Comparison of Neutral Wire Distribution Options for Data Centers

Overview

The paper examines what happens when a 4-pole transfer switch at the main LV switchgear changes the UPS and downstream loads from a TN earthing system to an ungrounded system during generator changeover. It describes the resulting power-quality disturbances. The introduction also compares the typical 400/230 V global architecture with the 480/208/120 V North American one. One example of the disturbances is given: 25 µH x 100 A / 5 µs on a PEN conductor induces about 500 V. Transients of this kind show that the handling of the neutral conductor during changeover matters in design.

It then compares eight neutral distribution solutions, including isolation and PDU transformers, tabulating losses, footprint and cost trade-offs. Losses for transformer-based options are quantified: the PDU transformer option adds about 3% losses, and neutral conductor losses are about 1.1%. An isolation transformer on the UPS input adds about 1.98% at 100% load and about 0.7% in eco mode. These transients matter for power quality at sensitive IT equipment, and the choice of solution has a lasting effect on both initial investment and operating losses.

A limit is that the paper is a comparison of options rather than a design procedure, so the choice has to be made together with local regulations and site details. Even so, the figures show that earthing and neutral decisions feed directly into the efficiency calculation. Selecting among them therefore means weighing power quality, efficiency and cost together, instead of treating the neutral as a purely wiring detail.

Key points

1. PEN conductor transient example: 25 µH x 100 A / 5 µs is about 500 V induced.
2. The PDU transformer option adds about 3% losses; neutral conductor losses are about 1.1%.
3. An isolation transformer on the UPS input adds about 1.98% losses at 100% load and about 0.7% in eco mode.
4. Eight neutral distribution solutions are compared on losses, footprint and cost.
5. The typical 400/230 V global architecture is compared with the 480/208/120 V North American one.

Using it in design

• In the single-line builder, raise a warning when the transfer switch has 4 poles and the UPS neutral is not separately derived.
• Show the options: a 3-pole switch with solid neutral, or an isolation transformer.
• When a transformer is chosen, add its 1-3% losses to the efficiency model.

Who should read it

Electrical engineers designing generator changeover, earthing and neutral distribution.

Related standards

• IEC 60364-1
• IEC 60364-4-44
• IEC 62040-3
• NEC 250

Publication details

Authors

Malik Megdiche; Avelino De Oliveira; Eric Brun

Publisher

Schneider Electric

Year

2022

Document number

WP 41

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation