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Current Transformer Open-Circuit Protection

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July 18, 2018


Purchase the OCP-35 Open Circuit Protector


A Current Transformer secondary should operate into a very low resistance load – ideally, a “short” circuit.

When a CT secondary encounters an open or high resistance circuit, it becomes a step-up voltage transformer. Saturation will ultimately limit the voltage but the terminals still present dangerously high voltages that are at a minimum, unsafe, and frequently destructive to the device insulation and surrounding components.

Safeguarding personnel and equipment leads most utility/industrial designs to require a protection device to limit the voltage in the event of an open circuit.

Older protection designs were bulky and expensive; however, the OSI OCP-35 is a compact and economical solution that creates a path for the secondary current to flow and rapidly limits the voltage to protect personnel as well as minimize the risk of damage to the Current Transformer.

The OCP-35 design quickly and effectively limits the voltage spikes of either polarity to a non-destructive level, and resets the limiting action at each half-cycle quickly restoring the measurement. The OCP-35 creates negligible leakage current and does not introduce phase or ratio errors into the measurement.

The OCP-35 also protects against higher secondary voltage transients. As transient voltage rises above 35 volts, the circuit clamps the voltage to less than 1.5 volts until the next zero crossing, at which time it either resets or clamps again if high voltage is still present.

The compact size and terminal wires permit easy mounting and connection in close proximity to the CT with 3M 110 tape (included). Proximity improves protection and avoids the cost of mounting and wiring to a remote protection device, while the form factor makes it simple to retrofit to existing installations.


There are several important parameters to consider when selecting an OCP. Here are the specifications for the OSI OCP-35 for comparison:


Speed of Operation Response time of 25μ seconds or less
Continuous Current Capacity 10A
Surge Capacity 200A (1 cycle)
Average Voltage While Clamping Less than 1.5V (average)
Breakover Voltage 35V peak
Frequency of Operation 20-500Hz
Weight (Typical) 1 Oz
Operating Temperature Range -40°C to 65°C


Click here to view the OCP-35 specification sheet

Questions about the OCP-35 can be directed to our sales team through chat or over the phone at 614-777-1005