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4 Operation overview
Operation overview continued
The DL420 will test for No-trip loop, High current True Loop Impedance, No-trip
PFC (Prospective Fault Current), High current PFC using True Loop
Impedance, mains voltage (L-N, L-E and N-E) and correct socket wiring, In
addition to the incoming supply polarity check described on the previous page
Another unique feature of your DL 420 tester is high current 2-wire
measurement of TLI (True Loop Impedance). Regulations and guidance books
refer to loop impedance measurements but up until now loop testers measured
loop resistance which is different from loop impedance, particularly when
testing close to the main supply transformer.
This function is therefore recommended when testing for external earth fault
loop impedance (Ze) at distribution boards on non TT systems and similar
points on the supply side of any Switch/RCD gear and when testing the
Live/Neutral loop to determine prospective short circuit current.
It is the variation in power factor that makes the TLI measurement of your
DL420 so much more accurate than older loop testing techniques. (Because of
this there may well be variations in readings compared to ordinary loop testers
or the no trip function of this tester, particularly when the measurement is made
near to the main supply transformer).
The DL 420 also has a No-trip test mode that is guaranteed not to trip a 30mA
or higher rated RCD providing that the circuit is otherwise healthy.
No trip vs High current testing
Although the DL420 High current TLI measurement mode is inherently more
accurate than conventional techniques when measuring close to the main
supply transformer, the difference between resistance and true impedance
reduces the further away from the transformer the measurement is made.
This is why using the No-trip mode is suitable for measurement on final circuits
and similarly remote points. It should be noted however that whilst No – trip
testing at these locations will normally function at a similar high level of
accuracy, the low current measurement technique used is more likely to be
adversely affected by external factors such as contact resistance and circuit
noise. This can result in the occasional erroneous reading.
For this reason it is recommended that multiple measurements are made when
using the No-trip mode and any isolated odd results are ignored. When taking
multiple readings the tester or the supply should be powered off between
consecutive tests.
For safety reasons No-trip mode is recomended for all measurements made
on TT systems.
