Grounding of Baldor Drives Document Reference AN97010The subject of electrical grounding is complex and often misunderstood. Proper grounding of Baldordrives (Motor + Control) is required to insure personnel safety, successful startup and maximumreliability.

In power systems, there are two generally recognized purposes for grounding. The first purpose is calledequipment grounding. This concerns personal safety. The second is called system grounding. This isconcerned the electrical characteristics of the system. The NEC (National Electric Code in the UnitedStates) contain the regulations pertaining to system and equipment grounding. Its counterpart in Canadais the CEC (Canadian Electric Code).

Equipment Grounding.Equipment grounding is the intentional connection to earth of non-electrical metallic objects andequipment. The purpose is to reduce electric shock hazard to personnel. It must provide sufficient currentcarrying capability to accept the ground fault current without creating a fire or explosive hazard. It alsoprovides a low impedance return path for ground fault current to successfully operate ground faultprotection devices. Refer to the current NEC or CEC for the specific requirements.

Electric shock injuries can result from contact with metallic components that are unintentionallyenergized. Effective equipment grounding practices can minimize risks of personnel injuries due toelectric shock.

ElectricalService

Entrance

Motor

PowerGenerating

Station

CircuitBreaker

AccidentalShort to

Enclosure

Metallic Enclosureor Machine Frame

GroundFault

Current

Supply voltageappears onenclosure,

presenting ahazard topersonnel.

WITHOUT PROPER EQUIPMENT GROUNDING

ElectricalService

Entrance

PowerGenerating

Station

CircuitBreaker

AccidentalShort to

Enclosure

Metallic Enclosureor Machine Frame

Fault current flowsthrough safety ground

and opens circuit breaker.Supply voltage does not

appear on enclosure.No safety hazard.

WITH PROPER EQUIPMENT GROUNDING

Safety Ground

Motor Control

T1T2T3L3

L1L2

Motor

Motor Control

T1T2T3L3

L1L2

GroundFault

Current

2

System GroundingSystem grounding is the intentional connection of a supply voltage phase conductor or neutral conductorto earth. Its purpose is to limit system voltage with respect to earth within predictable limits. Limitingsystem voltage with respect to earth reduces the possibility of failure of both conductor and componentinsulation. The selection, installation and maintenance of a proper system ground will insure maximumreliability and equipment life.

There are many causes of transient overvoltages. Transient overvoltages are temporary overvoltages ofshort duration. They are often associated with the interruption of current in an inductive circuit. Theinterruption of current and resulting transient voltage can occur anywhere within the connected system.This includes the utility grid, local distribution grid, within the end users facility or within a piece ofequipment. Typically caused by switching a power factor capacitor, SCR based commutation (DCDrives), a ground fault, a lightning strike, or during arcing ground faults on ungrounded systems.

Transient overvoltages are not normally present on a power system during steady state operation. Theyoccur with the interruption of current in an inductive circuit. This is commonly called inductive kickbackand is exactly the mechanism used to provide high voltage to spark plugs in gasoline engines. Transientovervoltages cannot be measured using a standard voltmeter. An oscilloscope is required to capture anddisplay the transient overvoltages.

The figures below are equivalent circuits of the power distribution system. In figure A, during steady statecurrent flow, the terminal voltage is equal to the source voltage minus the voltage drop of themiscellaneous inductance. The miscellaneous inductance is due to the wires used in the transformer tosupply the voltage, the wires to connect to the load and the load itself.

In figure B, opening the switch interrupts the flow of current, the voltage surges above normal values.The terminal voltage is equal to the source voltage PLUS the voltage of the miscellaneous inductance.This is how transient voltages are produced.

As shown in figure C, this can easily exceed 300% of normal voltage or more.

Proper grounding techniques can reduce the damaging effects of transient overvoltages. This is thepurpose of system grounding.

TerminalVoltage

Time

100%

200%

300%

Switch opens

VoltageTransient

MiscellaneousInductance

-+

Terminal Voltage

Figure A

+

- Load

Figure B Figure C

MiscellaneousInductance

- +

Terminal Voltage

SourceVoltage

SourceVoltage

+

-LoadCurrent

Flow

3

Grounding Methods

The use of different ground connections can affect the magnitude of the transient overvoltages. Seechart below for comparison of different grounding methods.

