Understanding standby power system grounding

Knowing how to apply proper grounding and bonding to electrical systems and transfer switches for common standby power system configurations can minimize power outages, equipment damage, and injuries.

12/14/2011


Electrical system grounding is an often misunderstood area of electrical system design and construction that can cause havoc when misapplied. This problem is even more pronounced with standby power systems. Circuit breakers trip, generators are dropped offline, and crucial standby and life safety loads are lost because of a hidden grounding problem made manifest during a power outage, just when backup power is needed most.

Despite this harrowing picture, most grounding issues can be resolved with attentive design, and then checked and avoided during the installation and testing stages of construction, eliminating most issues before a facility is occupied.

Know grounding and bonding terminology

Before discussing the challenges of grounding standby power systems, key terms are explained—as defined in the 2011 National Electric Code (NEC) (NFPA 70)—to offer a better understanding of why we ground electrical systems. Unless otherwise noted, the following definitions are cited from NEC, Article 100, “Definitions.”

Ground: The earth itself is taken as “ground”. Building grounding electrode systems are sunken into the earth.

The connecting of current-carrying equipment to the earth is the definition of grounding. NEC Article 250.4(A)(2) states:

Grounding of Electrical Equipment. Normally, non-current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.

This is different from bonding, where adjacent conductive surfaces that do not normally carry electricity (such as chassis on a toaster, an x-ray, or a milling machine) are connected to another conductive object (such as the metallic raceway containing power wiring, nearby building steel, or an adjacent metallic water pipe) to create an electrically continuous, low-impedance path. NEC Article 250.4(A)(3) states:

Bonding of Electrical Equipment. Normally non-current carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground fault-current path.

Grounding electrode: A conducting object through which a direct connection to the earth is established.

Grounding electrodes in practice includes ground rods and (conductive) metallic water piping, and concrete-encased building steel (e.g., steel rebar embedded in concrete foundation in direct contact with the earth).

Grounding electrode conductor: A conductor used to connect the system-grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system. This conductor connects the building grounding system to the earth by means of the grounding electrode.

Grounded (grounding): Connected to ground without inserting resistors or impedance devices. Grounded equipment or panels can trace a continuous conductive path from their chassis to an equipment grounding conductor, to the grounding electrode conductor, to the grounding electrode to the earth (ground) itself (see Figure 1).

Equipment grounding conductor (EGC): The conductive path(s) installed to connect normally non-current-carrying metal parts of equipment together and to the system grounding conductor or to the grounding electrode conductor, or both.

The EGC connects objects back to the grounding electrode system and to the earth.

Ground fault current path: From NEC, Article 250.2, an electrically conductive path from the point of a ground fault on a wiring system through normally non-current-carrying conductors, equipment, or the earth to the electrical supply source.

Effective ground fault current path: From NEC, Article 250.2, an intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground fault detectors on high impedance grounded systems.

All current tries to return back to its source to complete the circuit. A live conductor striking an ungrounded conductive metal surface will go through whatever, or whoever, connects the metal object to earth, and back to the power source.

Because having large amounts of electrical current unintentionally going through people is generally a bad thing, we want the overcurrent protection device to trip as quickly as possible, removing the fault current before damage, injury, or death can occur. The best means of doing this for ground fault current is to provide a low enough impedance path such that ground fault current is high enough in magnitude to quickly trip the overcurrent protective device.

Figure 1: Line-to-ground faults can return to the main service over both ECG and the grounding electrode system. Courtesy: Interface Engineering

Figure 1 shows this in practice. A ground fault in an appliance, rather than routing to the earth through our unsuspecting victim, routes through the EGC back through the panelboard, and ultimately to the main service (utility transformer), completing the circuit.

More importantly, because this is an effective ground fault current path, the fault current is high enough to allow the circuit breaker in the panelboard to open, cutting the fault current.

Separately derived system: A premises wiring system whose power is derived from a source of electrical energy or equipment other than a service. Such systems have no direct connection from circuit conductors of one system to circuit conductors of another system, other than connections through the earth, metal enclosures, metallic raceways, or equipment grounding conductors.

When using a backup source such as a generator, note that if a generator has a neutral conductor that is directly bonded to the utility service neutral (for example, a 3-pole transfer switch), the generator is not separately derived. On the other hand, if its neutral never comes in direct contact with the utility neutral (either because the neutral is switched at the transfer switch, or the generator is a 3-wire system without a neutral), it is a separately derived system.

System bonding jumper: The connection between the grounded circuit conductor (often called the neutral) and the supply-side bonding jumper, or the EGC, or both, at a separately derived system.

