Four factors can determine the level of reliability and effectiveness of redundancy of an STS manufacturer’s design and system integration (refer to Fig. 1 example):

1. The supplies to the STS must be independent with high MTBF and low MTTR.
2. The STS is a single point of failure.
3. For many applications, the STS must transfer within power waveform limits suitable to the load to

maintain AC supply.For example, transformer loads cannot accept DC supply without saturation causing loss of supply to following system loads. In these cases, it is essential that AC supply is maintained to such critical systems. Generally this can be achieved by ensuring STS supplies are in-synch, however this is not practically possible under all conditions.

4. For many applications the STS must transfer within a defined time period. In such cases, a fast

switching STS must be used or both supplies must always be in-synch. Fast switching cannot be guaranteed with a traditional thyristor-controlled STS as it can take up to 10ms to transfer.