Tips for Choosing Transmission Switches for System Reliability and Resilience

1. Resist the urge to consider it a commodity.

Because the disconnect switch is one of the least expensive elements in a utility system, it can be tempting to approach it merely in terms of cost. Yet, the right switch can play a vital role in isolating and protecting other mission-critical and costly major components. Switches provide tremendous value for their role in helping prevent outages on multiple lines. A quality switch can contribute significantly to the reliability of the system, enhancing the life of assets on the line and reducing maintenance costs.

2. Educate yourself on the attributes of a quality switch.

With transmission line outages becoming costlier and more difficult to schedule, the ease and speed of installation has become more important than ever. Ensuring that the switch arrives at site, on-time and with minimal components to assemble and adjust can cut installation times in half. This not only saves time and avoids costly assembly errors but ensures a factory-tested and inspected unit ready to provide superior performance in the shortest time possible.

Good design of critical adjustment areas is essential. The best switch designs are not finicky and don’t require a lot of adjustment during installation. Transmission switches are typically installed at large height tiers, which can reach up to 120 feet above grade. The presence of a forgiving contact geometry and robust operating linkage makes a big difference in maintaining proper adjustments in the presence of outside factors such as wind loading, ice, galloping lines, thermal cycles, etc. Over time, less robust designs will yield to these conditions and result in a switch failure. Similarly, a resilient over toggle mechanism design ensures the linkage locks securely so it cannot drift open and cause an outage.

An additional key attribute is ensuring that the switch blade contact and jaw remain aligned during closing operations. While this sounds simple, alignment can pose real challenges during both installation and normal operation over the life of the switch. This is due to the fact that some switch designs have to be operated in a specific manner to ensure the blade contact and closing. For example, utility workers may be installing one manufacturer’s switch that requires high speed and force to ensure it closes properly; then, with another manufacturer’s product they may face the opposite challenge -- if the switch is not closed with a slow and deliberate motion there is a good chance that it may not close properly. It is recommended that a switch be selected where reliable blade contacts and jaw alignment is not dependent on the operator’s speed or force during closing.

3. Know that someday, nearly every transmission switch will be called on to perform as an interrupter.

Due to the long lines they support and transmission system design factors, transmission disconnect switches typically require interrupting capability.

Transmission switches vary in what they are capable of and need to be carefully selected based on their intended duties -- as well as in anticipation of surprises. For example, selecting a disconnect switch mounted interrupting device that can perform duties in excess of the presently intended function can ensure that if an out-of-sequence switching operation occurs, it will not damage the switch or its interrupting device.

Transmission switch mounted interrupting devices can play many roles. They may be used for line dropping, bus dropping, cable dropping, loop splitting and load breaking. There are seven major types of products available, but not all are equipped to perform all these functions. These interrupting devices include: