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
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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/groundloop/index.html • http://www.leminstruments.com/grounding_tutorial/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 • 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.
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: • 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.
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.
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 • 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 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.
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.
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 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.
Additional Recommendations EEL 4010 BOOKLET • 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 • 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.
Additional Recommendations EEL 4010 BOOKLET • 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
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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