SMA SSM8-21-BS - Manual (Page 15)

SMA Solar Technology AG 4 Product Description
Installation Manual SSMxx-21-IA-BS-JP-en-20 15
4.4 String Fuses
The string fuses protect the PV modules of a PV system from reverse currents. Only install string fuses supplied or approved
by SMA Solar Technology AG as accessories for Sunny String-Monitors.
Figure 5: Principle of reverse current
A reverse current can occur when different open-circuit voltages are applied to PV modules connected in parallel. In this
event, the reverse current can be much higher than the short-circuit current of the PV modules. The higher the reverse
current, the more the PV modules of the faulty string will heat up. Strong heat build-up can destroy the modules of the
faulty string.
Reverse current can have the following causes:
• short circuit in one or more modules
• short circuit in one or more cells of a module
• double ground fault in a module
• double ground fault in the cabling
The string fuses trip when the string current is too high and protect the PV modules of the faulty string. The
Sunny String-Monitor informs the inverter of the string failure.
In the case of grounded PV arrays, the positive or the negative terminal can be grounded in the inverter. To do this,
cylindrical bridges are fitted in the Sunny String-Monitor instead of string fuses. Cylindrical bridges are available as
accessories.
4.5 Overvoltage Protection
Depending on the device type, the Sunny String-Monitor is equipped with one or two surge arresters for overvoltage
protection.
In order to guarantee the overvoltage protection function, the surge arrester must be connected to the external grounding.
Components to be protected SSMxx-21-BS SSMxx-21-BS-JP
DC main cable of the Sunny String-Monitor ✓ ✓
RS485 interface and RS485 communication
bus
✖ ✓