Re: [tlug] Grounding/Earthing

[jep200404@example.com]

On Tue, 10 Jan 2017 20:51:58 +0800, Stephen Lee <sl-tlug@example.com> wrote:

> Isn't Earth just used for grounding the housing of the appliance?

The ground/earth connection is only for the protection of people
when bad things happen. The classic bad thing is when a high voltage
touches a case. The ground/earth connection is supposed to keep
the voltage on the case low so that people are not shocked.
The ground/earth wire connects the conductive parts of an appliance
that one can touch (such as a metal case), to ground/earth.

The ground/earth connection is only an emergency backup.
It is not to be used for carrying current
because then one would not have the backup.

In practice, it is often used for return path for small electrical
signals, which is usually tolerable, but not really good.
This can cause other problems such as ground loops.
By the way, ethernet floats and avoids such problems.

> Or is it actually electrically connected to the PCB of a computer?

Conductive parts that you can touch must be grounded.
On most PCs, you can touch connector shells of the motherboard,
so those shells must be grounded.

> Why would a missing Earth cause a power supply not to turn on?

Some power supplies are able to detect some problems,
such as voltage between ground/earth and hot being low
or voltage between ground/earth and neutral being high,
recognize that as a dangerous situation,
and refuse to turn on.

Such fancy power supplies are not common.