Switching ground instead of positive lead

[jspriddy]

Re: Switching ground instead of positive lead

I wasn't contemplating putting the fuses in the ground, just posing a hypothetical question. Someone above said "So your actually better protected with the fuse near the - terminal so that it blows before your load reaches overcurrent. Just actual theory, never could figure out why everyone alwas fuses the +?"


I understand that the closer you have the fuse/cb to the power supply, the more hot wire it "kills".

John

 

[NetDoc]

Re: Switching ground instead of positive lead

never could figure out why everyone alwas fuses the +?"

I've been thinking about this some more, and I still see that the biggest reason we use negative as a common is consistency. In the 60s it was not uncommon to work on British vehicles that had a positive ground. In fact, I did a number of conversions from 6V positive earth systems (generators) to 12V negative earth with an alternator. We had only worry about polarity on batteries, starters and the alternator. Of course, we changed a boat load of bulbs that burned quite brightly for a bit and then went poof. I did not work on boats until recently, but I bet the Brits did the same on their boats.

As for galvanic action, anything that is positively charged will not be eaten. Zincs are called sacrificial anodes as I doubt we would have that issue as electrons move opposite of what most people assume: negative to positive. Of course, we could experience some weird plating as we attracted elements out of the ocean.

 

[jspriddy]

Re: Switching ground instead of positive lead

( pos+)-----------(10 amp load)---------(5 amp fuse)-----(- neg)

OK, here, and this is purely hypothetical now, so no one will get their drawers in a knot over it, in the above diagram, poor though it may be, God, or the Gods, depending on what...ever, willing it will serve in place of me having to draw and scan and attach something, and I know this sentence is running on long, and Mrs. Turner would twist my ear for that, IF the load drawing 10 amps did actually draw those 10 amps and prevent them from reaching the 5 amp fuse, would the fuse still blow? In other words, does the draw (in this case, 10 amps) go beyond the load to the fuse?

If I lie awake all night pondering this, I hope all of you do also.

John

 

[Fed]

Re: Switching ground instead of positive lead

Yes

 

[dewey0726]

Re: Switching ground instead of positive lead

The way you have the circuit drawn. You would never get 10 amps through it. The 5 amp fuse would blow at around 5 amps. In a series circuit as you have drawn here the current is the same everywhere. If you have 10 amps going out of the battery, it has to go back in the other side. So you would have 10 amps through the entire circuit. The current flows through every component of a series circuit at the same rate.

 

[jspriddy]

Re: Switching ground instead of positive lead

Fed: Yes.

The way you have the circuit drawn.... The current flows through every component of a series circuit at the same rate.

Ok, that's what I was looking for. So the fuse would protect the circuit regardless of its location. Thanks.

John

 

[j_martin]

Re: Switching ground instead of positive lead

The reason a fuse is put in the circuit first in the string at the positive side is this.

It can then protect from a short to ground at any point.

If the fuse is in the negative lead, and the wire between the load and the switch shorts to ground, the wire then burns out, protecting the fuse.

14 gazillion electrical engineers are not likely to be wrong. wire it:
Battery +
fuse
switch or load (doesn't matter)
The other one
ground.

hope it helps
John

 

[NetDoc]

Re: Switching ground instead of positive lead

In other words, does the draw (in this case, 10 amps) go beyond the load to the fuse?

That would be fine for a positive earth system. Most modern systems use a negative earth and it's best, as J.Martin pointed out to fuse the "Live" side and not the earth. Always fuse the live wire!!! You're not just wiring for the appliance, but the wires and switch going to the appliance as well. I actually use two fuses for all my electrical circuits EXCEPT the battery and the automatic bilge pump.

Battery switch (+)--------{60 amp breaker on transom}--------------------------{Fuse Panel on dash}------all appliances (lights, ignition, etc)------ (-) Common

 

[jspriddy]

Re: Switching ground instead of positive lead

Thanks, Netdoc and John. Thanks to all you guys I'm pretty well prepared to wire it. Got to gather a few supplies and I'll be good to go. As I said, it's a very simple system, lights, two small pumps, trolling motor, fish finders and that's about it. The lights are to comply with regulations and I doubt (hopefully) that I'll ever use them.

Thanks to everyone for their time and efforts.

John

 

[Fed]

Re: Switching ground instead of positive lead

For a small system like that John you really only need 1 fuse in the positive wire at the battery, 3 switches (lights, pump, pump) at the dash and a connector to link the negatives.
The trolling motor will run straight from the battery with its own fuse/CB and have nothing to do with the rest of the circuitry.