On ground,with packs off,all doors closed you can get negativ pressure via electrical
extraction fans from systems like avionic or galley/toilet air extraction.
I disagree. Air going via extraction fans will be balanced by air coming in from outflow valves and other vents such as Forward Overboard Valves. Even if these were sealed, you would still get air leaking past door seals, negative pressure relief valves, etc.
Outflow valves are bigger than equipment cooling outlets, etc
