Electrical Grounds

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Electrical Grounds

• By Professor Wilmer Arellano

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Overview

• Glossary
• References
• Definitions
• Measuring Soil Resistivity
• Recommendations
• FPL
• IEEE 142
• Humming a Noise Example
• IEEE 1100
• Printed Circuits
• Electrical Noise
• Special Applications

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Glossary

• NEC, National Electric Code
• FPL, Florida Power Light
• IEEE, The Institute of Electrical and Electronics
  Engineers

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References

• NEC, National Electric Code
• http//www.fpl.com/
• http//www.epanorama.net/documents/groundloop/inde
  x.html
• http//www.leminstruments.com/grounding_tutorial/h
  tml/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

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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).

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Definitions. NEC
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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 enclosuresconduit, boxes, cabinets,
  housings, frames of motors, and lighting
  fixturesand 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.

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Definitions. NEC
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Measuring Soil Resistivity
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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/h
  tml/soilresistivitytest.shtml

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Recommendations. IEEE-142

• When you design a grounding system, use these
  items first and bond them together
• Metal underground water pipe,
• Metal frame of the building (where effectively
  grounded),
• Concrete-encased electrode, and
• Ground ring. A ground wire of No. 2 size
  encircling or surrounding a building, tower or
  other above-ground 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.

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Recommendations. IEEE-142

• If these items aren't available, Standard 142
  says, "then and only then can you use any of the
  following"
• Other local metal underground systems or
  structures,
• Rod and pipe electrodes, and
• Plate electrodes. Rods or pipes can be driven
  into the ground or a flat plate of copper can be
  installed as an electrode.

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Recommendations. IEEE-142
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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

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Recommendations. IEEE 1100

1. Strictly following the requirements of the NEC.
2. Using solidly grounded AC power systems.
3. Using dedicated circuits for sensitive loads.
4. Using an insulated grounding conductor to
   supplement the Code-minimum raceway grounding
   path.
5. 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.

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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 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.

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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.

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Balanced Circuit
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Electrical Noise Sources
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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

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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.

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Reducing Noise

• The most sensitive signals in an MCU-based 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.

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Special Applications
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Special Applicationselectrical charge dissipaters
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Additional Recommendations EEL 4010 BOOKLET

1. The signal ground for all amplifiers should be a
   flat plane such as a large copper area of a
   printed circuit board.
2. Connect all system chassis grounds together with
   heavy wire or braid.
3. Make all grounds large (wire, braid, etc.) or
   wide (pc board runs) as practical.
4. Connect signal ground of lowest level amplifier
   in system to chassis ground. Make this as close
   as possible to actual input signal ground.
5. Connect ground return of source voltage (e.g.,
   external input) to the lowest (input) level
   amplifier to the same chassis ground in item 4.
6. 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.

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Additional Recommendations EEL 4010 BOOKLET

• 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.

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Additional Recommendations EEL 4010 BOOKLET

1. Reduce high impedance positive inputs to the
   minimum allowable value (e.g., replace I Meg
   biasing resistors with 47k ohm, etc.).
2. Add small (lt1OOpF) capacitors across feedback
   resistors to reduce amplifier gain at

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Review

• Definitions
• Measuring Soil Resistivity
• Recommendations
• FPL
• IEEE 142
• Humming a Noise Example
• IEEE 1100
• Printed Circuits
• Electrical Noise
• Special Applications

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Questions
Answers