Faulty electrical wiring

I have had a problem with melting and smoking electrical wiring at 2 house. The 100 amp isolator burnt out so the electrician had to cut the seal on the main fuse and the meter needs replacing. I have taken this seriously and it was dealt with urgently. However this incident at 2 different houses could have been grave. The houses are new builds Barratt and Persimmon Homes.

Please can you advice me on:
1.The action I can take to hold the builders accountable. One house was purchased new on April 2005 and the other Dec 2005.
2.What as a landlord do I have to do to prevent this happening again?
3.Do I need to get electrical safety checks done on each property?
Thanks