Megger LT320 and Megger LT330

Automatic derivation of R1 + R2 values

Traditionally the sum of the resistance of the

phase conductor R1 and the circuit protective

conductor R2 have always been obtained by

disconnecting the supply and connecting the

phase conductor to the protective conductor

at the distribution board, followed by a test

between phase and earth terminals at the

circuit’s extremity.

If the inspection is an initial verification of an

new electrical installation, then the value for

R1+R2 should be obtained by the above

method and using a continuity tester (i.e.

Megger MIT320) as per BS7671.

However, for a periodical inspection where

there is an existing previous report, after

verifying the protective conductor, R1+R2

values can be acquired using the Megger LT320

or LT330 loop impedance testers.

These instruments are able to calculate the

R1+R2 values from tests conducted on a live

circuit.

Basically a reference loop impedance reading is

saved from the distribution board (i.e. Ze), then

the instrument is switched to R1+R2 setting, then

all following loop tests, made at the extremity of

each circuit, will have the reference reading

subtracted and the instrument will display the

R1+R2 in ohms.

These readings can then be checked against the

previous report.

Note: Caution should taken with boiler radials &

circuits feeding bathrooms since parallel earth

paths via any bonding will affect readings.

Megger LT320 downloading loop tester

Megger LT320 and Megger LT330

MaxZ function

The MaxZ setting on these instruments is

very useful for finding the maximum Zs

reading in any circuit.

Simply the instrument does a loop test and

stores the highest reading, any higher

readings from other tests on the same circuit

are held, lower readings are only displayed

momentarily, ideal for finding the maximum

Zs of a ring main quickly.

LT320 Loop tester like all Megger loop testers it is 3-phase safe