Andrew Olson, md of Olson Electronics says watch out for high earth leakage currents.
The revised IEE Wiring Regulations include a number of major changes, with serious implications for all new installations. Among the important changes in the 2001 edition are the new earthing requirements for IT installations.

The 2001 edition of the IEE Wiring Regulations have been generally available since June 2001 and came into effect on 1 January 2002. The large number of changes resulted from meeting the statutory requirement for a full review in accordance with British Standards Institution rules. BS 7671: 2001 reflects the rapid changes that have taken place, principally within the working environment, and that have had an impact on best practice in the electrical installation industry.

It will come as no surprise to learn that the massive growth in the use of IT continues to have a profound effect on business and the electrical installation industry in particular. The increase in the use of computers in virtually every field of business and commerce has also had its own effect on power distribution systems.

The internal switched-mode power units built into every pc are highly compact and efficient, but in common with many other non-linear electronic systems and controls based on high-speed semiconductor switching technology, they generate characteristically high earth leakage currents. In a large installation, significant levels of current can flow in the protective conductor of the mains power distribution system. Associated earthing problems are further complicated by the ease with which modern office desking can be reconfigured to suit new layouts. In many cases, earth continuity can be compromised or lost, with potentially catastrophic consequences.

The problems surrounding the earthing needs of equipment that result in high currents in the mains supply earth conductor have become so acute that the IEE Wiring Regulations Committee was forced to rewrite Section 607: high earth leakage currents, completely. As the effect is technically not the result of earth leakage but largely due to transient and harmonic suppression circuits, the Regulations now refer to protective conductor currents.

To overcome this increasingly common problem, new installations that suffer from high current levels in the protective conductor will require a dual earthing system for mains socket outlets, the two ends of the protective conductor being connected to separate terminals on the earthing bar at the fuseboard. The use of modern, factory pre-wired mains distribution panels can provide a cost-effective solution. These are available with multiple outlets to suit virtually all types of mains plugs, and some incorporate high-integrity dual earthing as standard practice. This meets the needs of Section 607 and eliminates the possibility of loss of earth continuity when desks are moved. Rather than incur the difficulty and expense of installing floorboxes, the use of suitably wired power distribution panels in conjunction with modern busbar trunking or extruded track, has now become the system of choice with an increasing number of end-users.