Many electrical distribution systems contain vaults and equipment rooms with insufficient maintenance access and working clearances that may not comply with current codes and OSHA regulations. Such installations may be considered to be "grandfathered" by code officials, leaving the maintenance staff no choice but to live with them until equipment failure or system expansion provide a driver for an upgrade project. Older systems are often 2,400V or 4,160V, and as facilities grow it may be necessary to upgrade the system to a higher voltage to accommodate load growth without increasing the number or size of feeders. However, the larger dimensions of higher voltage ratings of standard switchgear may preclude this option when space is tight.

Technology solutions

In recent years, there has been an increase in the availability of switchgear utilizing alternatives to traditional air insulated designs to reduce the spacing between energized parts and decrease the size of equipment accordingly. Some of this technology has long been used in the domestic utility industry and is now applied to products for commercial application, and some is adapted from European designs, which have traditionally been more compact than ours. This new selection of available compact switchgear products allows the electrical engineer greater flexibility in optimizing electrical room design and meeting tight space constraints.

Figure 1 shows a lineup of six compact switches rated 15kV, 600A, installed in the 13,800V distribution system of a high-rise office building. This equipment uses about half of the floor space that would be required for traditional air insulated switchgear serving the same function. While this particular product is similar in appearance to, but smaller in scale than traditional switchgear, other designs are available that minimize vertical height, are fully weatherproof, or have other features that may be advantageous for a particular project.

SF6 insulating gas

A major factor in reducing switchgear size is the use of sulfur hexaflouride (SF6) to replace air as the primary insulating medium. SF6 is a nontoxic, nonflammable, and nonreactive gas with excellent electric insulation and arc-quenching properties. Its insulation strength allows spacing between energized parts to be reduced, and its arc-quenching ability permits load-break switching with very short contact separation distances, both of which result in switchgear designs that fit more voltage and current capacity into a smaller space.

This gas has been used for many years in high-voltage (over 35kV) circuit breakers and gas-insulated switchgear for utility substations, and it is now being used in several models of 15kV-class commercial switchgear.

In these applications, the switchgear requires an internal SF6 pressure of about 15 psig to attain the required electrical ratings. This internal pressure prevents entry of atmospheric air and subsequent loss of voltage rating in the event of a leak. Pressure switches or density sensors are used to detect any loss of gas and initiate a remote alarm before the internal pressure drops to an unacceptable level. Compact cylinders of gas and regulators are available as maintenance accessories to permit recharging if needed. SF6 has been identified as a greenhouse gas, but I'm not aware of any current regulations affecting its use in electrical equipment. Because the small quantities involved are sealed within the tank, environmental impact should be minimal.

Operational considerations

New designs often require adjustment in operating and maintenance procedures to accommodate differences from traditional equipment. Compact switchgear uses either fully shielded dead-front connectors such as elbow terminators, or special arrangements of cable connectors that permit reduced spacing and limit access to the terminals. With fully insulated cable connections and switches and bus sealed within the tank, circuits are not readily available for voltage testing or application of protective grounds. This is addressed by specifying the switchgear with test points and integral grounding switches, or by identifying other locations in the distribution system where these functions can be safely performed.

The future

I expect to see more use of these new switchgear products as engineers take advantage of them to design safer, more reliable and more space-efficient distribution systems, particularly when replacing aging equipment in tight spaces, and accommodating voltage upgrades. We should also expect to see more compact switchgear products enter the domestic market as worldwide manufacturers integrate their European and North American product lines.ES