User Guide

Appendix D
Voltage Surge Protection
Voltage Surge Protection D-1MN850
Grounding Proper grounding is extremely important. The symptoms produced by improper
grounding are obvious. Sometimes filters and other expensive devices are added to
reduce the effects of problems caused by poor grounding. There can be several
reference points (neutrals) in a circuit but there should always only be one ground point.
Neutral and ground are not the same. Neutral should normally be a non current carrying
conductor, but it should be sized to carry momentary current caused be short circuits in
the equipment. All of the neutrals in a system should connect at a central point and that
point should be connected to the system ground.
The goal is to minimize the current through the ground conductor. Circulating ground
current is a source of electrical noise normally associated with unbalanced voltages or
unbalanced loads. Capacitive or inductive coupling between power lines and the neutral
or ground conductors is another noise source. Currents that flow through capacitive
paths or from a magnetic field tend to change rapidly and produce high frequency
interference called RFI (radio frequency interference).
Good grounding is also important to minimize the effects of large voltage spikes that can
create significant current flow in the ground conductors. The source of these voltages
can be lightning striking the power lines, switching of large power loads and others. A
balanced three phase four wire system with a system ground as shown in Figure D-1 can
minimize noise problems normally associated with grounding. AC power and motor
leads should be as short as possible and enclosed in conduit or shielded cable should be
used. Power wires and Motor leads should never be in the same conduit.
Figure D-1
L1
AC Main Supply
Safety
Ground
Driven Earth Ground Rod
(Plant Ground)
Four Wire
“Wye”
L1
L2
L3
Earth
L2 L3 T1 T2 T3
Route all 4 wires L1, L2, L3 and Earth (Ground)
together in conduit or shielded cable.
Route all 4 wires T1, T2, T3 and Motor Ground together in conduit or shielded cable.
Connect all wires (including motor ground) inside the motor terminal box.
Ground per NEC and Local codes.
Note: Wiring shown for clarity of grounding
method only. Not representative of
actual terminal block location.
Control
Enclosure
MOV (Metal Oxide Varistor)
Baldor recommends that MOV devices be installed at the input power connections to the
Soft–Start control to provide “phase–to phase” and “phase–to–ground” voltage spike
protection. Three MOV devices can be connected in “WYE” configuration with an
additional MOV device connected to ground. Figures D-2 and D-3 shows how to connect
the MOV’s into the various power distribution system designs. Voltage spikes on any
phase will be conducted to ground by the lowest impedance path (phase–to–phase or
phase–to–ground).