The key elements that should be given thought to is those whaich are firstly safety critical and secondly finacially benificial in respect of the losses incurred for failure. Soem may not agree with this, but it's what the employer wants to hear and not always in that order.

The KPI should relate to those areas that are safety critical. Checking the competence of installers, who does this, using what and of course was it actually done.

1. What safety checks are carried out and how is this measured, make a list and give some priority to the most critical.
2. How are other areas checked to ensure finacial loss is avoided where possible. If 'A' fails, what would the consequence be for 'B' and 'C', etc.

KPI's are often measured to give either a score or rating, i.e. Good, Fair, Poor. these then would need to show the actions required to show improvement.