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  • Writer's pictureRenewSys India

Why Junction box of the solar module fails in the field?

The Junction box of solar modules fails in the field mainly because of fault current passing through the Junction box.

Analyzed failed Junction box:- After the reversed engineering by X-ray inspection, the breakdown point is found at the PN junction chip. It reveals the evidence of strong reversed current or voltage flowing from the system side to the module side.

The main failure of the Junction box:-

- Burnt bypass diode

- Burnt Junction box

PV modules are current-limited devices, the string in the PV array does not need to be protected against a fault current from modules within the same string. The need for fault current protection in PV array comes from the risk of other strings in the array feeding current into a single string of the array due to the shading or an earth fault.

How to check overcurrent protection is required or not?

String overcurrent protection shall be required if:

(Number of strings -1) x Isc of the module) > Module reverses current rating

For example:

If a PV module has a reverse current of 15A (series fuse rating) and a short circuit current Isc of 9.41A


So for this module, the maximum number of strings that can be connected without fault current protection is 1. That means if you connect more than one string fault current protection is required. The overcurrent protective devices of the DC side shall be either gPV fuses following the IEC 60269-6 or another device following IEC 60947 (all parts) or IEC 60898-2, selected such that the cable current carrying capacity, module maximum reverse current rating, and the maximum current of other equipment is not exceeded.

How to install a fault current protection device?

The fault current protection device is to be installed in all unearthed conductors for PV string cables. These fuses must be located as close as possible to the strings to protect DC cables.

IEC 62548

Overcurrent protection sizing

Where string overcurrent protection is required

In > 1.5 x ISC_MOD; and

In < 2.4 x ISC_MOD; and



In is Nominal rated current of the module/strings

If strings are grouped in parallel under the protection of one overcurrent device provided:

Ing > 1.5 × NG × ISC_MOD; and

Ing < IMOD_MAX_OCPR – ((NG – 1) × ISC_MOD)


NG is the number of strings in a group under the protection of one overcurrent device.

Ing is the overcurrent protection current rating of the group overcurrent protection device.

The factor of 1.5 considers a design allowance for high irradiance conditions. Individual designs should take into account local ambient irradiance and temperature conditions. Cycling load, the grouping of fuses, and unequal current flow through the parallel strings may lead to factors higher than 1.5.

Propose some action and suggestions to prevent Module Junction box failure in the field:

  • Install suitable protection devices like string fault current protection, lighting aster, earthing all modules in the system to prevent energy over stress.

  • Avoid tramping/rework for the cable and connector of the junction box.

  • Check the string polarity before electrical connecting.

  • A qualified person must do project installation.

Data Credits: IEC 62548

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