Friday, January 12, 2018

Electrical System Design - Grounded or Ungrounded?

Further to the discussion of Earthing & Grounding for UPS System, we are going to explore different cases making mistakes to ground everything by default which creates parallel paths and are strictly prohibited!

We all note that ungrounded electrical systems are not often employed due to real and perceived safety concerns. Predominately, commercial systems are solidly grounded (SG). SG systems are characterized by high line-to-ground fault current with reliance on quick overcurrent protection to limit the release of dangerous energy.

Alternatives to an SG system include low-resistance grounding (LRG), reactance grounding (RG), and high-resistance grounding (HRG). LRG or RG systems are recommended on medium-voltage systems to limit fault currents while overcurrent protection operates. HRG systems, which limit the fault current to a small value, were adopted by the mission critical data centers. Onsite power generation and uninterruptible power supply (UPS) systems are used extensively where equipment costs can be justified against the losses due to business continuity interruptions:-

The picture shows an UPS installed in a typical equipment room with associated switchgear. This is a good illustration where the user is planning for future growth of the UPS, and has allowed space for additional modules to add capacity or redundancy.

Transformerless UPS systems are preferred due to efficiency savings, lower thermal heat rejection, and a smaller footprint as compared with transformer-based UPS systems. These transformerless systems have been introduced in the past decade and are commonly employed on a large scale for data centers and critical manufacturing processes. For domestic, medium- to large-scale applications, engineers are specifying UPS distribution as a 480 V, 3-wire system with 208 V power distribution units (PDUs) at the point of connection. A PDU or isolation transformer is provided when single-phase loads are served. A neutral is not required or advised for this system until single-phase loads are required:-

This 3-wire UPS system depicts an ungrounded zone. Ungrounded operation occurs during battery discharge when the UPS isolates the incoming source.

For smaller systems, such as a 208/120 V UPS input source, a 4-wire system may be specified (see the following figure). Systems in both figures operate ungrounded during an event where power is lost. Whether a short circuit is flowing through the neutral or grounding conductor when the UPS is providing power, transistors in the UPS rectifier isolate the input power, opening the supply circuit and interrupting the return path:-

This 4-wire UPS system depicts an ungrounded zone. Ungrounded operation occurs during battery discharge when the UPS isolates the incoming source.

For applications that can’t tolerate an ungrounded zone within the electrical distribution system, an isolation transformer inside the UPS is an option. Without an isolation transformer, there is no safe way to connect the direct-current source to ground without introducing a parallel return path. With transformerless applications being the leading choice in the industry, it is important for engineers to mitigate and understand the risks of operating an ungrounded system during power transfer.

Careful application of grounding continues to rank No. 1 in safety priority. It is a mistake to ground everything by default. Grounding duplication creates parallel paths, which is strictly prohibited for neutral conductors.

By design, when connecting exposed metal cabinets and conduit to the grounding system, there are many parallel paths to the source. However, properly grounded systems are only connected once at the source. A grounded conductor is provided to intentionally return unbalanced current back to the source. These grounded conductors are separated from the grounding system to avoid a parallel return path. Most important, the isolation of a grounded conductor from grounding keeps these intended and unintended exposed metal paths from carrying current under normal conditions.

For critical applications, redundant components along with alternative utility and standby sources are normal practices. These separately derived systems are grounded at the source and interconnected by transfer-switch schemes. Grounding interconnection is required and care must be taken to avoid hazards, such as not being able to isolate a ground fault or circulating ground currents. Where 4-wire sources are required, auto-transfer schemes must consider switching the neutral.

About us

Strategic Media Asia (SMA) is one of the approved CPD course providers of the Chartered Institution of Building Services Engineers (CIBSE) UK. The team exists to provide an interactive environment and opportunities for members of ICT industry and facilities' engineers to exchange professional views and experience.

SMA connects IT, Facilities and Design. For the Data Center Design Consideration, please visit 

All topics focus on key components and provide technical advice and recommendations for designing a data center and critical facilities.