TT earthing systems & protection against over voltage

frambo5

in a TT earthing system the earth fault loop impedence is high why ? do u need an RCD as well employed with a TT earthing system so as to cut the disconection time down?

In protection against overvoltage & undervoltage where is the contractor coil situated in the transformer or in the consumer unit (if thats how protection is provided)? ( i feel kinda retarded asking this question)