Your questions answered: Circuit protection in electrical/power systems
Questions left unanswered during the Sept. 19, 2019, critical power webcast are answered here
During the live webcast Critical power: Circuit protection in electrical/power systems, the presenters didn’t answer every question. More detail on the topic is here.
- Danna Jensen, PE, LEED AP BD+C, Principal, Certus Consulting Engineers
- Rick Reyburn, PE, Executive Director of Engineering, NV5
Question: What is a 100% rated circuit breaker and how is it used?
Jensen: The standard rating of a breaker is 80% of its rated value at a continuous load. The term “continuous load” is key here where NFPA 70: National Electrical Code defined continuous load as operating more than three hours. For instance, a 100-amp standard rated breaker is only rated to continuously handle load up to 80 amps. In contrast, a 100% rated breaker is rated to handle the full load rating (or the full 100 amps in the previous example) continuously or longer than three hours.
Question: Can the maintenance mode arc reduction method also account to reduce the incident energy when defining the required the personal protective equipment?
Reyburn: No, it should not be utilized. Since the maintenance switch may not be engaged due to lack of the facility maintenance personnel, the hazards are still present and therefore, the level of PPE should be required. It could be argued that with an active safety and training program the level of PPE could be reduced during a maintenance procedure, as a specifying engineer I want the safest environment for personnel.
Question: Please provide a code reference or discussion of primary overcurrent transformer protection without secondary main protection?
Reyburn: As you are aware, there are specific instances when primary overcurrent protection of a transformer without secondary overcurrent protection may be utilized. Please refer to NEC Table 450.3(A) for these applications.
Question: What is the difference between ground-fault circuit interrupter and ground-fault protection of equipment, and how do they protect against various faults?
Jensen: GFCI and GFPE are very similar in that they both sense ground faults and protect the circuit by opening a breaker or contacts. The devices use current transformers to continually monitor the imbalance of current between the phase wire and neutral. If everything is operating properly, the current both entering and leaving the device will be the same (minus some minor losses).
The difference between the two is that in general, GFCI devices are end use devices such as a receptacle and are specifically put in place to protect personnel. The code requires that a GFCI device trips if the current transformer senses an imbalance that is between 4 to 6 miliamps.
GFPE, on the other hand, are primarily located inside of panels at the breaker level and put in place to protect equipment. The code requires that these devices trip at a slightly higher threshold of 30 miliamps. Examples include a heat trace circuit on a water pipe. There are GFCI circuit breakers too, which could be installed as opposed to a receptacle, so just be sure you are getting the right listing for the correct application.
Question: What are the recommended methods for arc flash protection?
Jensen: The code gives specifics on what will meet the requirement for arc flash protection, which include things like zone selective interlocking, adjustable instantaneous trips or a manual maintenance switch. It also says “other approved means,” leaving it open for engineers and manufacturers to continue to design better ways for a facility to be safer. What system you put in your designs is largely dependent on budget. Certainly, anything that is automatic, such as differential relaying or ZSI will lessen the human error factor, but these types of systems are more expensive.
If you do chose to put in a manual means such as a maintenance switch to save initial cost, I would also highly recommend that that switch is monitored by some type of monitoring system whether it be through the building management system or even a local alarm that will alert the operator if it is left in maintenance mode too long and remind them to switch it back. Otherwise, you could be completely negating any selective coordination that has been designed into the system and vulnerable for outages.
Question: Is the arc energy reduction another terminology to arc-flash resistant?
Reyburn: No, arc-flash resistant is how the product is constructed. This type of product will divert the energy created by the arc to a safe location such as up and out of the building. Arc energy reduction is to reduce the arc energy (see above).
Question: How do you clear a fault in order to avoid closing on it?
Reyburn: The OCPD closest to the fault should be the device that opens and clears the fault. In the case where the fault is directly after a transfer switch, it will not be cleared by both sources of power until both nonsimultaneously close into it.
Question: Isn’t ground fault a sub-category of short circuit?
