Grounding and Safety

Ground:

When voltage is spoken of it is always with reference to the electric potential at a reference point. In electronics ground (or earth) is a name we give to this reference point, and for a circuit with a battery for example this point is usually taken to be the negative terminal of the battery.

Ground is at a voltage of 0V with respect to itself as it is our reference point.

Ground is commonly shortened to just GND.

The two circuits shown below are equivalent circuits, the triangle symbol is one way of indicating a ground connection and all ground connections connect back to a common point which is the negative battery terminal in this case. This is why the bottom circuit is equivalent.

The most common circuit symbol for ground is the following:

Examples:

1. a) What is the voltage at node a in the circuit shown below?

Node a is connected to GND which is by definition 0V.

b) What is the voltage at node b?

From node a charges pass through a 3V voltage source hence they gain 3V of electric potential for a voltage of 3V at node b.

c) What is the voltage at node c?

From node b charges pass through a 2V voltage source hence they gain 2V of electric potential which means charges have gained 5V(3V+2V) of electric potential from node a for a voltage of 5V.

d) What is the voltage at node d?

At node a charges have 0V of potential which means if we go back through the supply under node a they must have 3V less of electric potential hence the voltage is -3V at node d.

Safety Grounding:

Safety ground is an electrical connection to ground which protects people from an electric shock by providing a low resistance return path. Recall that current seeks the path of least resistance.

Current ONLY flows when there is a potential difference between two points, hence if a ground connection is added to prevent a voltage when a person touches the circuit then current won't flow through the person.

Power electronics have a point that is connected to an earth stake buried in the dirt to ensure that one side of the circuit is always at 0V.

A common hazardous example is a circuit enclosed in a metal case without a ground connection. If live wires from the circuit by fault touch the metal case then if a person touched the case they would provide a return path to ground and be electrocuted!

The high current circuit shown below adds a ground wire connection to the metal case which would prevent a person from being electrocuted if the circuit were to short with the case.

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