Model Boat Mayhem - Forum

Please login or register.

Login with username, password and session length.
Pages: [1]   Go Down

Author Topic: Why Positive?  (Read 2176 times)

ianb

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 76
  • Location: Japan
Why Positive?
« on: November 02, 2008, 11:54:35 AM »

When I see suggestions or recommendations for fuses and switches between the battery and ESC or motor, its always "Put the fuse in the positive lead".

I can see this point when its a negative ground system such as in cars (full size) but can't figure out the reasons for doing so in a model.

It seems to be an important point. Can anyone enlighten me as to why? Please keep any explanation simple!

I always thought that electrons flowed from negative to positive in a DC circuit, but maybe this is just another thing that I have gotten wrong.

Cheers

Ian
Logged

malcolmfrary

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 5,563
  • Location: Blackpool, Lancs, UK
Re: Why Positive?
« Reply #1 on: November 02, 2008, 04:52:35 PM »

The negative is by common usage, common.  This applies when multiple batteries of various voltages are used.  If a battery is connected to give a neg supply (as in +/- 12 volts) then the non-common ends should be fused.
When measuring voltages, it is normal to refer to the ground line - a blown ground line fuse creates interestingly confusing readings.
The -ve line is normally the reference line for the receiver aerial, and using the +ve as ground has the potential to compromise radio performance.
Logged
"With the right tool, you can break anything" - Garfield

wideawake

  • Guest
Re: Why Positive?
« Reply #2 on: November 02, 2008, 05:40:54 PM »


I always thought that electrons flowed from negative to positive in a DC circuit, but maybe this is just another thing that I have gotten wrong.

Cheers

Ian

You're quite right Ian.   Conventional current flows from positive to negative but electrons (which are -ve particles) flow from negative to positive.   The decision about positve and negative and conventional current was taken long before anything was known about electrons or atomic physics generally.

Guy
Logged

Martin [Admin]

  • Administrator
  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 19,052
  • Location: Peterborough, UK
    • Model Boat Mayhem
Re: Why Positive?
« Reply #3 on: November 02, 2008, 09:30:11 PM »

Quote
I can see this point when its a negative ground system such as in cars (full size) but can't figure out the reasons for doing so in a model.

..... convention!  {:-{
Logged
"This is my firm opinion, but what do I know?!"    -   Mayhem FaceBook Group!

andygh

  • Guest
Re: Why Positive?
« Reply #4 on: November 02, 2008, 09:34:47 PM »

Quote
convention!


yes, though all the control systems I maintain at work are fused on both + & -
Logged

wideawake

  • Guest
Re: Why Positive?
« Reply #5 on: November 02, 2008, 09:57:53 PM »

Quote
convention!


yes, though all the control systems I maintain at work are fused on both + & -

That's interesting.  Presumably low voltage circuits.  Years ago, where I worked I seem to remember mains distribution panels were fused that way  but much effort was put into removing the fuses in the neutral side after someone realised that if one blew before the live side fuse it left an apparently dead unit with mains on the live side of the PSU mains transformer, nasty if you touched it while in contact with the chassis.

Guy
Logged

wombat

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 642
  • The view from the shallow end of the gene pool
Re: Why Positive?
« Reply #6 on: November 02, 2008, 10:19:37 PM »

Only place I have seen fusing on both ends of the circuit is on polyphase systems where a supply is taken from phase to phase rather than phase to neutral and both sides are equidistant from earth voltage. In the event of a fault to earth power is drawn from both phases, so a single fuse will not adequately break the fault current. In most systems it represents a health hazard.

OK the choice between positive and negative is nominally arbitrary - however it is helpful to define one as the source of energy and one of as the sink. These influence the layout of the circuit. It is better to fuse the positive as the current paths tend to be better defined and segregated

Wom
Logged
The prat in the hat - www.floatingwombat.me.uk - Have look & say Hi

andygh

  • Guest
Re: Why Positive?
« Reply #7 on: November 03, 2008, 01:36:09 AM »

Quote
That's interesting.  Presumably low voltage circuits.


yep, 24v control

PS. The + and - 24v is fused, zero volts is not
Logged

wideawake

  • Guest
Re: Why Positive?
« Reply #8 on: November 03, 2008, 08:39:23 AM »

Quote
That's interesting.  Presumably low voltage circuits.


yep, 24v control

PS. The + and - 24v is fused, zero volts is not

Right that makes sense of course.  Both supply legs fused, return leg not fused.

Cheers

Guy
Logged

sweeper

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 199
  • When all else fails, read the instructions
  • Location: On the edge of reason, in the state of Confusion
Re: Why Positive?
« Reply #9 on: November 03, 2008, 11:34:22 AM »

The use of double pole fusing dates back to the start of public electricity supply. Many places received their supply from the same source as the tramway systems (DC). A 3 wire layout was the norm, +ve outer, -ve outer and midwire (at or about 0v). As a hangover from this time many areas still use a three wire supply. The wiring of buildings allowed the supply to provide 240v (either outer to midwire) for lighting and heating or 480v (outer to outer) for motors. Due to the cost of replacing the networks, there are still in existence, large amounts of these networks. In my early career, I was involved several times in installing equipment for use on 480v. A real pain to do economically and an even larger pain if you forgot the double pole fusing. Many a finger has been warmed by the shock and many a screwdriver blade has been burned off because the neutral fuse had blown, leaving full mains voltage lurking for the unwary. (Been there and done it - several times).       
From the "standardisation" agreement in 1928 it was set that the public supply should be a 3phase 4 wire arrangement and that for PUBLIC supply one pole should be effectively bonded to earth (normally the star point of the supply transformer). This provides two functions (1) a reference point for all voltage measurement (2) a means of ensuring effective operation for protective devices in the event of an earth fault.
   For high voltage networks the rules are that one point in each network shall be earthed.

  As for the original point of "why fuse the positive". I was taught as an apprentice that because the popular analogy for electricity was the flow of water, most people can visualise the flow from a positive bucket (raised to a height) down to a negative bucket (at ground level). Try to convince people that water actually flows upwards from the negative bucket...
Even Tommy Cooper would struggle to get that concept across :}
 
Logged
Pages: [1]   Go Up