Have you ever wondered if you should be using Air Insulated Switchgear (AIS) or Gas Insulated Switchgear (GIS) on your project? Comparing the two can be confusing, but if you focus on the five major considerations below, you’ll have a better idea of which type of high-voltage switchgear will bring the most value to your next project.
The construction of each type of switchgear impacts its longevity and maintenance. GIS uses the gas sulfur hexafluoride for insulation, whereas AIS uses air insulation in a metal-clad system. Sulfur hexafluoride is five times heavier than air and offers excellent extinction behavior.
Another significant construction-based difference is that a metal-clad AIS uses three-position draw-out circuit breakers (on, off, and test). GIS systems use fixed, mounted circuit breakers. The sealed mounted breakers are a ‘sealed-for-life’ technology, whereas the AIS breaker can be removed for maintenance and troubleshooting.
Is your project schedule tight? What about the physical space you have available for switchgear? Installing GIS can be faster than installing its metal-clad AIS counterpart. This is mainly because GIS systems are significantly smaller and weigh less, despite the gas weighing more than air. Provided the technician doesn’t need to handle the gas itself, GIS is quicker to install. The average installation time is reduced by approximately 30% with a GIS installation. GIS systems also take up less space than AIS. The physical footprint of a GIS is about 35% less than AIS.
Safety should always be a priority, and you need to think about who will be operating your switchgear. GIS systems are significantly easier to operate on a regular basis, as they offer front instead of rear access. They also contain their own integrated testing instruments.
Arc flash incidents are rare in GIS because all the interior elements are insulated, with only the cable compartment being accessible. As the parts are fully insulated, no cables or linkage can contact the live elements.
As discussed above, GIS requires significantly less maintenance. On average, GIS systems need only be visually inspected every four years or more, depending on the specific manufacturer’s recommendation for your device. GIS drives only need to be re-greased after about 20 years.
AIS should be visually inspected every one to two years. When inspected, all compartments must be checked, unlike GIS, where the individual compartments and elements are fully insulated and can be monitored. On average, an AIS breaker will need about four hours of maintenance every two years.
In terms of maintenance, AIS requires significantly more effort, primarily due to the thoroughness of the inspections. Inspections include having a technician torque, draw-out, clean, lubricate, and vacuum the unit. They also need to be checked for visual signs of copper corrosion, which doesn’t occur in GIS due to the sealed units. That sealing also protects the components from environmental damage.
Total Cost of Ownership
On a high-level comparison of AIS and GIS, you need to weigh the upfront costs against the required lifelong maintenance. These units both generally have a 30-year minimum life span. GIS upfront costs are between 10% to 40% more than AIS.
Ultimately, AIS offers upfront cost savings; however, it requires more working hours over the long run. GIS’s advanced electrical engineering requires a larger upfront investment, but its sealed technology means improved safety and lower installation and maintenance costs, meaning that GIS will cost less over the life of the equipment.
If you want more information or have questions about which type of switchgear is best to use on your next project, give us a call.
Jaron Vande Hoef, P.E., Vice President of Client Delivery
This blog was originally published in the Current Connections Winter 2021 issue.