Electrical Engineering - arc suppression?

Author

arc suppression?

Gary Sr

2007-10-05, 9:25 am

We have railroad equipment with 12 and 24 volts direct current with negative
ground. I have junior mechanics asking why a positive engine starter post
is grounded to frame and it doesn't short out. I told him it was protected
with diodes somewhere and acted as an arc suppressor, but I was just
guessing. Does anyone know for sure or have information/schematics or
something that explains this?

thanx

Long Ranger

2007-10-05, 1:25 pm

"Gary Sr" <gbrower4@insightbb.com> wrote in message
news:4aCdnRh_ytB4k5vanZ2dnUVZ_tmhnZ2d@insightbb.com...
> We have railroad equipment with 12 and 24 volts direct current with
> negative ground. I have junior mechanics asking why a positive engine
> starter post is grounded to frame and it doesn't short out. I told him it
> was protected with diodes somewhere and acted as an arc suppressor, but I
> was just guessing. Does anyone know for sure or have
> information/schematics or something that explains this?
>
> thanx
You have a positive ground system. Works just like the negative ground we
have on cars today, except that the negative side is the insulated-isolated
switched side, and the chassis is the system common back to the positive
side of the battery/charging system. It is common in older diesel trucks,
and was used in some vintage American autos, and lots of British ones. It's
the same thing, only it is the opposite polarity.Your system probably uses
two 12 volt battery banks which are switched by a series/parallel switch to
make 24 volts across the starter coils when cranking the engine. After
starting, it reverts back to parallel for your 12 volt lights and controls.

Usual Suspect

2007-10-05, 1:25 pm

> We have railroad equipment with 12 and 24 volts direct current with negative
> ground. I have junior mechanics asking why a positive engine starter post
> is grounded to frame and it doesn't short out. I told him it was protected
> with diodes somewhere and acted as an arc suppressor, but I was just
> guessing. Does anyone know for sure or have information/schematics or
> something that explains this?

You don't have negative ground system. If the positive terminal is connected
to frame, it's positive ground.

Same as negative ground system, just invert your thinking... :-)

There may indeed be arc-suppression diodes in the circuit, but this has
nothing to do with a positive ground system. Same diodes may exist on a
negative ground system. They will protect switch contacts equally on both
types.
--
Al, the usual

daestrom

2007-10-05, 8:25 pm

"Usual Suspect" <reply@thegroup.net> wrote in message
news:0001HW.C32BB7C500288B55B051494F@news.sf.sbcglobal.net...
>
> You don't have negative ground system. If the positive terminal is
> connected
> to frame, it's positive ground.
>
> Same as negative ground system, just invert your thinking... :-)
>
> There may indeed be arc-suppression diodes in the circuit, but this has
> nothing to do with a positive ground system. Same diodes may exist on a
> negative ground system. They will protect switch contacts equally on both
> types.

If you remember to install them correctly :-)

Installing them backwards because you aren't aware it's a positive ground
system will let the 'magic smoke' out of them.

daestrom

Paul Hovnanian P.E.

2007-10-05, 8:25 pm

daestrom wrote:
>
> "Usual Suspect" <reply@thegroup.net> wrote in message
> news:0001HW.C32BB7C500288B55B051494F@news.sf.sbcglobal.net...
>
> If you remember to install them correctly :-)
>
> Installing them backwards because you aren't aware it's a positive ground
> system will let the 'magic smoke' out of them.
>
> daestrom

Not a problem. I have lots of spare magic smoke from some of my past
projects.

--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
SHIFT TO THE LEFT! SHIFT TO THE RIGHT!
POP UP, PUSH DOWN, BYTE, BYTE, BYTE!

LinkBot