homework 2

      This time the assignment was to figure out a little bit  on how a transfer switch works. A transfer switch is an electrical switch that reconnects electric power source from its primary source to a standby source. Two types of basic transfer switches, open transition (automatic transfer switch) and closed transition (static transfer switch).

      The next step in understanding these switch types was trying to figure out what “break before make” and “make before break” statements mean. An automatic transfer switch (ATS) system is an open transition circuit. It works with a break before make logic. What does this do? It simply means that the switch will break contact with the main source of power before it makes contact with the back up power. It is good in the sense that it prevents backfeeding from the generator into the utility line.

What about make before break? A closed transition circuit is also known as a make before break switch. An example is the static transfer switch. This means that it will make contact with the back up source before breaking contact with the main source. Why is this necessary? Well some loads, are affected by even the slightest loss of power. There are also operational conditions where it may want to transfer loads with 0 interruption of power. The switch will maintain a make before break mode as long as both sources are acceptable and synchronized. Parameters that need to be looked in synchronization are voltage difference ( <5%), frequency difference( <.2Hz) and relative phase angle between the sources of 5 electrical degrees.

It is generally required that the closed transition, or overlap time, be less than 100 milliseconds. If either source is not present or not acceptable, the switch must operate in a break-before-make mode to ensure no backfeeding occurs.
         Closed transition transfer makes code-mandated monthly testing less objectionable because it eliminates the interruption to critical loads, which occur during traditional open transition transfer.