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QUESTION 10b: HOW IS THIS TEST DONE AND WHAT VALUE OF MEASUREMENT IS ACCEPTABLE?
ANSWER: a) Thetestneedstobeperformedatthedistributionboard.
b) Connect the leads above the earth leakage circuit breaker to avoid it from tripping
(seeFig.E, F, G& H -Pg 7&8). FigE,F,G &Hshows typicalinstallationsthat canbe
encountered.
c) Makeuseofthetestleadssuppliedwiththetesteronlyasnonstandardleadsmaylead
toerroneousreadings.Noextensionleadsshouldbeused.
c)
Theearthloopimpedancetestwillbeconductedbetweenthephaseandearthconductors
and the prospective short circuit current (PSC) will be between the phase and
neutralconductor.
e) T1825isautorangingandwillautomaticallyselectthemostsuitablerange.
f) Pressingthetestbuttonwillmomentarilyinjecta16A(typicalloadcurrent)testcurrent
throughthecircuitandaloopimpedancereadingwillbeindicated.Recordthisreading
onthetestcertificate.
g) Itisacceptedthatacircuitshouldcarryafaultcurrentoftwicetheratedcircuitcurrent.
Tocalculatetheacceptablevalueofmeasurementproceedasperthefollowingformula:
V
Remax=2xI(Amps)rated
WhereRemax = Maximumearthloopimpedance
V = MeasuredVoltage
I = RatedCircuitCurrent
Example:60AmpCircuit(I)
230VACSystem(V)
V
Remas=2xI
230
=(2x60)
=1.92Ω
Therefore the loop resistance of the circuit must be below 1.92Ω to allow a fault
currentof120ampstoflow.
h) Set the tester on the 2000A range on the PSC scale and measure the prospective
shortcircuitcurrent.Theresultwillbeindicatedinamps.
i) The result should be entered on the front page of the certificate of compliance /
test report. An example can be found on page 282 of the SANS 10142-1:2003. (See
addendumB–Pg9).
j) Inthecaseofsupplysystemsratedatnotmorethan250Vtoearth,measurethePSCC
atthepointofcontrolwithacommerciallyavailableinstrument(faultcurrentmeter).
Before any instrument is connected, confirm that the instrument is rated for the
applicable current rating, in particular where the current rating at the main switch
disconnectorexceeds100AorthePSCCisexpectedtoexceed10kA.
Note 1: Do not measure three-phase PSCC if the meter is not specifically designed for
that purpose or for the capacity of supply (or both). In a balanced three-phase system,
the three-phase value can be estimated by multiplying the single-phase value by 1.73.
Note 2: Ensure that the instrument connections do not add impedance to the
circuit measured.
CAUTION: Verify the suitability and accuracy of the PSCC instrument with
the manufacturer.
k) To calculate prospective short-circuit current see page 271 of the SANS 10142-1:2003
(SeeaddendumC–Pg11).
l) Itisimportanttotakeintoaccountthedistancefromthetransformerandsizeofsupply
cablewheninspectingandacceptingthekAratingofthecircuitbreakerandinstallation.
