Questions about electrical systems, earthing, single phase...

Thread Starter


Joined Mar 17, 2010

I am just done studying my Master in electrical engineering and when I in my studies came across a ground connection, it has always been used as the return path for the +5 V input or whatever input being used.

However, now I am trying to understand the purpose of ground connection on a system level. For instance a system being fed with a single phase systems with phase, neutral and ground. I know that for instance in the car, the ground or protective earth that I understand it is called is connected to the chassis of the car. Can anybody explain to me why?

In the electronic systems we develop at work, I have also understood that the protective earth should be connected to the box surrounding the electronics. Why?

Can anyone explain or give me a hint on where to find a good book/electronic resource to read up on regarding this?
I've been reading on wikipedia for instance but I have a hard time to find what I am really looking for.



Joined Nov 6, 2005
There are two different things here:

Within a piece of electronics, the negative or zero volts of the power system is often called ground or reference ground or chassis ground, or also the 'ground plane' layer of a circuit board.

The protective ground in an AC power system is a different matter.
That is for safety and fault protection.

They may be interconnected, but they may also be completely separate, depending on the appication and regulations for that particular equipment.

The two have different symbols (below)

Chassis (internal) ground has a bar with near vertical 'teeth',
Protective ground has horizontal lines.


Thread Starter


Joined Mar 17, 2010
Thanx for the answer.

Do you have any recommended litterature to read up on AC power systems? Atleast basic enough to understand the purpose of the protective ground connection and its usage.

And when you mention it, I do have seen the two different symbols. Did you however mix them up? If that was the case, why do you use the ground with horizontal lines in electronics?


Joined Apr 20, 2004
In the electronic systems we develop at work, I have also understood that the protective earth should be connected to the box surrounding the electronics. Why?
One reason might be for shielding, so electrical noise generated by the circuitry does not affect other electronics (TV & radios, for instance). Another is that the grounded case will not create a shock hazard if there is a wiring fault inside the case. The ground connection will short the fault and cause the breaker or fuse to open. Without the connection, the case could be connected to AC hot -


Joined Oct 15, 2009
How do you get a Masters in Electrical engineering and not know the purpose of a ground/chassis ground,etc...?


Joined Oct 15, 2009
Without a metal enclosure being tied to chassis ground there is a possibility that an internal failure could cause the chassis to become energized to line/supply voltage. Now imagine the safety problems when you go grab your metal fridge that is not properly grounded and has failed internally. ZAP..


Joined Nov 9, 2007
Personally I distinguish three different uses for earths.

1) Protective earths

2) Reference

3) Screening and shielding

There may be more I haven't thought of so I welcome suggestions not covered by the above.

They all use the characterisic of an earth that

An earth is a body whose potential does not alter, regardless of the current flows into or out of it, within the design limits of the system.

Does this thread and in particular post#19 help?

Also the term earth or ground is used as part of a phrase in conjuction with other terms, for example:

Virtual ground

Ground lift

Ground plane

Unfortunately either sloppyness with words or a genuine lack of understanding can lead to incorrect statements.

For instance I have seen the incorrect statements

An earth has low, negligable or zero resistance.

An earth is a single point.

An earth requires a good connection.

There can only be one earth in a system.

An earth is at zero volts.

If you adopt the above as true how, for instance, would you go on to talk about earth resistance or wenner arrays?