Open Circuit Protection
Current Transformer secondaries are designed to operate into a very low resistance load - ideally, a "short" circuit.
When a CT secondary encounters an open or high resistance circuit, it quickly becomes a step-up voltage transformer. It then presents dangerously high voltages that are at a minimum, unsafe, and frequently destructive to the device insulation and surrounding components.
How high? Multiply the input voltage by the CT ratio. Yeah. That high.
Which is why most military applications, as well as many utility and industrial designs, require a current transformer protection device that limits the open circuit voltage.
Older protection designs are expensive and bulky. The OSI OCP-35 is a compact and inexpensive device that limits open circuit voltages to safe levels of 35V or less.