Current transformer protection

Metrosil provides an effective solution for safeguarding current transformers against secondary side open circuits.


A Current Transformer (CT) is used to measure the current of another circuit. CTs are used worldwide to monitor high-voltage lines across national power grids. A CT is designed to produce an alternating current in its secondary winding that is proportional to the current that it is measuring in its primary. In doing so, the current transformer reduces a high current to a lower value and therefore provides a safe way of monitoring electrical current flowing in an AC transmission line.

Operational Hazards

Operational hazards can occur if the secondary circuit of a CT is left open whilst the primary is energised. Open circuit conditions can happen inadvertently through routine maintenance of the burden or damage to the leads of the secondary circuit. High voltage transients may be generated in these situations and damage the CT winding insulation; possibly rendering it inoperable. In addition, these transients can cause high eddy currents in the CT core. These may be detrimental to the magnetising characteristics of the CT and result in errors in the measurement accuracy.

IEEE C57.13 advises that voltage limiting devices should be provided with secondary windings to protect against hazardous voltages. It states that the voltage limiting device should withstand an open-circuit situation for one minute without damaging the secondary circuit. Metrosil Current Transformer Protection Units (CTPUs) offer such protection and, unlike other voltage limiting devices, do not need to be replaced immediately after an abnormal condition. They can remain in-situ without the need for user intervention.

Current Transformer Protection Units

Under normal working or fault conditions, with the burden connected, the varistor is subjected to an applied voltage. It acts as a passive load and draws little current, thus preventing inaccuracies in the measurement of the CT. During an open circuit condition, the varistor is subjected to an applied current and acts as an active load. It therefore limits the voltage across the CT terminals and prevents any damage. The thermostatic switch manages the thermal cycling within the Metrosil CTPU when the CT is in an open circuit condition. A second thermostatic switch can be mounted on the heat sink plate for remote monitoring. Metrosil varistors can control the magnitude of a back EMF by dissipating stored energy in the coil to an appropriate load.

A Metrosil CTPU can be constructed in both single and three pole varieties for convenience of installation. All CTPUs are factory audited to ISO9001-2015. Metrosil CTPUs protect CTs from damage under open circuit conditions. They do not protect relay or CT systems from overvoltages created due to high secondary fault currents. For protection of high impedance relay systems against overvoltages during fault conditions, please refer to our Metrosil Relay Data Sheet. For assistance in using Metrosil CTPUs in conjunction with Metrosil Relays for High Impedance Relay Systems, please contact the Metrosil team.

CTPU Enclosures

Metrosil’s range of pre-assembled CTPU Enclosures provide advanced shielding against the damaging effects of secondary side open circuits. Type tested and independently certified to IEC 61439 Parts 1 and 2, the units are available in adaptable configurations and flexible fitting options.

Why Metrosil?

Metrosil Silicon Carbide varistors were borne out of the high-voltage department of Metropolitan-Vickers in 1936 and mass produced in 1937. As a major power house in the 20th century, Metrovicks was renowned for its industrial electrical equipment, which included generators, steam turbines, switchgear, transformers, electronics and railway traction equipment. Consequently, Metrosil resistors were specified into major flagship projects that paved the way for the efficient distribution of electricity. To this day our resistors remain in place within established power networks, which has instilled trust in major OEMs and utilities alike. As modern power infrastructures develop, we continue to innovate and be specified into substations on a global basis.

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Current transformer protection unit enclosures data sheet

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