What is GND and When Should We Connect Grounds

Ground or GND is the node we choose as the 0 V reference in a circuit. All other voltages are expressed relative to that point, unless we specify another reference.

We often hear “connect the black wire to ground” or “connect the grounds together”, but the physical reason behind it is rarely explained. And what’s more dangerous: the circuit’s GND is often confused with the physical Earth of your home’s electrical installation.

Let’s distinguish between GND and protective earth, and understand when two circuits need a common reference.

Voltage is relative

To understand GND, we first need to remember what voltage is. Voltage is not an absolute quantity, it is a potential difference between two points.

Suppose I ask you: “How high is a ball?” The question makes no sense unless we specify relative to what.

• Relative to the floor of the room: 1 meter.
• Relative to the street (if we are on the 5th floor): 15 meters.
• Relative to sea level: 500 meters.

The same thing happens in electronics. “5 Volts” does not exist by itself. It exists only as “5 Volts higher than this other point”.

What is GND or Ground

GND or Ground is simply the point in the circuit that we arbitrarily decide should be 0 Volts.

It is our reference, our “floor”. It’s the point where we place the black probe of the multimeter to measure everything else.

In many DC circuits, such as those powered by a single battery, GND coincides with the negative terminal of the source. Part of the ground wiring also acts as a current return path, although reference and return are not exactly the same concept.

Ground and Earth symbols

Although sometimes used interchangeably, different symbols can be found in schematics:

1. Protective Earth (PE): protective earth of the installation.
2. Chassis Ground: ground connected to the chassis or conductive enclosure of the equipment.
3. Signal Ground: reference used by the circuit signals.

Difference between GND and Protective Earth

• GND (Ground): This is the 0V of your electronic circuit. It’s the path through which electrons return to the battery.
• Physical Earth (Protective Earth - PE): This is the yellow/green wire in the wall outlets of your house. It is literally connected to a metal rod driven into the building’s ground.

The protective earth connects accessible metal parts to the grounding system. In the event of an insulation fault, it provides a path for fault current and facilitates the operation of protection devices.

Floating references and common ground

Here is the critical point. Suppose you have this setup:

1. An Arduino powered via USB (5V).
2. A 12V Motor powered by an external battery.
3. You want to control the motor with an Arduino pin.

You connect the Arduino’s digital pin to the motor driver and… it doesn’t work! Or the motor makes strange noises.

What happened? The Arduino outputs “5V” on its pin. But 5V relative to what? Relative to ITS GND (the USB’s). The motor driver receives this signal, but it measures the voltage relative to ITS GND (the 12V battery’s negative terminal).

Since the two circuits are not connected, their “grounds” are at different heights. It’s like I’m on a boat and you’re on a dock, and I tell you “throw the ball 2 meters high”. My height and yours don’t match because the waves are moving my boat.

This is called having a floating reference.

When to connect grounds

If two circuits exchange a non-isolated electrical signal, they usually need a common reference and a return path.

We must connect a wire between the Arduino’s GND and the external Battery’s Negative terminal.

By doing this:

1. We equalize the reference potentials (Arduino 0V = Battery 0V).
2. We create a common return path for the control current.
3. Now, when Arduino says “5V”, the motor perfectly understands “5V”.