What to know: ATS, UPS, coordination in mission critical facilities

Mission critical insights

• Ensuring proper selective coordination in electrical systems involves gathering detailed component-level information, making informed decisions on protective device ratings and settings and inputting accurate data, with a focus on manufacturer-specific parameters, such as time-current curves and reactances, to prevent overload and fault conditions and facilitate coordinated protection of critical equipment like generators and transformers.
• Selective coordination of UPS systems is crucial in ensuring uninterrupted power supply for mission critical facilities, such as data centers and health care facilities, as neglecting UPS behavior in fault conditions can lead to unnecessary load loss or outages.

With the need for extensive power, mission critical facilities often require the engineer to design for electrical reliability, which includes overcurrent protective device coordination in time ranges defined by codes and best practices. These ranges can be up to 0.1 seconds or up to the full ranges defined by manufacturers (selective coordination). Selective coordination, as defined in NFPA 70: National Electrical Code, is required for electrical systems as it relates to life safety.

This edited transcript of the Jan. 31, 2024, webcast Critical power: How to design for selective coordination in mission critical systems can be used to help electrical engineers understand the definitions.

Joshua Fluecke, PE, LEED AP, Senior Principal, Syska Hennessy Group, provided information. References to specific slides and figures can be seen in the on-demand webcast.

For automatic transfer switch (ATS) withstand ratings, the primary factor affecting ATS frame type selection is the maximum continuous current rating of the connected load.

However, the short circuit current that an ATS will need to withstand and the duration for which the ATS will need to withstand it are also significant factors, which are functions of the overcurrent protection devices upstream of that ATS. Consequently, ATS selection is dependent not only on the nominal ampacity of our loads but also on the withstand and close on ratings and short time ratings resulting from the short circuit testing required for the manufacturer’s qualification for UL 1008 or the ANSI standard for transfer switches.

Before evaluating withstand and close on ratings and short time ratings, it is important to distinguish the purpose of the transfer switch from the purpose of the overcurrent protective devices.

Unlike a fuse or a circuit breaker, an ATS must remain connected to an available power source even when a fault occurs. Overcurrent protected devices are designed to protect power conductors and equipment by disconnecting loads from power sources when faults occur. High current faults generate magnetic forces that oppose ATS contact closures and heat builds up due to resistance across the contacts within the transfer switch.

Without adequate design provisions, these high current faults would cause the contacts to open or produce heat that could degrade the contacts themselves and their materials and surfaces. For these reasons, UL 1008, NFPA 99: Health Care Facilities Code  and NFPA 110: Standard for Emergency and Standby Power Systems standards require that the ATS be electrically switched and mechanically held. Based on product testing, manufacturers can provide the UL 1008 ratings for the current that the ATS contacts can close on and withstand if a short circuit fault occurs.

UL 1008 testing can also provide optional short-time ratings, indicating the amount of current that the ATS can carry for a specific duration between a tenth of a second and half a second. Returning to selecting the appropriate frame, the available fault current and its expected duration need to be known for the location where the ATS is going to be installed in your electrical system.

This information is typically determined from a selective coordination study with the facility’s power distribution system. By comparing the magnitude of faults and the duration of these available fault currents based on published tables and ratings, we can appropriately select an ATS that can be coordinated with the overcurrent protective device provided.

Short time ratings are sometimes required for transfer switches where power systems use multiple levels of overcurrent protection. Selective coordination is required between different buses or it may be mandated by code, as the NEC requires. Selective coordination is necessary to ensure that the overcurrent device closest to a fault clears first, allowing the remaining upstream loads to stay active and connected to their power source for critical systems. Selective coordination requirements may result in removing instantaneous trip functions or settings for upstream circuit breakers, requiring a transfer source to endure a short circuit for an extended timeframe; all this needs to be looked at. But these ratings are developed in tables published by manufacturers in accordance with UL 1008.

Selective coordination input parameters

To get the proper output of our studies, we need to gather all the salient information at the component level to do a complete analysis and selective coordination look at our electrical systems. Often, selected ratings and settings for protected devices to establish proper coordination require decisions by the electrical engineer, sometimes among competing objectives. We need to input all the data correctly for this evaluation. At times, sacrificing selectivity may be necessary as you review certain systems to achieve required protection. The selection and settings for protective devices need to reflect the application of engineering judgment to achieve a compromise while meeting code requirements and accepted industry practice.

Manufacturer-specific breaker data is readily available, but we must ensure that when inputting the model, we have the correct time-current curves provided by the manufacturers. This information can generally be found in a generator submittal or switchboard submittal. In most cases, for a generator, the standby engine generator will not provide enough fault current greater than what can be seen from our utility source.

If a short circuit is applied directly to a synchronous generator’s output terminals, it will initially produce an extremely high current, but that current will gradually decay to its steady-state value over time. This change is represented by varying reactive components. Three specific reactances are used for the short circuit currents, which will be input into your model based on the data you have for your specific piece of equipment. We have subtransient reactance, which determines the fault during the first one to five cycles during a three-phase fault at the engine generator. We also have transient reactance, which determines the fault during the next roughly 5 to 200 cycles. Lastly, we have synchronous reactance, which determines the steady-state fault current if a fault occurs for a significant amount of time.

