Automatic transfer switches (ATS) are the devices that sense a loss of normal power and connect each branch of the Essential Electrical System to the generator source within seconds — and selective coordination is the overcurrent-protection discipline that guarantees a single downstream fault trips only the nearest device, never the whole emergency system. Together they determine how and how reliably emergency power actually reaches a patient at the bedside, which is why they are among the most heavily scrutinized electrical items in any hospital design review, inspection, and accreditation survey.

Where ATS and selective coordination sit in the power system

The generators and fuel supply provide the source of emergency power; the Type 1 Essential Electrical System (EES) defines the branch structure (Life Safety, Critical, and Equipment) that the source feeds. Automatic transfer switches are the switching layer between the two — they decide, automatically and within code-mandated timeframes, which source (normal utility or alternate generator) energizes each branch at any given moment. Selective coordination is the protection layer layered on top of that distribution, ensuring a fault does not cascade upstream and dark out healthy parts of the system.

This Article covers the transfer switches themselves and the coordination study that governs the overcurrent devices feeding and fed by them. Generator sizing, fuel storage, and N+1 source redundancy are addressed in the companion Article on generators; the static transfer switches inside UPS systems and isolated-power transfer in wet procedure locations are addressed in the UPS/critical/isolated-power Article. The acceptance and witnessed transfer testing of these devices is addressed in the commissioning Article.

Governing codes and standards

Reference What it governs for ATS / coordination
NFPA 99, Health Care Facilities Code Defines the Type 1 EES and its three branches; sets transfer-time expectations and the separation of branch transfer switches.
NFPA 110, Standard for Emergency and Standby Power Systems Defines system Type (max allowable transfer time — Type 10 = 10 seconds for hospital EES), Class (minimum run duration on stored fuel), and Level (Level 1 where failure risks life); governs ATS construction, monitoring, and testing.
NFPA 70, National Electrical Code (NEC), Article 700 Emergency systems — requires that emergency-system overcurrent devices be selectively coordinated with all supply-side devices (700.32 / 700.28).
NEC Article 701 Legally required standby systems — parallel selective-coordination requirement (701.27 / 701.32).
NEC Article 517, Health Care Facilities Wiring and source requirements for the EES; defines the Life Safety, Critical, and Equipment branches and their permitted loads.
NEC Article 708 Critical operations power systems (COPS) — where designated; also requires selective coordination.
NFPA 101, Life Safety Code / NFPA 110 Life-safety branch loads (egress lighting, alarms, exit signs) and their transfer behavior.
CMS Conditions of Participation; TJC / DNV / accreditation Adopt the NFPA suite by reference; surveyors verify transfer testing records and EES integrity.
AHJ / state amendments (e.g., HCAI/OSHPD in California) May add stricter transfer-time, seismic, or witnessing requirements; HCAI requires special-seismic-certification and field observation of EES equipment.
UL 1008 Product safety standard transfer switches are listed to; UL 1008S covers solid-state/static transfer switches.

A frequent point of confusion: the EES is never energized by a single transfer switch. NFPA 99 requires the Life Safety and Critical branches to transfer on separate ATS units from the Equipment branch (and the life-safety/critical loads to be on dedicated transfer equipment), so that a single switch failure cannot take down both egress lighting and patient-critical loads at once.

How an automatic transfer switch works

An ATS continuously monitors the normal source. On a sensed failure (loss of voltage, low voltage, phase loss, frequency excursion, or phase imbalance beyond setpoints), it initiates the transfer sequence:

  1. Sensing & delay. The controller confirms the outage is real, not a momentary dip, by waiting out a short engine-start time delay (typically 1–3 seconds) to avoid nuisance generator starts.
  2. Generator start signal. The ATS sends a start command to the generator(s) via the generator control system.
  3. Generator stabilization. The controller waits for the alternate source to reach acceptable voltage and frequency.
  4. Transfer. The switch transfers the load to the generator. For the hospital EES, the total elapsed time from normal-power loss to restoration of the Life Safety and Critical branches must not exceed 10 seconds (NFPA 110 Type 10).
  5. Retransfer. When normal power returns and is confirmed stable for a retransfer time delay (commonly 15–30 minutes, to ride out utility instability), the ATS transfers back to normal, then runs the generator unloaded through a cool-down time delay before shutdown.

Transfer mechanisms and transition types

Transition type Behavior Typical healthcare use
Open transition (break-before-make) Brief open interval; load momentarily de-energized during transfer. The norm for most EES branches — simplest, lowest cost; the sub-10-second outage is acceptable for code-defined loads.
Delayed transition Deliberate timed open interval (a "neutral" position). Used for large motor or transformer loads to allow residual voltage to decay before reconnecting, preventing damaging inrush/out-of-phase reconnection.
Closed transition (make-before-break) Momentary parallel of both sources (typically < 100 ms) so the load never loses power. Used for return-to-normal and for scheduled testing so critical loads (imaging, OR, data centers) are not interrupted during routine transfers; requires utility coordination/permission to briefly parallel.
Bypass-isolation ATS A maintenance bypass that lets the switch be isolated and serviced or replaced without dropping the load. Strongly recommended (and often required) for Life Safety and Critical branch switches so maintenance never forces a clinical outage.

Most ATS units in hospitals are electrically operated, mechanically held, and built around either contactor-based or molded-case/power-switch contacts; they are listed to UL 1008 and rated for the available fault current at their location (withstand and closing rating, or WCR) so they can survive a downstream fault without welding closed.

Selective coordination — the core requirement

Selective coordination means that for any fault, the overcurrent protective device (OCPD) immediately upstream of the fault operates before any device further upstream — so only the smallest possible portion of the system is de-energized.

NEC Articles 700 and 701 require that the OCPDs in emergency and legally required standby systems be selectively coordinated with all supply-side overcurrent protective devices. In a hospital this is not optional and not a "best effort" — it is a hard, enforceable design requirement, and inspectors increasingly ask for the coordination study as a condition of approval and final sign-off.