Junction Box

A junction box (JB) is an interface of a solar module with the external world, it channels the power generated by the solar module to the external world. It is an enclosure made up of Poly Phenylene Oxide (PPO) and attached to the back sheet of the solar module using silicone sealant glue. The JB which combines all the sub-strings of the cells in a module and also connects the bypass diode. A 4 sq-mm Cu multi-stranded cable is connected to the positive and negative sub-strings to extract power from the modules. The end interconnection is done using Multi-Contact (MC4) or compatible connectors. The length of the cable is typically 1000 or 1200 mm depends on the manufacturer’s choices. The module sub-strings are connected to the JB either through the wet soldering process or by a clamping mechanism.

Standard junction box consists of metal strips, Schottky bypass diodes, and main DC positive and negative output cables with connectors. These metal strips are connected to each other by bypass diodes. Each junction box has 2 output wires: DC positive and DC negative, which can be connected to other solar panels to form a string. The cables from a junction box conduct the electricity from the solar PV module to the string and MC4 connectors are used to provide simpler and faster array wiring. 

The junction boxes are certified under IEC61250, IEC 61730, and UL1703. Their ratings are of the format IP-n1-n2-XY, 

where,

• IP - Ingress Protection (mandatory)
• n1 - Single numeral (0-6) or letter X (mandatory)
• n2 - Single numeral (0-8) or letter X (mandatory)
• X - Additional letter: Other protections (optional)
• Y - Supplementary letter: Other protections (optional)

The most common rating for a junction box is IP65, IP67, and IP68. An IP65 rating junction box provides complete protection against dust and cannot be damaged by low-pressure water jets during cleaning but there is no guarantee of protection against high-pressure water jets which might corrode the boxes from inside.

Similarly, an IP67 rating junction provides complete protection from dust and will perform satisfactorily till 1m deep in water while an IP68 rating junction box can protect itself beyond 1m too. A sealant is added within the junction box to improve the IP rating, prevent moisture ingress and increase heat dissipation of the JB.

Case study

We conducted plant inspection on the 65 MWp plant in Telangana which had 3 types of modules makes. There were several issues with the junction box.

1. Melted junction boxes
2. Corrosion on contacts
3. Uncoupled from module back-sheet
4. Open covers/lids

Around 50%-60% of these issues were observed in one module make. It was observed that several junction boxes deviated from their position, in some conditions lost contact with the back sheets because of poor quality of RTV solvent (Room Temperature Vulcanizing silicone).

Since the covers of junction boxes were opened, the string contacts faced wet air, moisture, and water, which corroded them. This could have been avoided to great lengths if the maintenance team would have diagnosed the plant earlier and the JB was filled with sealant.

Another common issue observed was the melted junction boxes due to arcing. Arcing could happen due to improper contact, improper soldering (dry soldering) process in the factory, or use of clamp-type junction box (these have pressure type of contacts which can release their tension over the period of time).

The most astonishing part is that the plant is just 2-3 years old and these levels of defects should not occur, as modules are rated for 25 years life. 

Now to answer last week’s poll: The correct answer is Soldering/Soldering and Clamping. Kudos to everyone who got it right.

Let me know what you think of this article.