TECHNICAL TOPIC
Howtoavoidmistakeson
Bill Allan
suggests
ways of
tackling this
section on
certificates
and reports
with
confidence
In the Electrical Installation Certificate, the Periodic
Inspection Report and NAPIT’s popular Single Circuit/
Minor Works Certificate there is a box entitled Supply
Characteristics and Earthing Arrangements.
Within that box, there is a section called Nature
of Supply Parameters. This seems to cause more
problems than any other part of the above mentioned
certificates. So I’ll give some guidance on completing
this section.
Nominal voltage U/U0
The symbol U is the nominal voltage between phases.
The nominal voltage between phases in a threephase system is 400 volts. The symbol, U0 represents
the nominal phase voltage to Earth (for TN systems).
The nominal value of U0 in three-phase and singlephase systems is 230 volts. When completing the
nominal voltage line, nominal voltages should be
used rather than measured values of voltage.
For three-phase systems, the nominal voltage line
should be completed thus: 400/230. For single-phase
systems, it should be recorded as: 230. The Electricity
Safety, Quality and Continuity Regulations 2002
permit tolerances of +10% to – 6% of the nominal
supply voltage.
the energy let-through of the upstream device
doesn't exceed a value which can be withstood
by the device (or devices) on the load side.(see
Regulation 434-03-01).
The term prospective fault current includes the
prospective short-circuit current and the prospective
earth fault current. It is the greater of the two values
which must be compared with the breaking capacity
Fig 1
Measurement of prospective
short-circuit current
Note: The neutral and earth probes are
connected to the neutral terminal
Supply
Earth
connection
block
N
P
E
P
N
Consumer
unit
Nominal frequency
The nominal frequency of public supplies in the UK
is 50 Hertz and a tolerance of +/- 1% is permitted.
Therefore, in the case of a three-phase or singlephase system being referred to, insert 50 next to
nominal frequency.
E
P
N
Prospective fault current, Ipf
The design requirements of Regulation 434-02-01
are included in the testing section in Regulation
713-12-01. The requirements are that the prospective
short-circuit current and the prospective earth fault
current are to be measured, calculated or determined
by another method (e.g. enquiry of the supplier) at the
origin of the installation and also at other relevant
points in the installation.
A relevant point is a point where a protective device,
required to operate under fault conditions, has been
installed. It is necessary to ascertain that the breaking
capacity rating of each fuse and miniature circuit
breaker is not less than the prospective short-circuit
current or the prospective earth fault current at the point
at which it is installed. However, a lower breaking
capacity is allowed if another protective device which
has the necessary breaking capacity is installed on
the supply side. In this case, it is necessary that the
characteristics of the devices are co-ordinated so that
10 NAPIT 0870 444 1392 www.napit.org.uk
Main
earthing
terminal
(MET)
Main bonding
conductors
Test instrument
of the protective device and recorded on the
certificate. Measurement is the preferred method for
determining the value of the prospective fault current
using an instrument capable of giving a direct reading.
Fig. 1 shows how the prospective short-circuit
current is measured using a three-lead test
instrument. As many test instruments cannot measure
the voltage between phases, when working on threephase systems, the maximum balanced prospective
short-circuit is taken to be twice the single-phase
value. In three-phase systems, the prospective
short-circuit current will always be larger than the
prospective earth fault current.
Fig. 2 shows how the prospective earth fault
current is measured using a three lead test instrument.
supplyparameters
Prospective fault current rating of
overcurrent devices
and the means of earthing. It is most
important that the means of earthing is
disconnected for the duration of the test
to avoid false readings being obtained
through the main bonding conductors. These
are called parallel paths.
As the means of earthing is disconnected,
care must be taken to avoid any shock risk to
For a single-phase 100 A supply, the prospective
fault current may be as high as 16,000A (16 kA),
depending upon the external supply network
conditions. In practice however, it is more likely to
be less than 5 kA. BS EN 1361. Type 1 cartridge
fuses of the type
frequently found in
Fig 2 Measurement of
electricity suppliers’
prospective earth
cut-out boxes have a
rated breaking
fault current
capacity of 16.5 kA.
BS EN 1361, type 11
cartridge fuses are
Supply
Earth
also increasingly
connection
common and they
N
block
P
have a rated breaking
capacity of 33K.
Domestic mcbs are
commonly rated at 6
kA. Further
information on
breaking capacities of
protective devices can
be found in IEE
Guidance Note 1,
Selection and
Erection.
The external
loop
impedance Ze
E
P
Measurement of external
loop impedance, Ze
Note: The main earth is disconnected
from main bonding conductors and
circuit protective conductors for this test
Supply
Earth
connection
block
N
P
E
Main
switch
in ‘off’
position
Consumer
unit
N
Fig 3
P
N
Consumer
unit
N
N
P
P
E
Regulation 313-01-01
requires the value of
Test instrument
Ze to be determined
and this can be
achieved by
measurement, enquiry of the electricity supplier
or calculation. The main reason for requiring Ze
to be established is to confirm that the main
earthing terminal provided by the electricity
supplier is actually connected with Earth. Therefore,
however a value for Ze is arrived at, a test must
be made to ensure that a connection with Earth
has indeed been provided. This test can be carried
out using an earth fault loop impedance tester or
a test lamp. NAPIT recommends that the value
of Ze is determined by direct measurement.
Fig. 3 shows how Ze is measured using a threelead phase-earth fault loop impedance tester at
the origin of the installation. The measurement is
made between the incoming phase conductor
E
(MET)
(MET)
Main bonding
conductors
Main bonding
conductors
Test instrument
the tester and to others on the site. The main
switch must be switched to the OFF position.
The tester must ensure that the means
of earthing has been replaced following
the test. Values for Ze are normally about
0.35 ohms for TN-C-S systems and around
0.8 ohms for TN-S systems.
See overleaf for
Student Activities
Conclusion
I hope this article has been helpful but
please remember when using test
instruments such as those discussed in
this issue it is most important to read the
manufacturer’s instructions before use.
NAPIT 0870 444 1392 www.napit.org.uk 11