sparks
ELECTRICAL NEWS
october 2014
contractors’ corner
9
Mark Palmer, Gauteng Electrical Inspection Authority
The pitfalls of electrical installation testing
LASTmonth, I discussed in some detail various
aspects of testing that, I believe, are becoming
problematic – and perturbing – in the electrical
industry.
So, this month, I’mgoing to highlight inmore
details the more common problems associated
with electrical installation testing.
Prospective short circuit current (PSCC)
With the publication of Amendment 8 of SANS
10142-1 in 2012, an addition was made to Sec-
tion 2 of the test report, which included indicat-
ing the PSCC
at the point of control
. Attached
to this requirement is that details of how this
value was obtainedmust be indicated. It is clearly
evident tome that many Registered Persons are
not aware of the importance of this reading in
relation to the actual short circuit withstand rat-
ing of the protective device. In all toomany CoCs
that pass over my desk every month, I see the
reading at the point of control given as the same
as the actual withstand rating of the switch.
To further compound this error, it is then
declared
that the given valuewas obtained from
the supplier
. Now, unless tangible proof to this
effect is in the hands of the Registered Person
this could, on examination, be determined as
negligence on the part of a Registered Person
and, furthermore, would render an issued CoC
as invalid.
Generally, this is not the end of the problem
with this test. In Section 4 of the test report the
Registered Person is also required to perform
this test for all sub distribution boards as well
as any alternative supplies; and to add this test
as part of the required tests, as an additional
annexure. It is very seldom that this requirement
is complied with and I see very few CoCs where
these tests have been included.
It is also important whenmeasuring PSCC to
pay attention to the cautionary notes detailed in
SANS 10142-1:
Note 1:
Do notmeasure three-phase PSCC if
themeter 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 bymultiplying the singe-
phase value by 1.73.
This next sentence is of utmost importance:
In addition, before any instrument is connected,
confirm that the instrument is rated for the
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applicable current rating, in particular
where the
current rating at themain switch disconnector
exceeds 100Aor the PSCC is expected to exceed
10 kA.
Elevated voltage on neutral
This again is a test with serious consequences
for a user of an electrical installation, if not
performed properly. Instructions on performing
the test are included under SANS 10142-1 Clause
8.7.6 and I quote:
Clause 8.7.6. With themain switch off, measure the
voltage between the supply neutral and
any earth
external to the installation
. Notify the supplier if
the reading exceeds 25 V.
Disconnect the installation and notify the sup-
plier (see annex K) if the reading exceeds 50 V.
I have witnessedmany Registered Persons
performing the test from the supply neutral to
the earth bar of the installation. Many readings of
‘0V’given on the CoC can also be attributable to
the incorrect performance of the test.
My concern with this test is that many Regis-
tered Persons have a fundamental problemwith
understanding the reasoning behind the test.
In a nutshell, the elevated voltage on the supply
neutral is, in fact, an objectionable difference
in potential that may exist between the supply
neutral of an installation
and any earth point
external to the installation.
This elevated
voltage is more commonly referred to as‘a stray
voltage’or‘a floating neutral’.
I cannot overemphasise the potential hazards
for a user when this voltage exists and it exceeds
the safe limit. Instructions for disconnection of
the installation if a voltage of more than 50 V is
found are also clear.
Earth loop impedance
Inmy opinion regarding the performance of this
test is that (again) many Registered Persons are
not fully conversant with the reasoning for the
test and, indeed, what constitutes an‘acceptable
reading’once the test has been done. The earth
loop impedance at the main switchmust be such
that
an earth fault current double the rated
current (or higher) of themain protective
device automatically disconnects the supply to
the installation
. In simple terms, in determining
whether a reading is acceptable and, for example,
using a main switch with a rating of 60 A, this rat-
ing would be doubled (that is 120 A), divide it into
the standard supply voltage (that is 230 V single
phase) and obtain a figure of 1.92 Ω. This figure
would then represent the maximum impedance
for a protective device rated at 60 A.
Voltage at available load
The last problematic area I intend dealing with
this month is voltage at available load. This test,
unlike the voltage no load and voltage on load
(which are performed at the point of control) is
required to be performed at a point of consump-
tion furthest from the point of control. In essence,
the test is required to determine the voltage at a
point of consumption where the worst voltage
drop condition is expected.
Select the circuit and point of consump-
tion where the worst voltage drop condition is
expected. Switch on the maximum available load,
but at least 50%of the circuit load and not less
than 2 A
, andmeasure the voltage at that point of
consumption. Record the value on the test report.
The voltage drop fromthe point of supply to the
point of consumption shall not exceed5%
(see
annexure E).
Serious repercussions
So, it must now be clearly evident that testing is
absolutely crucial when critically evaluating the
safety of an electrical installation. It must also be
clearly evident that any non-performance or omis-
sion will have serious repercussions when it comes
to the validity of the CoC.
In conclusion, I must make particular reference
to the declarationmade by the Registered Person
in terms of Section 5.4 of the test report:
I, being the person responsible for the inspection
and testing of the electrical installation, particulars
of which are described in Section 3 of this form,
certify that the inspection and testingwere done
in accordancewith this part of SANS 10142
,
and
that the results obtained and reflected on this
report are correct.
the legal system in prosecuting offenders who
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In addition to these services, theWCAEIA pro-
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or email
.
Enquiries: +27 21 987 3010
