Inspect Solar Panel Bypass Diodes for Opens and Shorts in Broad Daylight Without Covering Panels

Traditionally, bypass diodes can only be inspected for good working condition at night or when power is not being generated by the solar panels in order to verify that any applied current is guided past the solar cells. With the FT4310 Bypass Diode Tester, you can detect for open faults even when the sun is out without covering the panels. Also test for short-circuit faults and cell string losses. Key Features • Test for open or short-circuit bypass diodes even during the day*1 • Easily test using the strings in the junction boxes*2 • Save time - simultaneously measure all electrical parameters*3 • Automatically transfer data wirelessly (Available for Android and iOS devices*4) *1 Testing can also be performed at night. Testing for short-circuit faults can only be performed during the day. *2 There is no need to climb onto the roof and dramatically improving work efficiency. *3 Measure open-circuit voltage, short-circuit current, and bypass route resistance and display all three values at once. *4 Automatically transfer data with Bluetooth® wireless technology Data can be downloaded to tablets and smartphones using Hioki’s dedicated apps available from the Google Play or App Store. Search for “HIOKI” and download the “GENNECT Cross” app. *Android, Google Play and the Google Play logo are trademarks of Google Inc. *iOS is a registered trademark of Cisco Technology, Inc. and/or its affiliates in the United States and certain other countries. *iPhone, iPad, iPad mini, iPad Pro and iPod touch are trademarks of Apple Inc. *Apple and the Apple logo are trademarks of Apple Inc. App Store is a service mark of Apple Inc. *Microsoft, Windows, Windows Vista, and Excel are either registered trademarks or trademarks of Microsoft Corporation in the United States and/or other countries. *Company names and Product names appearing in this catalog are trademarks or registered trademarks of various companies. *The Bluetooth® word mark and logos are registered trademarks owned by Bluetooth SIG, Inc. and any use of such marks by HIOKI E.E. CORPORATION is under license. *For the latest information about countries and regions where wireless operation is currently supported, please visit the Hioki website.

Model No. (Order Code)

FT4310 Built in Bluetooth(R) wireless technology

Note: The FT4310 cannot measure strings installed in parallel. Please contact Hioki for more information.


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How to Use the Hioki Bypass Diode Tester FT4310

The FT4310, a bypass diode tester ideal for maintaining solar power systems, can detect open faults and short faults during daylight hours without the need to cover the solar panels. Watch this video to see how it can be easily and conveniently used in the field.

Inspect solar panel bypass diodes for opens and shorts in broad daylight without covering panels

Traditionally, bypass diodes can only be inspected for good working condition at night or when power is not being generated by the solar panels in order to verify that any applied current is guided past the solar cells. With the FT4310, you can detect for open faults even when the sun is out without covering the panels. Testing can also be performed at night. *Testing for short-circuit faults can only be performed during the day.

Test for bypass diode open faults and other malfunctions using the strings in junction boxes

Easily test using the strings in the junction boxes, eliminating the need to climb onto the roof and dramatically improving work efficiency. *Disconnect the string being measured from the interconnect prior to measurement.

Save time - simultaneously measure all electrical parameters

Simply set the rotary knob to “BPD TEST” and press the “Measure”button to measure and display all parameters necessary for fault identification (open-circuit voltage, short-circuit current, and bypass route resistance). Measurement takes 2 sec. or less per string! RBPR: Bypass route resistance value Voc: Open-circuit voltage value Isc + α1: Measuring current value Isc: Short-circuit current value Judgment: Presence of a bypass diode open fault (when set to comparator, judgment based using RBPR value as well)

Automatically transfer data with Bluetooth (R) wireless technology

Measured values held on the display are sent immediately to a smartphone or tablet via Bluetooth (R) wireless technology. Save date in CSV or PDF format. Eliminate the need to take notes - particularly useful at sites with a large number of test points. *Available for Android and iOS devices *Use with the dedicated Hioki GENNECT Cross app

Be alerted to faults with an audible warning and by the instrument’s backlight

Red backlight and audible warning alert the user to possible faults. Ex) Open fault (Using BPD TEST mode) RBPR:The instrument indicates the open state (“opn”)

High-precision voltmeter ideal for detecting short-circuit faults

When a bypass diode experiences a short-circuit fault, the output voltage decreases (by about 10 V) because the corresponding solar panels are not contributing to the array’s generated power. By detecting this difference, it is possible to detect bypass diode short-circuit faults and cell string losses. The FT4310 incorporates a high-precision DC voltmeter so that it can detect a voltage difference of 10 V in a 1000 V system. When used in Voc mode, which is specifically designed to measure open-circuit voltage, the instrument displays the voltage difference in less than 1 sec. and tests for a short-circuit fault.

Convenient accessories

● Bundled case with neck strap Leave both hands free so you can precisely position test probes without worrying about dropping the instrument. ● Integrated "hold" button right on test leads A button right at your fingertip on the test leads lets you hold measured values easily, eliminating the need to operate a control on the instrument itself. They also incorporate a handy light.