Simply put, the jumper ties the ground and neutral together at the generator. When this occurs at a utility service, we now use a different term to mean the same thing: supply-side bonding jumper.

Supply-side bonding jumper: From NEC, Article 250.2, a conductor installed on the supply side of a service or within a service equipment enclosure(s), or for a separately derived system, that ensures the required electrical conductivity between metal parts required to be electrically connected.

This term, introduced in the 2011 NEC, applies to both utility services and alternate sources of power such as a generator. This conductor is how return ground-fault current goes from the EGC to the neutral and back to the power source, completing the circuit, and hopefully tripping the overcurrent protection in minimal time.

Grounding standby power systems

Grounding systems are created to allow overcurrent devices to quickly open when a line-to-ground fault occurs. For this reason, bonding between the neutral and ground bus (or chassis) in the generator should not occur when a 3-pole transfer switch is used that directly bonds the main service panel neutral to the neutral bus of the generator.

Figure 2: Bonding the neutral to ground at both the utility and the generator is forbidden by the NEC with good reason: return current under normal conditions would then travel over both the neutral conductor and the building ground system. Courtesy: Interface Engineering

Figure 2 shows the problem with this approach. Return current from the electrical load travels down to the transfer switch and then splits with part heading back to the utility service and part heading down to the generator. At the generator, this current then returns through the neutral-ground bond back over the grounding system to the utility service, leading to stray current potentially traveling over metallic raceway, piping, and even building structural steel to return to its source.

An added problem of connecting the neutral to ground at both the main service and the generator without transferring the neutral occurs with ground fault protection (GFP), which is required by NEC Article 240.13 on overcurrent protection at 480 V and more than 1,000 A.

GFP devices operate by measuring the outgoing and incoming current and looking at differences between the two, with the assumption that any current difference is stray return current that is traveling over the building grounding system. If the measured current difference is high enough, the GFP device is then set to trip open, cutting power to downstream loads and the ground fault causing the stray return current.

However, if stray return current is caused for other reasons, such as a condition where return neutral current from the load can travel over the neutral-to-ground bond at the generator, the GFP device can also trip, because the current difference can be the same as in an actual ground fault, especially if the generator is relatively close to the service.

If the GFP is then dialed to a higher current setting to minimize interruptions, one is left with current regularly running over the ground system from standby loads, and has reduced the protection from the GFP in the event of a real line-to-ground fault.

Note that the current routed from neutral to ground at the generator can be similar in magnitude to a line-to-ground fault at the standby load—especially if the impedance of the path from the load to the generator is relatively low. Thus, setting the GFP protection to a higher trip setting can mean that a real line-to-ground fault at the load may not cause the breaker to open, leading to a sustained ground fault in the electrical system.

This leads us to our first rule of standby power system grounding:

Rule No. 1: For a 3-phase, 4-wire system, do not bond the neutral and ground bus together at the generator unless the neutral is switched at the transfer switch together with the phase conductors.

Transferring the neutral can be accomplished by one of two means:

  • A 4-pole transfer switch that switches the load neutral between the utility source to the standby source
  • A 3-pole transfer switch with overlapping contacts for the neutral that overlaps the utility and standby neutrals very briefly at the time of switching.

Note that the bonding of a generator’s neutral and ground bus should not be confused with the question of whether a generator should have a grounding electrode system. If the generator is a separately derived system, the NEC requires a grounding electrode system at the generator per NEC Articles 250.30 and 250.52(A).

If the system is not separately derived, the grounding electrode system may be installed as a supplemental grounding electrode system, providing it is also bonded to the equipment ground conductor (NEC Article 250.54).

This leads us to the second rule of standby power system grounding:

Rule No. 2: A standby power source should have its own grounding electrode system to facilitate ground fault current returning to the generator if a line-to-ground fault occurs when a generator is powering load. However, this grounding electrode system must always be bonded to the equipment ground conductor that is also bonded to grounding electrode conductor at the utility service disconnect.

This raises the question of whether there are times by code or design where the engineer must design the generator as a separately derived system with a neutral-ground bond at the generator.

One of the most common instances occurs when more than one level of GFP is used. This often happens in health care facilities where more than one transfer switch is used to isolate life safety, critical, and equipment branches from one another, and a second level of GFP is used on the overcurrent protection. Even when the generator does not have a neutral-ground bond and multiple 3-pole transfer switches are used, return current from the life safety branch returns on both its own neutral and on the neutral used by the equipment branch (see Figure 3).

