Although I eventually got to grips with wiring my car, I've read quite a lot about vehicle electrics, most of which has told me how to do things,
but in many cases has not told me why.
Now, I understand that some things are switched live and some are switched earth. But why?
So if any electrical guru's out there could explain it to me, I would be one step nearer to enlightenment and therefore fulfilment.
David
Why? Money.
Costs less for one in some places, and less for the other elsewhere. Think about cable runs, if it's shorter or less complicated to have a
switched earth then it will have, and vice versa.
Does that make me an electrical guru?
No, still don't get it.
The chassis forms part of the circuit to the battery and the rest of the circuit is wiring from the battery to the switched component and back to the
chassis. Somewhere along the circuit there will be a switch, but I can't see how its position would affect the length of the wiring. So how does
that work.
Or is there another reason?
David
Alot of of things switch earth because the switch contacts last longer when theres a high start up current draw on the unit.
I don't know the answer, but it does make it hard work sometimes to work out how different things work or should work. My car's wired and
working now, but it took a while
I was wondering myself, but that makes sense as most of the energy has been used on the device before it goes through the switch reducing wear. Only
problem is even when switched off most of the circuit is still live. Don't know if I like the sound of that on a car, where faults could easily
occur.
[Edited on 5/1/07 by macnab]
I'm also puzzled by this. I don't think it can be down to cost as even Ford can't make up their minds - aren't some cars switched
live and others switched earth?
I also don't think the high start current can apply as most relays on my loom are switched earth but the ecu is switched live ......... very
confusing 
The logic is fairly straightforward. With the switch between V+and load, you are switching full current at 12+ volts.
If the swithch is after the load, you are switching less voltage, as the load has taken its share out. Current stays the same though. With less
voltage, there comes less arcing and contact wear, as someone said above.
I've found quite a few German designed circuits that switch the earth, more than any others.
Cheers,
Syd. 
I think there's a mix-up here.
Switched earth means things that have a contant +ve supply and the ground run is switched, typical example is interior illumination switches on older
tin-tops.
What seems to be being alluded to is the use of relays to handle high current switching - which is a cost save. You can have low-duty contacts and
thin wiring for the main bulk of the wiring and relays to switch the high current between items located relatively close to the battery (in most
cases, there are of course exceptions), such as starter motor and headlamps. You also reduce the fire risk, and get much less voltage drop so better
cranking force and brighter lights.
The only switched earths on my car are the reversing lights and the horn. I think this is more to do with moving parts than current consumption. You
don't want a wire chafing inside your steering wheel if it's got 12 volts on it - cue fire brigade or a the least blown fuse. If the earth
is switched, worst case scenario is permanent horn (ooeerr....)
Just read Syd's post - seems to make more sense 
[Edited on 5/1/07 by Marcus]
I think you will find it is mainly used for safety circuits.
Example is your brake warning light.
You normally run the ignition positive to the warning lamp then to the brake resevour and then handbrake.
The theory here is that the hand brake tests that the circuit is intact.
If a short occurs, then the lamp stays on instead of blowing a fuse, where you would get no warning.
As to saving switches, this is nonsense!
When a switch is open, 12 volts is across the switch irrespective of the load. The same current that flows through the load will flow through the
switch.
It is opening a switch where the most damage is caused and it doesn't matter which part of the circuit it is in.
Some circuits that need switching in different places, like interior light are far easier to do with a switched return (its not earth!) some sensors
switch to ground, oil pressure and water temp for example so again live needs to go to lamp then sensor to work. This is also a failsafe for some
other switches like brake lights and fluid level sensors.
I am not totally convinced about switches lasting longer because before the circuit is completed the full potential i.e. 12V exists between the
contacts I guess on motors the current spike will be there bu the back emf from swithcing off may not be present so that would reduce arc on switch
off.
Basically car electrics are horrible. It is not at all surprising there are a lot of car fires when you combine potentially very high currents on
circuits without fuses with single insulated cables sized to take maximum load current for brief periods of time in an environment with heat, solvent
chemicals and mechanical vibration!
I am contemplating my wiring at the moment and want it to be as simple as possible but I will try and fuse all circuits save the starter feed that
will be wired in seriously big cable double sheathed and run as short as possible, there will also be a battery isolator switch that will double as a
second imobiliser with a second set of contacts to cut ignition circuit probably via a relay to make sure the engine stops!
Regards
Caber
I think a lot to do with it is controlling the element of risk, If the live wires are in a reatively safe area ie dash there is less chance of a short
occuring here than there is on wires running under carpets or in door looms etc. And of course you have to have the what if this happened senario thus
minimising the risk, if a short occurs on earth wiring all it does is stays permanently on, if it occurs on the 12 volt wire then it's a blown
fuse at best or a fire at worst.
Power switching with relays has to be considered but that is usually to reduce the size of the switches and cables.
Regards Steve Evans.
Control systems engineer.