[Reference] Issues caused by faulty bypass diodes

Normal reading: When a solar panel is obscured by a partial shadow (or when it fails), the current bypasses the panel in order prevent any drop-off in generating efficiency. Short-circuit fault: When a short-circuit fault occurs, the generated current flows in a loop, making it impossible to capture the generated power, resulting in lowered efficiency. Open fault: When an open fault occurs, current is forced to flow to the defective cell when it’s covered by a shadow, causing the panel to heat up and posing the risk of fire.

[Reference] The FT4310’s impact on strings

The impact of measurement on strings has been investigated thoroughly since even if bypass diode open faults can be detected while solar panels are exposed to sunlight, the endeavor would be pointless if the panels were damaged in the process. When the FT4310’s measurement button is pressed, the instrument first measures ISC (the short-circuit current) and then applies a voltage such that a current that is 1 A greater than ISC flows. Because bypass diodes are required by the JIS to function without issue even if a current equal to 1.25 times the module’s rated short-circuit current flows for a period of one hour, because the FT4310 performs constant-current control, and because the time of application is 5 ms or less, the FT4310’s impact on strings does not differ from that of I-V curve measurement or insulation resistance measurement. The FT4310 can be used with confidence that it will not damage solar panels.

Basic specifications (Accuracy guaranteed for 1 year, Post-adjustment accuracy guaranteed for 1 year)

Measurement itemsOpen-circuit voltage, Short-circuit current, Bypass route resistor

[BPD TEST mode]

Measurement itemsBypass diode comparator judgment, Bypass route resistor, Open-circuit voltage, Short-circuit current, Measurement (applied) current
Measurement objectCrystal system string
Open-circuit voltage: 1000 V DC or less, Rated current: 2 A to 12 A DC
Measurement methodShort-circuit and pulse voltage application
Measurement accuracyOpen-circuit voltage: ±0.2% rdg. ±3 dgt. (at 0 to ±1000 V)
Short-circuit current: ±3% rdg. ±3 dgt. (at 0.0 to 15.0 A)
Bypass route resistance: ±5% rdg. ±5 dgt. (at 0.0 to 15.0 Ω, During pure
resistance measurement)
Measurement time2 s or less (3 seconds or less when measurement voltage is 10 V or less)
Possible number of measurements3000 times (Comparator, backlight, Bluetooth® OFF)
LR6 Alkaline battery × 6

[Voc mode]

Measurement itemsOpen-circuit voltage
Measurement range0 V to 1000 V DC (Displayed up to 1200 V DC), Accuracy: ±0.2% rdg. ±3 dgt.
Response timeWithin 1 sec.


Dustproof and waterproofIP40 (EN60529)
FunctionsDisplays the number of bypass diode measurements, Automatic polarity judgment
function, Comparison display, Auto hold, Live circuit indicator, Buzzer sounds,
Backlight, Comparator, Battery indicator, Auto power off, Bluetooth® Smart
InterfaceBluetooth® 4.0LE, Supports devices: iOS, Android TM smartphone/ tablet
Power supplyLR6 (AA) alkaline battery×6, Maximum rated power 18 VA
Continuous operating time: 45 hours (Comparator, backlight, Bluetooth® OFF)
Dimensions and mass152W×92H×69D mm (5.98 W × 3.62 H × 2.72 D in)
650 g (22.9 oz) (including batteries, excluding test leads)

Countries and regions where wireless operation is currently supported (FT4310)Last update: May 18, 2020

Countries and regions where wireless operation is currently supportedAustralia, Austria, Belgium, Bulgaria, Canada, Chile, China, Colombia, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Georgia, Germany, Greece, Hong Kong, Hungary, Iceland, Ireland, Italy, Japan, Kenya, Latvia, Liechtenstein, Lithuania, Luxembourg, Macao, Malta, Mexico, Myanmar, Netherlands, Norway, Peru, Poland, Portugal, Romania, Singapore, Slovakia, Slovenia, Spain, Sweden, Switzerland, Thailand, Turkey, United Kingdom, United States, Viet Nam
NoteUse of devices that emit radio radiation requires approval in the country of use. Thus, use of the instrument in a country or region where they have not been approved may be subject to penalty as a violation of law.

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Seminar Videos On Demand

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Seminar Schedule

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Catalog: BYPASS DIODE TESTER FT4310 Download PDF  [4MB] English
Catalog: GENNECT CROSS SF4071, SF4072 Download PDF  [3MB] English

User Guides

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Precautions Concerning Use of Equipment that Emits Radio Waves Japanese
Instruction Manual English
Instruction Manual Simplified Chinese

Outline Drawings

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Declaration of Conformity


Drivers, Firmware

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Test leads

The L9788-11 is bundled, others are exclusive options for L9788-11


Lighting LED lamp & comparator indicator (Operate only when main unit provides a comparator function), 1.2 m (3.94 ft) length


Bundled with Test Lead with Remote Switch L9788-10/ Earth lead, alligator clip, 1.2 m (3.94 ft) length

TIP PIN L9788-90

Spare parts for tip of the L9788/ L9788-10, Tip length 35 mm (1.38 in)/φ 3.2 mm (0.13 in)


For checking breaker terminal, Detachable for tip of the L9788-10, 65 mm (2.56 in) length, φ 2.6 mm (0.10 in)

Other option

*The C0206 is bundled


PC peripherals


• Mobile app for iOS, Android • Improve efficiency of operations that repeat measurement and recording • Analyze correlated data during issues and generate quick reports

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