Figure 3: Return current from the equipment branch also returns on the life safety neutral, possibly causing the life safety branch ground fault protective devices to trip. Courtesy: Interface Engineering

Since outgoing phase and incoming neutral current are not the same, the GFP for both branches may perceive a line-to-ground fault and trip one or more branches offline. Again, adjusting the trip setting to a higher amperage level compromises the effectiveness of the GFP when a real line-to-ground fault occurs.

This leads us to the third rule of standby power system grounding:

Rule No. 3: When using more than one transfer switch on a 3-phase, 4-wire system where any one transfer switch may have two or more levels of GFP protection upstream of itself, the generator should be grounded as a separately derived system, and transfer switches that can switch the neutral should be used for all transfer switches.

Because it is much less expensive to specify the generator with the proper grounding and install the correct transfer switches initially than do so as a retrofit, consideration of this approach should be used if there is a possibility that a future upgrade could create this condition that doesn’t exist initially in the building.

For example, if an owner advises the engineer on a project with a small life safety generator that it may be replaced in the future with a large generator capable of backing up most of the building, and that upgrade would require two levels of GFP for the non-life safety branch, the engineer will want to specify a 4-pole transfer switch for the life safety branch to avoid needing to replace that transfer switch during the future upgrade.

Because the problem of neutral current—whether under normal conditions or line-to-ground fault conditions—emerges where multiple transfer switches are used in one facility, we can generalize Rule No. 3 as follows:

Generalized Rule No. 3: When using more than one transfer switch on a 3-phase, 4-wire system where any one transfer switch may have two or more levels of GFP protection upstream of itself, all generators should be grounded as separately derived systems, and transfer switches that can switch the neutral should be used for all transfer switches. In the discussion to this point, all mention of GFP has been assumed to be on the utility side of the transfer switches. While generators can be specified with an output circuit breaker with GFP, this is normally avoided with good reason, because a ground fault trip would result in the loss of the standby power source when it is critically needed.

Instead, where a GFP would otherwise be required, generators should be specified with a ground fault annunciation feature on the output circuit breaker, to notify via the generator control panel and annunciator when a ground fault condition has occurred while the facility is running on backup power. This allows the facility manager to hunt down the source of the ground fault without cutting power to critical loads.

The oddballs you must know

There are numerous cases where a 3-phase, 3-wire system (no neutral) is used on the generator side.

Case No. 1—solidly grounded, 3-phase, 3-wire standby power system: In this case, a transformer may be used on the load side of the 3-pole transfer switch to derive a neutral for any line-to-ground loads on the standby power system (see Figure 4). However, the transfer switch sees only phase and ground connections from the utility, generator, and load connections to itself. Because there is no way for line-to-ground current to return on any path except the ground plane, we do not see a splitting of return ground fault current between a neutral and ground. Thus, a GFP can monitor just the phase conductors and trip properly if the current does not return through the same phase conductors.

Figure 4: The illustration shows a line-to-ground fault at the load. Fault current returns to the source through ground on the line side of the transformer. Courtesy: Interface Engineering

Case No. 2—solidly grounded, 3-phase, 3-wire standby power system with interlocked circuit breakers used for load transfer: By definition, because the circuit breakers (unless they are 4-pole) can only switch the phase conductors and not the neutral, we would have the same problem of neutral current from the standby load returning on two pathways. One way of avoiding this problem is to connect only 3-phase, 3-wire loads (no neutral) to the common distribution board served from both sources (see Figure 5).

Figure 5: Interlocked motorized circuit breakers in switchgear are often used to operate in tandem as a transfer mechanism, but they depend on a 3-phase, 3-wire system to avoid the problems of 3-pole transfer switches. Courtesy: Interface Engineering

Case No. 3—high-resistance grounded, 3-phase, 3-wire standby power system: In this case, a high-resistance source between neutral and ground at the generator is used to limit any return ground fault current that may occur. This is often used in industrial settings where the owner does not want to interrupt power in the event of the ground fault, but also wants to reduce the fault current magnitude (and thus its inherent danger) while troubleshooting the location and source of the fault. Conditions of NEC Article 250.36 must also be met.

Much of the control the building owner, engineer, and electrician have over resolving grounding issues with the generator can be resolved with careful planning at the generator and transfer switch. Careful forethought and planning can do much to maximize the ground fault protection of the power system, and minimize power outages due to stray currents.

Chesley is an associate principal and senior electrical engineer at Interface Engineering. He has been with the company since 1993. His engineering expertise includes building integration, renewable energy systems, telecommunications infrastructure, backup power systems, energy metering, and building dashboards.



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