Baldor MN850, Baldor manual Appendix D Voltage Surge Protection, Grounding

Appendix D

Voltage Surge Protection

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

		Control		Note: Wiring shown for clarity of grounding
		Enclosure
				method only. Not representative of
		L1 L2 L3	T1 T2 T3	actual terminal block location.
AC Main Supply		L1
		L2
		L3
Safety		Earth
	Four Wire
Ground
		“Wye”

Driven Earth Ground Rod	Route all 4 wires L1, L2, L3 and Earth (Ground)
(Plant 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.

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).

MN850 Voltage Surge Protection D-1