Why ac and dc system cannot share a common neutral?

Question

Why ac and dc system cannot share a common neutral?

Answer 1

Most uses of DC require low noise. AC lines inherently have 60 (or 50) cycle noise  on their grounds. That is why it is always good practice to keep those grounds separate.

Answer 2

If the DC is isolated from the AC such as through the power transformer in the DC supply is will function... but I doubt it will meet approval as far as NEC is concerned.

Answer 3

As ac has been changed from dc so how both can share a common neutral.  If they share common neutral then what will be benefit where as ac is the need of people every where.

Answer 4

They can.  There are many systems, mostly electronic, which have AC signalling with a DC power carried on the same wires.  Separating the two takes extra circuitry which until solid state became available was awkward and costly to maintain.   High power AC often involves switching transients and motor startup noise which can play havoc with DC.