Selective coordination: Your questions answered

NFPA 70: National Electrical Code defines selective coordination. Questions not addressed in the Critical Power: Selective Coordination webcast are answered here by Syed Peeran, PE, PhD, CDM Smith, and Rick Reyburn, PE, JBA Consulting Engineers.


Syed Peeran, PE, PhD, CDM Smith (left); and Rick Reyburn, PE, JBA Consulting EngineersQuestion: Who should do the coordination study, the consultant or the manufacturer?

Rick Reyburn: It is my opinion that the specifying consultant (the engineer of record) must perform the selective coordination study to confirm the equipment specified selectively coordinates. If the specified equipment is used for the project and not substituted, then all is good. If the contractor chooses an alternate manufacturer of that specified, the contractor shall provide a sealed and signed (by another engineer of record who is taking responsible charge for the alternate equipment being submitted) submittal to both the original engineer of record and the authority having jurisdiction (AHJ) for review and acceptance. The original engineer of record would then not be responsible for the alternate equipment/settings.

Question: How are engineering firms handling the need for a multiple manufacturer specification when designing a selectively coordinated system of overcurrent protective devices?

Rick Reyburn: See the response to the question about who should do the coordination study (above). In addition, the specifying engineer may list alternate manufacturers that are acceptable substitutions as long as they provide, as a part of the normal submittal process, a sealed and signed selective coordination study using the alternate equipment.

Question: What is implied by the word "selective" in this topic? Is there a difference between "coordination" and "selective coordination”?

Syed Peeran: It is not clear why NFPA 70: National Electrical Code (NEC) has used the term “Selective Coordination.” In Article 100 – Definitions, the word “selective” is used in parenthesis after the word “coordination.” This implies that there can be other types of coordination. However, in Article 700.28, the term “selective coordination” is used in its entirety. This article also states that “Selective coordination shall be selected by a licensed professional engineer or other qualified personnel ….” I think what this implies is that the processes of selective coordination has two parts. The first is the selection of the protective devices. For example two molded case breakers in one radial circuit may not be the best choice for providing satisfactory coordination. One time delay fuse and a molded case breaker may be the best combination. This selection must be made by a qualified person. The second part is the coordination of the selected devices by examining their time-current curves (TCCs) and adjusting them.

Question: For renovations/upgrades, please cover the interface between selective coordination, available fault current, and arc flash due to equipment changes, configuration changes, and reuse of existing protection devices.

Syed Peeran: During renovation or upgrade, either new equipment is added to the existing equipment or some existing equipment is replaced by new equipment. In both cases we must identify all the protective devices (new and existing) in the path of the fault current for all faults downstream of the new devices. We then must perform a short-circuit study followed by protective device coordination. Some of the existing devices may need to be reset to coordinate with the new devices. An arc flash study is then run to determine the arc flash hazard. If arc flash hazard mitigation is required then the coordination study is revisited to see if it is possible to reduce the hazard. If not the other means of reducing the hazards such as maintenance switch, etc. must be looked at.

Question: Please highlight a few affects of not providing coordination.

Rick Reyburn: No. 1: Violation of the law, arrest, prosecution, fines and jail time.

No. 2: The possibility of the loss of a complete emergency system due to a fault on a branch circuit.

No. 3: The failure of a fire pump system, which is the single most important piece of equipment in the entire building when it comes to the protection of life for humans.

No. 4: A maintenance personnel slips when re-torqueing a conductor termination in a powered condition. Assuming the maintenance personnel has been given the assumed personal protective equipment, because the coordination wasn’t performed the available fault at the point of fault is much higher than the expected and the maintenance personnel is critically burned because of the un-selectively coordinated system.

Question: Please explain the differences in coordination requirements between health care and other types of facilities per NFPA 99 and NFPA 70.

Rick Reyburn: In the past, prior to the adoption of selective coordination in Articles 700 and 701 of the NEC, selective coordination was required in NFPA 99 (NEC 517) systems for ground fault protection for a minimum of 6 cycles. That has since been removed, and NEC 700/701 are now recognized by both NFPA 99 and NEC 517 as requiring full compliance with selective coordination for these systems. The only application which adds to these requirements is the two levels of ground fault protection for NEC 517/NFPA 99.

Question: How do we use the up, over, and down method of current limiting fuses in coordination and arc flash calculations?

Syed Peeran: The same way as we use it for bolted fault current. Remember, however, that the magnitude of the arcing fault current is much lower than the bolted fault current. Arc flash study gives you the arcing fault current magnitude. You need to select a current-limiting fuse such that the calculated arcing fault current is in the current limiting range of the fuse.

