The mechanical and electrical engineering liability insurance policy protects you when claims are made against you if you are legally liable for personal injury or property damage by third parties.
Also, if you are legally liable for death, disease, illness or injury sustained by any of your employees while they are employed by you then the employers liability extension can help protect you in these situations. As a mechanical/electrical engineer there is the possibility of claims being made against you, especially from members of the public, and this is where it pays dividends to have this sort of cover.