Your questions answered: Critical power: Circuit protection

Questions not answered at the Nov. 3, 2016, circuit protection webcast are addressed here.


Tom Earp, PE, ATD, PageAlthough a building's entire electrical distribution system is important, its overcurrent protection is the most critical to safety. Overcurrent scenarios dictate the type of overcurrent protection that should be used. NFPA 70: National Electrical Code (NEC) has established basic power system overcurrent protection requirements and recognizes fuses and circuit breakers as the two of the basic types of overcurrent protective devices (OCPDs). Fuses and circuit breakers are available in a variety of sizes and ratings. Their similar-yet different-features and characteristics allow electrical system designers to choose devices appropriate for a facility's electrical system.

The Nov. 3, 2016, "Critical power: Circuit protection" Webcast presenters addressed questions not covered during the live event. The presenters:

  • Tom Earp, PE, ATD, principal/MEP engineering director, Page, Austin, Texas
  • John Yoon, PE, LEED AP ID+C, lead electrical engineer, McGuire Engineers Inc., Chicago

Question: Provide clarification on which specific pieces of equipment or systems that need to be fully coordinated per the current NEC standards?

John Yoon: Selective coordination requirements apply to all portions of an electrical distribution system that serve the following:

  • Health care (Article 517.30G)
  • Elevators (Article 620.63)
  • Fire pumps (Article 695.3)
  • Emergency systems (Article 700.28)
  • Legally required standby systems (Article 701.27)
  • Critical operations power systems (COPS) (Article 708.54).

These are systems that impact life safety for the occupants of a facility in one way or another. For COPS, it's helping ensure the safety of the public at large.

Question: How does the ambient temperature affect the rating of circuit breakers? And what's the impact of that rating on cable sizing per NEC Article 110.14(C)?

Tom Earp: Higher ambient temperatures will reduce the amount of current that thermal magnetic trip circuit breakers can carry continuously, and low ambient temperatures increase the amount of current they can carry continuously. However, this is not a change in rating. Circuit breakers listed to the UL 489 standard must be able to carry rated load in open air, 40°C ambient. This is their rating, and they do not have alternate ratings at other ambient temperatures. Solid-state trip devices are not affected by temperature variations.

Question: Do the numerical values for RK1and RK5 fuses indicate multiplier values in the time-current curves (TCC) and/or coordination curves?

Yoon: Not necessarily. Remember that RK1 and RK5 fuses are intended to be replacements for older style K1 and K5 fuses. There is still a 5X multiplier for RK fuses in clearing I2t threshold values. However, with some minor exceptions (0 to 40 amp fuses at 100 kA), that relationship is generally only valid for fault currents up to 50 kA and fuse sizes below 201 amps. That relationship is not valid for larger size fuses and other NEMA FU1-rated fault currents (100 and 200 kA).

Question: On TCCs, if you extrapolate out the wire, it will cross both the fuse and circuit breaker curves. So, at high overloads, is the wire not protected?

Earp: The region above 10 seconds on a wire's thermal damage curve is called the "emergency overload" region. Wires can carry much more than their rated current for a short period of time because the temperature of the wire will not stabilize for an hour or more. However, it is possible to damage the wire if the overload persists for a long period of time.

Question: Please define an LVPCB and differences with electronic-trip circuit breakers not considered to be LVPCBs.

Yoon: Low-voltage power circuit breakers (LVPCBs) that meet the ANSI C37 standard have two distinguishing characteristics that are pertinent to this question: they are constructed to withstand fault current for a greater amount of time than a standard UL 489 circuit breaker (30 cycles versus 3 cycles), and they have the ability to delay or defeat the instantaneous trip function of the circuit breaker. While you can find UL 489 circuit breakers that are similar in general appearance and have a two-step stored energy trip mechanism (i.e., insulated case circuit breakers, or ICCBs), they still are required by UL 489 to have an instantaneous trip function. UL 489 circuit breakers are designed to trip with no intentional delay and as such do not have to be designed to withstand rated fault current for an extended period of time.

Comment: I understand you're just showing the theory, but the transformer inrush point and damage curves were likely not manufacturer-specific and were ANSI/IEEE-typical. Using actual manufacturer data, you would likely note a standard thermal-magnetic circuit breaker would adequately protect the transformer. Attendees should not get the impression an electronic circuit breaker is required to protect all transformers, particularly small dry-type. Good presentation of the theory though.

Earp: Thank you for your comment. There are indeed some thermal magnetic breakers and fuses that adequately protect some transformers. A solid-state trip is not necessarily required.

Question: How does an electronic breaker differ from a thermal magnetic breaker or a fuse in terms of TCC tolerances?

John Yoon, PE, LEED AP ID+C, McGuire EngineersYoon: While circuit breakers with electronic trip units typically have tighter TCC tolerances than a standard thermal-mag breaker, it depends on what portion of the time-current curve is being evaluated. Most generic thermal-mag circuit breakers have reasonable ±10% tolerances in the overload/long time portion of the TCC. However, in the short time portion of the TCC, the area of uncertainty in the curve gets bigger as the mag portion of the breaker takes over.

For fuses, the minimum and maximum melt times usually are very consistent from the long-time to short-time portions of the TCC.

Of course, these are generic blanket statements. Your mileage may vary and you should get TCC data from the manufacturer for any particular overcurrent protection device to make a proper evaluation.

Question: What part of NEC Article 695.3 requires selective coordination?

Earp: Article 695 (fire pumps) requires selective coordination on multibuilding, campus-style complexes where the fire pump is connected to one or more feeder sources with more than one overcurrent device in series.

Question: Please provide examples of how to use fuse product sheets to determine fault current limiting.

Yoon: In addition to the TCC, you will also find "current limitation curves" on most manufacturers' fuse data sheets. The method used with these current limitation curves is called "up, over, and down." You should be able to find detailed information on this technique in the FAQs on the manufacturer's website.

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