From IEEE C57.105 §7:
The use of properly adjusted 3-pole switches and fault interrupters at the source end of the cable circuit will prevent single-phasing during switching and fault conditions and consequently will prevent sustained ferroresonance. However, this is a costly solution which most utilities reject from an economic standpoint. A similar solution is to use only a 3-pole switch at the remote location and fuses only at the transformer location. This is a workable solution, but it has the drawback of causing a line lockout for faults on the cable circuit. Fortunately, the compromise of a 3-pole switch and fuses at the source end of the cable will greatly reduce the probability of ferroresonance when the primary windings are not grounded. Sustained ferroresonance will not occur during switching operations by virtue of the 3-pole switch. If a fuse blows due to a fault on the cable, the capacitance to ground on the open phase is shunted by the impedance in the fault path, and ferroresonance would not be expected. But if a fuse or conductor in the primary circuit causes an open circuit in the absence of a shunt fault, and the transformer is unloaded, ferroresonance can occur.
Fortunately, when a fuse or conductor opens, the load connected to the secondary of the transformer usually is sufficient to prevent sustained ferroresonance. However, if all of the connected secondary load is fed through a breaker or breakers which have undervoltage tripping, then the total secondary load may be disconnected during an open circuit on the primary, and ferroresonance may occur.