Megger MIT221 User manual

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M
MFT1700 series
Multifunction testers
User manual

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Contents
SAFETY WARNINGS .................................................................................................................................................................3
1. Introduction.............................................................................................................................................................................4
2. Overview .................................................................................................................................................................................4
2.1 Front panel and controls .................................................................................................................................................6
2.2 Waste electrical and electronic equipment.......................................................................................................................6
2.3 Battery and fuse location, fitting and replacement ..........................................................................................................6
3. Operation .......................................................................................................................................................................7
3.1 General operation – all models........................................................................................................................................7
3.2 Mode button functions...................................................................................................................................................7
3.3 Test inhibit ......................................................................................................................................................................7
4. Voltage, frequency, current and temperature measurement..........................................................................................................8
4.1 Making a voltage measurement......................................................................................................................................8
5. Continuity / resistance measurement............................................................................................................................................10
5.1 Nulling test lead resistance (up to 9.99 ohms) ...............................................................................................................10
5.2 Making a CONTINUITY measurement..............................................................................................................................11
5.3 Storing / downloading results (MFT1731 only).................................................................................................................11
5.4 Continuity Buzzer ON/OFF...............................................................................................................................................11
5.5 Switch probe (SP5)..........................................................................................................................................................11
5.6 Buzzer threshold .............................................................................................................................................................12
5.7 Measurement methods and sources of error ...................................................................................................................12
6. Insulation resistance ...............................................................................................................................................................12
6.1 Making an INSULATION measurement.............................................................................................................................12
6.2 Insulation test lock..........................................................................................................................................................13
6.3 Measurement methods and sources of error ...................................................................................................................13
7. Loop Impedance testing...........................................................................................................................................................13
7.1 Range selection and test leads ........................................................................................................................................14
7.2 Making a loop impedance measurement.........................................................................................................................15
7.3 Phase to Neutral (or Phase to Phase) testing....................................................................................................................19
7.4 Prospective Fault Current and Short Circuit calculation (PFC & PSCC) ..............................................................................20
7.5 Zmax test mode..............................................................................................................................................................20
7.6 Making a Zmax loop impedance measurement ...............................................................................................................20
7.7 Making an R1+R2 loop impedance measurement (MFT1721 only)...................................................................................21
7.8 Measurement methods and sources of error ...................................................................................................................22
8. Residual Current Device testing................................................................................................................................................22
8.1 Making an RCD measurement ........................................................................................................................................23
8.2 Selecting RCD type .........................................................................................................................................................23
8.3 ½ x I RCD current rating (No-trip test) ...........................................................................................................................23
8.4 1 x I RCD current rating (Tripping test on 30mA RCD) ...................................................................................................24
8.5 5 x I RCD current rating (Tripping test on 30mA RCD) ...................................................................................................25
8.6 Ramp Test.......................................................................................................................................................................25
8.7 Type A (DC sensitive) RCD test ......................................................................................................................................26
8.8 Type B (Pure DC) RCD test...............................................................................................................................................26
8.9 Variable RCDs (not MFT1711) .........................................................................................................................................27
8.10 AUTO RCD testing ..........................................................................................................................................................27
8.11 3 Phase RCD testing........................................................................................................................................................27
8.12 Touch voltage display......................................................................................................................................................29
8.13 Measurement methods and sources of error ...................................................................................................................29
8.14 Useful information..........................................................................................................................................................29
9. Earth resistance measurement..................................................................................................................................................29
9.1 Connection terminals......................................................................................................................................................29
9.2 Touch voltage limit..........................................................................................................................................................29
9.4 Making a measurement – Three terminal resistance measurement ..................................................................................30
9.5
9.6
10. Setup options ..........................................................................................................................................................................31
11. Warning messages...................................................................................................................................................................32
11.1 Startup warnings ............................................................................................................................................................32
11.2 Battery............................................................................................................................................................................32
11.3 Battery charger ...............................................................................................................................................................32
11.4 Fuse warning ..................................................................................................................................................................32
11.5 Invalid rotary switch setting.............................................................................................................................................32
11.6 Continuity test................................................................................................................................................................32
11.7 Insulation test .................................................................................................................................................................32
11.8 RCD Test.........................................................................................................................................................................32
11.9 RCD range selection errors..............................................................................................................................................32
11.10 Loop Test.......................................................................................................................................................................32
11.11 Earth test.......................................................................................................................................................................32
11.12 Test will not start ...........................................................................................................................................................32

