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Ground Grid Integrity

by Jeff JowettMegger

T

he testing o grounding electrodesgrids, meshes, and the likeis most

oten considered in terms o a resistancetest. Tat is to say, the resistance

o the surrounding environment to current ow to some arbitrary orcareully plotted point typically reerred to as remote earth or infnite earth. Te

ground grid is intended to serve the dual purpose o carrying currents into the

earth without exceeding the operating tolerances o any protected equipment

while assuring that personnel in the vicinity are not exposed to electric shock as

would result rom excessive step or touch potentials. Resistance tests indicate

the overall capability o the grid in this regard: its electrical relationship to its

environment. But there remains the question o the internalcondition o the

grid itsel.

Out o sight, out o mind? Buried under ground, the grounding electrodedoesnt call attention by mere visual inspection, which is the rst step in mostelectrical maintenance. Tough they mayseem inert, grounding electrodes aresubject to their own unique set o stresses, just like other electrical equipment.Fault clearance and lightning protection can severely damage a grid or mesh,separating individual elements, interrupting continuity, and introducing highresistance across bonds. But in the meantime, the electrode may have cleared theault perectly well, leaving no obvious indication that it has been compromised.A subsequent event may not be aforded the same level o protection.

Furthermore, a less dramatic but more persistent orce o deterioration isthe incessant process o corrosion and weather. Freezing and expansion exert

pressures that can break apart a grid. Ironically, the best grounding soils are alsothe most corrosive. Low resistivity soil that acilitates the ow o ault currentalso promotes electrolytic current that eats away at the metallic structure o agrounding electrode. Use o dissimilar metals hastens the process. Rods have beenknown to last as little as two years, with typically a risk o at least some corrosionefects being present ater our.1A standard ground resistance test just looks atvoltage drop across the surrounding soil and gives no measure o the physicalcondition o the electrode itsel.

Te grounding electrode typically iscarrying onlynoise, but must be ableto accommodate worst case conditionso high current ow when called online during an event. Tereore, to testgrid integrity, the tester must be ableto produce high current. A grid testerworks similarly to a ground tester inthat it supplies current and measuresvoltage drop across the test item. Inthis case, the test item is the grid,whereas in a ground test, it includesthe surrounding soil. It is dissimilar inthat the grid tester typically employs

an industry standard o 300 amperes,whereas a ground tester operates onthe milliampere level. Rather thancalculating and displaying resistance,the grid tester evaluates the change incurrent ow.

est equipment consists o a vari-able current source requiring on theorder o a 10.5 kVA capability, oper-ated rom a 50 ampere, 240 Vac source.est leads can range anywhere rom10 to 100 eet o 2/0 welding cable. A

reerence ground is rst established,preerably a transormer neutral. Teleads are connected, one to the testground and the other to the reerenceground below any bonding connec-tions (Fig. 1). Te tester is then ener-gized and adjusted to pass 300 amperesvia the reerence ground through thegrid under test or a duration o threeminutes.

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Figure 1

D

A

B

C

Measurements after Test Hook-Up

A clamp-on ammeter is then used to measure currentat critical points around this system: through the reerenceground above (A) and below (B) the attachment o the testlead and on the opposite side o the system through the testground both above (C) and below (D) the lead connection.Current readings are recorded, and the tester indicatesvoltage drop across the system. Voltage drop o the leadsthemselves is also measured. Tis is done by disconnecting

rom the test item, shorting the leads together, and passing300 amperes or three minutes, noting the voltage. Tis valueis then subtracted rom the voltage drop taken during thetest to isolate voltage drop across the grid rom the leadcontribution. For an indication o acceptable continuity, avalue o no more than 1.5 volts per 50 eet o straight lineground path should be measured. Te straight line groundpath is the distance between the two lead connections.

Tough valuable, this method is not rigorously precise,and so a redundant system o evaluation exists based oncurrent return. For single driven electrodes, at least 200amperes should return to the source via the ground path.

For mats and grids, at least hal o the current must returnvia the ground path. I not, it indicates a potentially badconnection and should be dug up or repair.

What is the method that makes this procedure success-ul? What happened to Kirchhofs laws? Kirchhof s rstlaw states that the sum o the currents owing rom a pointin a circuit equals the current owing to that point, i.e.,current is a precisely measurable quantity that doesnt justdisappear into thin air. Operation o the high-current gridtester is based on an application o Kirchhofs rst law to

account or all the current that is injected into the systemBy injecting a substantial amount o current, it becomecomparatively easy to note its division along stress lines. Iis expected that most o the current will ollow the short-est, straightest path (least resistance) between the two tespoints. Te ammeter readings indicate to what extent thisis occurring. Discontinuity or high resistance connectionsanywhere between the test points will divert proportionate

amounts o current through the rest o the system.Substations are multiply-bonded into a Faraday cageconguration, and other acilities with complex or extensivegrids are also typically connected to the electrical systemat multiple points. Tereore, it cannot be presumed thaallcurrent is owing in a particular path. Current owingrom the tester must rst be measured or any diversion intothe system (point A, Fig. 1), and to determine the amounowing into the grid (B). Tis value is then compared tothe amount returning through the test ground (D), andthat which is diverted through parallel paths into the reso the system (C).

