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Electrical Grounds
By: Professor Wilmer Arellano
Overview
Glossary
References
Definitions
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Measuring Soil Resistivity
Recommendations
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FPL
IEEE 142
Humming a Noise Example
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IEEE 1100
Printed Circuits
Electrical Noise
Special Applications
Glossary
NEC, National Electric Code
FPL, Florida Power & Light
IEEE, The Institute of Electrical and
Electronics Engineers
References
NEC, National Electric Code
http://www.fpl.com/
http://www.epanorama.net/documents/groundloo
p/index.html
http://www.leminstruments.com/grounding_tutori
al/html/soilresistivitytest.shtml
System Design and Layout Techniques for Noise
Reduction in MCU-Based Systems. By: Mark
Glenewinkel. CSIC Applications, Austin Texas.
MOTOROLA AN1259
EEL 4010 Senior Design 1 Booklet
Definitions. NEC
Wiring system ground. This consists of grounding one of
the wires of the electrical system, such as the neutral, to
limit the voltage upon the circuit which might otherwise
occur through exposure to lightning or other voltages
higher than that for which the circuit is designed. Another
purpose in grounding one of the wires of the system is to
limit the maximum voltage to ground under normal
operating conditions. Also, a system which operates with
one of its conductors intentionally grounded will provide
for automatic opening of the circuit if an accidental or
fault ground occurs on one of its ungrounded conductors
(Fig. 250-1).
Definitions. NEC
Definitions. NEC
Equipment ground. This is a permanent and continuous
bonding together (i.e., connecting together) of all non
current-carrying metal parts of equipment enclosures—
conduit, boxes, cabinets, housings, frames of motors,
and lighting fixtures—and connection of this
interconnected system of enclosures to the system
grounding electrode (Fig. 250-2). The interconnection of
all metal enclosures must be made to provide a lowimpedance path for fault-current flow along the
enclosures to assure operation of overcurrent devices
which will open a circuit in the event of a fault. By
opening a faulted circuit, the system prevents dangerous
voltages from being present on equipment enclosures
which could be touched by personnel, with consequent
electric shock to such personnel.
Definitions. NEC
Definitions. FPL
The purpose of grounding
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Electrical grounding prevents shortages from
passing through electrical equipment.
The ground is the primary path through which
a surge protector dissipates energy from a
voltage spike.
The ability of grounding systems to
dissipate electricity is measured in ohms.
Properly installed grounding systems
require 25 ohms or less
Measuring Soil Resistivity
Measuring Soil Resistivity
The measuring procedure described below uses the
universally accepted Wenner method developed by Dr.
Frank Wenner of the US Bureau of Standards in 1915.
(F. Wenner, A Method of Measuring Earth Resistivity;
Bull, National Bureau of Standards, Bull 12(4) 258, s
478-496; 1915/16.)
p = 191.5AR
Where: p = the average soil resistivity to depth
in ohm - cm
A = the distance between electrodes in feet
R = the measured resistance value in ohms
from the test instrument
http://www.leminstruments.com/grounding_tutorial/html/s
oilresistivitytest.shtml
Measuring Soil Resistivity
The basic formula used for the design of a grounding system is:
RG = p x f
Ground Resistance = Soil Resistivity x Function based on electrode
type, size, and shape
Typically, the target resistance is dictated by company standards.
Less than 5 ohms is a common value used in the telecommunication
industry. Soil resistivity is a given based on site conditions and "f" is
a function based on the shape, size, type and layout of the
electrode. A good design engineer will ensure that the components
of the grounding system are configured to achieve the desired
resistance value throughout all the seasons.
Some basic formulas that are used to determine electrode
resistance can be found in the IEEE -"Green Book"IEEE
Recommended Practice for Grounding of Industrial and Commercial
Power Systems, Chapter 4, Table 13.
http://www.electricity-today.com/et/issue0502/i05_lightning.htm
Notation modified to coincide with previous page
Popular Definitions
For facilities engineers, grounds are a return for
lightning strikes (e.g., may be 100,000 A for a
few milliseconds).
Electricians see grounds as a return path for
fault currents (up to hundreds of A at 60 Hz).
EE see grounds as a way for current to return to
its source in such a way as to avoid/reduce
noise, interference, and oscillations.