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Appendix A – Sending, storing, deleting and recalling test results........................................................................................................33
Appendix B – Downloading data using Bluetooth®.............................................................................................................................35
Appendix C – Installation category definitions.....................................................................................................................................36
Appendix D – Safe working practice....................................................................................................................................................36
Appendix E – Cleaning and maintenance ............................................................................................................................................37
Appendix F – Earth resistance testing – Basic principles .......................................................................................................................37
General specification...........................................................................................................................................................................38
Repair and warranty............................................................................................................................................................................41
GSAFETY WARNINGS
Safety warnings and precautions must be read and understood before the instrument is used. They must be observed during use.
■The circuit under test must be switched off, de-energised and isolated before test connections are made when carrying out insulation
and continuity tests.
■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, RCD or earth testing
■Do not touch circuit connections and exposed metalwork of an installation or equipment under test. Under fault conditions the
system earth could become hazardous live.
■Do not touch the earth stakes, test leads and their terminations (including connections to the earthing system under test) if an
installation earth fault can arise, unless adequate precautions are taken.
■The ‘live circuit warning’ and ‘automatic discharge’ functions are additional safety features and should not be regarded as a substitute
for normal safe working practices.
■Do not move the rotary switch positions while a test is in progress.
■Do not operate the instrument or connect it to any external system if it shows any visible signs of damage or if it has been stored
for prolonged periods in unfavourable conditions.
■Do not operate the instrument or connect it to any external system if the battery compartment or casing is open or any parts of the
case (including keypad, selector switch, display window, etc.) are missing.
■Always disconnect the instrument from all systems while batteries are being changed or the fuse replaced
■Do not replace rechargeable cells in the instrument with non-rechargeable “dry” cells and attempt to charge the cells. This can
cause explosion or fire.
■Do not operate charging equipment supplied with the instrument in damp or wet environments or outdoors. All test leads must be
removed from the instrument while charging.
■After insulation tests, capacitive circuits must be allowed to discharge before disconnecting test leads. Locking the insulation test
ON should only be used where there is no risk of a circuit holding a charge.
■The instrument should not be used if any part of it is damaged.
■Test leads, probes and crocodile clips must be in good order, clean and with no broken or cracked insulation.
■All test leads supplied with the instrument form part of the measuring circuit of the instrument. They must not be modified or
changed in any way, or be used with any other electrical instrument or appliance.
■A plug severed from the power cord MUST be destroyed, as a plug with bare conductors is hazardous in a live socket outlet.
■Ensure that hands remain behind guards of probes/clips when testing.
■U.K. Safety Authorities recommend the use of fused test leads when measuring voltage on high energy systems.
■Replacement fuses must be of the correct type and rating.
■Failure to fit the correctly rated fuse will result in damage to the instrument in the event of an overload.
■Special precautions are necessary when operating in situations where “live” earths may be encountered: isolation switches and fuses
(not supplied with this instrument) must be used.
■Special precautions are necessary when working near high tension systems (MV and HV): rubber gloves and shoes (not supplied with
this instrument) should be worn.
■Special precautions are necessary when working in wet conditions or in agricultural areas: observe the local safety standards and
take all necessary special precautions applicable to the particular location and do not touch the test leads with bare hands.

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LIVE EARTH SAFETY PRECAUTIONS
A ‘Live’ earth is one that carries current from the mains supply, or could do so under fault conditions. The following warnings apply in
addition to those listed previously.
■All persons involved must be trained and competent in isolation and safety procedures for the system to be worked on. They
must be clearly instructed not to touch the earth electrode, test stakes, test leads, or their terminations if any ‘Live’ earths may be
encountered. It is recommended that they wear appropriate rubber gloves, rubber soled shoes, and stand on a rubber mat.
■The earth electrode under test should be isolated from the circuit it is protecting before testing commences. If this is not possible,
ART (attached Rod Technique) may be used to measure electrode resistance.
■The instrument terminals should be connected to the system under test through isolation switches that are rated to handle the likely
maximum fault voltages and currents that could be encountered at the installation. The isolation switch must be open whilst any
personal contact is made with the remote test stakes, or the connecting leads, e.g. when changing their position.
■The instrument terminals should be connected to the system under test through fuses that are rated to handle the likely maximum
fault voltages and currents that could be encountered at the installation.
NOTE
THE INSTRUMENT MUST ONLY BE USED BY SUITABLY TRAINED AND COMPETENT PERSONS
Users of this equipment and/or their employers are reminded that Health and Safety Legislation requires them to carry out valid risk
assessments of all electrical work so as to identify potential sources of electrical danger and risk of electrical injury such as inadvertent
short circuits. Where the assessments show that the risk is significant then the use of fused test leads constructed in accordance with
the HSE guidance not GS38 ‘Electrical Test Equipment for use by Electricians’ should be used .
This instrument is internally protected against electrical damage when used for the purposes of testing low voltage electrical installations
as defined herein. If used in a manor other than those defined in this user guide, the protection capabilities could be impaired with
potential risk to the operator and the instrument.
Symbols used on the instrument are:
GCaution: Refer to accompanying notes

