Open vs Closed Transition Transfer: Differences and Risks

Three ways to move the load from one source to another

A facility with an emergency generator (or generator set) has a transfer switch that moves the load from the utility to the generator and back. The question is what happens to the load during the change. There are three answers:

There is also the soft transfer, or transfer with prolonged paralleling, in which the generator picks up the load in a ramp over seconds or minutes. This is no longer a transfer: it is a generator in parallel with the grid, with all its requirements.

Open and delayed transition

This is the usual solution. The logic of the automatic transfer switch detects the utility failure, starts the generator, waits for it to stabilize and connects the load. When the utility returns, it waits for it to stabilize, transfers back and leaves the engine to cool down. The two sources are never together, so there is no need to synchronize and the facility does not become a generator as seen by the utility.

The weak point is motor loads. When the switch opens, the motors keep spinning and generate a residual voltage that drifts out of phase with the new source. If the new source is connected with that large phase difference, a high closing current appears that can trip protection or cause mechanical shock. There are two remedies:

• An in-phase monitor that closes when the two voltages pass close to coincidence.
• Delayed transition, which leaves the load without voltage long enough for the residual voltage of the motors to decay. It is the typical option when there are older drives, rectifiers or large motors.

On medium-voltage buses with large motors, the fast bus transfer is also used, which connects the new source while the phase difference with the residual voltage is still small. The complete logic of a main-tie-main switchboard is in automatic transfer at medium voltage and its interlocks in selective secondary.

Closed transition

Closed transition is only possible when both sources are live: in the retransfer from the generator to the utility, in periodic tests under load and in scheduled transfers before maintenance. When the utility fails suddenly there is nothing to synchronize with, and the transfer is open.

Its advantages: the process does not notice the retransfer, sensitive equipment does not restart, and the monthly generator tests can be done with the real load without cutting power to the facility. Its requirements:

• Verified synchronism, with a narrower window than the in-phase monitor of an open transition transfer. See generator synchronization and synchronism check 25.
• A time limit on the paralleling and a protection that opens one of the sources if the paralleling is prolonged by a failure to open.
• Short-circuit level: while the paralleling lasts, a fault in the facility receives the contribution of the utility and the generator at once. The equipment must withstand that sum.
• Governor and regulator able to keep the generator stable while the utility takes the load.

Risks of each

In open transition, the risk is the outage and the out-of-phase closing on motors that are still spinning. In closed transition, the risk is that the paralleling fails: a poorly set synchronizer, a crossed VT, a breaker that does not open, or a fault on the utility grid during the paralleling to which the facility contributes current. That is why closed transition is specified, set and tested with the same care as any generation interconnection.

One manufacturer’s synchronism windows

The synchronism limits are set by each manufacturer in its controller. As a reference, Schneider Electric’s frequently asked questions on ASCO equipment describe two different functions:

The same document explains that the in-phase monitor is useful with motor loads, because a large angle jump produces a high inrush current, and that the in-sync monitor is stricter because the live sources are paralleled for an instant. These are values from that manufacturer, not from a standard; the setting for a project is taken from the equipment specification and the agreement with the utility.

Withstand and close-on rating (UL 1008)

The transfer switch is a piece of equipment that can end up in the middle of a short circuit. That is why UL 1008 requires every transfer switch to have a withstand and close-on rating (WCR). It is the level of fault current that the equipment withstands and can close onto without creating a hazardous condition. According to the frequently asked questions of a Cummins technical seminar (2017):

• The rating is declared in two ways: with a time-based value (for example, so many amperes for 50 ms) or with a specific overcurrent device listed for that equipment. UL 1008 requires one of the two.
• If the equipment has no time-based rating, a listed overcurrent device must be in place on both sources, normal and emergency.
• The duration is chosen by the manufacturer. 50 ms is the most common, partly because of the UL 489 requirement that molded-case breakers of 400 A or more clear in that time. For those under 400 A the requirement is 25 ms. UL defines several durations up to six cycles or 100 ms.
• What is published is a point, not a curve: to check the protection, the maximum clearing time of the upstream breaker must be shorter than that of the declared rating.
• Fuses and current-limiting breakers clear faster and allow a higher withstand rating.
• In closed transition, the two sources are paralleled for instants, and there is disagreement in the industry on whether the rating must cover the sum of the two fault currents. The speaker’s explanation is that each set of contacts closes onto its own source and carries only the current of that source, and that withstand and close-on tests are done the same way on open and closed transition equipment. Using fast fuses is not a requirement, but they have advantages.

In a project this becomes a design input: the short-circuit current available at the line terminals of the transfer switch and the clearing time of the upstream protection must be compared with that rating, on both sources. The facility’s short-circuit and coordination studies are the input; see short-circuit study.

What local codes ask of a transfer panel

Local electrical codes often treat automatic transfer panels as a safety-critical product. Typical requirements are compliance with the standards for this type of product, such as IEC 60947-6-1 and UL 1008, minimum monitoring functions in the electronic transfer controller (high and low voltage, phase loss and phase sequence reversal), an operating manual that lists each function with its programmed values, and product certification of the components that are subject to it. The code does not usually distinguish between open and closed transition or set synchronism times or windows; those values come from the equipment specification and from the utility. The code that applies is the one of the country and the project, so the current wording must be confirmed.

Periodic tests and maintenance

The transfer switch is tested at the frequency set by the facility maintenance plan and, where it applies, the emergency systems standard adopted. A useful test is not just starting the generator unloaded: it is transferring the real load and checking the times of each stage, the retransfer and the cooldown. With closed transition, the test under load is done without cutting power to the facility, which is one of its advantages; with open transition, it is scheduled in a window of the process. In both cases the controller logs are reviewed to detect slow transfers, failures to start or out-of-window closings before they happen in a real outage. See emergency diesel generator.

The utility and the standards

A transfer that puts the facility in prolonged paralleling with the grid falls under the utility’s interconnection requirements for generating systems. For momentary paralleling, the prudent course is to agree the scheme with the utility before design and to put in writing the maximum paralleling time and the protections that limit it.

Internationally, transfer equipment is certified to UL 1008 and emergency and standby systems follow NFPA 110 or the equivalent IEC and local codes, which many projects adopt. Names vary by country: “transfer switch” or “ATS” and “standby generator” in North America, “changeover switch” and “generator set” in other English-speaking markets.

We design and configure your transfer scheme

Our team designs the transfer logic of your facility (open, delayed or closed), reviews the synchronism, the paralleling limit and the short-circuit level, and configures and tests the controllers and relays that carry it out. See our control, protection and communications service, and write to us through contact.

Related guides

• Automatic transfer switch (ATS)
• Emergency diesel generator
• Generator synchronization
• Synchronism check 25
• Generator in parallel with the grid
• PLC and substation automation
• AC station service
• Main-tie-main interlocking: selective secondary
• Automatic transfer at medium voltage: logic and blocks
• Fast bus transfer of motors: three methods

References

• UL 1008, Standard for Transfer Switch Equipment.
• NFPA 110, Standard for Emergency and Standby Power Systems.
• IEC 60947-6-1, Low-voltage switchgear and controlgear: transfer switching equipment.
• ASCO Power Technologies (Schneider Electric), documentation on closed transition transfer equipment and on sync and in-phase monitors.

Frequently asked questions