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User Manual
AC Circuit Load Tester
Model CT70
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Introduction
Congratulations on your purchase of the CT70 AC Circuit Load Tester.
This device can detect circuit and wiring problems such as: Poor ground impedance, false grounds,
missing ground fault protection, low voltage availability under load, and high ground-to-neutral
voltage. In addition, the CT70 tests GFCI and EPD circuits.
Circuit and wiring issues listed above can introduce shock hazards (from grounding issues) and can
comprise performance of machinery and equipment (from poor ground impedance, lack of sufficient
voltage under load and/or high ground-to-neutral voltage). In addition, fire can result from the heat
generated by high resistance points in a circuit.
Proper wiring habits have been shown to greatly increase power quality performance.
This meter is shipped fully tested and calibrated and, with proper use, will provide years of reliable
service.
Features
• True RMS measurements
• Voltage drop measurements on 12A, 15A, and 20A loads for 120 V circuits
• Voltage drop measurements on 5A, 8A, and 10A loads for 230 V circuits
• Voltage measurements: Line, ground-to-neutral, and peak
• Measures frequency of the voltage
• Checks 3-wire receptacle configuration
• Finds false grounds
• Tests GFCI and EPD circuits
Safety
International Safety Symbols
This symbol, adjacent to another symbol or terminal, indicates the user must refer
to the manual for further information.
This symbol, adjacent to a terminal, indicates that, under normal use, hazardous
voltages may be present
Double insulation
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.
WARNING
CAUTION
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Description
Meter Description
1. AC power cable connection
2. Measurement results menu
3. Hot-Neutral-Ground coded measurement result
4. Primary reading display
5. Secondary reading display
6. Down arrow button
7. Right arrow button
8. GFCI test button
Display Description
A Amperes (Current)
V Voltage
Vd Voltage Drop
% Percent Voltage Drop
VL Voltage load
Z Impedance
Hz Hertz (cycles per second)
Ohms (Resistance)
mS Milliseconds
ASCC Available Short Circuit Current
Peak Ground to positive peak measurement
RMS Root Mean Square
GFCI Ground Fault
EPD Equipment Protection Device test
NEUT Neutral
OL Overload
m, M, k Unit of measure prefixes: milli, mega, and kilo
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Operation Overview
The CT70 AC Circuit Load Tester can test outlets or circuits under load for proper wiring, reverse
polarity, and the presence of a ground. The CT70 uses a simple menu-driven display to allow the
user to quickly see line voltage, voltage drop under full load, ground-to-neutral voltage, and line
impedance. The GFCI testing utility is performed separately per UL-1436, disrupting the flow of
electricity if a functioning GFCI is present.
Warning: To avoid damage to the instrument, do not use this device on the output of a UPS
system, a light dimmer, or a square wave generator.
Warning: Use only the supplied power/test cord (CT70-AC) with this equipment
Interpreting Measurement Results
Important note: The CT70 is a microprocessor controlled device that prioritizes its tasks. Taking a
reading and analyzing the results are its top priorities. This is why, at times, the keypad may not
respond immediately. The internal computer places a higher priority on completing a test than on
recognizing a keystroke. To minimize this effect, press and hold a key until the display menu
changes.
In addition to the main Wiring Configuration icon, shown on the meter display, the measurement
modes are accessed using the four (4) menu items shown on the left side of the display. The menu
items are:
1. Voltage (V)
2. Voltage drop (Vd)
3. ASCC (Available short circuit current)
4. Impedance (Z)
Use the ▼ button to scroll the menu list.
The wiring configuration screen shows correct wiring, reverse polarity wiring, and ‘no ground’
conditions. The three circles on the wiring configuration icon indicate the wiring status. The circles
are coded, changing appearance (clear, solid, and flashing) to indicate the measurement results. A
table is provided below for interpreting the circle code.
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The Voltage menu displays the True RMS line voltage. Use the ► button to scroll the Voltage submenu (line voltage, ground-to-neutral voltage, Peak Voltage (P), and Frequency (Hz)).
The Voltage Drop (Vd) window indicates percent voltage drop (with 15A load) and the Loaded
Voltage (Vl). The voltage drop sub-menu offers a load voltage result for 20A and 12A loads. Use the
► button to scroll the sub-menu.
