Your questions answered: How to design for selective coordination in mission critical systems
Experts Josh Fluecke and Radames Cocco expand more upon mission critical power systems
Selective coordination insights
- Mission critical facilities, such as hospitals and data centers, demand meticulous electrical engineering to ensure reliability, safety and redundancy, with a focus on selective coordination for overcurrent protection according to codes and best practices.
- Designing overcurrent protection for mission critical facilities involves extensive planning to ensure the timely operation of protective devices, adhering to codes and standards while prioritizing life safety through selective coordination risk assessments and practical implementation strategies.
Mission critical facilities typically require electrical engineers to design and specify significant amounts of power to increase reliability, improve life safety, limit outages and provide for redundancy. Common examples of mission critical facilities include hospitals, laboratories, security operations centers and data centers.
The webcast on Jan. 31, 2024, left several questions unanswered. Read more here for the responses.
Subject matter experts:
- Radames Cocco, PE, LEED AP, Electrical Engineering Leader | Principal, DLR Group
- Joshua Fluecke, PE, LEED AP, Senior Principal, Syska Hennessy Group
What about the NFPA 70: National Electrical Code (NEC) 517.31(G) provision of 0.1 seconds for medical facilities?
Radames Cocco: Article 517.31(G) does limit the requirement for selective coordination of overcurrent devices to all periods of time when the fault duration exceeds 0.1 seconds. So, coordination below 0.1 seconds is not required in health care facilities.
Do you consider internal UPS breakers in the coordination study?
Joshua Fluecke: Internal uninterruptible power supply (UPS) breakers are intended to protect only the UPS module. Therefore, these internal devices do not need to be considered as part of electrical installation and coordinated with other protective devices in electrical system.
Will you speak to coordinating multiple levels of ground-fault protection?
Radames Cocco: Where multiple levels of ground fault protection are required (e.g., health care facilities), they are required to be coordinated. This is also good practice when additional ground fault levels may not be required, but are provided.
Do you have any recommendations for coordinating with an inverter source with fast acting trip? Would this be considered an unacceptable source for emergency power?
Radames Cocco: I have found luck employing fusible solutions downstream of an inverter. However, typically find we need to limit the number of device levels downstream
What about of using fuses for selective coordination?
Radames Cocco: Using fuses are a simple and clean approach to achieve selective coordination.
Please discuss coordination, especially in the instantaneous areas of the time current curves (TCC) and the NEC requirements
Radames Cocco: The NEC defines selective coordination as applying to the full range of operating times associated with the devices and fault values seen. This means it extends beyond 0.1 second. I have found circuit breaker-based systems, because of their responses in the instantaneous region, can be difficult and limiting to achieve the requisite coordination. One needs to look closely at the manufacturer charts and associated notes.
To what level of selective coordination (0.1 or 0.01 second) would you provide for data centers that fall under NEC 702?
Joshua Fluecke: Article 702 is for optional standby systems, which primarily provide the backup power supply for data centers and there is no prescriptive Code requirement for coordination during the duration of a fault. For these optional systems, the electrical engineer has complete latitude to determine the appropriate coordination of the electrical system during fault durations to influence system behavior during the fault.
For optional systems, will coordinating only faults longer than 0.1 second like health care facilities provide adequate coordination to reduce downtime over selective coordination as required for NEC Article 700?
Radames Cocco: Everything is a trade-off. Limiting coordination to 0.1 seconds, as permitted by NEC, can result in lack of coordination in the instantaneous region. However, it also does allow the use of easily resettable breakers and may also provide lower arc flash energy values (less damage from the fault).
Would you include fuse that is inside of medium-voltage (MV) transformers?
Radames Cocco: Yes, to assure coordination with downstream devices, which may be more easily replaced or reset.
Getting selectivity tables from a manufacturer sure seems to be a hassle. Is there a recommended place to look for these, maybe somewhere in SKM PowerTools or another resource?
Radames Cocco: Not that I am aware of. Recommend getting the charts from the manufacturer to assure you have the latest information.
So, with a large enough fault, selective coordination is rendered ineffective?
Radames Cocco: It depends on the overcurrent devices employed. Breakers may have overlap in the instantaneous region. But fuses tend to have thinner, more defined curves, which simplify coordination.