Maximum 300 V a.c. CAT IV to earth
> 600 V Maximum nominal system voltage of 600 V
fInstrument protected by 2 x F2 1000 V 30 kA fuses
cEquipment complies with current EU Directives
This equipment should be recycled as electronic waste
Equipment complies with ‘C tick’ requirements
12 V DC charger socket

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1. Introduction
Congratulations on your purchase of a genuine Megger Multifunction tester. The MFT1700 series Multi-function tester is a compact
instrument designed to perform all of the functions required by the electrical contractor to fully test domestic, commercial and industrial
wiring. Specially designed to comply with U.K., European and other International wiring regulations and standards, the MFT1700 may
be used on all single and three phase systems with rated voltages up to 300 V AC. rms to earth (ground).
2. Overview
2.1 Front panel and controls
2.1.1 Display
NOTE: Some features detailed within this user guide are model dependent. Not all features appear
on all models
Test button
Lead Null
Test lock
Setup selector
Results store
Test lock
Setup selector
Touch voltage
contact point
Mode
Display
backlight
Display
Primary
functions
Secondary
functions
Test button
Lead Null

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Display symbols
Symbol Meaning Symbol Meaning
lTest function locked on (also used to indicate a
change is saved in setup) GWarning triangle – instruction to refer to this user
guide
zTest lead null active fFuse blown
UL= 50V Touch voltage limit (and Earth test voltage) set to
50 V (change setup) Battery indicator
Buzzer enabled NiMH Battery type set to rechargeable NiMH - Change
in setup section 10
AUTO RCD test in AUTO mode >100 V
Indicates that the ground noise voltage exceeds
the instrument measurement capability (test is
inhibited)
Type AC RCD selected Rp (Rs)
Potential stake (P stake)
resistance exceeds range for accurate
measurement
Type A RCD selected Rc (RH)
Current stake (C Stake)
resistance exceeds range for accurate
measurement
SType S RCD (Type AC) V Ground noise voltage exceeds range for accurate
measurement of resistance
SType S RCD (Type A) V VCLAMP error
Type B RCD selected I ICLAMP error
Fast or Full RAMP test selected Bluetooth®enabled
TEST Instrument is running a test TInstrument is too hot, allow to cool
Earth loop noise detected.
N<->L Live and neutral connections reversed
Zref Reference loop measurement
R1+R2 Loop measurement with Zref value automatically
deducted
ZMAX Loop maximum measurement
2.2 Waste electrical and electronic equipment
WEEE
The crossed out wheeled bin placed on Megger products is a reminder not to dispose of the product at the end of its life with
general waste.
Megger is registered in the UK as a producer of electrical and electronic equipment. The Registration No is WEE/HE0146QT
2.3 Battery and fuse location, fitting and replacement
Battery type: 6 x 1.5 V Alkaline LR6 (AA) or NiMH HR6 rechargeable
Fuse type: 2 x F2 1000 V 30 kA fuses

