Megger LT7 User Manual

Digital Loop Tester

M LT7

User Guide

Guide de l’utilisateur

Gebrauchsanleitung

Guía del usuario

SAFETY WARNINGS

✱ Safety warnings and precautions must be read

and understood before the instrument is used.
They must be observed during use.

✱ Continuity of protective conductors and earthed

equipotential bonding of new or modified
installations must be verified before carrying out
an earth fault loop impedance test.

✱ Exposed metalwork of an installation or

equipment under test must not be touched.

✱ LT7 must not be connected across two phases

of a 3 phase supply.

✱ The LCD neon voltage indicators cannot reveal a

Neutral - Earth reversal.

✱ Loop impedance <0,01Ω must be investigated.
✱ Test leads, probes and crocodile clips must be in

good order; clean, and with no broken or cracked
insulation. Avoid drawing excessive arcs on high
current tests.

✱ The mains power cord forms part of the

measuring circuit of the instrument. This test lead
must not be modified or changed in any way, or
be used in any other electrical instrument or
appliance.

✱ The instrument must not be used if any part is

damaged.

✱ Use of the instrument in any unspecified manner

may impair the protection provided.

THE INSTRUMENT MUST ONLY BE USED BY SUITABLY TRAINED AND

Symbols used on the instrument

Caution: refer to accompanying notes.

Equipment protected throughout by Double Insulation
(Class II)

Equipment complies with relevant EU Directives

NOTE

COMPETENT PERSONS

.

GENERAL DESCRIPTION

The Megger LT7 Digital loop tester is a compact portable instrument
designed to measure earth loop impedance and prospective short circuit
current. A special range is provided to carry out loop measurement on
installations protected by Residual Current Devices ≥30mAwithout causing
the breaker to trip out. The resolution in this range (1Ω) is more than
adequate for this purpose. Should greater resolution be required (0.01Ω)
the conventional 23Arange can be used.

Digital LCD

Low Battery

Indicator

P h a s e / E a r t h

Vo l t a g e

I n d i c a t o r

Retest

Push button

4 Position

R o t a r y
S w i t c h

Battery
Compartment
C o v e r

Features

Thermal
Protection
Indicator

‘Excessive
Noise’
Indicator

Neutral/Earth
Vo l t a g e
I n d i c a t o r

Prospective
Short Circuit
Current
Measurement

High Current
(23A) Loop
M e a s u r e m e n t

Low current
(15mA) Loop
M e a s u r e m e n t

Battery Replacement:

For safety reasons, the battery compartment must not be opened if the test
lead(s) are connected. Undo the two screws in the instrument base to
access the battery compartment, and replace the battery cells. The
instrument will beep. Ensure that correct polarity is observed and that the
cover is correctly replaced.

Measurements - General

OPERATION

Live - Earth loop impedance measurement can be made via installation

sockets using the plug terminated test lead, or at any other convenient
point on the installation using the optional dual lead test set. The optional
dual lead test can also be used to carry outLive - Neutralloop impedance
measurement.

Refer to Safety Warnings before using the instrument

Switching ‘On’and testing

Turning the rotary switch will activate the instrument. Alternatively, pressing
the Retest button will re-activate the instrument after ‘Auto shut off’. The
activated instrument will momentarily display all screen segments, followed
by the software version number. When connected to mains voltage of the
correct polarity the L - PE ‘neon’indicator activates and supply voltage
is displayed. If out of voltage range, VOL flashes alternately with the actual
measured voltage.The word ‘null’ is then displayed as an 11 second self
calibration check (‘Auto’ range only) is completed. The instrument then
proceeds to automatically take a measurement. Two pin plug terminated
test leads should be reversed if only the N - PE voltage indicator is
activated. Repeat measurements can be made either by disconnecting
and then reconnecting the instrument (e.g. when using test probes) or by
pressing the Retest button. When operational, the LT7 will carry out a self
calibration every 5 minutes.

Circuit condition Indication

Testing is inhibited if:

1 ) Phase and Neutral supply connections are reversed. (N - PE‘ n e o n ’ activated).

2) Neutral - Earth voltage >50 V when performing a 3 wire test (L-PE
and N - PE ‘neons’activated).

3) Electrical ‘noise’exceeds rejection level on low current range.

( displayed).

4) Voltage is out of range. Flashing ‘VOL’ and actual Voltage
displayed.

5) Supply frequency out of limits (flashing ‘HZ’ displayed).

6 ) Thermal protection device operates on h i g h current range ( and

d i s p l a y e d ) .