The Impedance (Z) window indicates the impedance in ohms of the hot conductor. The impedance
sub-menu displays the neutral (N) and ground (G) conductor impedances. Use the ► button to move
through the sub-menu.
Note that testing ground impedance will trip a GFCI circuit.
The ASCC window indicates the Available Short Circuit Current that the branch current can move
through a breaker in a short circuit situation.
GFCI Button
The GFCI (Ground Fault Circuit Interrupt) feature performs two tests:
GFCI: Faults a circuit when 6 to 9mA from hot to ground is detected
EPD (Equipment Protective Device): For breakers equipped with an EPD, the breaker trips for
ground faults greater than 30mA.
To display the GFCI main menu window, press the GFCI button. To toggle the two tests use the ►
button. Once the desired test is selected, press the GFCI button to start the test. These tests are
further detailed below.
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Testing Procedures
Test 1: Wiring Verification
The wiring configuration is the first test result that is displayed. Refer to the table presented earlier in
the user guide for the test result key.
For wiring conditions other than normal, the CT70 is limited in the type of tests it can perform on a
circuit until the circuit’s wiring issues are resolved. For ‘no ground’ conditions, only the line voltage
and voltage drop tests can be made. For reverse polarity conditions, open neutral, or open hot
conditions the meter will not display since power will not be available.
Notes:
• The meter cannot sense two hot wires in a circuit
• The meter cannot display the results of more than one circuit issue at time
• The meter cannot sense ground reversals
Test 2: Voltage Measurements
Warning: Do not take measurements on circuits with voltages higher than 300VAC
(maximum voltage rating).
Line voltage measurements should be 120VAC ±10% at 60Hz, or 230V +/- 10% at 50Hz. For noisefree sine waves, the peak voltage should be 1.414 times the rms line voltage reading. Ground to
neutral voltage should be less than 2 VAC in which case the display backlight will appear blue in
color, if ground to neutral voltage is greater than 2 VAC the backlighting appears in red.
Higher ground to neutral voltages indicates excessive current leakage between the neutral and
ground conductors. Excessive ground to neutral voltage may result in inconsistent or intermittent
equipment performance.
Voltage Measurement Troubleshooting Suggestions
Problems
Likely Causes
Possible Solutions
Out of tolerance Line Voltage
(Line should be within ±10% of
the stated line voltage)
Overloaded circuit
Redistribute loads
Connection within circuit or at
the panel has excessive
resistance
Repair high resistance
connection
Utility company problem
Contact the power utility
company
High Ground to Neutral voltage
(Readings > 2VAC indicated a
problem)
Neutral to Ground current
leakage
Identify leakage, check for
multiple bonding points
Peak Voltage out of tolerance
(For 120V Line, Peak should
measure between 153 - 183V)
(for 230V Line, peak should
measure between 292 – 357)
Supply voltage out of tolerance
Contact power utility company
High peak loads on circuit
Redistribute electronic devices
Frequency out of tolerance
(50/60Hz)
Supply frequency out of
tolerance
Contact power utility company
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Test 3: Voltage Drop Measurements
To determine voltage drop, the CT70 measures the unloaded line voltage, then simulates a load at
the zero crossing of the AC signal by using a load pulse of sufficient duration, and then calculates
the voltage drop.
Results are provided for 120V circuits with 12A, 15A, and 20A loads and 230V circuits with 5A, 8A,
10A loads.
Warning: To avoid the buildup of heat during load testing, allow at least 20 seconds to elapse
between tests. In addition to the safety benefits, this will allow the meter to maintain its stated
accuracy during repeated tests.
For nominal efficiency, a voltage drop of 5% is the maximum recommended by the National Electrical
Code (NEC) board. When a voltage drop measurement of less than 5% is made, the meter’s display
backlight turns blue in color. If the voltage drop is higher than 5%, the meter display appears in red.
An efficient branch circuit should have less than 5% voltage drop at the furthest receptacle from the
breaker panel at the termination of the cable run. A steady decrease in the voltage drop should then
be measured for each receptacle tested in sequence towards the breaker panel.
If the voltage drop is higher than 5% and does not noticeably decrease as the testing moves closer
to the first device on the circuit, then the problem lies between the first device and the breaker panel.
Visually check the terminations at the first device, the wiring between the device and the panel, and
the circuit breaker connections.