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Battery condition is shown by the following display symbols:
Where NiMH rechargeable batteries are fitted, the battery condition display can be adjusted accordingly. Refer to section 10 Setup
options to change between alkaline and rechargeable batteries.
When set to NiMH batteries, the battery indicator in the display wil show NiMH under the battery status symbol as below: (Feature
available on all models).
To replace batteries or fuse:
Switch off the instrument.
Disconnect the instrument from any electrical circuits.
Remove the battery cover from the base.
For battery replacement:
a) Remove old cells and refit new batteries following correct polarity as marked on the battery holder.
c) Replace the battery cover.
Incorrect battery cell polarity can cause electrolyte leakage, resulting in damage to the instrument.
For fuse replacement
a) Withdraw each fuse in turn and check for failure. The blown fuse must be replaced with a F2 1000 V 30 kA fuse.
Rechargeable batteries and battery charging
Some models are supplied with rechargeable NiMH cells. These batteries can be charged in the instrument, using the supplied Megger
charger.
To charge the batteries:
Ensure fitted batteries are of the rechargeable NiMH type.
Connect the 12 V DC plug of the charger to the socket on the front of the MFT marked
Warning: Whenever battery cells are being recharged, there should be no connections to the instrument terminals and the instrument
should be switched off.
Warning: Do not attempt to recharge non-rechargeable (Primary) cells. Doing so may result in instrument damage and may
cause personal injury.
Ensure ambient temperatures are between 4 ºC and 40 ºC while charging the MFT.
Note: The crossed out wheeled bin placed on the batteries is a reminder not to dispose of them with general waste at the end of their
life.
Spent Alkaline and NiMH batteries are classified as portable batteries and should be disposed of in the UK in accordance with Local
Authority requirements. For disposal of batteries in other parts of the EU contact your local distributor.
Megger is registered in the UK as a producer of batteries.
The Registration number is BPRN00142

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3. Operation
3.1 General operation – all models
3.1.1 Switching on
Turn the rotary knob away from the off position.
The instrument will perform internal self tests then display the appropriate test screen, depending on the position of the function knobs.
3.1.2 Switching off
Turn the primary function knob to the OFF position.
The instrument will automatically turn itself off after 20 minutes* of inactivity. Press any button or turn either of the rotary knobs to turn
back on.
* 2 minute option in Setup, refer to section 10.
3.1.3 Backlight
Press the backlight button. The backlight will operate for 20 seconds.
3.1.4 Test buttons
Test buttons are duplicated on the left and right. Both buttons perform the same function except, when the is displayed, in this case
the right hand buttons perform a scrolling function. The left RED button also performs storage and lock functions on the MFT1731.
3.1.4 Test button lock
To lock the test button, hold down either of the RED test lock buttons with the Lsymbol, whilst holding down the test button. If is
displayed, the right hand buttons perform a scrolling function.
3.2 Mode button functions
The function of the mode button is dependent on the test function selected:
Test selected Function Options Comments
V/ºC Volts (mv model specific) Temperature requires suitable transducer
Continuity
RLO
Buzzer
ENABLE/DISABLE
Buzzer ON
Buzzer OFF
Buzzes on <2 Ω
May be changed in SETUP.
Refer to Section 10.
Insulation
RSIO Buzzer
ENABLE/DISABLE
Buzzer ON
Buzzer OFF
Buzzes on <1 MΩ
May be changed in SETUP.
Refer to Section 10.
Loop impedance (Z)
L-PE
3Lo - 3 wire no trip
2Hi – 2 wire high current
2Lo – 2 wire no trip
3Lo
2Hi
2Lo
3Lo default measurement
2Lo not available if 3 wire connection is
detected
RCD/GFCI
0º /180º selection 0º
180º (Press and release)
RCD Type
AC
A
AC(s)
A(s)
B
(Press and HOLD)
Type B available on MFT1721 and
MFT1731 only
EARTH (RE) Touch voltage limit 50 V/ 25 V (Press and release)
MFT1731 only
SETUP Refer to instrument setup
section 10.

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3.3 Test inhibit
Each test mode has conditions under which testing will be inhibited, as below:
3.3.1 Insulation
Detection of a circuit voltage above 50 V (a warning is displayed at 25 V)
3.3.2 Continuity
Detection of a circuit voltage above that used by the instrument will inhibit testing
3.3.3 Earth loop impedance
Touch voltage exceeds 50 V (or 25 V depending on instrument configuration)
Supply voltage over range or under range
Supply frequency out of specification
3.3.4 RCD/GFCI testing
Touch voltage detected or predicted to exceed 50 V (or 25 V depending on instrument configuration)
Supply voltage over range or under range
Supply Frequency out of specification
3.3.5 Earth testing
External voltage greater than 25 V present
Leads not connected as per the test requirements
Potential stake not within range (Rp)
Current stake not within range (Rc)
Other conditions that will inhibit testing include:
3.3.6 Battery exhausted
All testing will be inhibited in the event of a flat battery, refer to section 2.3.

