Your questions answered: Electrical: Circuit protection

The electrical engineer is responsible for designing power distribution systems for buildings. Understanding the full circuit-protection requirements will enable the engineer to design the safest and most reliable electrical distribution systems for buildings.

During the June 28 webcast, the presenters explained how circuit protection requires an understanding of the power distribution system as well as the development of a protection scheme for circuit overloads, ground faults, and short circuits. Circuit protection can be further enhanced by implementing selective coordination design techniques. This also was demonstrated with a case study explaining the circumstances in greater detail.

Register for the on-demand version of the “Electrical: Circuit Protection” webcast here.

Additional questions are answered by Randall Ehret, PE, technical director, Electrical, Environmental Systems Design (ESD) Inc. and John Yoon, PE, LEED AP, Lead Electrical Engineer, McGuire Engineers Inc.

Question: How do you prioritize selective coordination and reducing arc flash energy?

Randall Ehret: It depends. Selective coordination is the only code required for emergency systems, legally-required standby systems and healthcare. So unless there is an owner requirement, all other installations can prioritize reducing arc flash energy. t is definitely a balancing act and project dependent.

John Yoon: For arc energy reduction, the NEC requires that clearing time “be reduced.” However, it does not quantify what exactly that means. The informational note for 240.87 references NFPA7 0E (Standard for Electrical Safety in the Workplaces) and further clarifies that the devices shall trip “with no intentional delay” while a worker is within an arc-flash boundary. That functionality is intended to protect the worker and that work is only occasionally going to be within the arc flash boundary. So selective coordination, where required by the code, is only superceded by that energy reduction functionality when workers are present.

Question: What is the best way to coordinate fuses and breakers and what are the rules of thumb for sizing breakers so they coordinate?

Ehret: Breakers and fuses operate in different ways, thus, selective coordination between them can be difficult. In my opinion, there is no valid rule of thumb. The curves must be analyzed.

Yoon: Fuse manufacturers use to state that as long as a 2:1 ratio between upstream and downstream fuses was maintained, they were most likely coordinated. However, when intermixing fuses and circuit breakers, there’s no easy answer. The time-current characteristics of circuit breakers can be all over the place. The only true way to ensure coordination is to compare TCC’s for the OCPDs.

Question: Do you know of any new standards around BESS that could be released to Code?

Ehret: I am unaware of any eminent code changes. With the growing trend towards lithium-ion batteries, I recommend consulting the owner’s insurance provider.

Yoon: ESS standards are in flux and mostly focus on fire protection at this point. NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, was originally introduced in 2020. The 2021 IFC was significantly revised as well and probably has more pertinent information.

Question: How are you addressing the relatively low (5kA) interrupting rating that comes with many types of HVAC equipment?

Ehret: Start with your specifications. Many manufacturers can provide a higher rating at additional cost when required. Otherwise, a current limiting fuse is your best option.

Yoon: Yes, this is potentially an issue. The 5kA rating is generally attributed to UL508A standard for industrial control panels. If not tested and certified by the manufacturer, the component with the lowest rating typically determines the overall rating of the equipment. As such, the manufacturer has to provide a means to limit available fault (control transformers, fuses with low peak let through current, etc.).

Question: Do breakers lose their I-T curves over years of inaction?

Yoon: Circuit breakers are mechanisms with multiple moving components. If not exercised on an annual basis, there is no guarantee that the components will operate smoothly/not bind and impact its time-current response.

Question: What types of mid-construction design changes ought to trigger the need to update to the coordination study?

Yoon: Was the coordination study required by code or the client? The NEC specifically requires selective coordination for article 517, COPs, life-safety and elevators. As such, if you cannot be certain that changes to your design won’t impact coordination, you are obligated to update your coordination study.

Question: How do OCPD strategies vary between different building types?

Yoon: There are always prescriptive code requirements, but beyond that, it boils down to a cost-benefit analysis. Certain technologies (solid state trip units, ZSI, ICCB/LVPCB, etc.) while desirable from a performance standpoint, represent additional cost that may not be acceptable to the client. In those cases, developing and understanding the owners project requirements at the beginning of the project can go a long way in avoiding over specification.

Question: What are the current best strategies for selective coordination of existing panels/subpanels with a replacement switchboard?

Ehret: That is always a challenge. You need to start with a good understanding of the complete one-line diagram, existing breaker types/sizes and feeder lengths. If you can develop an understanding of the loads throughout the system, it may help when feeder sizes need to be adjusted. be sure and communicate early with the owner regarding the potential need to modify parts of the system not directly related to the switchboard replacement.

Yoon: You have to know the operating characteristics for all segments of that circuit. If you don’t know how an overcurrent protection device will respond to an overload/short, how can you even make an attempt at selective coordination?

Question: Can you briefly mention the electrical code, and how wire sizing plays in to circuit protection?

Yoon: Your goal is to interrupt the flow of undesirable current (overloads and faults). Aside from protecting the wire, the impedance of the wire can also impact the magnitude of available fault current at downstream equipment.

Question: On some new 3-phase circuit breakers there appear to be removable chips. If there are problems keeping a particular circuit from tripping, can the settings be modified by changing these chips?

Yoon: The first question should be, “why is the circuit breaker tripping?” Again, the function of the circuit breaker is to interrupt the flow of current during overloads and faults. Simple changing the rating plug and/or LSI settings on the trip unit without understanding why it is tripping in the first place could cause serious safety issues.

Question: How do you know when to use one overcurrent protection device over another, and why?

Yoon: For motors, see the diagram at the beginning of article 430. That helps explain the relationship between feeder short circuit protection and motor overload protection.

Question: What are the differences of relays versus circuit breakers?

Yoon: Protective relaying is an extension of what you already do with circuit breakers (50=instantaneous;  51=overcurrent/overload). While there are other additional protective functions such as over/undervoltage, synchonization, etc., that are not common.

Question: Arc flash relay: How effective is it in saving the SWGR?

Yoon: Arc energy mitigation strategies in metalclad switchgear are different from those in UL891 switchboards. The level of inherent hazard is dramatically greater. This is something that is not easily/quickly answered in this forum.

Question: How do you select the size of circuit breaker?

Ehret: In general, the overcurrent protection size should be no less than 125% of all continuous loads and 100% of all non-continuous loads. There are many factors that can necessitate adjustment to the OCP size: motor loads, transformer damage curves, selective coordination, future expansion and more.

Question: Can you elaborate on the ELR setting?

Yoon: I assume you’re talking about earth leakage relays (ground fault protection). There are prescriptive requirements for low voltage distribution equipment in the NEC for North America. In other parts of the world where the term “ELR” is commonly used, I expect the requirements will vary.

Question: Can you discuss new GFI requirements?

Yoon: Not sure what is meant by “new.” There are some expanded requirements for GFCI on branch circuits in the 2023 NEC. However, they are more expansions of existing requirements rather than brand new ones. Again, they are prescriptive requirements for low voltage distribution equipment in the NEC and whether the protection is intended for personnel or equipment.

Question: Can you discuss lug ratings versus wire ratings and how they are coordinated?

Ehret: Conductor insulation determines the temperature rating of the conductor. However, the actual ampacity of the conductors may need to be adjusted due to the temperature limitations of the lugs. So, if you a 90 °C-rated conductor and a 60 °C-rated lug, you will need to derate the conductor ampacity to the associated 60 °C rating.

Yoon: You are limited by the weakest link in your distribution. So, even if you have 90 °C-rated conductors, if your equipment/terminations are rated at 75 °C, you are forced to use the 75 °C ampacity charges.

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