Transcript Electrical Grounds

Electrical Grounds
By: Professor Wilmer
Arellano
Overview
• Glossary
• References
• Definitions
– Measuring Soil Resistivity
• Recommendations
– FPL
– IEEE 142
• Humming a Noise Example
– 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/groundlo
op/index.html
• http://www.leminstruments.com/grounding_tutor
ial/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 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
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/
soilresistivitytest.shtml
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
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.
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.
Balanced Circuit
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 highcurrent 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
Special Applications
electrical charge dissipaters
Additional Recommendations 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 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.
Additional Recommendations EEL
4010 BOOKLET
7. Make overall layout compact.
8. Keep all component lead lengths as short as
possible.
9. Route all inputs and input related components
away from any outputs.
10. Separate input and output leads by a ground
or supply trace where possible.
11. Low level high impedance signal carrying
wires may require shielded cable.
Additional Recommendations EEL
4010 BOOKLET
13. Reduce high impedance positive inputs
to the minimum allowable value (e.g.,
replace I Meg biasing resistors with 47k
ohm, etc.).
14. Add small (<1OOpF) capacitors across
feedback resistors to reduce amplifier
gain at
Review
• Definitions
– Measuring Soil Resistivity
• Recommendations
– FPL
– IEEE 142
• Humming a Noise Example
– IEEE 1100
– Printed Circuits
• Electrical Noise
• Special Applications
&
Questions
Answers