The more usual word for 'appareillage' would probably be 'fittings', as it doesn't really mean 'apparatus' or 'equipment' as much as it does 'accessories' in general electrical work; HOWEVER, in this specialist field, I'd be inclined to go rather for 'equipment', which is vague enough, but at least avoids suggesting 'appliances' or 'machines'

As for 'de puissance', I'd be inclined to go for 'high-power' rather than 'high-voltage', since [AFAIK] it usually means high-power, but that doesn't NECESSARILY also mean high-voltage (though of course that IS often the case!). Again, in general electrical installation work, it seems more often to be used to mean '(high) power' (as distinct from low-level or signals) circuits etc., rather than SPECIFICALLY referring to the voltage ranges involved.

So I'm suggesting 'high-power electrical equipment' to start you off...