19. System Grounding – Separating AC and DC Grounds
AC Distribution System Grounding (Continued). DC Grounding also Shown
PSC: Power Supply Common DC GND CP6103: DC power supply
MGB: Main Ground Bus LGB: Local Ground Bus
Typical DC Power System and Ground Connections for Cabinets with
Type CP6103 Power Supply Units
Separating AC and DC Grounds
Notes:
1 1/0 — 4/0 AWG cable. Conductor used to connect the grounding electrode to the neutral ground bond at the source of a separately derived instrumentation power system. (per NEC 250.26 Parts a and b)(CSA C22.1 Section 10)
2 1/0 — 4/0 AWG cable. The conductors used to provide a low impedance ground reference for the DC power system (Logic, Transmitter, Output) and/or a cabinet ground for EMI/RFI noise protection of the instrumentation cabinets, file, and field wiring shields.
3 Supplemental conductor used to connect the grounding electrode for the source of a separately derived instru-mentation power system directly to the plat ground grid system. This is used to provide low impedance ground reference to EMI/RFI noise. (per NEC
250.81/250.83)(CSA C22.1 Section 10)
4 If the PIG is not tied to an electrical ground steel column, the grounded conductor must be a continuous wire from the neutral-ground bond point to the ground grid point. The wire insulation must be stripped at the PIG and the wire clamped to the PIG to maintain the continuous ground. If the column is grounded, then terminations may be made at the PIG.
DC/Cabinet Grounding System
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Grounding
Good grounding is the one of the primary ways of minimizing noise and of producing a safe system! .
Kinds of Ground:
* Chassis ground, Earth ground
*Safety Ground: Green color. No current flow (i=0, except during fault!). Usually connected to the earth. Long (~2.5 m) conductor rod driven into the earth for lightning protect.
* Signal Ground: reference 0 V, small signal return current flows during normal operation. Not current sink, but current return path!
* Power ground: relatively large power return current flows during normal
operation. Not current sink, but current return path!
•
• •
•
• • 220 V
A B G(접지)
Ground Earth
220 V
7-1 AC Power Distribution and Safety Ground
A-B: 220 V A-G: 220 V B-G: 0 V
각 단자는 벽면 뒤에서 각기 연결되어 있다!
A-B: 220 V A-G: 110 V B-G: 110 V
•
110 V
220 V
110 V
•
•
•
A B
G
감전 !
안전 !
7-2 Signal Ground
Ideal voltage definition of ground: an equipotential line or surface that serves as a reference 0 V for a circuit or system.
Practical current definition of ground: a low impedance path for current to return to the source.
There is no such thing! (Why?)
Because current is flowing through some finite impedance, a difference in potential will exist between any physically separated ground points.
To reduce noise,
(1) Minimize the ground impedance! (wide or thick line or plane)
(2) Return the current through the smallest loop possible for the lowest inductance!
(3) Be aware of possible common impedance coupling in the ground!
1 / (for signal loop)
Z R j L j C R j L
Via hole ground plane
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Figure 3-21 Figure 3-21
For low frequency signal
☞ single-point ground
For high frequency signal
☞ multipoint ground
For high frequency signal
☞ single-point ground
For low frequency signal
☞ multipoint ground
Hybrid Ground
Single-point Ground Multipoint Ground
Best, but many lines Not good for high frequency or digital circuit - high inductance Simple, but noisy
Good for signal < 100 kHz Good for low current
Good for high frequency or digital circuit - low inductance
plane or grid!
separate or
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Series+ parallel Ground
Ground loop and lightning current!
Multi
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7-3 Ground Loop
Two physically separated ground points are likely to be at different potential!
(1) V
Gthat generates noise is results of current.
(2) Strong magnetic field generates V
G. (3) Current return is to be divided.
Solution to break ground loop and keep same potential – optical coupler!
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Q & A
