An ACDB looks like an ordinary AC distribution board. Same busbar, same breakers, same enclosure. The protection philosophy behind it cannot be the same, because the source feeding one side behaves nothing like the source feeding the other.

An inverter is a current-limited source

A transformer-fed supply will push tens of kiloamps into a bolted fault, limited mainly by its own impedance. A grid-tied inverter will not. It is a controlled current source, and under fault conditions it typically delivers somewhere between 1.1 and 1.5 times its rated output, for a few cycles, before its own protection detects the condition and shuts it down.

A breaker chosen against the prospective grid fault current may never see enough current from the inverter side to operate on its magnetic trip. It will fall back to the thermal element, slowly, or it will not trip at all.

Faults arrive from two directions

An ACDB sits between the inverters and the grid, which means it can be faulted from either end.

Grid side. Full prospective fault current. Conventional sizing and discrimination apply.

Inverter side. Limited current for a limited time. Conventional sizing may not operate at all.

A device selected only against the grid figure protects one of those two cases and leaves the other to chance.

What the AC side actually has to do

  • Isolate each inverter individually, so one can be worked on while the rest keep exporting
  • Carry AC surge protection on the grid side, not only the DC SPD on the array side
  • Carry continuous output current in an outdoor ambient, which usually means de-rating the busbar rather than sizing it at nameplate
  • Hold its IP rating through a monsoon and a summer, year after year
  • Meet the interface protection the DISCOM asks for, which is not the same in every state

The number to ask for

Every inverter datasheet states a maximum output fault current and how long the inverter will sustain it. Get that figure before the AC side is designed. It is the one input that decides whether the protection will actually operate, and it is the one most often left out of the enquiry.

The DC side gets the attention because arcs there are dramatic. The AC side is where a plant quietly fails to protect itself.

Frequently asked questions

How much fault current does a grid-tied inverter deliver?

Typically between 1.1 and 1.5 times its rated output, for a few cycles, before its own protection detects the fault and shuts it down. A transformer-fed supply can push tens of kiloamps into a bolted fault, so the two sources behave very differently.

Why can an ACDB not be protected like an ordinary AC distribution board?

Because it is fed by a current-limited source. A breaker chosen against the prospective grid fault current may never see enough current from the inverter side to operate on its magnetic trip. It then falls back to the thermal element, slowly, or does not trip at all.

Can an ACDB be faulted from both sides?

Yes. It sits between the inverters and the grid. A grid side fault carries the full prospective fault current, so conventional sizing and discrimination apply. An inverter side fault carries limited current for a limited time, where conventional sizing may not operate at all. A device selected only against the grid figure protects one of the two cases.

Which inverter figure do I need before designing the AC side?

The maximum output fault current and how long the inverter sustains it, both stated on the inverter datasheet. It decides whether the protection will actually operate, and it is the input most often left out of the enquiry.

What else does the AC side of a solar plant need besides breakers?

Individual isolation for each inverter so one can be worked on while the rest keep exporting, AC surge protection on the grid side, a busbar de-rated for outdoor ambient, an enclosure that holds its IP rating through monsoon and summer, and the interface protection the DISCOM asks for, which differs between states.