Quick Hits

Episode 3: Transfer Equipment and the 10-Second Rule (700.6, 700.12)

The Automatic Transfer Switch (ATS) is the system’s brain. It must be automatic and listed specifically for emergency use.

Technical Specifications

Automatic Transfer Switches must be electrically operated and mechanically held  (700.6(B)). Why? Because if control power is lost during a fire, we don’t want the switch "dropping out" or vibrating under fault conditions. It must stay locked in position.

The "So What?" Layer: The 10-Second Rule

Power must be available within  10 seconds  (700.12). However, you must manage "nuisance starts" with time-delay devices:

• The 1-Second Delay:  Overrides momentary utility dips.  Critically, the 10-second clock starts AFTER this time delay.  If the AHJ allows a 2-second recloser cycle, the 10-second restoration requirement begins once that cycle fails.

• The 5-Minute Cool-Down:  Vital for engine longevity. Running an engine unloaded for longer than 5 minutes can lead to performance deterioration.

Bypass and Isolation (700.6(C))

If the load cannot be dropped for maintenance, you must provide bypass/isolation means. This allows the ATS to be removed for service while a qualified person manually supervises the bypass to keep the emergency load energized.

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