Extech Instruments MG300 User manual

User's Guide
Wireless TRMS Multimeter and
Insulation Tester
Model MG300

2 MG300-EN V2.0 4/11
Introduction
Congratulations on your purchase of the MG300 True RMS MultiMeter Insulation Resistance Tester.
This meter measures AC/DC Voltage, AC/DC Current, Resistance, Capacitance, Frequency
(electrical & electronic), Duty Cycle, Diode Test, Insulation Resistance, and Continuity plus
Thermocouple Temperature. The MG300 can store and recall measurement data and features a
waterproof, rugged design for heavy duty use. This meter can transmit data wirelessly when linked to
a PC. Proper use and care of this meter will provide many years of reliable service.
Safety
This symbol adjacent to another symbol, terminal or operating device indicates
that the operator must refer to an explanation in the Operating Instructions to
avoid personal injury or damage to the meter.
This WARNING symbol indicates a potentially hazardous situation, which if not
avoided, could result in death or serious injury.
This CAUTION symbol indicates a potentially hazardous situation, which if not
avoided, may result damage to the product.
This symbol advises the user that the terminal(s) so marked must not be
connected to a circuit point at which the voltage with respect to earth ground
exceeds (in this case) 1000 VAC or VDC.
This symbol adjacent to one or more terminals identifies them as being associated
with ranges that may, in normal use, be subjected to particularly hazardous
voltages. For maximum safety, the meter and its test leads should not be handled
when these terminals are energized.
This symbol indicates that a device is protected throughout by double insulation or
reinforced insulation.
PER IEC1010 OVERVOLTAGE INSTALLATION CATEGORY
OVERVOLTAGE CATEGORY I
Equipment of OVERVOLTAGE CATEGORY I is equipment for connection to circuits in which
measures are taken to limit the transient overvoltages to an appropriate low level.
Note – Examples include protected electronic circuits.
OVERVOLTAGE CATEGORY II
Equipment of OVERVOLTAGE CATEGORY II is energy-consuming equipment to be supplied from
the fixed installation.
Note – Examples include household, office, and laboratory appliances.
OVERVOLTAGE CATEGORY III
Equipment of OVERVOLTAGE CATEGORY III is equipment in fixed installations.
Note – Examples include switches in the fixed installation and some equipment for industrial use with
permanent connection to the fixed installation.
OVERVOLTAGE CATEGORY IV
Equipment of OVERVOLTAGE CATEGORY IV is for use at the origin of the installation.
Note – Examples include electricity meters and primary over-current protection equipment
WARNING
CAUTION
MAX
1000V

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SAFETY INSTRUCTIONS
This meter has been designed for safe use, but must be operated with caution. The rules listed below
must be carefully followed for safe operation.
1. NEVER apply voltage or current to the meter that exceeds the specified maximum:
Input Protection Limits
Function Maximum Input
V DC or V AC 1000VDC/AC rms
mA AC/DC 500mA 1000V fast acting fuse
A AC/DC 10A 1000V fast acting fuse (20A for 30 seconds max
every 15 minutes)
Frequency, Resistance, Capacitance,
Duty Cycle, Diode Test, Continuity 1000VDC/AC rms
Temperature 1000VDC/AC rms
Surge Protection: 8kV peak per IEC 61010
2. USE EXTREME CAUTION when working with high voltages.
3. DO NOT measure voltage if the voltage on the "COM" input jack exceeds 1000V above earth
ground.
4. NEVER connect the meter leads across a voltage source while the function switch is in the
current, resistance, or diode mode. Doing so can damage the meter.
5. ALWAYS discharge filter capacitors in power supplies and disconnect the power when making
resistance or diode tests.
6. ALWAYS turn off the power and disconnect the test leads before opening the covers to replace
the fuse or batteries.
7. NEVER operate the meter unless the back cover and the battery and fuse covers are in place
and fastened securely.
8. If the equipment is used in a manner not specified by the manufacturer, the protection provided
by the equipment may be impaired.

