14
■
Make several tests and taking the average.
■
Ensure that potential sources of noise in the installation are isolated
(switched off), eg: automatically switched loads or motor controllers
■
Ensuring that the instrument is calibrated.
maxZ (LT320 and LT330)
The maximum loop impedance value of any final ring circuit (or any series
of loop measurements) can be derived by using the [maxZ] function:
To make a [maxZ] measurement:
1. Select the [maxZ] test range
2. Using either the [No Trip] or [Hi] loop tests, make a series of loop test
measurements as described on pages 11 and 12.
3. The display will hold the highest loop measurement, from any number
of loop tests. Lower values are displayed momentarily.
4. Turning the range knob away from [maxZ] removes the maxZ stored
value.
NOTE:
maxZ is not stored when the instrument is switched off (or auto-
powers-off), hence auto-power-off time is extended in the Zref and R1+R2
modes to 30 minutes from the standard 6 minutes.
Deriving R1 + R2 (LT320 and LT330)
Automatic Derivation of R1+R2 Values
The LT320 is able to derive the R1+R2 measurement reading from tests
made on a live installation.
NOTE:
Care should be taken as any parallel earth paths may effect this
result.
To make use of this feature, the reference/distribution board result must
be stored in the instrument’s memory (Zref), prior to measuring R1 + R2
Saving a Zref (distribution) result
1. Switch the range knob to [Zref].
2. Select a [No Trip] or [Hi] loop test as appropriate.
3. Connect the instrument to the circuit under test as described on pages
11 or 12 as required.
4. Press the [TEST] button to perform a loop test.
IMPORTANT NOTES:
When using Zref and R1+R2 functions, we DO NOT recommend changing
between [No-Trip] or [Hi] test ranges during the test. Always use the same
test method whilst using Zref and R1+R2.
Showing R1+R2
1. After saving a Zref result switch the instrument to R1+R2.
2. All subsequent loop test measurements will have the Zref distribution
resistance subtracted.
3. The Zref value can be updated at any time by returning the Range knob
to Zref and repeating the loop test.
NOTE:
Zref is not stored when the instrument is switched off (or auto
powers down), hence auto power-off time is extended in the Z ref and
R1+R2 modes to 30 minutes from the standard 6 minutes.
Application note for Zref and R1+R2 measurement:
On initial verification of a new electrical installation, the value for R1+R2
should be obtained by continuity testing methods (dead testing) as per BS
7671:2001
For periodic inspection reports (PIR) where it is not possible to isolate the
supply, the user should first verify the circuit protective conductor
(standard practice) prior to using the Zref and R1+R2 function on the
Megger LT320 or LT330.
