Rockwell Automation 7000 PowerFlex Medium Voltage Air-Cooled (B Frame) Installation - ForGe Control - Manual (Page 63)

Rockwell Automation Publication 7000-IN007E-EN-P - June 2014 63
Drive Installation Chapter 3
Guidelines for Drive Signal and Safety Grounds
When using interface cables carrying signals, where the frequency does not
exceed 1 MHz, for communications with the drive, follow these general
guidelines:
• Ground screen mesh around the entire circumference, rather than forming
a pigtail grounded only at one point.
• For coaxial cables with a single conductor surrounded by a mesh screen,
ground the screen at both ends.
• When using a multi-layer screened cable (that is, a cable with both a mesh
screen and a metal sheath or some form of foil), there are two alternative
methods:
–
Ground the mesh screen at both ends to the metal sheath. The metal
sheath or foil (known as the drain) should, unless otherwise specified,
be grounded at one end only, again, as specified above, at the receiver
end or the end that is physically closest to the main equipment ground
bus, or
– Leave the metal sheath or foil insulated from ground, and ground the
other conductors and the mesh cable screen at one end only, as stated
above.
Specifications for Customers and Power Integrators
Attach an external ground to the main ground bus, in compliance with applicable
local codes and standards. As general guidelines, the ground path must be of
sufficiently low impedance and capacity that:
• the rise in potential of the drive ground point when subjected to a current
of twice the rating of the supply should be no higher than 4V over ground
potential; and,
• the current flowing into a ground fault will be of sufficient magnitude to
cause the protection to operate.
Run the main grounding conductor(s) separately from power and signal wiring so
that faults:
• do not damage the grounding circuit, or
• will not interfere with or damage protection or metering systems, or cause
undue disturbance on power lines.
Electrical Supplies: Grounded and Ungrounded Systems
When dealing with an ungrounded, three-phase electrical supply system, the
cable insulation rating must handle both the phase-to-phase voltage and the
voltage-to-ground if one of the other phases develops a ground fault. In practice,
the cable insulation of an ungrounded, three-phase system must be rated for at
least a continuous voltage of root three (1.732) times (1.1) times the rated voltage
of the supply (1.732 x 1.1 = 1.9 times the rated line-to-line voltage).