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4. Voltage, frequency, current and temperature measurement
4.1 Making a voltage measurement
1. Set the Main rotary range knob to volts

(The position of the right hand rotary range knob does not matter)
2. Using two test leads, connect test leads to the L1 (+ve) and L2 (-ve) terminals
L1
L2
L3
N
E
L1
L2
L3
N
E
OR if Using the Mains plug Lead SAI10:
a. For Live to Neutral measurements, connect the Red connector to the L1 terminal and the Blue connector to the
L2 terminal
b. For Live to Earth measurements connect the Red test to the L1 terminal and the Green connector to the
L2 terminal
L1
N
E
L1
N
E
Connection (a) Connection (b)
Note: When connecting all three test leads (eg Phase, Neutral and Earth) or the mains plug test lead, the voltage displayed
is the highest of the three possible voltages.
Pressing either TEST button scrolls through L-E, N-E and L-N individual voltages. When the frequency of supply is shown, the voltage
displayed reverts to the maximum voltage across all 3 terminals.
On models with a dedicated mV range this is selected using the Mode button to select mV mode.
4.2 Frequency measurement
1. Automatically displayed when connecting to a live circuit as per 4.1 above
4.3 Phase rotation (Not MFT1711)
Display of Phase rotation is Automatic when all three test leads are connected to the 3 phase supply as below:
1. Set the main rotary range knob to volts V
(The position of the right hand rotary range knob does not matter)

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2. Using three test leads, connect test leads to the L1 to Phase1, L2 to Phase 2 and L3 to Phase 3. The MFT will display L1 L2 L3
or L1 L3 L2 depending on the direction of phase rotation.
L1
L2
L3
Normal rotation Reverse rotation
4.4 Leakage current measurement
Leakage current measurement uses the optional accessory current clamp (ICLAMP).
1. On the MFT1721, MFT1731, set the primary range knob to clamp position .
2. Connect to ICLAMP (part no. ICLAMP) to the ICLAMP socket on the MFT.
3. Connect the clamp to the circuit conductor. The instrument will display the AC current flowing in the conductor.
4.5 Temperature measurement (not on MFT1711)
1. Connect the thermocouple transducer to the L1 (+ve) and L2 (-ve) terminals
2. Press the Mode button to select ºC. (Pressing the mode button will cycle round the V, mV and ºC measurement modes)
The display will show the temperature at the tip of the temperature probe.
4.6 Switch probe
In the V/mV/ºC mode all measurements except temperature can be made with the remote switch probe. Tests are automatic and do not
require the test button to be pressed.
1. Connect the switch probe to the switch probe socket. The probe replaces the standard RED test lead and can now be used as
a normal test probe.
L1
L2
L3
N
E

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5. Continuity / resistance measurement
IMPORTANT
The continuity test will auto-range from 0.01 Ω to 99.9 kΩ. Circuits up to 2 Ω will be tested at >200 mA. To change the test current, go
to section 10 - Setup.
The continuity test is automatic. The test starts as soon as the leads are connected to a circuit. The TEST button is ONLY used to null the
lead set.
Warning: Prior to any continuity testing, ensure the circuits under test are isolated and not live.
SETUP allows the follow configuration options:
■Positive test current
■Bi-Directional test current
Bi-directional test current allows the automatic testing of the circuit in both directions and the highest measured value being displayed
See section 10 Setup options.
5.1 Nulling test lead resistance (up to 9.99 ohms) W
Before starting a continuity test, the test lead resistance should be nulled such that it does not add extra resistance to the
circuit being measured. Once nulled it does not need repeating for each test. Periodically it should be checked to ensure
nothing has changed.
The “Lead Null” value is retained even when the tester is switched off.
To null test leads:
1. Short test probes or clips together and press the TEST button. The null symbol zwill be displayed to indicate lead null is
active.
Lead null OFF
This null value is stored until the TEST button is pressed again.
2. To cancel the LEAD NULL, separate the test leads and press the TEST button.
5.2 Making a CONTINUITY measurement
1. Set the Primary (Left) range knob to Wrange. (The position of the right hand rotary range knob must not be in the
position).
2. Connect two test leads to the L1 (+ve) and L2 (-ve) terminals on the instrument.
A continuity measurement is made automatically.

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L
N
E
g
NOTES: Measurements are prevented when:
A resistance of > 99.9 kΩ is present
Circuit voltages in excess of 4 V are detected.
5.3 Storing / downloading results (Not MFT1711 or MFT1721)
For full details see Appendix A and B.
Once the display shows a value it will automatically be logged into temporary memory. Unless stored, this will be over written by the
next measurement.
To store this result or to send it to a PowerSuite compatible device, refer to Appendix A and B.
5.4 Continuity Buzzer ON/OFF
Whilst in the continuity range, press the MODE button . This will toggle the buzzer ON and OFF.

