7/29/2019 Electrical Business (Topic High Resistance Grounding)

1/2

High-resistance grounding isby far the most effectivemethod

BY ANDREW COCHRAN

The term grounding iscommonly used in the elec-trical industry to mean both

equipment grounding and sys-tem grounding. The two types,

illustrated in Figure 1, are definedas follows:

Equipment grounding: The con-

nection of earth ground tonon-current-carrying conductive

materials (e.g., conduit,

cable trays, junction

boxes, enclosures andmotor frames);

System grounding: Theconnection of earthground to the neutral

points of current-carry-ing conductors (e.g., theneutral point of a circuit,

a transformer, rotatingmachinery or a system),

either solidly or with acurrent limiting device.

Characteristics ofungrounded systemsAn ungrounded systemis one in which there is no

intentional connectionbetween the conductorsand earth ground.

However, as in any sys-tem, a capacitive cou-pling exists between the

system conductors andthe adjacent groundedsurfaces. Consequently,

the ungrounded sys-tem is, in reality, acapacitively grounded

system by virtue of thedistributed capacitance.See Figure 2.

Under normal operat-

ing conditions, this dis-tributed capacitance

causes no problems. Infact, it is beneficial

because it establishes, in effect, aneutral point for the system, asshown in Figure 3a. As a result, the

phase conductors are stressed atonly line-to-neutral voltage above

ground.However, problems can arise

under ground fault conditions. Aground fault on one line results infull line-to-line voltage appearingthroughout the system. Thus, a

voltage 1.73 times the normalvoltage is present on all insula-tion in the system, as shown in

Figure 3b. This situation canoften cause failures in oldermotors and transformers, due to

insulation breakdown.The interaction between the

faulted system and its distributed

capacitance may cause transientover-voltages (several times nor-mal) to appear from line to

ground during normal switchingof a circuit having a line-to-ground fault (short). These over-

voltages may cause insulation fail-ures at points other than the origi-nal fault. In addition, a second

fault on another phase may occurbefore the first fault can becleared. This can result in very

high line-to-line fault currents,equipment damage and disrup-tion of both circuits.

In addition to the cost of equip-ment damage, ungrounded sys-tems complicate locating fault(s),

involving a tedious process oftrial and error: first isolating the

correct feeder, then the branch,and finally, the equipment atfault. The result is unnecessarilylengthy and expensive down-

time.An ungrounded system, despite

the drawbacks, does have onemain advantage. After the firstground fault, assuming it remains

as a single fault, the circuit maycontinue in operation, permitting

continued production until a con-venient shutdown for mainte-nance can be scheduled.

Benefits of propersystem groundingSystem grounding the intention-al connection of the neutral points

of transformers, generators androtating machinery to the earthground network provides a ref-

erence point of zero volts, whichoffers many advantages over anungrounded system, including:

Reduced magnitude of tran-sient over-voltages;

Simplified ground fault loca-

tion; Improved system and equip-

ment fault protection;

Reduced maintenance time andexpense;

Greater safety for personnel;

Improved lightning protection; Reduction in fault frequency;Solidly neutral grounded sys-tems offer partial protectionIn solidly grounded systems, theneutral points have been intention-

ally connected to earth groundwith a conductor having no inten-tional impedance. See Figure 4.

This reduces the problem of tran-

sient over-voltages found on theungrounded system and speeds

the location of faults.However, solidly grounded sys-

Vol. XXXVIII No. 5 A C L B M E D I A P U B L I C A T I O N May 2002

Reprinted with permission from ELECTRICAL BUSINESS CLB Media Inc., Mississauga ON, Canada 2002

Without proper protection, ground faults canhave serious, damaging effects on your processesand equipment.

Protect your business with a ground fault applicationsolution from IPC. Designed to detect, locate andprotect against ground faults, our family of productsminimizes downtime, prevents damage to equipment

and reduces the risk to personnel.

As the single source for ground fault applicationsolutions, IPC gives you the power to protect. Callus or visit our Web site today for more information.

IS YOUR BUSINESS

PROTECTED?

7615 Kimbel St., Unit 1, MississaugaOntario Canada L5S 1A8Phone: 905-673-1553Toll Free: 1-888-737-4787Fax: 905-673-8472E-mail: info@ipc-resistors.comwww.ipc-resistors.com

Grounding of industrial power systems

Figure 1

Figure 2

7/29/2019 Electrical Business (Topic High Resistance Grounding)

2/2

2 ELECTRICAL BUSINESS May 2002

G R O U N D I N G A N D B O N D I N G

tems lack the current-limiting abili-

ty of resistance grounding and theextra protection this providesagainst equipment damage and

arcing ground faults.The well-known and well-docu-

mented destructive nature of arcing

ground faults in solidly groundedsystems is caused by the energydissipated in the fault. Ameasure of

this energy can be estimated fromthe formula:Kilowatt cycles = V x I x T1000.

where V and I are the voltage andcurrent respectively of the arc, and T isthe duration of the arc in cycles.

