The importance of SCCR and UL for equipment in district energy systems

SCCR insights

• New system designs, as well as those with relocated equipment installed before NFPA 70: National Electrical Code (NEC) 2005 enforcement, must scrutinize the short-circuit current rating (SCCR) and UL listing of equipment.
• Short-circuit current rating (SCCR) protection is crucial for ensuring that electrical equipment can safely withstand and interrupt fault currents without causing equipment damage or creating a safety hazard.

Short circuits have existed since the inception of electricity. Faults can happen for assorted reasons, be it wildlife, lack of proper maintenance or even misplaced tools within equipment. For district energy plants, which often include large boilers and chillers requiring equally significant power sources, the consequences of a short circuit can be severe.

NFPA 70: National Electrical Code (NEC) introduced new equipment rating requirements in the mid-2000s to reduce the risk of full destructive short-circuit events, but the enforcement of rule changes in some states can take multiple years.

As a result, district energy projects completed as late as 2012 may have equipment that is not in compliance with this NEC requirement. Considering that standard chillers and boilers also have life cycles ranging from 15-30 years, the likelihood of encountering noncompliant equipment is high and design engineers must be prepared.

Understanding short-circuit ratings and risks

Equipment that uses power can interact with a fault in two ways. Protective devices, such as fuses and circuit breakers, are designed to interrupt and stop the short circuit up to a certain threshold; they possess an interrupting rating for this purpose. Nonprotective devices such as chillers and boilers with proper short-circuit current rating (SCCR) protection can experience an electrical fault and live to tell the story; this is defined as the withstand rating.

Without the appropriate ratings, as evidenced in this video example, a short circuit can trigger an immediate, subsequent arc flash event; this causes high currents to develop, producing extreme heat and strong magnetic forces at the location of the fault. This all happens instantaneously, often within 50 milliseconds or three electrical cycles, and poses a serious risk to personnel working on or with the equipment. In addition to safety concerns, these events can lead to significant downtime for the district energy plant while the damaged equipment is repaired or replaced.

Figure 1: The view of the North Carolina Department of Administration’s new chiller plant from its east side includes double-height windows that offer a look inside from the street and its 2.76-million-gallon thermal energy storage tank. Courtesy: Flad Architects

As such, when designing modifications to existing plants and new facilities, engineers must consider arc flash mitigation strategies and ensure that equipment is provided with appropriate SCCR. Equipment with proper SCCR possesses a withstand or interrupting rating that is higher than the available fault current that can be experienced at that point in the electrical distribution system. A short-circuit study performed by an engineer can determine the available fault currents at different points of the distribution and help dictate the SCCR requirements of the plant’s equipment and protective devices.

The history behind SCCR

The NEC was first introduced in 1897, but NFPA first officially defined SCCR in 2008 in Article 100. In 2005, the NEC first required manufacturers to mark the SCCR of their equipment. Given the amount of time it can take for updated code requirements to be adopted and enforced, the likelihood of encountering a district energy project that’s noncompliant makes these ratings that much more important for design engineers to understand and be prepared for.

At baseline, the NEC requires that equipment be labeled and listed by an Occupational Health and Safety Administration (OSHA) Nationally Recognized Testing Laboratory (NRTL). This confirms the products have met OSHA requirements for performing safety testing and certification.

One of the most common NRTLs recognized by OSHA is UL. A UL label signifies that products will not cause fire, burns or electrical shock. This label has formed the baseline standard for safety for many years and remains an important qualification for equipment.

UL 508A is the standard used by manufacturers to determine the SCCR of their equipment. The third edition of UL 508A, the most recent, was published in 2018. The standard was first released in 2001, followed by the second edition in 2013. The NEC relies on this standard for the construction of industrial control panels, which, in a district energy plant, includes boilers and chillers.

Why district energy equipment needs SCCR

The timeline for these new rating requirements and their roll-out is critical when working on district energy equipment. Water-cooled chillers have a median life cycle of 20-30 years, air-cooled chillers 15-20 years and boilers around 15-20 years — though owners sometimes extend their lifespans.

This is important for both designers reusing equipment installed in the 2000s and 2010s, and owners looking to directly replace equipment one for one, when SCCR was not deeply scrutinized.

For example, a medium-to-large boiler for a district energy plant will most likely require 480 volts (V) for its power supply, which is often fed from the service, if not the first or second level of feeders. Being this close to the source of power often results in available fault currents above 10 kilo amperes (kA), but the standard SCCR from manufacturers is only 10 kA, and even 5 kA in some instances. This would result in inadequate protection based on the SCCR for the equipment. Equipment with higher SCCR can be specified, but it’s not the default offering from manufacturers.

