What is the difference between open-transition and closed-transition transfer switches?

Open-transition switching briefly disconnects the load from both sources during transfer (break-before-make), causing a momentary interruption, and is standard for responding to an unplanned utility failure. Closed-transition switching briefly parallels utility and generator sources (make-before-break) for a seamless, zero-interruption transfer, but requires the generator to already be running and synchronized, so it is used for planned transfers such as scheduled testing rather than initial outage response.

Why do generators need to be synchronized before paralleling?

Closing a generator breaker onto a shared bus while its voltage, frequency, or phase angle does not match the bus produces a severe transient current and mechanical shock that can damage the generator and switchgear. Automatic synchronizing controls monitor the incoming generator relative to the bus and only permit breaker closure once they match within a tight tolerance, with a check-synchronizing relay as a hardwired backstop.

What is the difference between NFPA 110 Level 1 and Level 2 EPSS?

Level 1 Emergency Power Supply Systems are required where failure could result in loss of human life or serious injury — for example, life safety loads in hospitals — and carry the most stringent NFPA 110 installation, testing, and maintenance requirements. Level 2 systems protect property or business continuity rather than life safety directly, and carry somewhat less stringent requirements, though still substantial compared to purely optional standby power.

Why use a day tank if there is already a bulk fuel storage tank?

A day tank is a smaller tank local to the generator that supplies the engine directly, automatically refilled from bulk storage by a transfer pump system. It provides a short-term buffer that keeps the generator running even if the transfer pump or bulk tank has a brief issue, and simplifies fuel piping and quality management at the engine, while the bulk tank is sized for the facility’s actual required run-time.

What is load shedding on a generator system and when is it used?

Load shedding automatically drops lower-priority loads if the generator approaches its capacity limit, protecting it from overload while preserving power to the highest-priority circuits. It is used when a generator is intentionally sized below the full connected building load — a common cost-control choice — with loads organized into priority tiers so life safety loads are never shed and lower-priority loads are shed first.

Electrical·11 min read·July 22, 2026

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Topics covered

generator system designautomatic transfer switch typesgenerator parallelingNFPA 110 EPSSgenerator fuel system designload shedding generatorclosed transition transfer switch