So I bought a motorcycle that doesn’t work, and I’m trying to fix it up. I’m pretty new to electrical work, and having trouble with the concept of the ground. On my bike, the electrical components are grounded to the steel frame, and the negative battery terminal is also grounded to the frame.. so is the frame basically a conductor back to the negative terminal? Why is it not dangerous to be exposed like that unlike the insulated wires??
In general the concept of ground is a little hazy for me.
Thanks
In: Physics
Yes, the frame is a conductor back to the negative terminal.
The idea is that if a fault develops then it goes through the frame back to the battery (and probably blows a fuse) rather than going through you and giving you a shock.
Think of voltage like water pressure in a pipe. The electricity wants to flow from high pressure (the positive terminal) to low pressure (negative terminal).
If the pressure between two points is the same electricity won’t flow. The negative is called “ground” because it’s the same voltage (pressure) as the earth. If you’re touching the earth and the negative there’s no voltage/pressure difference so nothing flows. If you touch the positive cable then there is and you get a shock.
Most car and bike electrical systems use the chassis / frame as the negative, and yes it’s the conductor back to the negative terminal on the battery to complete the circuit.
So there are a couple of concepts in play here. First there is potential, voltage is a measure of how much potential energy there is in an electrical system, higher the voltage the greater potential. To jump through the human body, there needs to be a decent amount of potential. 12v you find in a car battery is not enough to jump through you without mitigating cases (wet hands, open wound, ect). Now the ground side of a battery is not at the same voltage as the ground you stand on, but the difference in that voltage very, very rarely gets above a full volt. Because of that, you can have the ground side exposed with no issue (certain systems actually use a ‘hot’ chassis instead of a grounded one, because again, 12v really won’t jump through you).
‘Ground’ is really misused in electrical systems. Earth ground is any point connected to the physical ground outside. However, circuit ground is the lowest voltage potential point in a circuit. Earth ground and circuit ground can and usually are at different voltage potentials from each other, in fact earth ground can be different from earth ground at a different point.
Ground is a common concept used from the tiniest of electrical circuits to electrical grids. Let’s stick to the motorcycle’s electrical system to look into it. Ground is one section of the whole installation that is (for practical purposes) all at the same voltage and is set as a reference point. In an absolute majority of cases, the negative terminal of the battery is connected to ground. Also connected to ground is all the electrical equipment: lights, horn, starter, instrument cluster, etc. When you turn on the headlight, (simplified) you basically close a path from the battery’s +12V to one side of the lamp, while the other side is at ground -> you now have 12V across the bulb, the intended current flows from the + of the battery through the bulb back into the – of the battery. Now, you could run a wire from every single electrical component to the negative terminal, or have some sort of tree structure. Since you already have a conductive piece of metal spanning across most of the vehicle, it is however convenient to use just that. So all the bulbs in the rear light assembly may just have their negative on a common metal baseplate, which is in turn screwed to the frame, or connected to it through a couple of inches of wire. This basically cuts the amount of wire needed in half and one can measure pretty much any voltage referencing it (aka holding the black probe to) an exposed part of the chassis. Yes, the frame is the return path for the current in this scenario. It’s not dangerous because the battery’s internals “work” to create a voltage difference of 12V across the terminals, not to establish any kind of potential difference between the chassis you’re touching and the garage floor you’re standing on. The Cables are insulated because copper is sensitive to corrosion and because no other voltages should touch each other or ground. There are generally fuses to prevent anything too bad from happening, but it can still damage stuff or cause unfunny sparks.
Ground in vehicles and ground in your house serve a different purpose
Things labeled “ground” are mostly so there’s an agreed upon reference for 0V in the system. Just about every system has a “ground” terminal somewhere just so we can agree what all voltages should be measured to
The ground pin on outlets in your home is a *safety* ground, it is not supposed to carry currents under normal circumstances, its just there to make sure that parts of the device you can touch remain safe at all times. Back in your breaker panel all the ground wires and neutral wires and tied together and hooked to a long metal rod that goes into the dirt, this is the actual ground reference, the ground in all your outlets is just for safety
For vehicles and other relatively low voltage DC applications, you don’t need a safety ground. There isn’t enough voltage available for anything really shocking to happen so ground is simple the reference 0V point for everything in the system. In general the negative terminal of the battery is connected to the body of the vehicle so we can define the negative terminal as 0V and the positive as +12V to keep things simple.
Having the frame tied to ground also greatly simplifies wiring of the vehicle as you only have to route a single +12V wire to many systems as they can use the bolts that connect them to the frame to provide their return path so you have half as many power wires running around which is a lot cleaner and cheaper.
Since the frame of the vehicle is all nice conductive metal it has a very low resistance, so while there may be a fair amount of current running through it there is almost no voltage difference across it so every point on it is very close to that 0V reference of the – terminal. Since there’s basically no voltage across it there’s no way to generate a potential difference across you by touching two points on the frame so its safe to make contact with.
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