What does happen if neutral conductor is cut or broken? Then what should be predicting the necessary measures to prevent cutting the neutral conductor?

jcm:

. . . I got the torch and switched it on , lights came on , one minute later darkness, this time RCD was up/on but no supply, noticed all the street lights were off the whole countryside was off . Candles and after while looked up on laptop to see any information , sure enough whole postcode for my area was off, they seemed to break the post code  into blocks but all was off , they said it was equipment fault , power back on after about 2 hours . Why did this fault trip my main RCD before the power failure I don't know.

jcm

Though RCDs are supposed to protect the circuits they supply, I have known them to be tripped by external faults. A friend once asked me for advice on nuisance tripping of a RCD at her home. She pointed out tree branches that had become tangled round the overhead supply wires to her house; sparks would fly during windy conditions. An electrician had said, no, RCDs detect faults in only the circuit within the house. I advised her to report the extraneous tree branches to her electricity supplier. She did, the branches were removed and the nuisance tripping stopped. 


Faults of this nature cause dirty incoming mains supply full of high-frequency transient spikes which are not balanced between live and neutral and fool the RCD into thinking there is a fault. In your case, JCM, it sounds as though there was a fault brewing up in the distribution network, causing dirty mains which tripped your RCD (and possibly a few others in other houses, unknown to you) before the network protection tripped the substation, taking the lot out.

jcm:

. . . I got the torch and switched it on , lights came on , one minute later darkness, this time RCD was up/on but no supply, noticed all the street lights were off the whole countryside was off . Candles and after while looked up on laptop to see any information , sure enough whole postcode for my area was off, they seemed to break the post code  into blocks but all was off , they said it was equipment fault , power back on after about 2 hours . Why did this fault trip my main RCD before the power failure I don't know.

jcm

Though RCDs are supposed to protect the circuits they supply, I have known them to be tripped by external faults. A friend once asked me for advice on nuisance tripping of a RCD at her home. She pointed out tree branches that had become tangled round the overhead supply wires to her house; sparks would fly during windy conditions. An electrician had said, no, RCDs detect faults in only the circuit within the house. I advised her to report the extraneous tree branches to her electricity supplier. She did, the branches were removed and the nuisance tripping stopped. 


Faults of this nature cause dirty incoming mains supply full of high-frequency transient spikes which are not balanced between live and neutral and fool the RCD into thinking there is a fault. In your case, JCM, it sounds as though there was a fault brewing up in the distribution network, causing dirty mains which tripped your RCD (and possibly a few others in other houses, unknown to you) before the network protection tripped the substation, taking the lot out.