Your questions answered: Circuit protection in health care facilities
Dwayne Miller, PE, RCDD, CEO, JBA Consulting Engineers, Las Vegas, and Danna Jensen, PE, LEED AP BD+C, Vice President, WSP + ccrd, Dallas, present additional information about circuit protection in health care facilities. Engineers must understand and apply circuit protection best practices—especially when designing electrical systems in hospitals and health care facilities.
The Nov. 12 "Critical power: Circuit protection in health care facilities" webcast presenters addressed questions not covered during the live event.
Question: Is selective coordination required for the normal branch that serve critical/life
safety branch ATSs?
Dwayne Miller: Yes, selective coordination is required for both normal and emergency feeders that serve the ATSs. Refer to NFPA 70: National Electrical Code (NEC) Articles 700.27 and 701.27 for coordination. The emergency system overcurrent protective devices (OCPDs) must coordinate with all supply-side overcurrent devices per the NEC, hence, the normal OCPDs upstream of transfer switch must be coordinated with automatic transfer switch (ATS) load side devices. Remember, NEC Article 700 applies to Article 517 facilities, except where specifically amended.
Q: What is your opinion about using high-resistance grounding of the neutral?
Danna Jensen: It is a great application for medium-voltage systems, but not very common on the 480-V level in hospital distribution systems.
Q: Do you know where I can find the requirements for testing an emergency power system (both startup testing and annual testing)?
Miller: NEC Article 700.3 covers this and also references NFPA 110: Standard for Emergency and Standby Power Systems; and NFPA 99: Health Care Facilities Code. Both NFPA 99 and NFPA 110 address the requirements.
Q: Is ground-fault interrupting (GFI) overcurrent protection required on any of the three branches, or just GFCI indicating but not tripping?
Jensen: GFI is required on any branch circuit per NEC, where it states within 6 ft of a sink, in a kitchen, etc., regardless of which emergency branch it is on. GFI protection is NOT required or permitted on the emergency branch (life safety, critical, or equipment) from a feeder perspective.
Q: Define NEC, NFPA code relationships.
Miller: NEC is included in the body of the NFPA codes and standards. For electrical design, the NEC is the primary code I start with and then I reference other NFPA documents pertinent to the type of project I’m working on. NFPA 101: Life Safety Code, NFPA 110, NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection, NFPA 72: National Fire Alarm and Signaling Code, NFPA 99, and others are referenced in the NEC and should be relied upon to ensure the design complies with applicable standards and codes. The NEC is compulsory as are other codes within NFPA, whereas the standards serve more as a guideline, unless adopted by regulating bodies/authorities. Having said that, compliance with both the codes and standards is recommended. The NEC and other NFPA codes and standards are complimentary documents and support each other. A key issue is to find out which edition (year) has been adopted and what amendments have been published by the local regulating bodies. Sometimes, local NEC amendments may be in conflict with other NFPA documents and the local authority having jurisdiction (AHJ) has the final say.
Q: What is the latest decision on 0.1 sec for coordination?
Jensen: NEC Article 700.28 requires "full range" coordination as selected by a licensed professional engineer. This is often interpreted to mean down to the 0.01 sec instantaneous range. NEC Article 517 and NFPA 99, however, amended this to state that coordination for a period of time that a fault’s duration extends beyond only 0.1 sec. However, it is still ultimately the local AHJ’s decision, so be sure to check for any amendments based on locality.
Q: Discuss selecting 3-pole or 4-pole ATSs.
Miller: It depends on the application. I’m a proponent of setting generator systems up as a separately derived system. In my experience, it simplifies grounding and reduces errors for both the installation and future changes to the system.
This results in the load referencing the service neutral-to-ground bond when on utility, and transferring to the generator system neutral-to-ground bond when on the generators. Then, generally speaking, you have 4-pole transfer switches for the portion of the system serving both 3-wire and single-phase loads, and 3-pole transfer switches for 3-wire loads (3-phase loads, no line-to-neutral loads). Practical examples would be a motor control center (MCC) served from a transfer switch. I would use a 3-pole transfer switch to serve the MCC. If I have single-phase loads in the same area that I need to serve, I can either specify a small transformer within the MCC to create a separately derived system for single-phase loads, or I can serve the loads from a nearby 4-wire source. In addition, 4-wire loads use a 4-pole transfer switch so the neutral switches during transfer. Overlapping contacts in the 4-pole switch is my personal preference to reduce concern with voltage instability during the transfer when both sources are available.
The other option is to not bond the generator and then the system always references the service neutral-ground bond. My preference is to stay away from this arrangement unless there are circumstances that make it a prudent approach. In this arrangement, 3-pole transfer switches with solid neutral can be used. However, ground fault protection (GFP) can become problematic. See NEC Handbook 700.5 commentary.
Q: Any thoughts on the ground fault pick-up (GFPU) setting for emergency branch circuit, particularly for branch circuits downstream of 2 tier system?
Jensen: When you have two levels of ground fault (on your normal power system, because the GFPU is not permitted on the emergency side), the safest setting is to set the breaker farthest downstream at its lowest setting. Then adjust the next breaker up in the distribution system so that it selectively coordinates with the one you just set. In other words, provide just a sliver of white between the curves with no overlap. This will provide the safest system.
Q: If I have a generator that is a separately derived system, should the generator ground be bonded to the main service equipment ground electrode system?
Miller: The grounding system (grounding electrode) will be bonded through raceway, enclosures, etc. The emergency grounded (neutral) conductor should not be bonded to the service grounded (neutral). If you use 3-pole SN transfer switches in a system with a bond at the service and at the generator, the systems reference two bonds and GFP becomes an issue. But 4-pole transfer switches transfer the neutral-to-ground bond reference for the system.
Q: Is it less effective grounding to use 3-pole transfer switches with 4-wire generators?
Jensen: It is not necessarily less effective to use 3-pole transfer switches with a 4-wire generator, but with a 3-pole system, you could see nuisance tripping of your ground fault protection. Say for instance, there is a ground fault at a point on the emergency system while in operation. The current will need to find a way back to the source (the generator in this scenario). Its only option is to flow along the ground, but the ATS has a neutral/ground bond (no switched neutral), so it can also flow back to the service. The ground fault current will flow along the neutral to the utility and may cause the fault to be incorrectly sensed, or trip the ground fault on the normal power side. It is recommended to always use 4-pole switches when your system requires ground fault protection.
Q: In paralleling switchgear, does the NEC require the life safety, critical, and equipment distribution breakers to be in three different sections, or can you stack these breakers?
Miller: The devices need to be in separate isolated sections to comply with NEC-2011 Article 700.10(5). Also refer to Exhibits 700.3, 700.4, and 700.5 in the 2011-NEC Handbook. NEC Article 700 applies to NEC Article 517 systems, except where specifically amended in 517, so separation and isolation of the devices is required in the paralleling switchgear.