Can you use one ground for multiple circuits?

The code requires each branch circuit to have an equipment ground (either a wire, or conduit, or cable tray as in 250.120A), they can be shared when they are in the same raceway. If all the 20A circuits are in one raceway then you just need one ground.

What is the purpose of multiple earthed neutral system?

The multiple earthed neutral (MEN) system of earthing is one in which the low voltage neutral conductor is used as the low resistance return path for fault currents and where its potential rise is kept low by having it connected to earth at a number of locations along its length.

Can I separate ground and neutral in main panel?

The answer is never. Grounds and neutrals should only be connected at the last point of disconnect. This would be at main panels only.

Can two circuits share a neutral?

If the two circuits are from the same line, the main danger is overloading the neutral with too much current. A fire hazard as the neutral is not connected to a breaker. In short, stay safe and run separate neutrals for each circuit. you definitely do not want to share neutrals from different circuits.

Can you borrow an earth from another circuit?

Its not a good idea to borrow an earth from another circuit, that circuit could be disconnected/stripped out or rewired at some stage in its life.

Can I connect ground and neutral together?

No, the neutral and ground should never be wired together. This is wrong, and potentially dangerous. When you plug in something in the outlet, the neutral will be live, as it closes the circuit. If the ground is wired to the neutral, the ground of the applicance will also be live.

How many earthing systems are there?

BS 7671 lists five types of earthing system: TN-S, TN-C-S, TT, TN-C, and IT. T = Earth (from the French word Terre) N = Neutral S = Separate C = Combined I = Isolated (The source of an IT system is either connected to earth through a deliberately introduced earthing impedance or is isolated from Earth.