Buzzer ON =
Buzzer OFF = No symbol
5.5 Switch probe (SP5)
In the CONTINUITY/RESISTANCE mode all measurements can be made with the remote switch probe (SP5). Tests are automatic and do
not require the TEST button to be pressed.
1. Connect the switch probe to the switch probe socket L1 (+ve). The switch probe replaces the standard RED test lead. Test as
in 5.2 above.

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5.6 Buzzer threshold
If the measured resistance is less than the buzzer threshold, the buzzer will sound. The resistance at which the buzzer stops sounding
can be changed to meet individual test requirements. Refer to the Setup section 10 of this guide.
Selectable limits of 0.5 Ω, 1 Ω, 2 Ω, 5 Ω, 10 Ω, 20 Ω, 50 Ω, 100 Ω, (depending on model) are available.
This setting is stored even when the instrument is switched off.
5.7 Measurement methods and sources of error
Method of measurement
The 2-wire lead set must be used for this measurement. A DC voltage of nominally 4.4 V with a current limit of >200 mA is used to
measure resistance less than 2 Ω.
Possible sources of error
Measurement results can be affected by the following:
■The presence of circuits connected in parallel.
■Presence of AC voltages on the circuit being measured
■A poor connection to the circuit under test
■Incorrectly nulled test leads
■Use of fused leads

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6. Insulation resistance 500
V
IMPORTANT:
The insulation test is protected by a live circuit warning. Detection of a voltage over 50 V will inhibit testing. This applies whether or not
the insulation test is locked on.
6.1 Making an INSULATION measurement
1. Set the left hand rotary range knob to the 500
Vinsulation test voltage required:
2. Connect two test leads to the L1 (+ve ) and L2 (-ve) terminals on the instrument.
L1
L2
L3
N
E
3. To start test, press and hold either of the TEST buttons, or , on the instrument.
Release the test button after the displayed reading has settled. The circuit will now discharge safely.
Note: A 1000 V warning is displayed whenever the 1000 V range is selected for the first time and the TEST button is pressed.
6.2 Insulation test lock
To lock an insulation test ON, hold down either of the TEST buttons followed by either of the RED LOCK buttons.
To release the “Locked on” insulation test, press the TEST button.
Warning: The test voltage will be permanently present on the test probes or crocodile clips when in the locked position.
Warning: Auto discharge - Auto discharge facility automatically and safely discharges the circuit at the completion of an
insulation test.
Live circuit warning - operates when connected to Live circuits > 25 V. Testing is still permitted.
Test inhibited - Live circuits greater than 50 V will inhibit testing.

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6.3 Measurement methods and sources of error
Method of measurement
The selected DC test voltage (current limited to less than 2 mA DC).is applied to the circuit under test and the resistance is calculated
from measurements of the resulting voltage and current.
Capacitive circuits can take some time to charge. This is displayed as an increasing voltage that takes longer to reach its maximum than
normal.
The reading is stable with a circuit capacitance less than 5 µF.

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7. Loop impedance testing (Not MFT70)
IMPORTANT
This measurement requires both selector knobs to be set to the Loop testing mode (GREEN RANGES).
This is a live circuit test. All precautions relevant to working on live circuits, to ensure the safety of the operator and any
other personnel should be in place.
Overview of the LOOP IMPEDANCE test
A Loop impedance test is the measurement of the impedance of a circuit whilst the circuit is electrically live.
Unlike a continuity test, a loop impedance test applies a load to the circuit and measures the change in the circuit voltage, from which
the loop “resistance” is calculated.
For those circuits protected by an RCD the load that is connected Phase to Earth must be small enough not to trip the RCD.
Consequently there must be many tests performed to establish the loop impedance of the circuit. These are automatically performed
and the end result is displayed.
Test Lead Null:
The MFT does not need the resistance of the test leads to be nulled for this test. They are already calibrated into the measurement
circuit at 0.07 Ω.
However if using fused leads or 3rd party test leads, the resistance of these leads may be different. In this case they can be measured
using the continuity test and the resistance can be compensated for in the Setup options, see section 10.
Circuit connection:
The MFT is designed to test the L-PE and the L-N (and L-L) part of the circuit. Selecting the L-PE range on the MFT will enable testing of
the Live to Earth circuit as below:
7.1 Range selection and test leads
7.1.1 Phase to earth L-E circuits:
The main range knob and test leads should be connected as shown:
L1
N
E
The right hand knob should be set to “Z”
Connecting the 3rd (Blue) lead enables the “3 wire loop test 3Lo, as above and “reverse polarity detection”.
Test Options in L-PE mode:
In L-PE mode the MFT1700 series offer 3 types of loop test:
3Lo – A 3-wire low current loop impedance test. This test requires all three connections.
Where to use:
For making L-E measurements on circuits where all three conductors are available AND the Phase – Earth circuit is RCD protected.
Requires all three test leads to be connected.
2Hi – A 2-wire high current test. A fast 3-4 second test using high test currents.
Where to use:
On ALL circuits except Phase – Earth measurements on RCD protected circuits.

