In case of DC system design, Why fuses are mandatory on both poles in ungrounded system & in grounded system only on ungrounded pole?

This is very basic question arises in our mind during finalization of SMB/SCB and also in case of inverter. Why fuses are mandatory on both poles in ungrounded system & in grounded system only on ungrounded pole?

For answer above we have to know DC side faults will occur majorly due to three conditions:

• Positive to Ground
• Negative to Ground  
• Positive to negative

Suppose 20 modules in series so Vmp becomes 700V. 

1) Positive to Ground = 350V

For this case fuses are mandatory on the positive pole for overcurrent protection and also insulation monitoring relay in inverter will check  & trip the inverter if fault is there.

2) Negative to Ground = 350V

For this case fuses are mandatory on the negative pole for overcurrent protection and also insulation monitoring relay in inverter will check & trip the inverter if fault is there.

3) Positive to Negative = 700V

SC happen between two cables then current will flow through the close path of cable itself and due to this may be cable will burn.

Conclusion: When arrays are floating. The direction of fault current will be through the cable of faulty string to panels. If fuses are not placed in the circuit this fault current can damage the cable and modules. Hence in this case fuses are mandatory on both poles.

Case 2: When one pole is grounded through GFDI

1) Positive to Ground = 700V

For this case GFDI in inverter will check & trip the inverter if fault is there.
The fault current will flow through GFDI. If this current exceeds the threshold rating of GFDI interrupter, the interrupter will trip the Inverter by opening AC and DC breakers. As a result path of fault current will break. Secondly if this current will not exceeds the threshold rating of GFDI interrupter fuses are there for protection purpose.

2) Negative to Ground = 0V

Since the Negative pole is grounded and leakage current will follow the path of low resistance so no over current will flow.
Since all the negative strings are connected to ground (same potential) hence no overcurrent occurs in single faulty string.

3) Positive to Negative = 700V

SC happen between two cables then current will flow through the close path of cable itself and due to this may be cable will burn.

Conclusion: When one pole is grounded through GFDI, fuses are mandatory to ungrounded pole in grounded system as explained above. 

Conclusion mentioned above is as per my opinion, I am welcoming the inputs from each individual so that everyone can learn something new