Dual service with double-ended switchgear from different manufacturers presented a grounding system that would not function correctly due to the connection’s configuration.
When the owner of a major office building decided to renovate the building’s interior space, the job became more complicated than planned. Due to finances, the owner had to keep the building’s existing main electrical service, service switchboard, and interior power distribution. However, the existing service switchboard had inherent problems, specifically with the installed ground fault detection system. The problem was a dual feed arrangement consisting of multiple grounding points with a ground fault detection system that would not function properly.
The challenge was to install a functional, Code-compliant, ground fault protection scheme—free of nuisance tripping, yet sensitive enough to detect ground faults. This system would have to coordinate with other protective devices to selectively isolate a ground fault while maintaining service to other switchboard loads.
The existing building electrical service. The main electrical service of the building consisted of two outdoor utility-owned service transformers feeding a 480/277V doubled-ended switchboard with two 2000A main service switches and a bus-tie switch. Installed in the switches was the Code required ground fault protection, which included current sensors on the switchboard neutral bus in each main switch compartment. Upon sensing ground current, the sensors would energize a ground fault relay at each main and tie switch.
Here’s where it became more complicated: There were no ground fault sensing devices installed on any of the branch feeder circuits from the switchboard. This emphasized the need to have a functional detection system at the mains to minimize interruption of service in case of a ground fault. Local utility requirements required grounding of the neutral of each outdoor utility service transformer at the transformer. A cable connection grounded the switchboard neutral bus to the building’s water main per National Electrical Code (NEC) mandate.
Grounding and the NEC. The NEC requires installation of ground fault protection on all solidly grounded wye electrical services rated at more than 150V to ground for each service disconnect rated at 1000A or greater. Art. 250-24(a) requires installation of this type of grounding on the supply side of the service disconnect(s). This article implies that systems with multiple electrical services would have multiple grounds. However, item No. 3 of the same article includes provisions for establishing a single grounding point on the load side of service disconnects for dual-fed systems. This provision, when applicable, would eliminate the need for multiple grounding. Keep in mind: The local utility has its own regulations that require grounding the neutrals of their service transformers at the transformer secondary. There are no exceptions to this local utility requirement.
Ground fault protection for electrical services comprising a single incoming service is simple. Typically, the installation consists of ground sensor (zero sequence) relays installed on the main conductors and neutral. These relays initiate opening the main protective device (circuit breaker or fusible switch) upon detection of current flow. The typical ground sensor relay consists of a current transformer and overcurrent relay combination. This provides an effective means for detecting the presence of ground fault currents.
When electrical systems consist of more than one incoming service and system ground point, standard ground fault protective schemes cannot assure correct action of the ground relays. Also, gone is the desired selectivity between the tie and main protective devices under arcing ground fault conditions.
An example of the above arrangement is the double-ended substation or dual-fed switchboard. These typically consist of two main protective devices and a tie device with each service transformer’s neutral solidly grounded at the transformer. The problem created by such an arrangement is the multiple paths for the ground fault current to flow. In this common-type arrangement, a bond exists between the solid neutral bus and ground bus inside the switchboard enclosure. The NEC requires a grounding connection between the neutral bus and water main. Therefore, a ground fault that occurs in a solidly grounded system would have multiple paths, including:
• Current flow through the neutral and ground bus to ground at the building water main.
• Current flow back through the neutral bus to ground at the associated source transformer
Current sensing problems to address. The above arrangements create problems for proper sensing of ground fault current. The main reason is all the current may not flow through a sensor installed at either main protective device location. It also effectively eliminates the ability to isolate and confine an interruption to a small part of the system. Since there is a solid (unswitched) neutral in the system, the multiple paths for current flow would exist for all switchboard operating configurations.
Alternative grounding solutions. To meet the need for a functional ground fault protection system, engineers examined two solutions.
• A single ground point approach. This requires establishing a single neutral-to-ground bus connection at the center of the switchboard (in accordance with NEC Sec. 250-24a, item No. 3) with a current sensor installed on that connection. Installed on the neutral bus are current sensors either side of the neutral-to-ground bus connection along with the associated ground relays (See Fig. 2, on page 42).
This approach would deliver some problems from the utility. If a ground fault occurs while the tie device is closed, the tie device would open first due to the interlocking of the ground relays.
If while the tie device is open, the ground fault current will flow from the switchboard ground bus to the neutral bus through the associated neutral current sensor and finally back to the service transformer’s neutral-to-ground connection. Since only one of the neutral current sensors sees the fault current, only one main device relay activates and only the associated main device opens. The other main device remains closed and maintains power to the other half of the main switchboard. The main device ground relays will not operate upon unbalanced line-to-neutral load current flowing in the neutral. This is because a normally open contact in the tie device relay supplies the control voltage.
Although this scheme is simple and straightforward, the requirement for a single point ground was a major problem. This requires disconnecting the neutral-to-ground connections at the utility service transformers.
• Summation of currents approach. With this method, ground current sensors are installed on each phase and neutral of each set of main service conductors at the main protective devices. Current sensors are also installed on each phase and neutral at the tie protective device along with two ground relays interconnected. A ground fault occurring on either side of the tie device on the main bus or feeder circuit from the bus will produce a proportional amount of secondary current in the corresponding current sensor. This current then flows through the associated relay coil.
If this current is above the preset pickup setting of the relay and persists beyond the selected time delay settings, the relay will initiate opening of both the tie device and the corresponding main protective device. The ground fault current may return over either of the two previously described ground paths and neutral conductor. It might also flow in any proportion between these various paths without disturbing the current flow through the relay coils. Normal load currents can flow in the system without activating the relays. Any unbalanced line-to-neutral current on a feeder circuit has dual paths by which it may return to its source at one or both of the solidly grounded service transformers.
The unbalanced neutral current will not add to or subtract from the ground fault current flow in the relay coils, nor will it contribute any current flow through the relay coils during normal operating conditions. This would apply for any combination of system operating configurations (i.e., main protective devices and tie device open or closed). This scheme allows for grounding both the utility service transformers and the switchboard neutral.
Conclusions and recommendations. The summation method accomplishes all the objectives of a properly designed and functional system. It provides both adequate ground fault pickup and most importantly, selectivity and the capability to isolate a ground fault while maintaining power to as much of the system as possible.
Although the single point ground method is much simpler, the utility requirement for the service transformer ground connection prohibited its use. Note that if the facility transformers were not utility-owned, the single ground point method would have been a viable and Code-compliant solution.