Recommendations FPL
Since the 1980s National Electrical Code has
required the bonding and grounding of all lines
(power, phone, cable TV, communications lines)
together before they enter the building. This is
typically done at the ground by the electric meter
where all lines should be bonded to the wire
leading to the driven ground rod. If you have a
centralized grounding system and the cable or
phone lines are not bonded to it,
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contact the appropriate utility and
have them check the system.
Recommendations NEC
See definitions NEC
Recommendations. FPL
Ground fault interrupt A special GFI outlet is
designed to shut off electricity to the entire circuit
in order to prevent electrical shock. To restore
electricity to the circuit, the GFI outlet must be
reset
Ground Fault Interrupt (GFI) circuits most often
provide power to outlets located wherever water
can be a threat, near
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sinks
tubs
garages and
on the exterior of your home.
Recommendations, NEC
The ground fault senses a difference in the flow of
current from the hot wire through the neutral, if that
difference is about 5 milliamps or more the ground fault
will trip the circuit out. It actually assumes that if the
current is not flowing in the neutral it is flowing through
something else. Some motor windings have sufficient
losses to cause one to trip out so don't use a gfi circuit
for a refrigerator or washer outlet. You should use (and
the NEC requires) the use of gfi protected outlets within
6 feet of a sink, anywhere in a bathroom, in a garage or
outside; anywhere an outlet can be reached from a water
source, a wet area, or earth ground, you should use gfi
protection.
Recommendations. IEEE-142
When you design a grounding system, use
these items first and bond them together:
1. Metal underground water pipe,
2. Metal frame of the building (where effectively
grounded),
3. Concrete-encased electrode, and
4. Ground ring. A ground wire of No. 2 size encircling
or surrounding a building, tower or other aboveground structure. Usually the ground ring should be
installed to a minimum depth of 2.5 ft. and should
consist of at least 20 ft. of bare copper conductor.
Recommendations. IEEE-142
If these items aren't available, Standard
142 says, "then and only then can you
use any of the following:"
1. Other local metal underground systems or
structures,
2. Rod and pipe electrodes, and
3. Plate electrodes. Rods or pipes can be
driven into the ground or a flat plate of
copper can be installed as an electrode.
Recommendations. IEEE-142
Humming, a Noise Example
Hum and buzz (50Hz/60Hz and it's harmonics) occur in
unbalanced systems when currents flow in the cable
shield connections between different pieces of
equipment. Hum and buzz can also occur balanced
systems even though they are generally much more
insensitive to it.
The second most common source of hum and buzz is
the voltage difference between two safety grounds
separated by a large distance or the voltage difference
between a safety ground and an "Earth" ground (such as
a grounded satellite dish or cable TV source). This
problem is usually called "ground loop". This is the most
common one in severe humming problems.
Recommendations. IEEE 1100
A recent addition to the Institute of
Electrical and Electronic Engineers (IEEE)
color book series, IEEE Standard 1100
(Emerald Book), Recommended Practice
for Powering and Grounding Sensitive
Electronic Equipment, seeks to bring order
to the apparent chaos of power quality
assurance by doing exactly what its title
says
Recommendations. IEEE 1100
1.
2.
3.
4.
5.
Strictly following the requirements of the NEC.
Using solidly grounded AC power systems.
Using dedicated circuits for sensitive loads.
Using an insulated grounding conductor to supplement
the Code-minimum raceway grounding path.
Using a separately derived source close to the
sensitive loads. Separately Derived Sources may
include: shielded isolation transformers, power
conditioners, voltage regulators, UPS systems, rotary
power conditioners, and motor generators.
Electrical Noise
Noise is any electrical signal present in a circuit other
than the desired signal. This definition does not apply to
internal distortion, which is a by-product of nonlinearities. Noise is not a problem until it interferes with
system performance. Noise sources can be grouped into
three different categories:
1) Man-made noise sources — digital electronics, radio
transmitters, motors, switches, relays, etc.
2) Natural disturbances — sunspots and lightning
3) Intrinsic noise sources — related to random
fluctuations from physical systems such as thermal and
shot noise. Noise cannot be eliminated totally. However,
the magnitude and impact of noise can be reduced.
Electrical Noise Sources
Reducing Noise
Separate the Components in the
circuit according to their function, low
level analog, high speed digital and
noisy circuits.
