Operation - Megger DET62D Handbuch

OPERATION

LIVE EARTH SAFETY PRECAUTIONS
It is preferable that the earth electrode to be tested is first isolated from the circuit it is protecting, so that only the earth is
measured and not the complete system. When this is done, the circuits and equipment must be de-energised. If however
this is not possible, the earth electrode should be duplicated so that when it is disconnected for test purposes, the other one
provides the necessary circuit protection. The following safety precautions are essential when working near high tension
systems where any unintentional 'Live' earths may be encountered between the site earth and remote earths established
for test purposes. A 'Live' earth is one that carries current from the mains supply, or could do so under fault conditions.
1.All persons involved must be trained and competent in isolation and safety procedures for the system to be worked on.
They must be clearly instructed not to touch the earth electrode; test spikes; test leads, or their terminations if any 'Live'
earths may be encountered. It is recommended that they wear appropriate rubber gloves, rubber soled shoes, and stand on
a rubber mat.
2.The 'P' ('S') and 'C' ('H') terminals should be connected through a double pole isolation switch, the rating of which will
cope with the maximum fault voltage and current. The isolation switch must be open whilst any personal contact is made
with the remote test spikes,or the connecting leads, e.g. when changing their position.
Note: If a fault occurs while a test is being made the instrument may be damaged. Incorporating fuses at the isolation
switch, rated at 100 mA, and able to cope with the maximum fault voltage will provide some protection for the instrument.
TEST PROCEDURE FOR EACH MEASUREMENT
Connect the instrument terminals to the respective earth electrode and test spikes. See 'Initial Test Set Up and
Connections'.
1. Press the push button. This will begin the test sequence and include the circuit condition, and 'noise' monitoring. The
instrument will automatically switch 'Off' after 30s of inactivity. Alternatively, press the Test push again to switch 'Off'.
2. Check the circuit condition warning LEDs. If any adverse conditions are indicated, the cause should be rectified (to
prevent a false reading), before the test continues. Note: If the Current spike resistance is too high for the required
measurement range, the instrument will autorange to a range which can tolerate a higher Current spike resistance. This
results in a loss of resolution.
3. If conditions for the test are satisfactory, the display will stabilise, and the reading given may be accepted.
4. Several individual readings are taken, in order to locate the Potential spike in its optimum position, relative to the Current
spike and the Electrode under test. Measurements are plotted and the 'Plateau' region of the curve is identified to confirm
the real resistance value.
5. Calculate the average of ALL measurements which lie on the horizontal plateau of the curve.
6. If the maximum deviation from the average of all measurements is better than 5%, then their average may be taken as
the resistance to earth of the electrode under test.
Note: To determine 'X' ('E') terminal lead resistance, firmly connect the lead to the 'X' ('E') terminal, and its free end to the
'P' ('S') terminal. Firmly connect terminals 'P' ('S') and 'C' ('H') together with a short length of lead. Press the push button,
and note the reading of the instrument. This is the resistance value of 'X' ('E') terminal lead ONLY. If desired, subtract this
value from the overall measured resistance.
CONTINUITY TESTING
The instrument will measure metallic resistances of low inductance or capacitance. The circuit under test MUST be
switched off, de-energised and isolated before the test is made. Firmly connect terminals 'P' and 'C' together with a short
length of lead, and connect a short test lead from these terminals and another from 'X' terminal. Connect these two test
leads across the test points, press the push button, and note the instrument reading. Due to the inherent high accuracy of
the instrument, and the low resistance to be measured, contact resistance between the test lead clips and the conduit could
be a factor in the measured value. Contact and test lead resistance should therefore be kept as low as possible.
BATTERY INSTALLATION
It is advisable that new battery cells are available before embarking on a test sequence, as it can be extremely inconvenient
if the battery life expires while a field test is in progress. Caution: Whenever battery cells are being fitted or replaced, there
should be no connections to the instrument terminals. The cover to the battery compartment lifts off the front of the
instrument, and is held in position by two captive screws in the base of the instrument. To fit or replace battery cells, loosen
the two captive screws, and lift away the cover. Fit new cells as indicated on the battery compartment moulding. Replace
the cover and tighten the securing screws. To avoid damage by leaking electrolyte, do not leave cells fitted in an instrument
which will remain unused for extended periods of time.