High resistance points can be identified as hot spots using an infrared (IR) thermometer or by
measuring the voltage across the breaker. If a voltage drop measurement exceeds 5% but noticeably
decreases as the testing moves closer to the panel, then the circuit may have an undersized wire,
too long of a cable run, or excessive current on the circuit. Check the wires to ensure that they are
sized per code and measure the current on the branch circuit. If a voltage drop reading changes
significantly from one receptacle to the next, then the problem could be a high impedance point at or
between two the receptacles. It is usually located at a termination point, such as a bad splice or
loose wire connection, but could also be a faulty receptacle.
Voltage Drop Measurement Troubleshooting Suggestions
Problems
Likely Causes
Possible Solutions
Voltage drop > 5%
Overloaded circuit
Redistribute loads
Wrong wire gauge size for
length of cable run
Check code and rewire if
necessary
High resistance connection in
the circuit or at the panel
Locate bad connection and
rewire or replace
Test 4: ASCC Measurements
The CT70 calculates the ASCC (Available Short Circuit Current) that a branch circuit can deliver
through a breaker in a dead short circuit condition.
The ASCC is calculated by dividing the line voltage by the circuit’s line impedance. See equation
below:
ASCC = Line Voltage / Hot impedance + Neutral impedance
Use the ► button to simulate a situation where all three conductors (hot, neutral, and ground) are
shorted together. Note that this second test will trip a GFCI.
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Test 5: Impedance (Z) Measurements
The impedance measurement capability of the CT70 is used to check Hot and Neutral impedance
when voltage drop measurements are too high (greater than 5%).
The Line impedance measured for Hot to Neutral and Hot to ground is calculated using the difference
between the unloaded and loaded line voltages (Voltage drop test) applied to each pair of wires using
a limited current pulse. With the voltages known and the current known, the line impedances can be
calculated for both the hot to Neutral lines and the Hot to ground lines.
To determine where the problem is, measure the impedances and analyze the data as follows:
• If one impedance measurement is exceedingly higher than the other, then the problem is
with the conductor that shows the higher impedance.
• If both impedances are high the problem could be an undersized conductor, a faulty load,
or poor connections.
Ground impedance should be less than 1 Ω, preferably in the 0.25 Ω region to ensure that the
ground conductor can safely return current when necessary.
Surge suppressors require good grounding to adequately protect against transient voltages.
Note: A small amount of current is applied to the ground conductor during impedance measurements
and can trip a GFCI circuit.
If the circuit does not trip, it may indicate a False Ground condition.
High Impedance Troubleshooting Suggestions
Problems
Likely Causes
Probable Solutions
High hot and/or neutral impedance
(Limit: 0.048Ω / ft of 14 AWG wire)
Excessive loading
Redistribute loads
High hot and/or neutral impedance
(Limit: 0.03 Ω / ft of 12 AWG wire)
Undersized wiring
Check code and rewire if
necessary
High hot and/or neutral impedance
(Limit: 0.01 Ω / ft of 10 AWG wire)
High resistance connection in
the circuit or at the panel
Locate bad connection and
rewire or replace
High Ground impedance (Limit: 1Ω
for personal protection)
Undersized wiring
Check code and rewire if
necessary
High Ground impedance (Limit:
0.25Ω for equipment protection
High resistance connection in
the circuit or at the panel
Locate bad connection and
rewire or replace
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Test 6: GFCI (Ground Fault Circuit Interrupter) Testing
A ‘GFCI’ can protect personnel from shock hazards. The CT70 tests GFCI circuits by forming a HotNeutral imbalance, leaking small amounts of current (6 to 9 mA) from Hot to Ground through a fixed
resistance.
A good GFCI circuit will sense this imbalance and switch off the power. The CT70 displays the
current value in mA. To test a GFCI circuit:
1. Plug the meter into the receptacle under test.
2. Press the CT70 GFCI button to enter the GFCI test mode menu.
3. GFCI is the default test and the letters ‘GFCI’ should appear on the lower side of the
meter’s display. If not, press the ► button once to switch to ‘GFCI’.
4. Press the GFCI button again to start the test. The current that is leaked to ground will
display. The rotating display affect lets the user know that the test is in progress.
5. The GFCI circuit should typically trip within 200ms (the meter’s display will switch off
because the power has been removed by the GFCI circuit).