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Controls and Jacks
1. 40,000 count LCD display
2. MAX/MIN ( - ) button
3. STORE (<RECALL) button
4. RANGE(SETUP) button
5. INSULATION TEST button
6. MODE button
7. Function switch
8. 10A input jack
9. mA, µA, Insulation (-) jack
10. HOLD (PEAKHOLD>) button
11. REL (+) button
12. EXIT (AC+DC) button
13. Backlight button
14. Positive and Insulation (+) input jack
15. COM input jack
Note: Tilt stand and battery compartment are on rear of unit.
Symbols and Annunciators
•))) Continuity
Diode test
Battery status
n nano (10-9) (capacitance) 
µ micro (10-6) (amps, cap)
m milli (10-3) (volts, amps)
A Amps
k kilo (103) (ohms)
F Farads (capacitance)
M mega (106) (ohms) RF transmitter active
 Ohms PEAK Peak Hold
Hz Hertz (frequency) V Volts
% Percent (duty ratio) Relative
AC Alternating current AUTO Autoranging
DC Direct current HOLD Display hold
ºF Degrees Fahrenheit ºC Degrees Celsius
MAX Maximum MIN Minimum
No. Serial number S second
SET Setup parameter AC +DC Alternating current + Direct current
TRMS True RMS STO Store
RCL Recall AUTO Auto Range
Auto Power off enabled Backlight
Auxiliary Displays
1
8
7
6
5
23
4
9
10
11 12
13
14
15

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Operating Instructions
WARNING: Risk of electrocution. High-voltage circuits, both AC and DC, are very dangerous and
should be measured with great care.
1. ALWAYS turn the function switch to the OFF position when the meter is not in use.
2. If “OL” appears in the display during a measurement, the value exceeds the range you have
selected. Change to a higher range.
DC VOLTAGE MEASUREMENTS
CAUTION: Do not measure DC voltages if a motor on the circuit is being switched ON or OFF.
Large voltage surges may occur that can damage the meter.
1. Set the function switch to the VDC position.
2. Insert the black test lead banana plug into the negative COM jack.
Insert the red test lead banana plug into the positive Vjack.
3. Touch the black test probe tip to the negative side of the circuit.
Touch the red test probe tip to the positive side of the circuit.
4. Read the voltage in the display.
AC VOLTAGE (FREQUENCY, DUTY CYCLE) MEASUREMENTS
WARNING: Risk of Electrocution. The probe tips may not be long enough to contact the live parts
inside some 240V outlets for appliances because the contacts are recessed deep in the outlets. As
a result, the reading may show 0 volts when the outlet actually has voltage on it. Make sure the
probe tips are touching the metal contacts inside the outlet before assuming that no voltage is
present.
CAUTION: Do not measure AC voltages if a motor on the circuit is being switched ON or OFF.
Large voltage surges may occur that can damage the meter.
1. Set the function switch to the VAC/Hz/% position.
2. Insert the black test lead banana plug into the negative COM jack.
Insert red test lead banana plug into the positive Vjack.
3. Touch the black test probe tip to the neutral side of the circuit.
Touch the red test probe tip to the “hot” side of the circuit.
4. Read the voltage in the main display and the frequency in the right
auxiliary display
5. Press and hold the MODE button for 2 seconds to indicate “Hz”.
6. Read the frequency in the main display.
7. Press the MODE button again to indicate “%”.
8. Read the % of duty cycle in the main display.
9. Press EXIT to return to AC volts display
10. With ACV in the main display, press and hold EXIT for 2 seconds to measure
AC+DC.