Question: What happens when two LTD sections of the curves for two circuit breakers (one immediately upstream of the other) are overlapping?

Syed Peeran: The LTD sections begin right after the LTPU sections (the uppermost vertical sections) and end at the STPU vertical section. Therefore, if the fault current is between the LTPU and STPU settings, both breakers will trip at the same time if the curves overlap. It is not likely that the fault current is in this region. Nevertheless, one must avoid this situation.

Question: In system studies, which should be conducted first: arc flash analysis or coordination of protective devices? And, in case of conflict what would be recommended practice, focus on mitigating arc fault current or obtain coordinated system?

Syed Peeran: Actually, the studies must be performed in the following sequence: Short-circuit, coordination, arc flash. In case of conflict, focus should be on arc flash if there are many areas where the personal protective equipment (PPE) is dangerous. If the maximum PPE is 2 or even 3, no mitigation or repetition of coordination study is required unless the client want reduction of the PPE to 2 or below.

Question: Where did the time data come from on the CTI chart?

Syed Peeran: IEEE Standard 242-2001: Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems Table 15-3.

Question: On the low-voltage system in the emergency system where panelboards are made of molded case circuit breakers, how do you meet the coordination required by NEC?

Rick Reyburn: There may be alternate methods, however the method I use is to provide fused coordination panelboards.

Question: Are static trip devices the same as solid state trip units?

Syed Peeran: Yes.

Question: How do you coordinate surge arresters, MOV or TVSS?

Syed Peeran: This coordination is known as insulation coordination. This is important in high-voltage (HV) substation design. Instead of the time-current curves, we look at the impulse characteristic of the protective devices and the BIL of the equipment. The protective devices are the surge arresters, expulsion tubes, low-voltage (LV) protective devices such as TVSS and MOVs. The last two are used in LV switchgear. MOVs are generally used in electronic equipment such as rectifiers, VFDs and others to protect the power electronic devices. The MOVs and TVSS can be applied almost independently without any need for coordination. The only thing we need to look for is the energy rating of the two. The TVSSs, which are normally connected to the LV bus in the switchgear, must have an energy rating of more than that of the MOVs. The electronic equipment manufacturer selects the MOVs while the TVSSs are selected by the consultant. The high-voltage surge arresters, which are outdoor-mounted, need coordination. This is a vast subject itself. Please read Chapter 18 of the T&D Handbook published by Westinghouse/ABB.   

Question: Can you speak to selection of emergency generator main breaker or main emergency bus breaker with respect to selective coordination? This should be done early in the design in my experience, to avoid problems or having to upsize the generator alternator.

Rick Reyburn: Agreed. The selection of this breaker is very important. When used to serve downstream overcurrent protection and transfer switches, the adjustability and full flexibility of this breaker is needed.

Question: Are automatic transfer switches rated for 30 cycles a necessary requirement?

Rick Reyburn: No. As a matter of fact the use of 30 cycle transfer switches pose a dilemma when in the instantaneous region of the TCC. The quantity of fault current allowed to flow for ½ second to the downstream equipment could potentially cause a component/system to see higher fault currents than what they are listed for.

Question: How bad of an assumption is it to use infinite bus model for utility when information cannot be obtained?

Syed Peeran: Only in two cases this assumption will create problems. The first is harmonic analysis. Assuming infinite bus gives us very optimistic harmonic results. Therefore, in harmonic analysis, one should use a realistic short circuit MVA (SCMVA) instead of the infinite bus. The second case is the arc flash study. The results are questionable. However, in many instances, the arc flash study gives conservative results. In other words infinite bus may be OK.

Question: Should the kAIC rating of a circuit breaker always cascade downstream from the main circuit breaker or is it allowable to increase back up at some point?

Syed Peeran: In all radial systems, the kAIC rating should reduce as we go further downstream from the main circuit breaker.

Question: Do you believe life safety coordination is best achieved with fuse selection in lieu of circuit breaker selection?

Rick Reyburn: Not necessarily. While fuses sometimes are easier to selectively coordinate since the width of the characteristic TCC for fuses is much narrower than circuit breakers the use of adjustable trip fuses allows for flexibility in the system if changes are made. I will say, however, that when below 0.1 seconds fuses are superior to circuit breakers.

Question: When was selective coordination first introduced to the National Electrical Code?

Rick Reyburn: Selective coordination was first introduced in the 1993 NEC with regards to multiple elevators. The change was proposed by Ed Lawry and it was also mentioned it was for correlation with the Canadian Electrical Code.

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