High-frequency, low-inductance axial
glass or multi-layer ceramic capacitors
should be used for decoupling ICs.
Use a 0.1µF capacitor for system
frequencies up to 15 MHz. If the
system frequency is above 15 MHz,
use 0.01µF capacitors. Place the
capacitor as close to the IC as
possible.
After laying down the power and
ground system traces, signal layout
follows. When laying out mixed-signal
boards, do not mix digital and analog
signals together. Try to route sensitive
lines first and be aware of potential
coupling paths
Reducing Noise
The IC decoupling caps used for
current glitches often deplete their
charge reservoirs and must be
recharged. This is done by using a
bulk capacitor placed as close to
the PCB power terminals as
possible. The bulk capacitor
should be able to recharge 15 to
20 ICs. If more ICs are on the
PCB, bulk capacitors can be
placed around the PCB. The
capacitor should have a small
series inductance. Use tantalum
electrolytic or metalized
polycarbonate capacitors. Do not
use aluminum electrolytic
capacitors.
A small 0.1µF capacitor also
should be used to decouple high
frequency noise at the terminals.
Reducing Noise
The most sensitive signals in an MCUbased system are the clock, reset, and
interrupt lines. Do not run these lines
in parallel with high-current switching
traces.
The crystal or ceramic resonator clock
is an RF circuit. The clock must be
layed out to decrease its emission
levels and susceptibility. Figure 11
shows an example of a crystal or
ceramic resonator layout with a DIP
package. Always place the circuit as
close to the MCU as possible. If the
crystal or ceramic resonator has a long
body, lay it down flush with the PCB
and ground the case. The ground
signal of the crystal circuit should be
connected to the ground pin of the part
using the shortest trace possible. The
power and ground pins should be
routed directly to the power posts of
the PCB.
Special Applications
RULES TO REDUCE NOISE
(GROUNDING, ETC.) EEL 4010 BOOKLET
1.
2.
3.
4.
5.
6.
The signal ground for all amplifiers should be a flat plane such as
a large copper area of a printed circuit board.
Connect all system chassis grounds together with heavy wire or
braid.
Make all grounds large (wire, braid, etc.) or wide (pc board runs)
as practical.
Connect signal ground of lowest level amplifier in system to
chassis ground. Make this as close as possible to actual op amp
input signal ground.
Connect ground return of source voltage (e.g., external input) to
the lowest (input) level amplifier to the same chassis ground in
item 4.
Power ground and + power leads may be “daisy-chained”
between amplifiers. Make only one connection between power
ground and signal grounds. One connection should be as close as
possible to the cluster of grounds in items 3 and 4 above.
RULES TO REDUCE NOISE
(GROUNDING, ETC.) EEL 4010 BOOKLET
7.
Three separate returns to power ground:
a)
b)
c)
8.
9.
10.
11.
12.
Power line bypass cap’s (<1” from IC), protection circuits, (all
together) should have a separate return to ground (rarely done).
Signal grounds, separate return to ground.
Output load ground, and power leads (power amp) separate returns
to power supply.
Make overall layout compact.
Keep all component lead lengths as short as possible.
Route all inputs and input related components away from any
outputs.
Separate input and output leads by a ground or supply trace
where possible.
Low level high impedance signal carrying wires may require
shielded cable.
RULES TO REDUCE NOISE
(GROUNDING, ETC.) EEL 4010 BOOKLET
13. Bypass caps are required (on each device or 5 max),
within one inch from chip power leads.
14. At the power input to the board add from + and - power
connections to ground, >10 µF capacitor, used to
absorb low frequencies and .1 µF disc paralleled
across the
15. >10 µF caps, to prevent high frequency feedback
through the power supply lines.
16. Reduce high impedance positive inputs to the
minimum allowable value (e.g., replace I Meg biasing
resistors with 47k ohm, etc.).
17. Add small (<1OOpF) capacitors across feedback
resistors to reduce amplifier gain at
Special Applications
Special Applications
Review
Definitions

Measuring Soil Resistivity
Recommendations


FPL
IEEE 142
Humming a Noise Example


IEEE 1100
Printed Circuits
Electrical Noise
Special Applications
&
Questions
Answers