Parallel Paths
https://www.youtube.com/watch?v=NrHy9pwv1iQ

This demonstration shows why we do not want to connect grounds and neutrals together downstream of the service equipment.

http://www.buellinspections.com/grounds-and-neutrals-bonded-in-sub-panels/
http://www.buellinspections.com/bonding-neutral-and-ground-in-a-sub-panel/

Transformerless uninterruptible power supply (UPS) systems operate ungrounded during power transfer to a backup source, but a robust grounding design can accommodate the requirement of both grounded and ungrounded systems.

In any facility containing critical loads, whether related to life safety or sensitive computer loads vital to facility operation, one of the most important pieces of equipment specified in the design is the uninterruptible power supply (UPS), which uses stored energy to supply power to these critical loads when normal power is lost and a backup power source is starting up to supply the building loads.

When selecting UPS modules to power critical loads in a facility, one key decision to make is whether to use a UPS with or without input and/or output transformers.

A UPS without transformers can see efficiency advantages of 5% or greater, as compared with those with transformers. Not only does this mean lower electricity bills, but it also represents lower heat loads in the room housing the UPS, resulting in reduced HVAC requirements.

In facilities with a large amount of critical load, the savings can be dramatic. Additionally, transformerless UPS systems reduce the weight and footprint of each UPS module when compared with transformer-based systems, reducing the size and structural requirements of electrical rooms and leaving more room for white space or other portions of the building.

However, the output transformer of a transformer-based UPS does provide an option that is not available for transformerless UPS systems: The electrical isolation provided by a transformer gives the opportunity to create a separately derived neutral-to-ground connection at the output of the UPS. In certain situations—such as a system served by an ungrounded delta service, a service grounded through a high-resistance ground, or systems in which there is the potential that the two sources of a dual-input UPS may come from two independent sources—it may be desirable to derive a neutral at the UPS without a transformer, to provide the UPS with a stable ground reference that it can use for voltage regulation at its output and on its dc bus.

If such a neutral is not derived in a transformerless UPS system, then while the UPS battery is discharging during an input power failure and the UPS input circuit breaker is open, the downstream system is operating ungrounded. In most installations, there will be one or more downstream transformers, external to the UPS, served by the critical power system. These downstream transformers are usually housed in a power distribution unit, and on their secondary side a grounded system can be derived, but that portion of the system on the primary side will nonetheless be ungrounded during this period.

Most design engineers are used to working with grounded systems, and the prospect of leaving a portion of the building ungrounded, even during a generally brief transition period between input power failure and the facility backup power system starting up, may seem worrisome. However, creating a safe, robust, and code-compliant ungrounded power system is relatively simple, requiring only minor modifications from the grounding and bonding systems required in any grounded power system.

Grounding the system

UPS manufacturers have a variety of solutions for the issue of how to ensure the UPS maintains a reference to the ground during ungrounded conditions, to ensure that the UPS voltage regulation remains stable. Some manufacturers derive a so-called “virtual ground” at the common point of the input and output filters of the UPS to achieve this purpose. This is often a standard feature, especially on newer UPS models, but an optional accessory is required in some cases. When specifying a transformerless UPS, especially in a 3-phase, 3-wire system, take care when considering how it will operate under ungrounded conditions.

No matter the size of the system, the grounding electrode conductor must always be at least as large as #8 AWG for copper or #6 AWG for aluminum, and unless superseded by local amendments or authority having jurisdiction (AHJ) requirements, the grounding electrode conductor is not required to be larger than #3/0 AWG for copper or 250 kcmil for aluminum.

Ungrounded systems

Thus far, the grounding rules discussed covering ungrounded systems are very similar to those covering grounded systems. Indeed, if one employs a robust grounding design for a normally grounded system and ensures that the UPS and battery-cabinet enclosures are connected to the building’s grounding-electrode system through appropriately sized grounding electrode conductors, almost all requirements for an ungrounded system will be met when the UPS discharges its batteries and becomes an ungrounded system during power transfer.

However, there is a key difference between the behavior of grounded and ungrounded systems that imposes an additional requirement on ungrounded systems. This difference appears when a single line-to-ground fault occurs in the system.

In a solidly grounded system, the connection of (usually) the neutral wire to ground at the supply source means that a complete circuit will be formed when a line-to-ground fault occurs. This allows a large amount of fault current to flow through the low-impedance path created by the fault, causing an overcurrent protective device (OCPD) equipped with ground-fault detection to operate and quickly isolate the fault.

In an ungrounded system, though, there is no circuit created when a single line-to-ground fault occurs through which fault current can flow. Instead, the faulted conductor simply becomes grounded and the line-to-line potentials between the faulted phase and the other unfaulted phases become line-to-ground potentials. The value of the potential difference between the phases, however, does not change. This will not have a noticeable effect on the system’s performance when it occurs, but if the fault is left unrepaired and a second line-to-ground fault occurs, this will result in a double line-to-ground fault, drawing larger fault currents and creating the potential for greater damage to electrical equipment and greater risk to personnel safety. As in grounded system, a phase-to-phase fault in an ungrounded system will generate fault current and will typically cause an overcurrent protective device to operate and isolate the fault.

To ensure that single line-to-ground faults do not go undetected, NEC 250.21(B) requires that ungrounded systems be outfitted with ground detectors at a point as close as practicable to the system supply source.

For example, it may be costly to initiate a shutdown of a critical computer system due to the presence of a ground fault on the system, but it will certainly be less so than an abrupt disconnection of power to those same computers. Most UPS systems will contain a ground-detection mechanism, but it is important to verify this component is included to ensure compliance with this requirement.

Detection of ground faults is especially important when a system becomes temporarily ungrounded, such as while a transformerless UPS is discharging its battery due to an input source failure, because it is likely to become grounded again when the input power returns. When power is restored, either through a return of the utility source or due to a generator source coming online, the UPS input circuit breaker will close and the system will once again be grounded. If a ground fault is still present in the system when this occurs, ground-fault current will flow through the fault. A ground detector in the UPS can prevent this situation through a pre-emptive shutdown before fault current has a chance to flow.

Vishay Intertechnology, Inc. announced that Vishay Milwaukee (a product line of Vishay Dale Resistors) NGR series neutral ground resistors are now available for CSA code special inspection.

NGR series resistors are designed to provide ground fault, overvoltage, and short circuit protection for generators and transformers in wye (star) configurations without exceeding the temperature limitations outlined by IEEE-32. Devices available for CSA code special inspection combine high line-neutral voltages to 8 kV and system voltages to 13.8 kV with high-temperature performance to 760 °C. Offering a tied live design to eliminate floating voltages in the assembly, the resistors feature current ratings from 100 A to 1000 A and a resistance range from 1.39 Ω to 80 Ω.