6. When the GFCI circuit is reset, the CT70 will display the elapsed time from start of test to
power down.
7. Press any button to return the meter to normal operation mode.
8. If the GFCI circuit fails to respond in 6.5 seconds, the CT70 stops the test automatically
and ‘OL’ will display on the meter.
Notes:
1. To test a GFCI circuit on a two-wire system, a three-to-two wire adaptor must be used with the
adaptor manually connected to ground (cold water pipe, for example).
2. Appliances connected to the circuit under test should be disconnected to avoid measurement
errors.
Note: If a False Ground is present, the GFCI test will not trip the GFCI circuit.
Test 7: EPD (Equipment Protective Device) GFCI Testing
An EPD device can protect equipment as well as personnel. The CT70 tests EPD circuits by forming
a Hot-Neutral imbalance, leaking current from Hot to Ground through a fixed resistance. A larger
amount of current (30mA) is used than would normally be used to test a standard GFCI (6 to 9mA). A
good EDP/GFCI circuit will sense this imbalance and switch off the power. The CT70 displays the
current value in mA.
To test an EPD/GFCI circuit:
1. Plug the meter into the receptacle under test.
2. Press the CT70 GFCI button to enter the GFCI test mode menu.
3. GFCI is the default test and the letters ‘GFCI’ will appear on the lower left side of the
meter’s display. Press the ► button once to switch to ‘EPD’.
4. Now follow steps 4 through 8 in Test 6 (GFCI) above.
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Specifications
General Specifications
Display 128 x 64 LED with backlighting
Voltage display update rate 2.5 seconds max.
Over range indication ‘OL’ display
Equipment Power Rating 100 to 250 VAC 3.9 VA, 45 to 65Hz, 18.0mA
Operating Temperature 32F to 122F (0C to 50C)
Storage Temperature 32F to 122F (0C to 50C)
Operating Humidity Max 80%
Storage Humidity Max 80%
Case construction ABS UL 94V/0/5VA rated
Altitude 6561.7 ft. (2000m)
Dimensions 8 x 2.8 x 2” (203 x 71 x 51 mm)
Weight 11.2 oz. (317.5g)
Safety approvals CE, ETL
General safety For indoor use and in accordance with the requirements for
double insulation to IEC1010-1 (2001): EN61010-1 (2001)
Overvoltage Category II 300V Pollution Degree 2.
Measurement Specifications
Range and Resolution
Accuracy
Line Voltage (Single phase)
100.0 to 250.0 VAC
± (1.0% + 0.2V)
Peak Line Voltage
121.0 to 350.0 VAC
± (1.0% + 0.2V)
Frequency
45.0 to 65.0 Hz
± (1.0% + 0.2Hz)
Voltage drop (%)
0.1 to 99.9%
± (2.5% + 0.2%)
Voltage (under load)
10.0 to 250.0 VAC
± (2.5% + 0.2V)
Neutral to Ground Voltage
0.0 to 10.0 VAC
± (2.5% + 0.2V)
Impedance
0.00 to 3.00 Ω (Hot)
± (2.5% + 0.02Ω)
>3 Ω (Neutral, Ground)
Unspecified
GFCI Trip Current
6.0 to 9.0mA
± (1.0% + 0.2mA)
EPD Trip Current
30.0 to 37.0mA
± (1.0% + 0.2mA)
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Two-year Warranty
Teledyne FLIR LLC warrants this Extech brand instrument to be free of defects in parts and workmanship for
two years from date of shipment (a six-month limited warranty applies to sensors and cables). To view the full
warranty text please visit: http://www.extech.com/support/warranties.
Calibration and Repair Services
Teledyne FLIR LLC offers calibration and repair services for the Extech brand products we sell. We
offer NIST traceable calibration for most of our products. Contact us for information on
calibration and repair availability, refer to the contact information below. Annual calibrations
should be performed to verify meter performance and accuracy. Product specifications are
subject to change without notice. Please visit our website for the most up-to-date product
information: www.extech.com.
Contact Customer Support
Customer Support Telephone List: https://support.flir.com/contact
Calibration, Repair, and Returns: repair@extech.com
Technical Support: https://support.flir.com
Copyright © 2022 FLIR Systems Inc.
All rights reserved including the right of reproduction in whole or in part in any form
www.extech.com
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