Sensitive Electronic Equipment

Article 647 allows a technical power system where a 120-volt supply is derived so that each leg sits about 60 volts from ground instead of the usual hot-and-neutral arrangement. That symmetry cancels common-mode noise beautifully for recording studios, post houses, and labs. In exchange the article demands GFCI protection, permanent wiring, strict labeling, and use only in commercial or industrial settings by qualified people.

This is a niche system and it can confuse anyone who does not know it is there, so I label it loudly and treat the conditions as mandatory. Done right it is a clean, quiet supply; done sloppily it is a nasty surprise for the next tech who assumes a normal neutral.

When it applies

This permits a special technical power system for sensitive electronics — recording studios, broadcast, medical and lab instrumentation — where you want the quietest possible ground reference. It runs 120 volts balanced as 60 volts above and below ground to cut the hum and noise that a standard system couples into the gear.

Common mistakes

• Installing it in ordinary occupancies when the article restricts it to specific commercial and industrial conditions with qualified maintenance.
• Forgetting that both lines are live relative to ground, so both need overcurrent protection.
• Skipping the required labeling and treating a technical outlet like a normal 120 volt receptacle.

In the field

This is a specialty tool, not a general fix. I only put it in where the conditions are met and qualified people maintain it, and I make sure everyone downstream knows both conductors are hot. Done right it cleans up the noise floor; done casually it just confuses the next person into a shock.