Page 300 - Mechatronics with Experiments

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electrical outlet in a room, and we have an electrical device with a three-prong power cord.
If a three-prong connector is connected to another adaptor that has three prongs on one side
and two prongs on the other side (hot and neutral), the third prong is not connected to the
electrical circuit, and the net result is that the chassis of the device is not grounded.
If there is a short from power lines to the enclosure or some charge has accumulated
on the enclosure, having it (the enclosure) grounded provides a conductive path to the
ground so that there will not be a high voltage built up on the enclosure. If a person touches
such an enclosure, there will not be an electric shock. If the enclosure is not grounded,
and there is a leak to the enclosure, it will result in a high potential build-up. When a
person touches it, the person’s body will provide a conductive path which is dangerous. For
example, the frame of an electric motor should be grounded. In case the windings make
contact with the frame due to some damage, the ground connection provides a path for the
current to ground. If a person touches the motor frame under that condition, there would not
be any danger. If the motor frame was not grounded, then there would be a serious danger
of electric shock. The same applies for home appliances such as electric heaters, electric
ovens, washers, and so on. The so-called ground fault interrupter (GFI) type receptacle
works on the principle that it monitors any voltage leakage and voltage build-up on the
connected equipment enclosure. If there is voltage build-up, the GFI receptacle disconnects
the power from the line to the device, effectively behaving like a circuit breaker. Hence, it
provides a safety measure against such voltage leakage and potentially harmful conditions.
Signal ground (also referred as COMMON ground or just COM in circuits) is used
as the zero voltage reference for sensor signals.
Voltage is a relative quantity measured always between two points. Ground in any
electrical circuit provides the local zero voltage level against which other voltages are
referenced. Ideally, there would be one common ground point in all circuits and all voltages
would be referenced to it. However, practical circuits often have more than one ground
point. When the actual voltage level of these multiple ground points are different from each
other, two different kinds of errors occur:

1. The voltages measured against different grounds will not be compared correctly,
since their values are referenced against different “zero” levels.
2. If the different ground points are electrically connected to each other via a conductive
path and they have different voltage levels, there will be a current flow between
the grounds. This is called the ground loop, and results in error (noise) on the
signal. Therefore, in circuits with multiple grounds, there should not be a conductive
path between the grounds. The key to avoiding ground loops is to make sure that
there is no conductive path between different grounds in a circuit. For instance, as
shown in Figures 5.25 and 5.26, the shield should be grounded only at one end,
not both. Similarly, signal common wire in single-ended signals and ground wire in
differential-ended signals should be grounded at one end only.

Single-Ended Voltage Signal and Differential-Ended Voltage Signal
When a signal is presented and measured as a voltage difference between a conductor and
a ground, it is called a single-ended signal. When a signal is presented and measured as
the difference in voltage between two conductors both of which are referenced to the same
ground, it is called a differential-ended signal. In order to conduct a single-ended signal
over a conductor, one signal wire and a ground wire are needed. One ground wire may
be shared by multiple single-ended signals. In order to conduct a differential-ended signal
over a conductor, two signal wires and a ground wire are needed. Again, the ground wire
may be shared by multiple differential-ended signals (Figure 5.26a and b). For example,