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2Lo – A 2-Wire low current loop test for L-E measurements where the 3rd conductor in not available.
Where to use:
On RCD protected circuits where access to all three conductors is not possible
Note: 2Lo is not available when all three leads are connected, as the 3Lo is the preferred measurement mode.
Selecting the test mode:
To switch between Loop test modes press the Function button, as below:
The test mode is displayed as below:
Default mode 1st Press 2nd Press
Note: RCDs can still trip when performing a “non-trip” loop test if there is an existing high level of fault current flowing in the Earth
conductor, or the RCD is not operating within specification.
7.1.2 L-N or L-L circuits:
L1
N
E
L-N (or L-L) selected Test performed
7.1.3 Z, Zmax, Zref and R1+R2 (MFT1721)
The right range knob has additional options.
Z - Standard loop impedance measurements
Zmax - For multiple loop measurements where the worst case value is required
Zref - the Zeor Zdb value used when calculating the R1+R2 value
R1+R2 - The loop impedance less the Zref value.
For general loop impedance testing the Z setting should be selected.
Refer to section 7.5 to 7.7 for additional functionality.

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7.2 Making a loop impedance measurement
7.2.1 Zemeasurements at the origin (Phase to Earth)
1. Set the LEFT rotary range knob to the L-PE range.
2. Set the RIGHT rotary knob to Z.
½II5I
AUTO
L-PE
L-N
LOOP
RCD(mA)
W
OFF
500
30
100
300
MFT1711
250V 500V
V
Hz
1kV
RCD TEST
MW
Z
ZMAX
10
VAR
TRUE RMS
The MFT automatically uses the Phase and Earth terminals.
3. Connect test leads as below, with the Red test lead connected to the L1 (Red terminal on the MFT and the Green test lead
connected to the Green (L2) terminal.
L
N
E
MCB RCD
g
4. Press the Function key to select the “2Hi” mode. The RCD will not trip, so there is no need to use the 3Lo and 2Lo
modes.
5. Press TEST to start the test sequence. This can be automated in SETUP so the test starts when contacting the circuit. See
section 10 – Setup.
6. On completion of the test, the display will show the loop resistance on the large display segments, and the fault current on
the small display segments.
Reverse Polarity warning:
The 3rd test lead can be connected to Neutral (L3) but is not used in the ‘2Hi’ Phase-Earth measurement.
However the MFT will show a Phase-Neutral reversed connection if present.

20
7.2.2 Zsand Zdb loop measurements without RCD - eg Zs, Zdb etc.
1. Set the LEFT rotary range knob to the L-PE range
2. Set the RIGHT rotary knob to Z.
½II5I
AUTO
L-PE
L-N
LOOP
RCD(mA)
W
OFF
500
30
100
300
MFT1711
250V 500V
V
Hz
1kV
RCD TEST
MW
Z
ZMAX
10
VAR
TRUE RMS
3. Connect test leads as below, with the Red test lead connected to the Red (L1) terminal on the MFT and the Green test lead
connected to the Green (L2) terminal.
L
N
E
MCB
g
L
N
E
MCB
g
The Blue (L3) test lead can be connected to enable “reverse polarity” warnings
L
N
E
MCB
g
4. Press the Function key to select the “2Hi” mode.
5. Press ‘TEST’ to start the test sequence. This can be automated in SETUP so the test starts when contacting the circuit. See
section 10 Setup.
6. On completion of the test, the display will show the loop resistance on the large display segments, and the fault current on
the small display segments.