Download Electrical Grounds

Electrical Grounds
By: Professor Wilmer
Arellano
Overview
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Glossary
References
Definitions
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Recommendations
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Measuring Soil Resistivity
FPL
IEEE 142
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Humming a Noise Example
IEEE 1100
Printed Circuits
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Electrical Noise
Special Applications
Glossary
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NEC, National Electric Code
FPL, Florida Power & Light
IEEE, The Institute of Electrical and
Electronics Engineers
References
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NEC, National Electric Code
http://www.fpl.com/
http://www.epanorama.net/documents/groundl
oop/index.html
http://www.leminstruments.com/grounding_tuto
rial/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
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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
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Equipment ground. This is a permanent and continuous
bonding together (i.e., connecting together) of all non currentcarrying 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 low-impedance 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
Popular Definitions
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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.
Definitions. FPL
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The purpose of grounding
• Electrical grounding prevents shortages from
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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
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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
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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
Recommendations FPL
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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. FPL
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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
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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
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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
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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
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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
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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
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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
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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 non-linearities.
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
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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 mixedsignal boards, do not mix digital and
analog signals together. Try to
route sensitive lines first and be
aware of potential coupling paths
Reducing Noise
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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
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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
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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
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Three separate returns to power ground:
a)
b)
c)
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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
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Bypass caps are required (on each device or 5 max),
within one inch from chip power leads.
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
>10 µF caps, to prevent high frequency feedback
through the power supply lines.
Reduce high impedance positive inputs to the minimum
allowable value (e.g., replace I Meg biasing resistors
with 47k ohm, etc.).
Add small (<1OOpF) capacitors across feedback
resistors to reduce amplifier gain at
Special Applications
Special Applications
Review
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Definitions
•
Measuring Soil Resistivity
Recommendations
•
•
FPL
IEEE 142
•
•
IEEE 1100
Printed Circuits
• Humming a Noise Example
• Electrical Noise
Special Applications
&
Questions
Answers