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mV VOLTAGE MEASUREMENTS
CAUTION: Do not measure mV voltages if a motor on the circuit is being switched ON or OFF.
Large voltage surges may occur that can damage the meter.
1. Set the function switch to the mV position.
2. Press the MODE button to indicate “DC” or “AC”
3. In AC mode, press and hold EXIT for two seconds to select ”AC+DC”
4. Insert the black test lead banana plug into the negative COM jack.
Insert the red test lead banana plug into the positive Vjack.
5. Touch the black test probe tip to the negative side of the circuit. Touch the red
test probe tip to the positive side of the circuit.
6. Read the mV voltage in the main display.
DC CURRENT MEASUREMENTS
CAUTION: Do not make 20A current measurements for longer than 30 seconds. Exceeding 30
seconds may cause damage to the meter and/or the test leads.
1. Insert the black test lead banana plug into the negative COM jack.
2. For current measurements up to 4000µA DC, set the function switch to the
µA position and insert the red test lead banana plug into the µA/mA jack.
3. For current measurements up to 400mA DC, set the function switch to the
mA position and insert the red test lead banana plug into the µA/mA jack.
4. For current measurements up to 20A DC, set the function switch to the
10A/HZ/% position and insert the red test lead banana plug into the 10A jack.
5. Press the MODE button to indicate “DC” on the display.
6. Remove power from the circuit under test, then open up the circuit at the
point where you wish to measure current.
7. Touch the black test probe tip to the negative side of the circuit.
Touch the red test probe tip to the positive side of the circuit.
8. Apply power to the circuit.
9. Read the current in the display.
AC+DC
In the VAC, mV(AC), 10A(AC), mA(AC) and uA(AC) measurement modes, press the EXIT button
for 2 seconds to enter AC+DC test mode. The LCD shows the AC+DC icon. Press EXIT to exit this
mode.

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AC CURRENT (FREQUENCY, DUTY CYCLE) MEASUREMENTS
CAUTION: Do not make 20A current measurements for longer than 30 seconds. Exceeding 30
seconds may cause damage to the meter and/or the test leads.
1. Insert the black test lead banana plug into the negative COM jack.
2. For current measurements up to 4000µA AC, set the function switch to the
µA position and insert the red test lead banana plug into the µA/mA jack.
3. For current measurements up to 400mA AC, set the function switch to the
mA position and insert the red test lead banana plug into the µA/mA jack.
4. For current measurements up to 20A AC, set the function switch to the
10A/HZ/% position and insert the red test lead banana plug into the 10A
jack.
5. Press the MODE button to indicate “AC” on the display.
6. Press and hold EXIT for 2 seconds to select AC+DC.
7. Remove power from the circuit under test, then open up the circuit at the
point where you wish to measure current.
8. Touch the black test probe tip to the neutral side of the circuit.
Touch the red test probe tip to the “hot” side of the circuit.
9. Apply power to the circuit.
10. Read the current in the display.
11. In the 10A range:
a) Frequency is displayed in the upper right display.
b) Press and hold the MODE button to display “Hz” in the main display.
c) Momentarily press the MODE button again to display “%” in the main display.
12. Press and hold the MODE button to return to current measurement.
RESISTANCE MEASUREMENTS
WARNING: To avoid electric shock, disconnect power to the unit under test and discharge all
capacitors before taking any resistance measurements. Remove the batteries and unplug the line
cords.
1. Set the function switch to the ΩCAPposition.
2. Insert the black test lead banana plug into the negative COM jack.
Insert the red test lead banana plug into the positive jack.
3. Press the MODE button to indicate “”on the display.
4. Touch the test probe tips across the circuit or part under test. It is best to
disconnect one side of the part under test so the rest of the circuit will not
interfere with the resistance reading.
5. Read the resistance in the display.