Chillers of large tonnage, around 1,200 tons or larger, have high power demands and often are fed by medium voltage at 4,160 V. When using medium-voltage systems, fault currents are lower and SCCR is less of a concern.

However, when using a large tonnage machine at 480 V, the power demand must be supplied by switchboard or switchgear that is directly fed from the utility or campus-owned transformers. This line carries high fault currents of 65, 85 or even 100 kA, depending on the configuration of the electrical system. Some large-tonnage chillers only have a standard SCCR of 30 or 42 kA.

For this reason, standard equipment offerings or quick-ship options from boiler and chiller manufacturers must be scrutinized by the engineer for SCCR and coordination of protection.

SCCR mitigation strategies

There are a number of electrical mitigation strategies, however, in the event that SCCR was overlooked in the plant’s design or in the purchase of replacement equipment. Special attention should be paid to maintaining the UL listing of the equipment. UL 508A gives direction on how to determine the SCCR in Supplement SB, which allows for the modification of the available short circuit current with the presence of current limiting components.

Transformers: Adding an “isolation transformer” is one solution for limiting the fault current during a short circuit. These transformers will have a similar voltage on the primary and secondary, but the secondary is isolated — a 480 V delta primary to a 480/277 V wye secondary isolation transformer, for example.

This is the simplest mitigation strategy when equipment SCCR is too low but comes with small energy losses due to the added impedance and can be a challenge to install if the space available isn’t large enough for additional components. This strategy carries added cost in the short term and the long-term energy cost should be evaluated. Additionally, the current supply chain issues for large transformers may add an unreasonable time delay to the project.

Current limiting: Current-limiting breakers and fuses offer another solution. Despite their widespread use, this type of fuse comes with major caveats that cannot be overlooked.

Over the years, the industry has employed the up-over-and-down method for using current-limiting fuses to reduce the available fault current to a piece of equipment, but it is only effective if both the peak let-through and root mean square let-through of the fuse and equipment are coordinated.

Engineers must be careful when applying this method because the peak let-through data is related to the X/R ratio of the system. Fuses are tested in laboratories by manufacturers at 6.6 X/R ratio and published let-through charts from manufacturers are based on that number. In district energy plants, the X/R ratio is always different and dependent on the utility serving. The peak let-through of the fuse must be recalculated based on its real-world application and compared with the tested figure provided by the manufacturer of the equipment with improper SCCR. In most cases, however, the data needed to run this recalculation won’t be readily available and, in some instances, is proprietary to the manufacturers.

One option is to bring in an NRTL to provide the third-party field certification of the equipment. The company can test, evaluate and verify the fuse and equipment components meet the UL standards and, upon certification, apply the appropriate third-party approval of the equipment. If an engineer self-performs the calculations without engaging a field certification service, that engineer will own the liability of their non-UL labeled product.

Figure 2: The west side view of the plant shows its outdoor cooling towers and access staircase. The first floor houses the plant’s electrical room and pumping area, with the second dedicated to the chillers. Courtesy: Flad Architects

UL has a library of tested fuses in combination with contactors, starters, power distribution blocks, etc., that serves as a useful tool when trying to use a current limiting device to reduce the available fault current. The field evaluation entity employed may use that tested data to apply to the specific project.

However, in some instances, the field devices may not have tested data disclosed to UL, so they may run the tests on-site themselves or send the device into a laboratory, which makes this mitigation solution less practical.

Using current limiting fuses shouldn’t be the first solution to resolve improper SCCR, but rather a last resort with the engineer and owner understanding that it may not work. Current limiting breakers and fuses are primarily used and intended for manufacturers when they are initially designing and assembling their panels to get an overall SCCR rating for their equipment. They’re not meant to be a long-term strategy for operational district energy plants.

SCCR allows for reliability

Proper SCCR applies to all equipment installed inside district energy plants — from the large boilers and chillers to the ancillary items such as condensate receiver units, water heaters, heating, ventilation and air conditioning units and even disconnect switches for pumps and cooling towers.

Close coordination with the mechanical engineer can ensure that the required SCCR is included in the equipment procurement specifications, which enables the manufacturer to provide the appropriate product for each specific project.

Given the importance of central energy plants to university campuses, state and federal buildings and large industrial processes, it is crucial that design engineers review, evaluate and verify that all equipment installed has an SCCR greater than the available fault current at the equipment. Proper attention to this must be part of any strategy for optimizing plant safety and operational reliability.

See the related case study for more information: Case study: Successful equipment relocation and reuse with new compliant electrical

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