There are four main competent persons schemes. NICEIC, ECA/ELECSA, both now part of Certsure. NAPIT, & Stroma, there are others, but these are the 4 big players. They are mainly geared around the domestic "Part P" market. You must be aware of the limitations of all of them, they only assess against BS7671 works. So, you need to be sure that it is BS7671 works you are looking for them to be competent in. Electrical work is much bigger than just that covered by BS7671. None are compulsory. They are an easy way of self-certifying compliance with electrical work in dwellings as required by Building Regulations, e.g. Part P. They show that the company is willing to be subjected to external audits. However, do they all guarantee that the individual that comes on site to do the work for you is competent in all aspects of the work they do. No they don't.