What is the easy way to get the selective coordination? Using fuses or breakers?
Radames Cocco: Because they have thin, defined curves, extending into the instantaneous region, I find fuses are easier to selectively coordinate. However, one needs to be mindful that fuses do have to be replaced after an event. This takes more time than simply resetting a breaker.
Do all health care facilities require two levels of ground fault protection? I believe it is just those with Category 1 and 2 patient care spaces (critical care).
Radames Cocco: Correct. NEC Article 517.17 requires ground fault in systems supplying Category 1 spaces.
How many cycles withstand for an automatic transfer switch (ATS) would generally allow coordination, if 3 cycles will not be enough?
Radames Cocco: This is dependent on the time current curves of upstream overcurrent devices and levels of such devices. Can be 6 cycles, 18 or more.
Why is a health care facility with selectivity required beyond 0.1 seconds (ignoring the instantaneous response), but a commercial building with an emergency generator requires a much more stringent level of all current and all times? Will the NEC ever fix this discrepancy?
Radames Cocco: I understand the point. And, it does seem counterintuitive. The comment from the code panels at the time the limit was adopted was to correlate with NFPA 99: Health Care Facilities Code.
Regarding ATS withstand ratings, it should be noted it is likely that available fault current will differ on the normal versus emergency source sides. Any advice/best practices you can recommend there?
Radames Cocco: The designer needs to look at both sides and select the ATS with ratings to survive the worst case.
For ATS units, what is the key understanding for 3-cycle versus 30-cycle?
Radames Cocco: The longer duration that the ATS is submitted to the destructive energy of a fault, the much more robust it must be. That means stronger contacts, bracing, etc. A 100-ampere (A) ATS may need to be 800 A just to attain this rating.
For low amperage situations, are there any specific design practices that can be employed? For example, I have a 100 A emergency panel that feeds a 50 A sub-panel but the breaker curves are too close to coordinate. This comes up for emergency panels that only service fire alarm/emergency lighting,
Radames Cocco: You sometimes must oversize the distribution to maintain overcurrent device ratios as you work your way upstream. Consider smaller branch circuits to keep each step up the distribution overcurrent devices smaller. Also, solutions employing fusible coordination panels simplify the approach,
Withstand current is referring to asymmetrical or symmetrical current?
Joshua Fluecke: NEC defines short-circuit current rating as the prospective symmetrical fault current at nominal Voltage to which an apparatus or system can be connected without sustaining damage exceeding defined acceptable criteria.
When health care facility is on 100%, 10 second backup power, do we still need minimum three branches.
Radames Cocco: Yes. Putting everything on the generator does not negate the requirements of separate branches.
If the whole building electrical service is generator power backed, does NEC 700 and 701, 702 still apply?
Radames Cocco: Yes. Putting everything on the generator does not negate the requirements of separate branches.
I have seen arc flash studies with high incident energies and coordination studies that are not coordinated after the design is complete. What is a good balance to between the engineering design and account for short circuit and arc flash results?
Radames Cocco: Where required by code, the system must be selectively coordinated. For optional standby, this is a conversation to be had with the owner. One needs to help the owner understand the trade-offs for full selectivity.
Would it be better to SKM or ETAP or Easy Power for preparing a power study?
Radames Cocco: I think this is more of a personal preference.
Doesn’t the generator manufacturer provide a generator circuit breaker that protects the generator alternator?
Radames Cocco: The breaker supplied with a generator does not necessarily protect the generator. One needs to look at the generator breaker as well as the alternator damage curves. For some units, the protection is provided by the onset controller. Contact your specific manufacturer for information on how they achieve the requisite protection.
Do you have any recommendations on coordinating breakers downstream of emergency lighting inverters. With the limited short circuit current available from the inverter breaker selective coordination is very challenging.
Radames Cocco: I find the use of fuse-based solutions simplify this process
Is the code open to the end user defining the time level for the selective coordination for the project? Or is it always all times for all sources regardless
Radames Cocco: The definitions in NEC Article 100 are clear regarding the definition. The other articles for emergency systems, legally required, etc. set the requirements. For normal or optional standby systems, it would be a determination of the engineer in consultation with the owner.
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