Figure 1 shows alternator manufacturer-published data for input into an analysis software. This decrement curve represents the current output of the engine generator considering a three-phase fault applied at or near its output terminals. The short circuit current will decay over time, hence the name decrement curve.

Figure 1: This shows alternator manufacturer-published data for input into an analysis software. Courtesy: Consulting-Specifying Engineer

The subtransient reactance’s impedance value determines its contribution during the first cycle after the fault occurs. In approximately a tenth of a second, this reactance increases to the transient reactance, typically used to determine the fault current after several cycles. In approximately half a second to two seconds, the generator’s reactance increases to the synchronous reactance, which determines the current flow after a steady-state condition is reached should a fault occur for that duration.

Watch the webcast to see the alternator thermal damage curve, generally shown as a percentage of the generator’s full load current provided by the manufacturer. This data is manufacturer-specific. It’s important not to exceed these overload values for time duration to avoid damage to the generator’s alternator during a fault condition. This data is another input parameter needed from manufacturers for our protection analysis of the engine generator system.

When talking about component overcurrent protection for generators, here are some general guidelines: Most critical facilities have a generator. The properties we populate into our information at SKM will determine the settings for our protected device. For longtime settings, protection is set above the generator full load amps to allow the generator to operate at its full load if necessary and be protected from damage during an overload.

Note that the current on the generator may approach the limits of the overload curve, affecting alternator insulation life. For the instantaneous region, protection is set above the decrement curve shown previously and below the short circuit withstand point, to facilitate coordination with other devices in our system. In other words, a fault in the middle region will operate this device; now that protection is set above this point, the device will never trip.

Figure 2 depicts generator overcurrent protection. Some suggestions here: Set these protection limits as we described earlier. You want to make sure this aligns with some of the input data you put into SKM and try to adjust the ratings on a circuit breaker with selectable settings to intersect or avoid these points and these curves here.

Figure 2: This figure depicts generator overcurrent protection. Courtesy: Consulting-Specifying Engineer

For low-voltage protection, there is another component often found throughout our critical facilities. Here are some industry-standard overcurrent protection schemes for low-voltage transformers fed from circuit breakers and equipped with long time, short time and instantaneous functions. The circuit breaker characteristics are plotted on this phase-time current curve along with the transformer and feeder damage curve. You will see those lines on the right of the chart; that is what those represent. The purpose of the circuit breaker for this component is to allow for full use of that transformer and to protect the transformer and cable from overloads and faults.

To accomplish this, a circuit breaker curve should be to the right of the transformer full load current. The full load that the transformer. And importantly, the inrush point and to the left, this protection needs to be set to avoid the cable damage curves and those cable amp ratings for that specific transformer. Some suggested margins are listed in the table that you see here that have historically allowed for safe operation of transformers and protection of cables while reducing instances that you may have nuisance trips.

This is the same philosophy as any of the other components. We are going to protect the equipment first, see how that component is protected and integrate it with the other components in the system and look at that selective coordination. But in the end, we are looking to allow this component to operate as intended and be protected properly.

UPS protection in mission critical facilities

Unique component, we will find this in a lot of data centers plus some other facilities. But the behavior and capabilities of the uninterruptible power supply (UPS) under short circuit conditions differ from other power distribution components. It’s important to note that neglecting the UPS in a selectivity study can turn a fault that occurs in any load or branch circuit into a single point of failure resulting in unnecessary load loss or outage or operational system for an extended period.

In bypass mode for the UPS, the critical load is fed for the UPS’s static bypass switch. Typically, the UPS’s static bypass switch is used under fault conditions as designed by the manufacturer. When an inverter is not capable of either producing or maintaining proper voltage at the system output, it will transfer to bypass. Or if the inverter does not have sufficient capacity to support the load due to overload condition or reduction in the inverter capacity, it will go to bypass.

For selectivity purposes, understanding the UPS system behavior under a downstream short circuit condition is important; it should always be verified with the UPS manufacturer. When the UPS bypass feed is not available and a fault occurs at the UPS system output, the UPS will use the inverter to clear the fault. In this case, inverters will feed as much current as they can feed limited by their inherent current limit until that fault is cleared and the system voltage restored. If the inverter is unable to do this, it will shut down after usually a timer will expire and, in this case, UPS is turned off and the load is interrupted.

To estimate some of the selectivity between load distribution breakers and UPS system static bypass, you need to look at some basic questions: What is the estimated fault current in the load branch device for coordination evaluation? We need to see what is downstream and what that fault is and how long is it going to take for that breaker to trip or clear for the branch circuit protected device. And then we also want to look at is that UPS system, is that system able to feed the current long enough for this unit to be selective? Different types of evaluation that need to take place for the UPS component in a mission-critical system.

In summary, selective coordination, which is essential to ensuring mission critical facilities (data centers, the health care facilities, facilities with important human life or business function) behave as they are intended during fault conditions. We understand through some of the examples and some of the one-lines can be very complex systems with many different types of equipment and perhaps numerous failure scenarios. That all will require evaluation, ensure that we have our mandated code requirements met and optional selective coordination is achieved for these cases and per owner or client requirements for those optional systems.

It is important during the design that we establish these project selectivity requirements as part of our contract documents for design, including the specification and confirm viability of these parameters with the manufacturers and power system analysis during the design stage. We must make sure this is going to work as intended with the information we know when we complete our design.

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