HPS Rotary Screw Compressor Units
070.700-IOM (MAR 21)
Page 16
Installation
Figure 20: Electronic control wiring
Grounding
Grounding is the most important factor for successful
operation and can be the most overlooked. The NEC states
that control equipment may be grounded by using the
rigid conduit as a conductor. This worked for the earlier
relay systems, but it is in no way acceptable for electronic
control equipment. Conduit is made of steel and is a poor
conductor relative to an insulated stranded copper wire.
Electronic equipment reacts to very small currents and
must have a correct ground in order to operate
correctly.
This means stranded copper grounds are required for cor
-
rect operation.
For correct operation, the control power ground circuit
must be a single continuous circuit of the correctly sized
insulated stranded conductor, from the electronic control
panel to the plant supply transformer (see
).
Driving a ground stake at the electronic control may also
cause additional problems because other equipment in the
plant on the same circuits may ground themselves to the
ground stake causing large ground flow at the electronic
control panel. Also, running multiple ground conductors
into the electronic control panel from various locations
can create multiple potentials resulting in ground loop
currents. A single ground wire (10 AWG or 8 AWG) from
the electronic control panel, that is bonded to the control
power neutral at the secondary side of the control power
transformer in the starter and then to the 3-phase ground
point, yields the best results.
Figure 21: Electronic control panel and plant supply
transformer
Notice
Structural grounding can also result in multiple
ground potentials and is also a relatively poor con-
ductor. As a result, this is not an acceptable method
for correct operation of electronic equipment.
There must be a ground for the three-phase power wir
-
ing. This must be sized in accordance to the NEC and any
local codes relative to the highest rated circuit overload
protection provided in the circuit. The manufacturer may
require a larger ground conductor than what is required by
the NEC for proper steering of EMI from sensitive circuits.
This conductor must also be insulated to avoid inadvertent
contact at multiple points to ground, which could cre
-
ate Ground Loops. In many installations that are having
electronic control problems, this essential wire is usually
missing, is not insulated, or in
correctly sized.
NEC size ratings are for safety purposes and not necessar
-
ily for adequate relaying of noise (EMI) to earth ground to
avoid possible interference with sensitive equipment. Siz-
ing this conductor 1 to 2 sizes larger than required by code
provides better transfer of this noise.
Johnson Controls-Frick requires that the ground conductor
meet the following:
• Stranded Copper
• Insulated
•
One size larger than NEC requirements for conventional
starters
•
Two sizes larger than NEC requirements for VFD starters
•
Conduit must be grounded at each end
• This circuit must be complete from the motor to the
starter continuing in a seamless manner back to the
plant supply transformer (power source).
For direct coupled, package mounted starters, the ground
-
ing between the motor and the starter may need to be
made externally (see
). The connection on the
starter end must be on the starter side of the vibration
isolators. Be certain the connection is metal to metal. You
may need to remove paint to ensure a suitable conductive
circuit. Using counter-sunk star washers at the point of
connection at each end maximizes metal to metal contact.