The faulted circuits need to betripped, with conventional time-current coordination. Often, it is

difficult to selectively trip the circuitwith minimum time delay andhigh equipment damage due to

arcing ground fault is tolerated.Ground fault relays with zone-

selective instantaneous protectionneed to be used to provide maxi-

mum equipment protection, whileretaining selectivity and coordina-tion.

Advantages of resistance-grounded neutral systemsResistance-grounding is by far themost effective and preferred

method. It solves the problem oftransient over-voltages, therebyreducing equipment damage. It

accomplishes this by allowing themagnitude of the fault current to bepredetermined/limited by a simple

ohms law calculation:I = ER

Where: I = Limit of fault current.

E = Line-to-neutral voltageof system

R = Ohmic value of neutral

rounding resistorIn addition, limiting fault cur-

rents to predetermined maximum

values permits the designer toselectively co-ordinate the opera-tion of protective devices, which

minimizes system disruption andallows quick location of the fault.

There are two broad categories ofresistance-grounding: low-resist-ance and high-resistance. In both

types of grounding, the resistor isconnected between the neutral of

the transformer secondary and theearth ground, as shown in Figure 5.Low-resistance-grounding of theneutral limits the ground fault cur-

rent to a relatively high level (typi-cally 50 amps or more), in order tooperate protective fault-clearing

relays and current transformers.These devices are then able toquickly clear the fault, usually

within a few seconds. This fast re-sponse time is important, since itlimits damage to equipment, pre-

vents additional faults from occur-

ring, provides safety for personneland localizes the fault.

The limited fault current and fastresponse time also prevent over-heating and mechanical stress on

conductors. It must be noted that,like the solidly grounded neutralsystem, the circuit must be shut

down after the first ground fault.Low-resistance-grounding resis-

tors, typically rated 400 amps for

10 seconds, are commonly foundon medium- and high-voltage sys-tems.

High-resistance-grounding of theneutral limits the ground fault cur-

rent to a very low level (typicallyunder 25 amps). It is used on low-voltage systems of 600 volts orless, under 3000 amps. See Figure

6. By limiting the ground faultcurrent, the fault can be toleratedon the system until it can be locat-

ed, and then isolated or removedat a convenient time. This permitscontinued production, provided a

second ground fault does notoccur.

H i g h - r e s i s t a n c e - n e u t r a l

grounding can be added to exist-ing ungrounded systems withoutthe expense of adding fault-clear-

ing relays and breakers. This pro-

vides an economical method ofupgrading older, ungrounded

systems.The resistor must be sized to

ensure that the ground fault cur-

rent limit is greater than the sys-tems total capacitance-to-groundcharging current. If not, then tran-

sient over-voltages can occur.By strategic use and location of

ground fault sensing relays,

troubleshooting can be greatlysimplified. High-resistance neutralgrounding, combined with sensi-

tive ground fault relays and isolat-ing devices, can quickly detect and

shut down the faulted circuit. Thisprovides operating personnel withthe added safety thats essential inhostile operating environments

(such as in mining applications).Another major advantage is the

elimination of dangerous and

destructive flashovers to ground,which can occur on solidly

grounded systems.As is the case with most systems,

there are some disadvantages to

high-resistance neutral grounding: After the first ground fault, the

two un-faulted phases rise to

the line-to-line voltage, asshown in Figure 7. This createsa 73 per cent increase in voltage

stress on the insulation of the

system; When a ground fault occurs,

the neutral point of the systemrises to line-to-neutral voltageabove ground. As a result, the

neutral cannot be used in thesystem for load connectionssuch as single-phase lighting;

Should a second ground faultoccur on another phase beforethe first ground fault is removed,

a line-to-line fault is created.

Summing upUngrounded delta systems havemany operating disadvantages.

High transient over-voltages can

occur that are not immediatelyevident. In addition, ground

faults are difficult to locate.Solidly grounded neutral sys-

tems provide greater safety for

personnel, limit the systempotential to ground, and speedthe detection and location of the

ground fault. However, the sys-tem must be shut down after thefirst ground fault.

Low-resistance-grounded neu-tral systems only limit the mag-nitude of the ground fault cur-

rent so that serious damage doesnot occur. The system must still

be shut down after the first

ground fault. This level of resist-ance-grounding is generally used

on medium and high-voltagesystems.

High-resistance-grounded neu-

tral systems limit the fault currentto a tolerable level, permittingcontinued production, until the

fault can be located and correctedat a convenient time. It also pro-vides an economical method of

upgrading older, ungroundedsystems without expensive addi-tion of fault-clearing relays and

breakers.For a detailed comparison of

the performance of the differentgrounding methods under avariety of operating conditionsand characteristics please contact

Andrew Cochran at (905) 673-1553. EB

Andrew Cochran is president of

Mississauga, Ont.-based IPC

Resistors Inc.

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8