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Chapter 26: Floating Metal and Guard Electrodes
ELECTROMAGNETIC COMPATIBILITY HANDBOOK

1


Copyright
©
2002 by

Kenneth L. Kaiser, Version 08/17/04

Chapter 26: Floating Metal and Guard Electrodes

26.1
Provide three examples of floating metal objects not already given in this
chapter.
26.2
For a single-line telephone, the red and green lines are used as the tip and ring.
For a second-line telephone, the yellow and black wires are used as the tip and
ring. If only one phone is needed in a home, what should be done with the
remaining wires? Explain
26.3
The batteries for a small aircraft are placed in the rear end of light-weight plane
to help counter balance the weight of the engine. Discuss the routing of the
power lead(s) between the batteries, in the rear of the plane, and the electronics,
located mostly in the front of the plane. Should the metal body of the plane be
used as the power return?
26.4
Explain how the movement of a corrugated iron roof on a wooden shack near a
high-voltage power line can produce radio and TV interference. What type of
climate could increase the noise level?
26.5
A large floating conducting plane is placed parallel and above a signal trace
already located above a large return plane. Therefore, the signal trace is located
between the return plane and the floating plane. Will the inductance between the
signal trace and the return plane decrease, increase, or remain the same after
adding the floating plane? Explain.
26.6
Around some ferrite-based toroid transformers an external conducting shield is
provided. Should this shield be left floating? Assume that electrostatic charge
buildup is not an issue.
26.7
A portable signal generator with a single output lead is powered entirely by a
battery. No part of the generator or battery is connected to any external ground
or other conducting objects. The generator and battery are floating. If the signal
input terminal of an oscilloscope (powered by the ac power line with its chassis
grounded) is connected to the single output lead of the signal generator, will it
measure the output of the generator? State all assumptions.
26.8
A sensor is used on the engine of a truck. The distance between the test
equipment and sensor is 50'. Power-frequency noise is definitely a concern.
Why should the sensor not be floated? What happens if it is single-ended
grounded? Suggest a method of avoiding this noise during the measurement.
26.9
The output signal of bridge transducer powered by a dc voltage source is strongly
interfered with by a 60 Hz signal. Only inexpensive twisted pair is available.
Grounding at various locations does not cure the problem. How can the supply
voltage (amplitude, frequency, or reference level) for the bridge be changed to
reduce this noise problem? Assume the upper frequency response of the
transducer is 100 Hz.
26.10
A low-level transducer, grounded, is connected to shielded twisted pair to a
floating amplifier. The floating amplifier is to be powered by the ac line. How
should it be connected to the ac line? The shield of the twisted pair is connected
to the ground at the transducer. Why is the amplifier floating? At the amplifier, 2
ELECTROMAGNETIC COMPATIBILITY HANDBOOK


Copyright
©
2002 by

Kenneth L. Kaiser, Version 08/17/04

a ground strap is connected between the return lead of the twisted pair and the
shield. Why?
26.11
Are there any benefits in using a bridge when the amplifier connected to the
bridge is single-ended?
26.12
Neither supply voltage leads to a motor are grounded. By connecting the chassis
of the motor to the more negative lead, the noise emissions decrease. Explain.
26.13
A capacitor transducer with a source impedance of 400 M at 20 Hz is
connected to a coaxial cable. Assuming a typical input capacitance for 10 ft of
the coax, determine the major negative consequence of this capacitance. What if
the inductance of the line is included in the analysis? Would a piezoelectric
transducer have a low or high source impedance?
26.14
A high-megaohm (1 G or more) resistor inside a metallic case is to be
measured. The resistor is supported by two, high quality insulators mounted to
the case. What are a few possible candidates for these insulators? Some of the
current through the high resistance, during the testing, passes through the
insulators! What should be done? Hint: think of the metallic case as a guard
electrode or add a guard electrode around one of the input leads. Carefully
follow the path of the current to ensure that the resistance measured is due to the
current through the high-impedance resistor.
26.15
Some test instruments are guarded. There are three output leads for guarded
instruments: High, Low, and Guarded. When using a guarded instrument for
measuring voltage across a resistor, where should the three leads be attached if
the one side of the resistor is grounded? What if neither side of the resistor is
grounded? What occurs if the High and Low leads are interchanged in both
cases? [Keithley]
26.16
A floating microphone is connected to a differential amplifier through shielded
twisted pair. The microphone is enclosed in a shield. Where should this shield
be connected? The amplifier is not shielded, but it is enclosed in a chassis that is
connected to ground for safety purposes.