The nature of the earth-fault loop and its significance have
been considered in detail in {5.3}. Since the loop includes the resistance of phase and
protective conductors within the installation, the highest values will occur at
points furthest from the incoming supply position where these conductors are
longest. A measurement within the installation will give the complete
earth-fault loop impedance far the point at which it is taken (Zs), or the
earth-fault loop impedance external to the installation (Ze) may be measured at
the supply position. Internal loop measurements should be taken at points
furthest from the intake to give the highest possible results.
In simple terms, the impedance of the phase-to-earth loop is
measured by connecting a resistor (typically 10 Ohms) from the phase to the
protective conductor as shown in {Fig 8.17}. A fault current, usually something
over 20 A, circulates in the fault loop, and the impedance of the loop is
calculated within the instrument by dividing supply voltage by the value of
this current. The resistance of the added resistor must be subtracted from this
calculated value before the result is displayed. An alternative method is to
measure the supply voltage both before and whilst the loop current is flowing.
The difference is the volt drop in the loop due to the current, and loop
impedance is calculated from voltage difference divided by
current.
Since the loop current is very high, its duration must be
short and must be limited to two cycles (or four half-cycles) or 40 ms for a 50
Hz supply. The current is usually switched by a thyristor or a triac, the firing
time being controlled by an electronic timing circuit It is very important to
have already checked the continuity of the protective system before carrying out
this test. A break in the protective system, or a high resistance within it,
could otherwise result in the whole of the protective system being directly
connected to the phase conductor for the duration of the test. Commercial
testers are usually fitted with indicator lamps to confirm correct connection or
to warn of reversed polarity. {Fig 8.18} shows a typical earth-fault loop tester
connected to a socket outlet so that its loop impedance can be measured. If the
circuit to be measured includes socket outlets, the tester is connected as
indicated in {Fig 8.18}. Special leads for connection to phase and to earth are
provided by suppliers for all other circuits.
Fig 8.18 - Earth-fault loop tester connected for
use
Before testing, the main equipotential bonding conductors are
disconnected (BUT NOT THE CONNECTION WITH EARTH) to prevent parallel earth
return paths and to ensure that there is no reliance on the service pies for gas
and water for effective earthing, (REMEMBER TO RECONNECT THE MAIN EQUIPOTENTIAL
BONDING AFTER THE TEST).
Tests must be carried out at the origin of the installation,
at each distribution board, at all fixed equipment, at all socket outlets, at
10% of all lighting outlets (choosing points farthest from the supply) and at
the furthest point of every radial circuit. The test should be repeated at least
once to allow for the effect of transient variations in the supply
voltage.
A modified version of the earth-fault loop tester, which
effectively measures the phase to neutral impedance and calculates then displays
the value of the current which would flow if the supply voltage were applied to
this impedance are readily available. The principle of such a PSC tester is
described in {3.7.2}.
Since the test result is dependent on the supply voltage,
small variations will affect the reading. Thus, the test should be repeated
several times to ensure consistent results. The test resistor will be connected
across the mains for the duration of each test. and will become very hot if
frequent tests are made. Some testers will then 'lock out' to prevent further
testing until the resistor temperature falls to a safe value.
The earth fault
loop impedance measured as described will be for installation cables at ambient
temperature, unless the circuit concerned has been in use immediately before the
test, when it will be the impedance at normal operating temperature. Under
normal operating conditions, cable temperature will rise, and so will the
resistive component of the impedance. This effect is difficult to calculate, and
a practical alternative is to ensure that the measured values of earth fault
loop impedance do not exceed three quarters of the maximum values shown in {Tables 5.1, 5.2
or 5.4} as appropriate.
A circuit protected by
an RCD will need special attention, because the earth-fault loop test will draw
current from the phase which returns through the protective system. This will
cause an RCD) to trip. Therefore, any RCDs must be bypassed by short circuiting
connections before earth-fault loop tests are carried out. It is, of course, of
the greatest importance to ensure that such connections are removed after
testing. One manufacturer supplies a patented loop tester which does not require
RCDs to be short circuited and which will not cause them to trip
when the earth-fault loop test is made. Some instruments limit the test current to 15
mA so as not to trip RCDs with ratings of 30 mA and above. Whilst such tests may
often be useful, they do not test the integrity of the system under fault
current conditions.
When loop testing at lighting units controlled by passive
infrared detectors (PIRs), there may he damage to the associated electronic
switches unless they are short-circuited before testing.
An alternative to the use of a dedicated earth-fault loop
impedance tester is to measure the combined resistance of the phase and
protective conductors from the incoming position to the point for which
earth-fault loop impedance is required (this is R1 + R2 - ) and to add to it the external
earth-fault loop impedance (Ze) which can be obtained from the electricity
supplier. All earth-fault loop impedance test results should be carefully
compared with the data in [Tables 41B and 41D], adjusted to allow for ambient
temperature, or with figures provided by
the designer. To ensure that ambient temperature is taken into account, the
results should never exceed three quarters of the values given in [Tables 41B
and 41D].
