Out of personnel experience I would say a DB must have a mains smaller then the supply mains. I recently had a call out about a light not working. After testing I decided to investigate the I ceiling for loose wires. I switched the earth leakage off, since there was no mains and looked for the fault in the ceiling. When I touched a metal conduit I received a severe shock
Back on the ground I found out that the lights were not on earth leakage but was fed from the TOP of the earth leakage. So having a mains, prefabil a two pole is best.

Just for interest sake: 30A and 40A mains and even smaller is pretty common in my valley. That however is a story for another day