16
5.6 Possible sources of error
The displayed reading depends on a measurement of the supply voltage and therefore noise or transients caused
by other equipment during the test could cause an error in the reading.
One way to check for these is to do two tests and look for any difference in value. The instrument may detect
some sources of noise and warn the user.
Test results may be adversely affected, by supply voltage fluctuations, voltage glitches, spikes or electrical ‘noise’
during a measurement. It is recommended that tests are repeated and the results verified. If measurements are
far apart results should be considered abnormal.
Errors can be reduced by:
Use the two-wire lead set with prods and a firm connection to clean conductors.
Making 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.
Ensure the 4mm test probe sockets on the instrument are clean and free of grease and dirt by using a
cotton bud dipped in Isopropyl alcohol or similar solvent cleaner.
Further errors can be caused by proximity to transformers:
Testing in the close proximity of a transformer can cause errors in measurement because of the worsening of the
power factor caused by the large proportion of reactance in the measurement.
Worst case operating error calculated for Non Trip measurement mode in accordance with
IEC 61557-3:1997:
Intrinsic error or
influence quantity
Reference conditions or specified
operating range
Designation Code
Error
Intrinsic error
Reference conditions
A
0.05
Position
Reference position ± 90°
E1
0
Supply voltage
Battery voltage at 8 V to 13.2 V
E2
0.0042
Temperature
0°C and 40°C, using 10 loop measured at
230V in Non Trip mode
E3
0.023
Phase angle
At a phase angle 0° to 18°
E6
0.0489
System frequency
Frequency of 49.5 Hz to 50.5 Hz
E7
0.024
System voltage
Voltage of 195.5 V to 253 V
E8
0.00038
B
±0.12
B[%]
±12%
Operating error:
)
15
.
1
(
2
8
2
7
2
6
2
3
2
2
2
1
E
E
E
E
E
E
A
B
+
+
+
+
+
+
±
=
5.7 maxZ (LTW325, LTW335 and LTW425)
The maximum loop impedance value of any final ring circuit (or any series of loop measurements), can be
derived by using the [MaxZ] function:
1.
Select the [MaxZ] test range on the top range knob.
2.
Make a loop test measurement as described in section 5 above. The instrument will display and hold the
loop resistance value.
3.
For subsequent loop tests the instrument will display the new loop value.
If this is higher than the previous result it will be held on the display. Lower values will only be displayed for
2 seconds prior to reverting to the higher result.
5.8 Deriving (R
1
+ R
2
) (LTW325, LTW335, LTW425)
Automatic Derivation of an R1+R2 value
The LTW325, LTW335 and LTW425 are able to derive the (R1+R2) circuit impedance from tests made on a live
installation. It is not possible to isolate R1 or R2 separately.