Grounding MethodCondition Ungrounded Solid Ground Resistance Ground

Safety to personnel Worst(shock hazard)

Best Good

Immunity toTransientOvervoltages

Worst(Surges over 300%

normal)

Good(Surges kept under

200% normal)

Good(Surges kept under

250% normal)

Line to ground fault Worst(Line to earth voltages

173% of normal)

Best(Line to earth voltages

100% of normal)

Good(Line to earth voltagesup to 173% of normal)

Protection againstcontinuous arc faults

Worst(fire hazard)

Poor Good

Ease of locatingground fault

Worst Good Good

Notes Usually found inolder installations(from 1935 to 1975)

Typical of newerinstallations under 600amps.

(Normally onlyinstalled on systemsof 600 amps or larger)

Note: resistance grounding is the intentional placement of a resistor between the neutral connection andground. Standard practice is to select a resistor such that ground fault current is no less than 100 amps.Typical currents being in the 200-1000 amp range are more usual. If the ground current were less, thiswould be equivalent to an ungrounded system.

Ground FaultsA ground fault is the unintentional connection of a supply voltage conductor to ground. This unintentionalconnection results in an unwanted flow of current. In a properly grounded system, this current can bedetected and used to activate protection equipment. The protection equipment can then interrupt thesource voltage from system conductors.

Baldor AC motor controls incorporate a ground fault current sensor. In the event of a detected groundfault, the control will immediately remove the voltage to the motor terminals. The control will then indicatea ground fault condition. This reduces the amount of potential damage to the equipment due to arcingfaults. For the ground fault detection to operate reliably, a grounded system is required. Operation from aungrounded system can defeat the reliable detection of ground fault current.

4

Recommended Electrical Installation

Article 250-5 of the NEC specifies the method for grounding electrical systems. It isrecommended that Baldor AC and DC motor controls are supplied a three phase voltage that isbalanced (same voltage phase to ground) with respect to earth.

Recommended grounding method.

Note: The ground wire from the motor to the control should not be used as the machine ground.

H Series Control

PE

T1T2

L3

L1L2

OptionalLine

Reactor

Route all 4 wires together inconduit or metallic cable(T1, T2, T3, and ground).Do not run multiple motorcables in same conduit or

metallic cable.

Route all 4 wires together inconduit or metallic cable(L1, L2, L3, and ground).

Drivenearth

groundrodor

buildingground

Ground per NEC andlocal electrical codes.

ACMain

Supply

OptionalLoadReactor

Route all 4 wires together inconduit or metallic cable(L1, L2, L3, and ground).

Drivenearth

groundrodor

buildingground

ACMain

Supply

T3

Ground per NECand local codes

5

Three Phase Ungrounded SystemsElectrical power systems having no intentional connection from system conductors to earth are calledungrounded systems. Even though there is no physical connection to earth in these systems, groundcurrents circulate due to distributed system capacitance. These ground currents are normal in all powerdistribution systems but are more troublesome in ungrounded systems.

The possibility of injury due to electric shock is greater from ungrounded system than a groundedsystem. System overvoltages can occur during arcing and resonant or near resonant ground faults. Aresonant condition can arise when the system capacitive impedance approaches the inductiveimpedance of the ground fault connection.

The supply of power from ungrounded general distribution systems to Baldor drives is not recommended.Personnel working around ungrounded systems suffer increased risk of electric shock. Ungroundedsystems also suffer increased transient overvoltage conditions.

To operate a Baldor drive from an existing ungrounded system, the installation of an isolation transformeris recommended. The transformer secondary wye winding must have an accessible neutral point suitablefor connection to earth. The neutral of the wye secondary is connected to earth and serves as the localsystem ground.

In a special case, the NEC allows for an ungrounded transformer secondary to supply power to a Baldor drive. Itcan be used on separately derived systems when the transformer supplies power to the drive exclusively. Refer toOSHA, NEC or CEC and local electrical codes for the most recent information.

