arpadveress 
HAC stands for hazardous area classification. EN IEC 60079-10-1 goes for gas hazardous areas; -10-2 for dust hazardous areas.
60079-10-1 4.6 says:
The hazardous area classification should be carried out by persons who understand the nature of flammable substances, gas dispersion and ventilation and are familiar with the process aspects for the plant under consideration. It may be beneficial for other engineering disciplines, e.g. electrical and mechanical engineers, and personnel with specific responsibility for safety to be part of and have an input to the hazardous area classification process. The competency of the person shall be relevant to the nature of the plant and methodology used for carrying out the hazardous area classification. Appropriate continuing education or training should be undertaken by personnel on a regular basis where required.
NOTE 1 Competency can be demonstrated in accordance with a training and assessment framework relevant to national regulations or standards or user requirements.
NOTE 2 Elements of competency are covered in several personnel certification schemes, such as the IECEx Unit of Competence Ex 002 according to IECEx OD 504.
60079-10-2 4.3 says:
The area classification should be carried out by those who are competent and understand the relevance and significance of the characteristics of dust and those who are familiar with the process and the equipment, along with safety, electrical, mechanical, and other qualified engineering personnel.
NOTE These elements are covered in several personnel certification schemes, such as the IECEx Unit of Competence Ex002 according to IECEx OD 504.
In both cases it says competency shall be proven. IECEx CoPC unit 002 is a possible choice (not the only one). But important – Ex competency and proven technological knowledge must be there. Again HAC is a teamwork.
NOTE if this above not well confirmed by the owner or responsible one of project or operation the whole might face trouble.
Keep up good work!
Arpad
