Fused Disconnect Vs. Non-Fusible Disconnect

[TheElectrician]

Hey Guys, I wanted to have a discussion on the Alternating Current Disconnect Requirements for PV systems. This is the disconnect between the Inverter output and Load Center. I hear people telling me that it needs to be fused and some tell me it need not be. I think, since there is already a breaker OCPD for each inverter in the load center, it is not necessary for this disconnect to have fuse in it as it just serves the purpose of disconnecting the supply from the inverter for emergency/maintenance. According to 690, it says "The PV system disconnecting means shall be an externally operable general-use switch or circuit breaker, or other approved means." Does this mean either is fine? Let me know your thoughts

 

[LarryFine]

It says fuses not required.

 

[ggunn]

Hey Guys, I wanted to have a discussion on the Alternating Current Disconnect Requirements for PV systems. This is the disconnect between the Inverter output and Load Center. I hear people telling me that it needs to be fused and some tell me it need not be. I think, since there is already a breaker OCPD for each inverter in the load center, it is not necessary for this disconnect to have fuse in it as it just serves the purpose of disconnecting the supply from the inverter for emergency/maintenance. According to 690, it says "The PV system disconnecting means shall be an externally operable general-use switch or circuit breaker, or other approved means." Does this mean either is fine? Let me know your thoughts

First of all, OCPD on inverter output circuits is to protect the conductors from fault current from the service, not from the inverter; in a properly designed PV system the conductors are sized to handle the maximum current that the inverter is capable of producing. That's why the OCPD is on the opposite end of the feeder from where it would be in a supply circuit. But to your question - if an inverter output is landing on a breaker, that's all the OCPD it needs. If it is landing on a supply side connection, it needs to be fused.

 

[pv_n00b]

It depends....

In general, a non-fusable disconnect protected upstream by any correctly rated fuse or circuit breaker will give an SCCR of 10kA. If you have more available fault current than that then you either need to series rate the disconnect and the upstream OCPD or use a fusable disconnect with current limiting fuses to increase the rating above 10kA. Each disconnect manufacturer will have guidance on how to select the correct fusable disconnect for the available fault current or select the correct series rates components.

 

[electrofelon]

Another way to think about it: note there are multiple requirements that are often met with the same device. You typically have an ocpd to protect the inverter output conductors, an ac disconnecting means requirement, and a max ocpd size per the inverter manufacturer. Those might all be the same device, maybe not. The disconnecting means is just that, a disconnect.