Jensen: Yes. Ground faults and arc faults are both types of short circuits.
Question: NFPA 130 requires egress lighting to be a one-hour rated circuit protective assembly. How does the panel recommend to accomplish this with limited choices available?
Reyburn: Use one-hour circuit protective assemblies complying with UL product categories FHIT, FHIY and FHJR in accordance with UL 2196: Standard for Fire Test for Circuit Integrity of Fire-Resistive Power, Instrumentation, Control and Data Cables.
Question: Why the code limits a ground fault protection of a feeder or service to 1,200 amps? Why not lesser value?
Jensen: Code requires ground fault protection at the service for services that are more than 150 volts to ground and not exceeding 100 volts phase–to–phase for each service disconnect rated 1,000 amps and greater and it stipulates that the maximum setting of ground-fault protection is 1,200 amps. It is important to notes that codes are minimums, so there is nothing wrong with designing a system with a lower setting. But note that settings at low levels increase the likelihood of unwanted tripping of the device or nuisance tripping. The code council included this requirement to make systems safer, but set a limit at 1,200 amps as opposed to lower to help the engineer use the proper judgment on what is right for their system without the added nuisance tripping that could be a side–effect of settings at lower levels.
Question: Was there legal liability for the engineer of record in the case of the 40-year-old hospital?
Jensen: NEC 517.17(B) now states “the additional levels of ground-fault protection shall not be installed on the load side of an essential electrical system transfer switch.” However, it did not always state that. In the case of the facility that was designed in the early 1960s, GFPE was not even yet required by the NEC, therefore the engineer of record from that time was not liable for any code violations since they did not exist.
Question: It should say 100-amp overcurrent protection device, not 10-amp.
Reyburn: You are correct. A typographical error.
Question: What is a 100% rated circuit breaker and how is it used?
Reyburn: The standard rating of a breaker is 80% of its rated value at a continuous load. The term “continuous load” is key here where the NEC defined continuous load as operating more than three hours. For instance, a 100-amp standard rated breaker is only rated to continuously handle load up to 80-amps. In contrast, a 100% rated breaker is rated to handle the full load rating (or the full 100-amps in the previous example) continuously or longer than three hours.
Question: You stated that an energy-reducing maintenance switch lower the trip setting. That’s not strictly true. When activated, some manufacturers’ products replace the normal trip curve with the instantaneous-only function at whatever its pickup is set at. If it is inadvertently set higher than the normal trip curve, that will still be the trip function. In other words, if not set correctly the maintenance mode device can result in increased arc energy.
Reyburn: Understood. The example and discussion was geared more toward an example of how it is done with some manufacturer’s equipment and with the time limitations of the discussion was not geared toward all manufacturers. The intent is the same and requires an engineer’s interface to oversee the settings that are being used to minimize arc flash energy during maintenance, i.e., a “maintenance switch.”
Question: Can a generator be installed without ground fault?
Jensen: The load side of a transfer switch (which would be downstream of a generator) should not be provided with any sort of ground fault trip (NEC 517.17(B)). Rather, a better option is to provide a ground fault alarm where it would notify the facility engineer there is a ground fault on the system, but not trip any devices.
Question: For GFCI personal protection, for loads within approximately 40 feet of a panel what is a preferred method of protection of a refrigeration unit located in a kitchen? Utilizing a wiring device or a breaker? Why do GFCI breakers tend to be more sensitive than wiring devices?
Reyburn: There are several methods of providing GFCI protection for this application. A simple, readily GFCI receptacle would suffice. An inline GFCI module in conjunction with a standard receptacle could be used where the module is ready accessible and the receptacle is accessible. A GFCI rated circuit breaker could be used. All GFCI devices are tested in accordance with UL standards for compliance of current and time characteristics. The method utilized should not provide different results.
Question: What have you found to be the most cost-effective means to accomplish arc energy reduction?
Reyburn: My experience has been to provide the maintenance switch as the most cost-effective method, short of turning the equipment off during service.