Grounding is often misunderstood, especially when multiple sources of power supply are present.
Does your uninterruptible power supply (UPS) system allow you to transfer load to a bypass source so you can perform maintenance on system components, totally isolate a UPS from external power sources for maintenance, replace a failed inverter without interruption of power to the load, and functionally test the static transfer switch? If you answered no to any of the above, you'd better keep reading.
To ensure quality power to critical loads, it's important to understand the basics. The fundamental components of a UPS include a charger/rectifier, battery, and inverter. The charger/rectifier converts the normal AC source that serves the UPS into DC power. The DC output of the charger/rectifier normally provides DC power to the inverter, and also charges the battery. Upon loss of normal AC input, the battery automatically discharges to provide DC power to the inverter. The inverter converts DC power to regulated and uninterrupted AC power serving the critical load.
Performance improves if you add a static transfer switch and maintenance bypass switch (Fig. 1). The static transfer switch transfers the critical load to a bypass source upon loss of power at the output of the UPS. The static switch senses the power loss and completes the transfer within 1/4 cycle; fast enough to be invisible to nearly all loads. The static switch also prevents the UPS from reaching or exceeding its overload rating, which can happen when a fault occurs at the critical load.
Proper design and installation are essential, but don't forget about maintenance. A bypass source serves several functions, the most obvious is facilitating maintenance. The maintenance bypass switch allows transfer of the critical load to a bypass source to electrically isolate the UPS equipment for maintenance, without interrupting power to the load. But be aware that while using the bypass source, you expose the critical loads to voltage transients or power interruptions from the source.
Using an isolation transformer provides some isolation of the load from the source and mitigates the passage of transients. A regulating (ferroresonant) transformer offers increased protection as an isolation transformer, but may not be suitable for all applications.
Many manufacturers offer a maintenance bypass switch internal to the UPS enclosure. However, the internal switch in bypass position leaves energized terminals inside the UPS equipment enclosure. Using an external maintenance bypass switch is a better choice if you need complete isolation of your UPS equipment. The same is true of the battery disconnecting means. You generally need total isolation of UPS equipment for complete maintenance. The externally mounted maintenance bypass switch can even allow complete replacement of UPS equipment without interruption of power to critical loads.
Schematically shown in Fig. 2, the maintenance bypass switch may either be a 2-position (2PS) or 3-position switch (3PS). Although the switching positions shown use contact closure representation, the maintenance bypass switch is typically a rotary make-before-break type switch. Note the additional "test" position of the 3-position switch. There is a different contact closure arrangement for the test position of the 3PS as compared to the 2PS. The test position allows you to functionally test the static switch without the risk of dropping the load due to static switch failure.
Grounding systems are vital
Using multiple power sources imposes special requirements on grounding design. First, you must determine if sources are separately derived or service supplied. The NEC defines a separately derived system as
"A premises wiring system whose power is derived from a battery, a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system."
On the other hand, it defines a service point as the point of connection between the facilities of the serving utility and the premises wiring.
The NEC requires bonding of the neutral to ground at a service point and where a separately derived system exists. The two one-line diagrams (Figs. 3a and 3b), involve identical equipment, except the transformer in Fig. 3a is utility-owned, and the one in Fig. 3b is user-owned. The grounding requirements of each vary based on who owns the transformer. A utility-owned transformer requires classification of the power entrance into the building as a service point. As shown in Fig. 3a, the required grounding locations of the neutral are at the transformer's secondary windings and at the service equipment. The user-owned transformer in Fig. 3b requires bonding of the neutral to ground at a single location, either at the transformer's secondary windings or at the equipment that provides disconnecting means and overcurrent protection.
Introducing an automatic transfer switch (ATS) obscures the criteria used to designate a source as separately derived. An ATS affects the designation of multiple sources differently, depending on whether or not the switch contains a switched or solid neutral. Figs. 4a and 4b show how this happens. The ATS in Fig. 4a has a switched neutral. The ATS in Fig. 4b has a solid neutral. Assume all equipment shown in both figures is user-owned. The switched neutral of the ATS in Fig. 4a creates a separately derived system. In any switch position, the neutral circuits of each transformer remain separate. In contrast, the solid neutral of the ATS in Fig. 4b permanently connects the neutrals of both transformers. This creates only one separately derived system. You should bond the neutral to ground in only one location within the separately derived system. Since the normal supply was selected as the location for the bonding jumper, it is considered the separately derived source. You should not bond the neutral to ground at the alternate supply.
Automatic transfer switches with switched neutrals are often a better choice. Assume the transformers in Figs. 4a and 4b are utility-owned. This means each power entrance into the building is classified as a service point, and the NEC requires bonding the neutral to ground at the service equipment (see Fig. 3a). Therefore, you must bond the neutral of both sources to ground. Using the ATS with the switched neutral keeps the neutrals of the two systems separate.
We consider the output of the UPS inverter a separately derived source since the inverter output isolates from the UPS charger/rectifier input, similar to how the secondary of a delta-wye transformer isolates from the transformer primary. However, since the static and maintenance bypass switches generally have solid neutrals, consideration of the UPS inverter output as a separately derived source depends on the arrangement of the bypass source. Figs. 5a and 5b explain how this works.
In Fig. 5a, the bypass source comes from a feeder breaker in the user's distribution system. Per the NEC, bonding of the neutral to ground should occur at or ahead of the first disconnecting means downstream of the secondary windings of the transformer. Therefore, you must locate a neutral to ground bond at the bypass source. Since the neutral from the user-owned transformer is continuous to the maintenance bypass switch, where it is solidly bonded to the neutral of the UPS output, there is only one separately derived system. You should only bond the neutral in a separately derived system to ground in a single location. Since NEC requires the bonding jumper in the bypass source, the output of the UPS should not have a neutral to ground bonding jumper. Thus, the source from the user-owned transformer is considered the separately derived source, and the output of the UPS is not.
In Fig. 5b, an isolation transformer is added to the bypass source, ahead of the UPS bypass switch. This transformer isolates the neutral of the user's distribution system from the neutral of the UPS output. Therefore, there are two separately derived systems. The first separately derived system extends from the user-owned transformer to the high-side of the isolation transformer. The second separately derived system extends from the low side of the isolation transformer to the UPS. You must bond the neutral of each separately derived system to ground in one location. For this system, the bonding jumper may be located either at N1 (at the isolation transformer) or at N1 at the UPS inverter. In Fig. 5b, the UPS inverter location was chosen. Therefore, the UPS inverter is considered a separately derived source and the output of the isolation transformer is not.
You can find a discussion of grounding requirements for emergency and standby power systems in Chapter 7 of IEEE Standard 446-1995.