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

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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® Smart

Model No. (Order Code)

FT4310 Built in Bluetooth® Smart

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

Guarantee 3 Years



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.
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*For the latest information about countries and regions where wireless operation is currently supported, please visit the Hioki website.

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 items Open-circuit voltage, Short-circuit current, Bypass route resistor

[BPD TEST mode]

Measurement items Bypass diode comparator judgment, Bypass route resistor, Open-circuit voltage, Short-circuit current, Measurement (applied) current
Measurement object Crystal system string
Open-circuit voltage: 1000 V DC or less, Rated current: 2 A to 12 A DC
Measurement method Short-circuit and pulse voltage application
Measurement accuracy Open-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 time 2 s or less (3 seconds or less when measurement voltage is 10 V or less)
Possible number of measurements 3000 times (Comparator, backlight, Bluetooth® OFF)
LR6 Alkaline battery × 6

[Voc mode]

Measurement items Open-circuit voltage
Measurement range 0 V to 1000 V DC (Displayed up to 1200 V DC), Accuracy: ±0.2% rdg. ±3 dgt.
Response time Within 1 sec.


Dustproof and waterproof IP40 (EN60529)
Functions Displays 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
Interface Bluetooth® 4.0LE (Bluetooth® SMART)
Supports devices: iOS8 or later, Android™ 4.3 or later smartphone/ tablet
Power supply LR6 (AA) alkaline battery×6, Maximum rated power 18 VA
Continuous operating time: 45 hours (Comparator, backlight, Bluetooth® OFF)
Dimensions and mass 152W×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 up date: Apr. 12, 2017

Wireless certification Japan, United States, Canada, Austria, Belgium, Bosnia and Herzegovina, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey, United Kingdom, and Chile
Exempt from wireless regulations Viet Nam, Hong Kong, Macao, India, Russian Federation, Colombia, Pakistan, Vanuatu, and Peru



Need an instrument for photovoltaic system maintenance?


BYPASS DIODE TESTER FT4310 (Precautions Concerning Use of Equipment that Emits Radio Waves) [409KB]


Technical Notes

FT4310 Bypass Diode Tester Measurement Principles

Declaration of Conformity



Other option

*The C0206 is bundled




*For the BT3554-01 only


GENNECT Cross (free app)

• Streamline tasks that require repeated measurement and recording of values • Link data and create reports to troubleshoot problems • Available for Android, iOS and Windows

Test leads

Options for the L9207-10 (L9207-10 is a bundled accessory)



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)


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


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

There are no drivers, firmware or software available for this product.