If on-site power is your sole source of power and you follow the NEC Art. 250, then grounding is straight forward. But, if it serves an alternate source, then grounding becomes much more complex.
Be careful when grounding on-site power systems: Simply conforming to minimum NEC requirements won't necessarily assure the required degree of reliability. You must consider several factors, including protecting against power disruption within the building or facility and providing adequate ground-fault protection.
Basically, you must use techniques for both equipment and system grounding that provide optimum safety while assuring maximum continuity of power to essential loads. This includes proper grounding and ground-fault sensing when the transfer switch is in the emergency position as well as the normal position. In most applications requiring ground-fault sensing, area protection, multiple transfer switches, or multiple engine gensets, you should consider the on-site power as a separately derived system. In such cases, you ground the neutral of the engine genset at its location.
Grounding particulars. You should permanently bond together engine generator frames, automatic transfer switch enclosures, conduits, and other exposed components of an emergency power system to the grounded conductor of the service equipment providing normal power and/or the grounded conductor of the genset. Connect these grounded conductors to grounding electrodes by means of grounding electrode conductors.
The NEC permits the grounding electrode to be a metal underground water pipe. However, don't use buried portions of a water pipe system less than 10 ft in length or one that includes plastic or cement materials. For small emergency power systems where the ground currents are of relatively low magnitude, designers prefer existing buried water pipe systems as electrodes because they're economical. Nevertheless, before relying on any existing electrodes, measure their resistance to earth.
Larger emergency power systems in industrial and commercial buildings often use the grounded metal frame of the building and concrete-encased metal below ground level as a grounding electrode. The concrete encasement of steel, along with contributing to low-grounding resistance, provides some corrosion protection compared to the earth's contact. Consider the ampacity of the equipment-grounding path and verify it can handle the higher available ground-fault current.
Another alternative is to use "made" electrodes such as driven ground rods, buried cables, and plates. The type you select depends on the soil and available depth. Although the NEC permits up to 25 ohms resistance to ground for "made" electrodes, keep grounding electrode resistance for gensets in industrial/commercial installations below 5 ohms. Where you have the building steel bonded to the ground grid as well as most pipe lines, it's not unusual to get an impedance as low as 0.1 to 0.2 ohms. Although Code requires only one electrode, sometimes you need more than one.
Meeting NEC requirements for transfer switching. You can use three approaches in meeting current NEC requirements. For a 3-phase, 4-wire grounded system, you should:
• Use a 3-pole transfer switch when considering the genset as a non-separately derived source;
• Use a 4-pole transfer switch when considering the genset as a separately derived source; and
• Use a 3-pole transfer switch with overlapping neutral contacts when considering the genset as a separately derived source.
However, just meeting the minimal Code requirements doesn't necessarily provide the reliability you need for a good emergency power system. Consider the following design practices and their results.
Designers often locate a genset remotely from the grounded utility service entrance. Thus, the ground potentials of the two locations may not be the same. Designers should locate the automatic transfer switch as close to the load as possible for maximum protection against power failure. The result? The distance of cable between incoming service and the transfer switch and then to the genset may be substantial.
Consider what would happen if a cable failed, with the engine generator not grounded to its own grounding electrode. The load automatically transfers to an ungrounded emergency power system. This can jeopardize emergency service continuity and lead to additional failures. You may be unable to detect concurrent failure of equipment or cable failure after transfer to emergency.
Some local codes require ground-fault protection while the engine generator is in operation. This may present a sensing problem if you don't provide proper isolation of neutrals or connect the neutral conductor of the generator to a grounding electrode at the generator site.
When the transfer switch is in the emergency position, other problems may occur if you haven't properly grounded the genset. For example, a ground-fault condition can cause nuisance tripping of a normal source ground-fault circuit breaker, even though load current is not flowing through the breaker.
The normal and emergency neutral conductors are simultaneously vulnerable to the same ground-fault current. Thus, a single fault could jeopardize power to critical loads, even with utility and emergency powers available. This may violate codes requiring independent wiring and separate emergency feeders.
Three-pole transfer switching. Using 3-pole transfer switches is the least expensive approach and requires less space. Yet, ground-fault currents may trip the normal source breaker when the transfer switch connects the load to the genset. Using a 3-pole transfer switch doesn't provide area protection within the facility. Finally, with the neutral conductor connected to the incoming normal service, it's difficult to provide ground-fault sensing on the emergency side to meet Sec. 700-7(d).
Four-pole transfer switching. A better solution is to use a 4-pole transfer switch, which provides isolation of service and generator neutral conductors. This eliminates improper ground-fault sensing and nuisance tripping caused by multiple neutral-to-ground connections. Here, the generator complies with the NEC definition of a separately derived system. With the neutrals thus isolated, you can add ground-fault protection with conventional sensors to the generator output.
Overlapping neutral contacts. Another method for isolating the normal and emergency source neutrals is for a 3-pole automatic transfer switch to include overlapping neutral transfer contacts. This provides isolation between neutrals. At the same time, it minimizes abnormal switching voltages. By this means, the only time the neutrals of the normal and emergency power sources connect together is during transfer and retransfer. With a solenoid-operated conventional double-throw transfer switch, this duration can be less than the operating time of the ground-fault sensor, which is usually anywhere from six to 24 cycles (100 milliseconds to 400 milliseconds).
Multiple transfer switches. Designers often use multiple transfer switches close to the loads, rather than one transfer switch for the entire load. In such cases, consider the possibility of equipment failure between the service equipment and transfer switches, which can cause an emergency system to become ungrounded. This is important if a solidly interconnected neutral conductor is grounded only at the service equipment. With this, there's a possibility of tripping the ground-fault circuits when no ground fault exists.
Multiple gensets. With multiple gensets connected in parallel, serving as a common source of power, you should connect each generator neutral to a common neutral bus within the paralleling switchgear; which, in turn, is grounded. Locate the associated switchgear containing the neutral bus in the vicinity of the gensets. One system-grounding conductor between the neutral bus and ground simplifies the addition of ground-fault sensing equipment.
You can argue by using individual grounding resistors, circulation of harmonic currents between paralleled generators isn't a problem. If you suppress third harmonics, circulating currents aren't a problem.
Upgrading transfer switching equipment. Upgrading older transfer switch equipment to present requirements can be a challenge. It hasn't been too apparent in the past that 3-pole transfer switches may not always provide complete isolation and proper ground-fault sensing. With more end users wanting to update their emergency power systems, you can't treat the concern for safety and product liability too lightly. Regardless whether or not you need ground-fault sensing, proper switching of the neutral conductor is a good practice because of the system isolation, improved grounding, and the extra safety it can provide.
Castenschoild is President of LCR Consulting Engeers, P.A., Green Village, N.J. Gordon Johnson is Technical Advisor for Electrical Generating Systems Association, Boca Raton, Fla. Both are Fellows of the IEEE.