Zimmy: part of the process of being a good H&S practitioner is the art of knowing what you don't know. It is fundamental to the training and to being accepted as a Chartered professional. I don't know much about electricity, but I know a man who knows an awful lot about it and so I ask him.
As a H&S professional I need to be able to identify issues which may be harmful and how to go about fixing them, I don't actually need to fix them myself. I have a massive list of professionals: electricians, air monitoring specialists, plumbers, machine guarding experts, builders, chemical experts, fire experts, asbestos professionals, gas fitters, etc, and when I need advice I pick up the phone. As one goes along one picks up fundamentals about each profession, what to look for in each job, strength of concrete, when you can put steel reinforcement in, what are the inspection intervals for electrical fixed installations, portable appliances, fixed appliances, who sets the recommendations and so on and so forth for each topic one is involved in. However, this does not make us experts in gas, concrete, electricity or whatever.
When a contractor needs a permit to work I will ask him for a risk assessment and a method statement for the job. My training and my expertise comes in knowing whether what he has said he will do on this documentation is:
a] feasible
b] safe
c] legal;
in order to do this I need to know the laws and some basic information about his business, but I don't need to carry out the job myself.
Additionally, the contractor may need to use, for example, chemicals as part of his process and I need to ensure that he is aware of the risks associated with said chemicals (even if this is just an aerosol or wipe). This may be overlooked by your average electrician, builder, gas fitter, or other trade, but what they are using may set off a chemical reaction with what we are using ..... BOOM! As a H&S practioner I have to project all possible consequences and ensure that these do not cause accidents or incidents as part of the contractor's processes. I use chemicals as an every day example, this could be work at height or vibrating tools or noisy equipment, entering confined spaces, etc - there are any number of risks which are outside the normal remit of our "expert" but which do need addressing.
The main thing to remember is that no single person is an expert on everything, we need to work together, all the professions, to ensure that tasks are carried out safely and without risk to health.