C-4
Electromagnetic Compatibility Guidelines for Machine Design
Grounding
High frequency (HF) grounding is different from safety grounding. A long wire is sufficient for a safety
ground, but is completely ineffective as an HF ground due to the wire inductance. As a rule of thumb, a
wire has an inductance of 20 nH/in regardless of diameter. At low frequencies it acts as a constant
impedance, at intermediate frequencies as an inductor, and at high frequencies as an antenna. The use of
ground straps is a better alternative to wires. However the length to width ratio must be 5:1, or better yet
3:1, to remain a good high frequency connection.
The ground system’s primary purpose is to function as a return current path. It is commonly thought of
as an equipotential circuit reference point, but different locations in a ground system may be at different
potentials. This is due to the return current flowing through the ground systems finite impedance. In a
sense, ground systems are the sewer systems of electronics and as such are sometimes neglected.
The primary objective of a high frequency ground system is to provide a well defined path for HF currents
and to minimize the loop area of the HF current paths. It is also important to separate HF grounds from
sensitive circuit grounds. “Single Point Ground Types” shows single point grounds for both series (daisy
chain) and parallel (separate) connections. A single point, parallel connected ground system is
recommended.
A ground bus bar or plane should be used as the “single point” where circuits are grounded. This will
minimize common (ground) impedance noise coupling. The ground bus bar (GBB) should be connected
to the AC ground, and if necessary, to the enclosure. All circuits or subsystems should be connected to
the GBB by separate connections. These connections should be as short as possible, and straps should be
used when possible. The motor ground conductor must return to the ground terminal on the drive, not the
GBB.
Intro
F
IGURE
11.3
Single Point Ground Types
CIRCUIT
1
CIRCUIT
2
CIRCUIT
3
CIRCUIT
1
CIRCUIT
2
CIRCUIT
3
PARALLEL CONNECTION
SERIES CONNECTION
Ground Bus Bar
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