Panel hotspots and bypass diodes

A PV hotspot is a small area operating at a higher temperature than the surrounding module. It can occur when a shaded, cracked, mismatched or damaged cell is forced into reverse bias by current from the other cells in its series path. Local electrical resistance in an interconnection, junction box or connector can also create abnormal heating.

A hotspot is a symptom, not a diagnosis. Some temperature variation is normal, and a thermal image needs suitable operating conditions and interpretation before a module is condemned.

Why one cell can heat

Cells in a module substring are connected in series, so substantially the same current passes through them. If one cell can no longer produce that current, the rest of the string can drive it in reverse. Electrical energy is then dissipated in the affected area as heat.

Possible causes include:

• hard local shade or opaque fouling
• cell cracks and damaged interconnects
• manufacturing mismatch or degradation
• moisture or corrosion
• failed solder joints
• impact or thermal stress

Heating can discolour encapsulant, damage the backsheet, worsen cracks or, in severe cases, contribute to a fire hazard. The outcome depends on the fault, module construction and operating current.

What a bypass diode does

A bypass diode is connected across a group of cells. In normal generation it does not conduct. When the voltage of that cell group becomes sufficiently negative, the diode provides an alternative current path around the group.

This can:

• limit reverse voltage across shaded cells
• reduce local heating risk
• allow the rest of a series string to continue operating

It does not preserve the lost output from the bypassed cells, and it does not guarantee that every partial-cell shade is harmless. The diode needs enough reverse condition across its whole protected group before it conducts.

Bypass diode failures

A diode can fail short circuit, open circuit or with an abnormal resistance.

• A shorted diode can permanently bypass its cell group, reducing module voltage and energy.
• An open diode removes its intended protection during shading.
• A poor diode or junction can heat locally and damage the junction box.

Several other faults create similar monitoring symptoms, so a reduced module voltage is not enough to identify the diode without electrical diagnosis.

The module junction box is sealed product equipment. Opening it or substituting a diode can compromise insulation, weather sealing and product certification. Follow the manufacturer repair or replacement route.

Shade and dirt patterns matter

A narrow shadow, leaf or bird dropping across part of a cell can be more significant than a light uniform film. Repeated fixed shading from a vent, aerial, parapet or nearby structure should be addressed in design rather than left for the bypass diodes to manage every sunny day.

Clean an opaque deposit using the approved method if it can be reached safely. Do not scrape the glass or climb onto the array. If the hot pattern persists after the obstruction is removed, the module needs competent assessment.

Monitoring clues

Possible indicators include:

• one optimiser or microinverter channel persistently below comparable modules
• a string voltage step or current mismatch
• a repeated loss at the same solar condition
• inverter insulation or arc-fault alarms
• visible browning, blistering, cracked glass or a damaged junction box

Monitoring can locate an area to inspect but cannot see every cell temperature or connection. Shading, orientation and equipment clipping must be excluded before interpreting a lower energy channel as damage.

Thermal imaging

Outdoor infrared thermography is covered by the IEC TS 62446-3 framework. A useful inspection needs adequate, reasonably stable irradiance and load, suitable viewing angle, correct equipment and awareness of reflection, wind and cloud effects.

Cells and modules need to be compared under similar conditions. The absolute temperature and temperature difference require interpretation against the inspection method, product and visible pattern; no universal failure threshold applies.

A bright thermal spot can indicate a reverse-biased cell, high-resistance joint or reflection. A cool substring can indicate bypass operation or lost current. Electrical tests are often needed to distinguish them.

Electrical diagnosis

A competent PV contractor may use:

• string and module voltage and current comparison
• current-voltage curve tracing
• insulation-resistance and polarity checks
• optimiser or microinverter channel data
• diode or junction-box tests permitted by the manufacturer
• visual and thermal inspection under load

Results should be compared with commissioning data, module tolerances and the actual irradiance and temperature. Disconnecting live DC connectors to test a theory can create an arc and should not be attempted by the owner.

When to stop and escalate

Treat scorching, melting, smoke, arcing sounds, a strong electrical smell or a fire alarm as urgent. Move away and call the fire and rescue service if there is immediate danger. Do not approach a roof array to isolate it during a fire.

For a non-emergency suspected fault, record the inverter message, time, weather and monitoring channel, then contact a competent PV contractor. Follow the manufacturer’s shutdown instruction only if it can be done safely from the designated controls.

An AC shutdown does not remove daylight voltage from the modules and DC cables.

Replacement and warranty evidence

Record thermal and electrical evidence before replacing a module. Check product warranty terms, serial number, approved connector and electrical compatibility with the remaining string.

A visually similar module can have different current, voltage, dimensions or fire classification. The replacement needs a system design check, and connectors should be a matched approved combination rather than merely fitting together.

Prevention

• design out predictable shade where practical
• keep cables and connectors supported and correctly mated
• follow clamp zones and handling instructions to avoid cell damage
• remove condition-based opaque fouling safely
• monitor for sustained changes
• inspect after impact, severe weather or roof work
• keep commissioning values and module layout

Bypass diodes are protective components, not a substitute for sound design and maintenance.

Related entries

• PV soiling and cleaning
• Shading analysis
• Power optimiser
• Microinverter
• PV fault codes