Low Current (Auto) measurement (0,1 or 1 resolution)

This mode measures Live to Earth loop impedance via installation
sockets. Live to Earth or Live to Neutral loop impedance can also be
measured using the optional dual lead test set. The measurement
sequence takes 11 seconds. Residual Current Devices (R.C.D.s) ≥30mA
will not trip.
LT7 is supplied as standard with a display resolution of 0,1 Ω. In areas of
exceptional electrical noise, and where improved repeatability may be
desired, it is possible to change the display resolution to 1 Ω in the
following manner:-

1) Turn rotary switch to the ‘OFF’ position.

2) Press and hold the Retest push button whilst turning the rotary
selector switch to any other position.

3) Still holding the Retest button, the display will cycle between 1 Ω and
0,1 Ω resolution.

4) Release the Retest button when the desired resolution is displayed.

To operate LT7:-

1) Turn rotary switch to ‘Auto’.

2) Connect LT7 by mains cord, or using the dual test lead firmly

connect red test probe to Live and the black test probe to Neutral
or to Protective Earth (ground). Mains (line) voltage is displayed.

3) Test progress is indicated by a sequential display of vertical bars.

4) Loop impedance value is displayed directly in . (>2 k displayed if
resistance exceeds 2 kΩ).

High Current (20 ) measurement (0,01 resolution)
This mode measures L i v e t o E a r t h loop impedance via installation sockets o r t h e
L i v et oE a r t h or the L i v et oN e u t r a l loop impedance using the optional dual lead test

set. N o t e: This mode may cause RCDs to trip. The measurement sequence takes
5 seconds.
1 ) Turn rotary switch to ‘ 2 0 ’.
2 )

Connect LT7 by mains cord, or using the dual test lead firmly connect
red test probe to Live and the black test probe to Neutral or to
Protective Earth (ground). Mains (line) voltage is displayed.

3 ) Test progress is indicated by a sequential display of vertical bars.
4 ) Loop impedance value is displayed directly in . N o t e : Any measurement

<0,01Ω will cause the display to flash, and should be investigated.
displayed if resistance exceeds 20Ω).

High Current (PSCC) m e a s u r e m e n t

In this mode, LT7 displays the Prospective Short Circuit Current (calculated at the
nominal supply voltage of 230V divided by loop impedance). This mode can be used
for L i v eto Earthconnection via unprotected installation sockets, o rfor Live toE a r t h ,
o r L i v e to Neutral connection on unprotected circuits using the optional lead set. T h i s
mode may cause RCDs to trip. The measurement sequence takes 5 seconds.
1

) Turn rotary switch to ‘PSCC’.

2) Plug in the LT7, or firmly connect red test probe to Live and black
test probe to Protective Earth (ground) or to Neutral. Mains (line)
voltage is displayed.

3) Test progress is indicated by a sequential display of vertical bars.

4) PSCC value displayed directly in kA. (>20kA displayed if value
exceeds 19,9kA).

Earth Bond Testing

By using the earth bond test lead available as an optional extra, the LT7
can measure the quality of Equipotential bonding within an installation.

1) Turn rotary switch to ‘Auto’or ‘20 ’.

2) Firmly connect the flying lead probe to the metalwork to be tested
and plug the LT7 into a convenient installation socket.

3) Reading displayed will consist of socket supply (live conductor)
impedance plus the impedance of the equipotential earth bonding.

(>20

SPECIFICATION

Ranges:
20 (23Anominal) 0,01Ω - 19,9Ω ±2% ±3 digits (230 V supply)

200 /2k (15mAnominal) 1Ω -1,99kΩ ±2% ±1 digit (230 V supply)

When set to 0,1 Ω resolution ±2% ±3 digits (230 V supply)

PSCC (23Anominal) 0,01 - 0,99 kA
Note: Calibration includes the test leads and plug. To maintain accuracy,

these must not be changed. See note overleaf.
Nominal System Voltage: 110 V / 230 V at 50 Hz
Voltage Accuracy: ±2% ±2 digits (when L-PE neon indicates).
Power supply: 4 x 1,5 V Alkaline cells IEC LR6 type

4 x 1,2 V NiCd or NiMHrechargeable cells

Auto shut off: After 5 minutes of instrument inactivity
Battery life: Typically 1200, one minute tests

Low Battery Indicator: The symbol will appear when A l k a l i n e

battery cells are almost exhausted.
Note: Battery cells should not be left in an instrument which may remain
unused for extended periods of time.

±5% ±6 digits (110 V supply)

±5% ±2 digits (110 V supply)

(±3 digits at 1σ typical)

1,0 - 19,9 kA

or