Text from Article 250-5 of the 1996 NEC:

Article 250-5: Alternating-Current Circuits and Systems to be Grounded. AC circuits and systems shall begrounded as provided for in (a), (b), (c) or (d) below. Other circuits and systems shall be permitted to begrounded.

(b) Alternating Current Systems of 50 volts to 1000 volts. AC systems of 50 volts to 1000 volts supplyingpremises wiring and premises wiring systems shall be grounded under any of the following conditions:

Exception No. 2: Separately derived systems used exclusively for rectifiers supplying onlyadjustable speed industrial drives.

Definition of Separately Derived Systems: A premises wiring system whose power is derived from generator,transformer or converter windings and that has no direct electrical connection, including a solidly connected groundedcircuit conductor, to supply conductors originating in another system.

H Series Control

PE

T1T2

T3

L1L2

L3

Existing transformerfloating or ungrounded

connection

Isolation Transformer

OptionalLine

Reactor

ACMain

Supply

Route all 4 wires together inconduit or metallic cable(L1, L2, L3, and ground).

Ground per NEC andlocal electrical codes.

Drivenearth

groundrodor

buildingground

Route all 4 wires together inconduit or metallic cable(T1, T2, T3, and ground).Do not run multiple motorcables in same conduit or

metallic cable.

OptionalLoadReactor

Ground per NECand local codes

6

Underwriters Laboratory (UL) listing.Safety in the workplace has made the grounding issue even more important. Baldor electronic powerconversion equipment sold in the US is listed by UL (Underwriters Laboratory) and CUL (CanadianUnderwriters Laboratory). Each product carries a UL and CUL listing and label.

The purpose of UL listing for a product is to certify the equipment meets stringent safety requirements,such as resistance to fire and reducing electrical shock hazard. This safety performance can only beachieved when the product has been installed in accordance manufacturer recommendations.

Baldor "H" series motor controls conform to UL508C guidelines. These guidelines require that all of theinternal components in the control be protected from transient overvoltages to prevent dangerouscomponent failures. In Baldor products, this protection is achieved with the use of an MOV (Metal OxideVaristor) module. The MOVs absorb excess energy from transient overvoltages. These transientovervoltages are typically in excess of 2 to 3 times normal operating voltages.

The MOV is limited in the amount of energy it can absorb from transient overvoltages. The Joule rating ofthe MOV defines how much energy it can safely absorb. The higher the Joule rating of an MOV, thehigher voltage surge or more transient overvoltage surges it can safely withstand.

The following chart outlines the maximum operating limits for the MOVs in the Harmonized productfamily. Exceeding these values may eventually cause damage to the MOVs. A MOV typically will failafter 1 surge at 1.5 times the voltage values listed in the table below.

Control Voltage 230vac Joules 460vac Joules 575vac JoulesLine to Line TurnOn Voltage @1ma

A & B size = 466vC, D, E size = 1034v

75J300J

A & B size = 881vC, D, E size = 1034v

F size = 1119v G size = 910v

110J300J220J620J

A & B size = 1175vC, D, E = 1364v

150J300J

Line to GroundTurn On Voltage@ 1ma

A & B size = **C & D size = 1034v

**300J

A & B size = **C, D, E size = 1034v

F size = 1119vG size = 910v

**300J220J620J

A & B size = **C, D, E = 1364v

**300J

** = No MOV connection to ground

Most grounded systems should not experience any problems with the MOVs. Ungrounded systems orcorner grounded delta systems without an isolation transformer should have a power quality checkperformed. The power quality check will determine the level of transient overvoltages present on thesystem. If the transient overvoltages exceed the level in the chart above, an isolation transformer, linereactor or other device to reduce the transient overvoltages will be required.

Additional transient overvoltage protection should be considered if the following equipment is suppliedpower from the same local power distribution system. This could be an isolation transformer, line reactoror additional transient overvoltage suppression devices:

DC drives, SCR based motor soft starters or other SCR based switching devices.Power factor capacitorsLarge motor startersReversing motor contactors

References

IEEE 142-1991 Grounding of Industrial and Commercial Power SystemsIEEE 1100-1992 Powering and Grounding Sensitive Electronic EquipmentANSI/NEC 70 - 1996 National Electric CodeUL-508C Underwriters Laboratories, Power Conversion Equipment