8.4.1 - Protective conductor continuity
All protective and bonding conductors must
be tested to ensure that they are electrically safe and
correctly connected. {8.7.1}
gives test instrument requirements. Provided that the supply
is not yet connected, it is permissible to disconnect the
protective and equipotential conductors from the main earthing
terminal to carry out testing. Where the mains supply is
connected, as will be the case for periodic testing, the
protective and equipotential conductors must not be disconnected
because if a fault occurs these conductors may rise to a
high potential above earth. In this case, an earth-fault
loop tester can be used to verify the integrity of the protective
system.
Where earth-fault loop impedance measurement
of the installation is carried out, this will remove the
need for protective conductor tests because that conductor
forms part of the loop. However, the loop test cannot be
carried out until the supply is connected, so testing of
the protective system is necessary before supply connection,
because connection of the supply to an installation with
a faulty protective system could lead to danger.
There are three methods for measurement
of the resistance of the protective conductor.
1. - Using the neutral conductor as
a return lead
A temporary link is made at the distribution board
between neutral and protective conductor systems. Don't
forget to remove the link after testing. The low resistance
tester is then connected to the earth and neutral of the
point from which the measurement is taken (see {Fig 8.2}).
This gives the combined resistance of the protective and
neutral conductors back to the distribution board. Then
| where |
| Rp
- is the resistance of the protective conductor |
| R -
is the resistance reading taken |
| An - is the cross-sectional
area of the neutral conductor |
| Ap - is the cross-sectional
area of the protective conductor. |
Note that the instrument reading taken
in this case is the value of the resistance R1 + R2 calculated
from {Table
5.5} (see {8.4.4}).
This method is only valid if both conductors have the same
length and both are copper; in most cases where steel conduit
or trunking is not used as the protective conductor, the
test will give correct results.
Fig 8.2 - Protective
conductor continuity test using the
neutral conductor as the return lead
2. - Using a long return lead
This time a long lead is used which will stretch from
the main earthing terminal to every point of the installation.
First, connect the two ends of this lead to the instrument
to measure its resistance. Make a note of the value, and
then connect one end of the lead to the main earthing terminal
and the other end to one of the meter terminals.
Second, take the meter with its long lead
still connected to the point from which continuity measurement
is required, and connect the second meter terminal to the
protective conductor at that point.
The reading then taken will be the combined resistance
of the long lead and the protective conductor, so the protective
conductor value can be found by subtracting the lead resistance
from the reading.
| Rp = R - RL |
| where |
| Rp
- is the resistance of the protective conductor |
| R - is the resistance reading
taken |
| RL
- is
the resistance of the long lead |
Some modem electronic
resistance meters have a facility for storing the lead resistance
at the touch of a button, and for subtracting it at a further
touch.
3. - Where ferrous material forms
all or part of the protective conductor
There are some cases where the protective conductor is made
up wholly or in part by conduit, trunking, steel wire armour,
and the like. The resistance of such materials will always
be likely to rise with age due to loose joints and the effects
of corrosion. Three tests may be carried out, those listed
being of increasing severity as far as the current-carrying
capacity of the protective conductor is concerned. They
are:
1 - A standard ohmmeter test as
indicated in 1 or 2 above. This is a low current test which
may not show up poor contact effects in the conductor. Following
this test, the conductor should be inspected along its length
to note if there are any obvious points where problems could
occur.
2 - If it is felt by the inspector
that there may be reasons to question the soundness of the
protective conductor, a phase-earth loop impedance test
should be carried out with the conductor in question forming
part of the loop. This type of test is explained more fully
in {8.4.4}
3 - If it is still felt that the
protective conductor resistance is suspect, the high current
test using 1.5 times the circuit design current (with a
maximum of 25 A) may be used (see {Fig 8.3}. The protective
circuit resistance together with that of the wander lead
can be calculated from:
| voltmeter reading (V) |
| ammeter reading (A) |
Fig 8.3 - High-Current
ac test of a protective conductor
Subtracting wander lead resistance from
the calculated value will give the resistance of the protective
system.
The resistance between any extraneous conductive part and
the main earthing terminal should he 0.05 Ohms or less;
all supplementary bonds are also required to have the same
resistance.
