Positive ground. 7

[itsmoked]

Got a positive ground on an instrumented rail car. The car was designed to do coupling based tension measurements, (think how many tons of pull are there between 7 locomotives and the 125 coal hoppers behind).

The car has racks and racks of ancient electrical measurement stuff in it. Looks like those panels in front of the phone operators of old.

Besides all this instrumentation there are house lights, dual 3-71 diesel generators, blowers, battery charger, air conditioning and some controls.

Well like I say, the dang car is wired with a positive ground!!

All the instrumentation is going to be yanked and the car is going to be converted into an escort car for things like Space Shuttle Booster motor sections.

Questions:
1) Why would someone have made this car positive ground?

2) What would be stumbling blocks to converting it to negative ground?


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[RRaghunath]

positive ground is the preferred arrangement in low voltage DC systems.
The entire DC system, thus earthed, will be negative with reference to earth, thus repels water (moisture) and helps prevent corrosion.

What are your reasons for changing the system to negative ground?

 

[waross]

Hi Keith.
Ford used positive ground with six volt batteries. They changed to negative ground about the same time as they went to twelve volt batteries, sometime in the fifties.
How old is the rail car?
respectfully

 

[itsmoked]

Hi rraghunath;
Can you provide more information as to how either of your points actually work?


Yo waross!
Not sure 40-50s?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

I think that British cars had plus grounded as late as in the sixties. Maybe even later.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

I can understand a car having a + ground from a strictly random selection by some designer but for someone to stick this into a 90ft railcar.. I just don't understand.

This is a system that had a single 1,800 pound 30V FLA battery. We have just pulled the dead battery and replaced it with five 6volt batteries. In the midst of this we discovered the positive ground.

The reason we'd like to change it is because all the myriad things that go into making a comfy, high end, "motor home" are generally built assuming negative cases, a car stereo for example.

I suspect that if we don't change it we'll have to worry about:
Stereos.
Laptops.
Every technician who approaches it.
High end communications equipment.
?

If we do change it we'll have to worry about:
Fans turning the wrong way.
AC Generator controls?
Battery charger.
Engine starters?
A/C controls?
?



Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[bimr]

I had one of those positive ground brit cars and tried to mount a negative ground radio in it. I kept the radio isolated for the most part, but I can still remember when the case of the radio accidentally grounded to the chassis. The power wiring instantaneously burned out.

 

[itsmoked]

Yes it would not be good to have the car escorting a million pounds of rocket fuel catch on fire. lolo

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jghrist]

I wouldn't expect any high-end equipment with a dc supply to have the case ground connected to any internal circuitry. In power systems, it is most common to have the dc system ungrounded. Connecting the case ground to the negative dc bus of the equipment would cause an unwanted dc system ground.

I'd say you would have less chance for problems if you kept the dc positive ground. If anyone wants stereo or laptops when riding the instrument car, make them bring their own battery operated ones.

 

[waross]

Hi Keith;
It may not be that hard to change. Fans that old will probably have wound fields and will not reverse direction with a change in polarity. The starter motors should also accept either polarity of current. It is only small equioment such as riding tractors and smaller outboard motors which have permanent magnet fields in the starters.
If you have old brush type battery charging generators, they can be polarized to generate either polarity.
I saw the battery installed backwards in an old genset one time. When the engine was started, the output of the generator built up in the wrong polarity. When the output of the generator reached the point that the cutout relay closed, the 12 volts coming from the battery met the 12 volts going from the generator. The battery overcame the generator and there was an instantaneous surge of current. This re-polarized the generator in the opposite direction and everything worked fine, except that the ammeter indicated discharge when it was charging and vice versa.
We changed it back the same way.
I don't however, recommend this method of polarizing. If the wiring does not withstand the surge, or if the battery cannot overcome the generator there will be interesting issues. The point is: the polarity of DC generator is easily reversed. The exact procedure varies with the type and connection of the voltage regulator, and the internal connection of the generator field.
respectfully

 

[RRaghunath]

What I wrote (practice of positive ground) is widely practiced in industrial environment for 24V batteries etc.

I am not much conversant with the electrical systems in automobiles / rail cars.

While the positive grounded system suffers less with corrosion related problems, I am yet to see any benefit with the negative grounding in DC systems.

 

[BJC]

Just for interest,may not have anything to sya about the negative ground google up "dynamometer car" . Heres one hit.

http://en.wikipedia.org/wiki/Dynamometer_car

Dynometer cars were state of the electrical art in the 20s and 30s.
That car may be worth something a museum piece, check before you tear it up.

 

[itsmoked]

BJC:
That IS what the car is. I'll pass this to the owner he'll get a kick out of it.

Thanks.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BJC]

Itsmoked
I would call the California RR museum. The owner might make a swap for pullman car or something closer to what you want.

 

[itsmoked]

Thanks BJC it's not my car, I just provide technical advice. It's a very complex business that people execute on a shoe string. All of the small group of private car owners generally know the others and what they're up to. Just moving a car to the museum would take thousands of dollars and take probably two months of lobbying. Not worth it.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BJC]

I was curious enough to write the California RR museum, his is the answer I got. Im anot trying to mettle jus wanted to see how rare dynamometer cars were.


''Well, I'd hate to see one torn apart. at the very least, I'd like to
see the equipment saved. What company owned the car, and where is it
now?


Kyle

Kyle K. Wyatt
Curator of History & Technology
California State Railroad Museum
111 "I" Street
Sacramento, CA 95814
Phone: (916) 324-7660"


Never discount trading to a museum or donating untill you talk to your accountants. All is not as it may seem.

 

[itsmoked]

Hmmm.. most interesting.

I shall pass this on.
(a star for the extra effort)
Thanks!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Warpspeed]

Positive ground systems were fairly common in days of old with dc distribution systems, and the reason is corrosion of the wiring due to electrolytic action. Telephone exchange battereies are still positive side grounded in many parts of the world.

The reason being that leakage current to ground tends to plate delicate wiring with all sorts of mysterious ionic growths, instead of eating away precious copper wiring. Back in the days before semiconductor electronics, when much insulation was paper and wood, cotton covered wire, and other pre space age materials, humidity, condensation, and creeping moisture was a real problem.

Negative grounding probably started around the turn of the century, when valves required positive supply potentials. But large dc distribution systems still preferred positive grounding wherever possible, especially with buried cables.

These days electronic circuits (mostly) run off a positive main supply rail, so most dc distribution systems are now negative ground, and it would be very impractical to do it the other way.

 

[Compositepro]

I would guess that perhaps negative grounding became standard because vacuum tube filaments are negative and also required a heater power supply. Filament transformers were typically around 6 volts and would have required high voltage insulation without a negative ground system.

 

[itsmoked]

Thanks guys for finally putting the bell on that cat.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com