o illustrate, an example o an acceptable test is shownin Fig. 2. Pretest conditions indicate typical values o cur-rent owing on the system. Te distance between the testconnections is measured, and the voltage drop across theleads is taken rom the tester. Perormance o the test thenindicates 270 amperes owing into the grid, with somediversion through the reerence ground back into the electrical system. Te ground connection being measured thenshows 280 amperes returning (test current enhanced bysome noise on system). Since this is a grid, the industrystandard calls or at least a 150 ampere return, so this is welexceeded. Voltage drop across the test was measured at 7.9but as 7.5 o this was lead resistance, only 0.4 volt is acrosthe tested path. Tis alls within the allowance or 1.5 voltper 50 eet (1.5/50 x 15 = 0.45). Te tested ground pathpasses both criteria with acceptable values.

A ailed test is outlined in Fig. 3. Here, only a negligibleamount o current returns through the tested ground con-nection, while 280 amperes ow through building structurevia a parallel connection. Voltage drop calculates to 8.1 (15.6 7.5), which ails the requisite criterion (100 eet allows 2x 1.5 = 3 volts). I the test setup were switched to the otheleg o the structure, results would be essentially reversed, so

the ground connection on the let would have to be dug upand inspected or a ault in continuity.Similarly, ground cables, clamps and errules can be

tested prior to installation using the same equipment andparameters. Cable manuacturers specications shouldprovide proper voltage drop. For instance, 300 ampereson 100 eet o cable yields 30,000 ampere-eet. For 4/0bare copper, the voltage drop should be 4.1 volts. For a 10oot section, thereore, the voltage drop would be 0.41. Imanuacturers guidelines are not available, the ollowing

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Figure 3

Measurement before Test Hook-Up

1. Current in transormer neutral (reerence) = 82 amperes2. Current in rame ground = none3. Distance between reerence ground and rame ground = 100 eet4. Voltage drop o the test leads with 300 amperes = 7.5 volts

Measurement after Test Hook-Up and with 300 Amperes Flowingin the Test Circuit (i.e., test set meter reading)

1. Current ow in reerence ground to grid = 270 amperes2. Current ow in reerence ground to (X) = 50 amperes

3. Current in rame ground rom grid = 2.5 amperes4. Current ow in rame ground rom the structure = 280 amperes5. Voltage reading at the test set meter = 15.6 volts

Hook-Up and ConnectionsMeasurements before Test Hook-Up

Figure 2

Measurements before Test Hook-Up

1. Current in transormer neutral (reerence) = 82 amperes2. Current in the post ground wire = 6 amperes3. Distance between reerence ground and post ground = 15 eet4. Voltage drop in test leads with 300 amperes = 7.5 volts

Measurements after Test Hook-Up and with 300 Amperes Flowingin the Test Circuit (i.e., test set meter reading)

1. Current ow in reerence ground to grid = 270 amperes2. Current ow in reerence ground to (X) = 50 amperes

3. Current ow rom grid to post ground = 280 amperes4. Current ow rom the structure to post ground = 1 ampere5. Voltage reading at the test set meter = 7.9 volts

ormula can be used to get an approximation o voltagedrop, bearing in mind that manuacturers specicationsare always preerable:

V = (2 x I x L x R)/1000 where,

I = test current

L = length

R = resistance per 1000 eet

Specic code requirements are not in efect, but stan-dards exist that provide guidelines or grid testing. Notably,NFPA70E-1983, Part I, Chapter 2, Section F, Item 4 out-lines low-impedance continuity, and Part III, Chapter I,Section B, Item 1 calls or continuous maintenance. OSHAhas adopted this as a saety requirement, and IEEE 81reerences testing o grid structure. By this method, eachground connection around a substation or other acility canbe tested. Faults are not preciselypinpointed, but by isolating

a aulty currentpath, the work o excavation and repair ismarkedly reduced.

1 Lyncole XI GroundingElectrical Equipment Testing and Maintenance, A. S. Gill, Prentice

Hall

Jefrey R. Jowett is Senior Applications Engineer or Megger in ValleyForge, Pennsylvania, serving the manuacturing lines o Biddle, Meg-ger, and Multi-Amp or electrical test and measurement instrumenta-tion. He holds a BS in Biology and Chemistry rom Ursinus College.He was employed or 22 years with James G. Biddle Co. which becameBiddle Instruments and is now Megger.