# What is the voltage potential of ground during a short

During a short, is it possible for the voltage potential of the ground to change? Say for example, we short a 9V battery, how does the ground stay at 0V and not become 9V?

• 0V in relation to what? 9V in relation to what? In relation to ground, ground is always 0V because ground is identical to ground. – Dampmaskin Mar 7 '18 at 14:27
• @Dampmaskin that started out a great comment then got lost in well... gibberish to the uninitiated. – Trevor_G Mar 7 '18 at 14:44
• @Trevor_G It well grounded gibberish! – StainlessSteelRat Mar 7 '18 at 15:07
• This is a very interesting question.I suspect it can have two answers: one for the ideal world of instantaneous fields and KCL-KVL ruled lumped circuit theory. And another one for the real world of light-speed propagation, distributed resistance and Maxwell's equations ruled EM theory. Food for thought. – Sredni Vashtar Mar 7 '18 at 21:55

Ground is a reference point so it is always regarded as 0 volts.

Of course in a lightning strike the localized area around the strike can create massive potentials per metre so if you value your life (and the lives of any future children you wish to bear), keep your feet close together if there's a risk of lightning.

Picture source

• +1 for nice graphic.. I always had issue with that though. They tell you in a lightning storm to take shelter but also tell you to keep your feet together. Since you have no idea when it's going to strike in your vicinity that's a bit of a catch-22. Maybe they should tell us to hop on one foot to safety ;D – Trevor_G Mar 7 '18 at 14:48
• @Trevor_G prisoners on chain gangs manacled at the ankles had their prize possessions much more protected! – Andy aka Mar 8 '18 at 15:43

Circuit ground is just some point from which we measure or reference voltages. If you like, it's the point where you connect the black lead of your multimeter. All other voltages can then be measured with respect to that point.

It should be clear from above that if you connect the red lead of your multimeter to the circuit ground that it will always read 0 V no matter what is happening to the battery. (In real life it can be a little more complex if there is a current flowing in the ground circuit and there is some resistance between the two points.)

simulate this circuit – Schematic created using CircuitLab

Figure 1. Both VM1 and VM2 will read zero volts. Battery internal resistance limits the current flow.

The resistance of standard PCB foil is 0.0005 ohms (500 microOhms) per square of foil ---- for any size square of foil.

simulate this circuit – Schematic created using CircuitLab

Each square is 0.0005 ohms. To exit the entry point, there are 8 surrounding squares, thus R of that first ring of square is 0.0005/8 = 0.00006 ohms. Then the next (3x larger) ring adds another 0.00006 ohms. Etc. But in the larger rings, the current is mostly heading toward the DESTINATION output node, and not all squares get equal current density.

• What are you basing this on? I agree changing the width and length together maintaining a square makes no difference because the length without doubling the width you have doubled the resistance but no-longer have a square to make it square again you half the resistance so are back where you started but depending on what I need I use between 1/4oz and 4oz (per square foot) copper and that makes a big difference. What in this context is standard? – Warren Hill Apr 10 '18 at 17:13

Ground is just a reference point, it does not need to be an actual connection to the earth.

When you short a 9 V battery, things which you can normally ignore (like the battery's series resistance) come into play. This prevents the ground potential from changing depending on what you choose as your ground reference point.

In the left circuit, the 1 ohm series resistance of the battery can be ignored as the resistance of the load is 100 times higher. R1 doesn't do much really.

In the right circuit R3 does come into play, it limits the current which will flow. The full 9V will be across R1. The ground will still be 0 V, it cannot even change as ground means that it is 0 V by definition.

Theoretically if you had an ideal battery with zero series resistance, an infinite current would flow if you shorted that ideal battery. But also then ground remains 0 V.

simulate this circuit – Schematic created using CircuitLab

The term "Ground" has historically meant ground literally: an electrical connection to the earth itself, typically via a rod driven into the earth. This was used in power and telecom systems to obtain a voltage reference, and sometimes as a current return path. In electronic circuits, the term has persisted and is typically used to denote the negative side of the power supply, even in cases where the circuit is isolated from the actual earth ground. In circuit analysis, "Ground" is often used to specify a reference point from which other circuit voltages are measured; therefore it is by definition at zero volts.

In the 9V battery example, suppose you specify the negative terminal as Ground. If you short the positive terminal to Ground, a current flows through the short circuit, and the positive terminal will be (nearly) at zero volts, since a physical battery, unlike an ideal voltage source, has an internal resistance.

Ground and Earth are often confused. Personally I only refer to Earth as a connection to to the local electricity supply earth which is literally connected to the planets ground as pointed out by others.

Ground on the other hand means a circuit reference and may, but is not required to, be connected to earth.

Consider the circuit below we have two volt meters what do they measure?

VM1 if sensitive enough should measure a positive voltage because there is current flowing in the connection between its inputs. Theoretically we have zero resistance so no matter how many amps are flowing out of the shorted battery it should be zero but we never, except possibly with super-conductors, really have zero resistance.

VM2 does not see the drop from the battery current so sees 0V as expected.

We talk about 'local grounds' for this reason each bit of circuit cares about the point it is referenced too which may not be the same everywhere if significant currents are involved or at 'high' frequencies due to inductive effects.

simulate this circuit – Schematic created using CircuitLab