
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