RWB II ROTARY SCREW COMPRESSOR UNITS
PROPER IN STAL LA TION OF ELECTRONIC EQUIPMENT
S70-200 IOM
Page 48
GROUNDING
Grounding is the most important factor for suc cess ful op-
er a tion and is also the most over looked. The NEC states that
control equipment may be grounded by using the rigid con duit
as a conductor. This worked for the earlier relay sys tems, but
it is not acceptable for electronic con trol equip ment. Conduit
is made of steel and is a poor con duc tor rel a tive to a copper
wire. Electronic equipment reacts to very small cur rents and
must have a good ground in order to operate prop er ly; there-
fore, copper grounds are re quired for proper operation.
NOTE: aluminum may be used for the large three-phase
ground wire.
The ground wire must be sized the same size as the
sup ply wires or one size smaller as a minimum. The
three phase power brought into the plant must also
have a ground wire, making a total of four wires. In many
in stal la tions that are having elec tron ic control problems, this
es sen tial wire is usually missing. A good ground circuit must
be continuous from the plant source trans form er to the elec-
tron ic control panel for proper operation. (Figure 45) Driv ing
a ground stake at the electronic control will cause ad di tion al
prob lems since other equipment in the plant on the same
cir cuits will ground themselves to the ground stake causing
large ground fl ow at the electronic equipment.
ALUMINUM
OR COPPER
3-PHASE
BUS
PLANT SUPPLY
TRANSFORMER
COPPER
ELECTRONIC
CONTROL
ART2
Figure 45
WIRING PRACTICES
Do not mix wires of different voltages in conduit. An
ex am ple of this would be the in stal la tion of a screw com-
pres sor package. The motor voltage is 480 volts and the
panel control pow er is 120 volts. The 480 volt circuit must
be run from the motor starter to the motor in its own
con duit. The 120 volt circuit must be run from the mo-
tor starter con trol trans form er to the control panel in its
own sep a rate con duit. If the two cir cuits are run in the same
conduit, tran sients on the 480 volt circuit will be in duct ed
into the 120 volt circuit causing functional prob lems with the
elec tron ic control. Dividers must be used in wire way sys tems
(con duit trays) to separate unlike voltages. The same rule
ap plies for 120 volt wires and 220 volt wires. Also, never
run low volt age wires in the same con duit with 120 volt
wires. (Fig ure 46)
Never run any wires through an elec tron ic control pan el
that do not relate to the function of the panel. Elec tron ic
control panels should nev er be used as a junc tion box.
These wires may be carrying large tran sients that will in ter fere
with the operation of the control. An extreme ex am ple of this
would be to run the 480 volts from a motor starter through
the control panel to the motor.
When running conduit to an electronic control pan el, take
notice of the access holes (knockouts) provided by the
man u fac tur er. These holes are strategically placed so that
the fi eld wiring does not interfere with the electronics in the
pan el. Never allow fi eld wiring to come in close prox im i ty
with the controller boards since this will almost always
cause prob lems.
Do not drill a control panel to locate conduit con nec -
tions. You are probably not entering the panel where the
man u fac tur er would like you to since most man u fac tur ers
rec om mend or provide prepunched con duit con nec tions.
Drill ing can cause metal chips to land in the elec tron ics
and create a short circuit. If you must drill the panel, take
the fol low ing precautions: First cover the elec tron ics with
plas tic and tape it to the board with masking or elec tri cal tape.
Second, place masking tape or duct tape on the in side of the
panel where you are going to drill. The tape will catch most
of the chips. Then clean all of the remaining chips from the
panel before removing the protective plastic. It would be a
good idea to call the man u fac tur er before drill ing the panel
to be sure you are entering the panel at the right place.
When routing conduit to the top of an electronic control pan el,
condensation must be taken into con sid er ation. Water can
condense in the conduit and run into the panel causing cat-
a stroph ic failure. Route the conduit to the sides or bottom
of the panel and use a con duit drain. If the conduit must
be routed to the top of the panel, use a sealable conduit
fi tting which is poured with a sealer after the wires have
been pulled, terminated and the control func tions have been
checked. A conduit entering the top of the enclosure must
have an “O” ring-type fi tting be tween the conduit and
the en clo sure so that if water gets on top of the en clo sure
it can not run in between the conduit and the enclosure. This
is extremely im por tant in outdoor ap pli ca tions.
480 VOLT
3-PHASE
BUS
GROUND BUS
ELECTRONIC
CONTROL
120 V
480 V
MOTOR
SEPARATE
CONDUIT
MOTOR
STARTER
PANEL
ART3
Figure 46
