Intentionally Insulated Ground.pdf

7/27/2019 Intentionally Insulated Ground.pdf

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Winter 2002-2003 1

Discharging stored capacitance through a fingertip generally results

in at least two things: an audible snap and a loud statement to the

effect of “Dang that hurts.” Anyone who has experienced this

knows the importance of verifying proper test set connection prior to

performing any tests. Due to capacitive coupling to components adja-

cent to the test specimen that are not adequately grounded, discharge is

still possible. During the testing of a 25 kV circuit breaker last year a

worker touched the outer “grounded” tube of a section of an isolated-

phase bus and was shocked.

Figure 1

Figure 1 shows the end view of an isolated-phase bus system. Theeight-inch aluminum tube in the center is the conductor and the outer36-inch diameter tube makes up the skin of the pressurized housing. The

by Paul Hartman Advanced Electrical Testing

Safety Corner

Intentionally Insulated Ground

question to address is how did

stray electrons get transferred tothe isolated-phase outer tubing if it was grounded properly? Theanswer is with a 100 kV dc high-potential test set. Not everythingwas as grounded as it seemed.

The SituationThe high-potential test set op-

erator was performing an insula-tion resistance test on a 25 kV SF

6

circuit breaker that had an isolated-

phase bus attached to the line side.The high-voltage test lead wasproperly isolated from the ground,the test set was grounded to thefacility ground wire, and a drainwire was attached to the case of the25 kV circuit breaker.

The isolated-phase bus had beenpreviously tested at a voltagegreater than that being used dur-ing the SF

6 circuit breaker testing.



2 NETA WORLD

That being known, it was correctly deduced that notechnical problem should exist when energizing thelength of the isolated-phase bus during the 25 kV cir-cuit breaker testing. From a safety perspective it wasalso assumed that since the energized isolated-phase bus was inside the grounded pressure case and notaccessible to workers that there would not be an is-sue.

However, during the C-phase testing, a construc-

tion worker reached from the scaffolding and touchedthe isolated-phase case about 50 feet from the SF

6 cir-

cuit breaker — only to quickly draw back his armwhen he discharged the built-up charge. Fortunately,the worker did not sustain any injuries. The construc-tion worker, who was 20 feet up in the air when theincident occurred, was wearing all of the proper per-sonal protective equipment (PPE), including fall pro-tection.

All testing was immediately stopped until the causeof the charge build-up could be determined. It turnedout that the isolated-phase pressure housing was notgrounded. This at first seemed impossible becauseevery 15 feet there was a 3/0 ground wire attached tothe steel isolated-phase bus support structure.

The ProblemBy design the isolated-phase housing has insula-

tors at every mounting point, thus isolating the hous-ing from the structure (see Figure 2). In addition, eachend of the isolated-phase bus housing has an insu-lated, flexible gasket that provides for ground isola-tion from the equipment at either end (see Figure 3).

Figure 3

The isolated-phase bus is designed to have a singlepoint ground attached to a ground lug connection onthe underside of one of the isolated-phase braces. Thissingle point design ensures that there are no circulat-ing currents from a fault or system leakage that couldtravel down the length of the isolated-phase housing(for a properly installed single point ground see Fig-ure 4). But at the time of testing the single point groundwires had not been installed.

Figure 2

Figure 4



Winter 2002-2003 3

PreventionWas this preventable? Absolutely. Two things

should have been done. First, the ground systemshould have been inspected prior to testing. If it wasincomplete then temporary ground wires should have been attached to those parts that were not yetgrounded. Second, the area around all the equipmentto be tested should have been secured. Even though

the parts being energized with test voltage were be-hind metal housings, the area around the isolated-phase bus and the circuit breaker should have beensecured (i.e., barricaded from other workers) for theduration of the test.

Paul Hartman has over 16 years experience in start-up, com-missioning, maintenance, and training in power generation, in-cluding international projects in Pakistan, Indonesia, Thailand,Brazil, and Korea. He has been an instructor for state certifiedcontinuing education programs as well as an associate instructorwith the San Francisco State University program. Paul is cur-rently Vice President of Advanced Electrical Testing. He is a regu-

lar contributor to NETA World and a frequent speaker at NETA’sAnnual Technical Conference.

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Intentionally Insulated Ground.pdf