Uneven loading is quite likely in such operations. It is probable that the majority of the motors failing in a single-phasing type pattern actually failed because of voltage unbalance. Even when voltage unbalance is suspected as the cause of a high motor- mortality rate, it is difficult to detect because of its erraticism. In such cases, a 3-phase recording ammeter can be a valuable tool in determining if unbalance is, in fact, the problem. In the past, two overload protectors were usually considered adequate for most motor applications.
Motor is in isolated, inaccessible, or unattended location.
Motor drives critical equipment. Transformer connections are unknown. Motors are operated in parallel with other motors, which might cause circulating currents or permit sustained operation under single-phasing conditions. Local electrical codes require three overload protective elements. With the new National Electrical Code, 3-phase protection will be provided on all new motor installations, and eventually, motor starters with only two protectors will become rare.