Your question describes two different electricians: "registered person" and also "trade apprenticeship served with NTC 3, trade tested electrician with 18 years experience".
"Registered person" yes he may. "Trade apprenticeship served with NTC 3, trade tested electrician with 18 years experience" no he may not. Only an "Accredited/Licensed" person may sign a COC.
I guess it might be easier to start at the top. The 3 types of registered electricians may carry out work and sign off as can be expected. Below them we have an "Accredited" or "Licensed electrician" These two top tiers in the hierarchy are the only "regulatory acceptable competent electricians". All the others below them may not carry out any electrical work unless "under the direct supervision" of one of the above. Not all "accredited" electricians need to be registered. A company only requires one full-time "Accredited" electrician to be registered in order for the company to operate as an Electrical Contracting Company". An "Accredited" electrician who is not registered. while still being "competent" may not sign a COC in his own capacity, however, he may in a large company sign off under the "umbrella" DOL registration of the "full-time employee or owner if he himself meets the requirements". An accredited electrician must be registered in his own right if he wishes to sign off any work outside the company.
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