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CONTINUITY CHECK
WARNING: To avoid electric shock, never measure continuity on circuits or wires that have
voltage on them.
1. Set the function switch to the ΩCAP position.
2. Insert the black lead banana plug into the negative COM jack.
Insert the red test lead banana plug into the positive jack.
3. Press the MODE button to indicate“ ” and “Ω”on the display
4. Touch the test probe tips to the circuit or wire you wish to check.
5. If the resistance is less than approximately 35, the audible signal will sound.
If the circuit is open, the display will indicate “OL”.
DIODE TEST
1. Set the function switch to the ΩCAP position.
2. Insert the black test lead banana plug into the negative COM jack and the red
test lead banana plug into the positive Vjack.
3. Press the MODE button to indicate “ ” and “V” on the display.
4. Touch the test probes to the diode under test. Forward voltage will typically
indicate 0.400 to 0.700V. Reverse voltage will indicate “OL”. Shorted devices
will indicate near 0V and an open device will indicate “OL” in both polarities.
CAPACITANCE MEASUREMENTS
WARNING: To avoid electric shock, disconnect power to the unit under test and discharge all
capacitors before taking any capacitance measurements. Remove the batteries and unplug the
line cords.
1. Set the rotary function switch to the ΩCAP position.
2. Insert the black test lead banana plug into the negative COM jack.
3. Insert the red test lead banana plug into the positive Vjack.
4. Press the MODE button to indicate “F”
5. Touch the test leads to the capacitor to be tested.
6. Read the capacitance value in the display

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TEMPERATURE MEASUREMENTS
1. Set the function switch to the Temp position.
2. Insert the Temperature Probe into the input jacks, making sure to
observe the correct polarity.
3. Press the MODE button to indicate “ºF” or “ºC”
4. Touch the Temperature Probe head to the part whose temperature you
wish to measure. Touch the part under test with the probe until the
reading stabilizes.
5. Read the temperature in the display.
Note: The temperature probe is fitted with a type K mini connector. A mini
connector to banana connector adaptor is supplied for connection to the input banana jacks.
FREQUENCY (DUTY CYCLE) MEASUREMENTS (ELECTRONIC)
1. Set the rotary function switch to the Hz/% position.
2. Insert the black lead banana plug into the negative COM jack and the red test
lead banana plug into the positive Hz jack.
3. Touch the test probe tips to the circuit under test.
4. Read the frequency on the display.
5. Press the MODE button to indicate “%”.
6. Read the % duty cycle in the display.

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% 4 – 20mA MEASUREMENTS
1. Set up and connect the meter as described for DC mA measurements.
2. Set the rotary function switch to the 4-20mA% position.
3. The meter will display loop current as a % with 0mA=-25%, 4mA=0%, 20mA=100%, and
24mA=125%.
INSULATION RESISTANCE MEASUREMENTS
Note: Disconnect the unit under test from all power sources and isolate it from any stray
resistance.
1. Set the rotary function switch to the INSULATION position
2. Press the RANGE button to select the test voltage as indicated on the top left display.
3. Connect the red test lead to the meter’s INSULATION ( + ) jack and the black test lead to the
INSULATION ( - ) jack. Connect the probe end of the test leads to the circuit under test.
4. Press and hold the TEST button to test (alternatively, press and hold the LOCK key until the
display shows LOCK and then momentarily press the TEST button to start a hands free test).
Note: If the circuit under test is live and has a voltage potential (AC/DC) over 30V, the meter it
will not test (the display will show “>30V”, the symbol will flash, and the buzzer will sound). If
the circuit under test is not live or if its voltage is less than 30V, the meter will begin applying
high-voltage to the circuit under test.
5. The primary display and the analog bargraph will show the insulation resistance in MΩ.
6. The test voltage (VDC) value will be indicated in the right-most auxiliary display, the symbol
will flash and the caution buzzer will sound.
7. Release the TEST button to stop the test (to stop a hands free LOCK test, press and hold the
release button for 2 seconds). The high voltage will switch off and the resistance values
indicated in the primary display hold.
8. Subsequently, the meter will internally discharge the balance of the insulation test voltage.
Note: Turning the function switch to another test position or pressing the EXIT button will abort an
Insulation resistance test.
RANGE