Hi,
My tuppence worth:
I believe it is mainly down to power distribution and convenience, I don't buy the bit about wearing out the switch less if the switch is after
the load at all. Most damage to switches is done by back EMF on inductive loads (motors, coils etc.) which is why old points-based ignition circuits
had a capacitor across the points to absorb the back EMF which can be much higher than 12 volts. A load only reduces the voltage across the switch
once it is running at full current.
Regardless of which way you switch the circuit the back EMF actually adds itself to the battery voltage and this is the voltage which is
"felt" across the switch and which causes the arc.
As for convenience, the controlling switch for most loads is usually in or around the dashboard and is not usually close to the load itself. Since the
chassis is like a large capacity wire connected to battery earth, there is little point in running earth wires from a load (eg. a horn) all the way
back to the dashboard just to switch them when you can use a local earth point or even just the mounting bolt for the load itself instead. Obviously
the load does need a supply wire and this will usually originate from the dashboard area via an appropriate fuse and/or relay. It is therefore usually
easier to switch the feed and have one cable run from control to load rather than having an unswitched feed followed by a return wire and
switched earth. Remember wires cause voltage drops, especially on high current loads which can be quite significant unless you use oversize wire.
A switched feed has the added advantage that the feed wire is only supplied with power when the control for the load (ie. usually a switch) is active.
If an unswitched feed wire is damaged and shorts out it will blow a fuse. Usually more than one function is fed from a given fuse so you would lose
more than just the faulty function. At least with switched feeds you can choose (in most cases) not to use the faulty function and just replace the
fuse until you can get it repaired. Imagine if you lost windscreen wipers just because your interior heater fan feed has a short?
Switched earth circuits are normally only used when the controlling device is near to the load. For example a thermostatic switch next to the cooling
fan. Another example is dashboard warning lights such as handbrake light or oil pressure light where the load (bulb) is at the dashboard and the
controlling device (oil pressure switch or handbrake) is remote. Here you can reduce wiring runs by switching the earth. There are other examples but
usually (in my experience) there is a logical reason why the designer has deviated from using a switched feed.
I hope this helps,
Craig.
Just to add to the topic,
The current actually flows from the negative to the postitive, hence the negitive earth so that electrons are suplied to the chassis.
Cheers
Ian
Ian,
I know what you are getting at but you might want to read this article before making sweeping statements like that.
http://amasci.com/amateur/elecdir.html
Cheers,
Craig.
ps. I agree that by supplying electrons to the chassis is a good thing as it helps to prevent corrosion a little. Better to use a chunk of Zinc
though.
[Edited on 5/1/2007 by craig1410]
quote:
The current actually flows from the negative to the postitive
I agree it's probably down to convenience. If you ave a horn button in the centre of the steering wheel, you have to provide one sliding contact if it's the earth that's switched. If you switched the feed to the horn, you would need two. Similarly, a reversing light switch will be simpler if you switch the earth.
quote:
Originally posted by Marcus
quote:
The current actually flows from the negative to the postitive
If I remember my physics, current flows positive to negative, electrons flow the other way. An electron is a negatively charged particle.
thats what marcus said! The concept of current is pos to neg, but the actual medium is electrons flowing the other way. Hence when someone says
current, they dont mean electrons.
quote:
Originally posted by JoelP
thats what marcus said! The concept of current is pos to neg, but the actual medium is electrons flowing the other way. Hence when someone says current, they dont mean electrons.
To be precise, "current" can travel in both directions as discussed in the link I provided earlier. However, "conventional
current" travels from positive to negative because it defines the flow of positive charge as per Benjamin Franklin's work on the subject. It
is just a convention which is used commonly and is neither right nor wrong.
If you are talking about just "current" then you cannot say which direction it is flowing as you are not being specific enough.
Read the article I linked to in my last posting for more details. It shows that in some cases current is travelling in both directions at the same
time and this is by no means unusual.
Cheers,
Craig.
[Edited on 5/1/2007 by craig1410]
You will find that any current flowing the "wrong" way is more to do with the battery's internal composition as a battery is in
it's self a resistance and will consume some of the current. While this may occur it is not measureable without specaialist equipment. Stick a
conventional ammeter in the circuit with a centred zero point and you will find that it will only point over one side when the switch is on.
In the case of cars, (which is what this forum is all about) this is all most people will ever need to know, especially in charging circuit ammeters
[Edited on 5/1/2007 by nitram38]
It wasn't me who started talking about electron flows versus conventional current flows...
I just provided a link to a more complete description of what is going on once the subject was raised. This forum regularly talks about more than just
cars and car technology after all and many people on the forum like reading a bit beyond the basics. If we didn't then I guess we'd all just
buy our cars at a motor dealer and then complain if one of the gauges points the wrong way...
Cheers,
Craig.
quote:
Originally posted by Marcus
quote:
The current actually flows from the negative to the postitive
If I remember my physics, current flows positive to negative, electrons flow the other way. An electron is a negatively charged particle.
quote:
Originally posted by Syd Bridge
The logic is fairly straightforward. With the switch between V+and load, you are switching full current at 12+ volts.
If the swithch is after the load, you are switching less voltage, as the load has taken its share out.