In such cases, the open phase may remain at approximately full potential. And, as pointed out in NEMA Standard MG 1-14.33, voltage unbalance of only 3.5 per cent will cause an increase in temperature rise of about 25 per cent. If investigation reveals that single-phasing did not occur, the failure is often attributed to a faulty motor. One electric Utility reports that among its customers there were 300 confirmed cases of motor burnouts caused by single phasing or voltage unbalance within a one-year period. Because large industrial plants seldom report motor failures to the utility company, it follows that the reports of failure came from operators of commercial buildings and small plants which do not have their own electric department.
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.