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Measurements (CT, PT, and Tore)

This article will discuss about the three common Electrical Measurements we can found in Switchgears and Panel boards.

I learned this electrical measurements when I worked with a gas manufacturing plant as a Cadet Engineer. However, this is just few of the measurement devices that we have in the field of Electricity. This article will focus on Current Transformer, Potential Transformer and Tore that are used as metering device.

Table of contents

1. Current Transformer
2. Potential or Voltage Transformer
3. Tore or Residual Current Sensor

Current Transformer

The current transformer makes it possible to reduce a high intensity current to a value of 1A or 5A exploitable by the measurement apparatus and the protection relays. (1A and 5A being standardised values). It includes a primary circuit which collects the high intensity current and a secondary of the same kind as the primary current but of lower value.

Their role is also to separate the power grid of the measurement, protection and control grid, which have to know the current. In fact, particularly important electric and electromagnetic disturbances happen on the grid in high voltage.

These disturbances are connected with the use of devices (disconnecting switches, switches, circuit breakers, contactors), with the atmospheric discharges, and with the appearance and elimination of the defects. The transmission must be compatible with the levels the apparatuses can stand up to. The level of this transmission depends on the quality of the galvanic insulation.

Typical symbol

The values 1000/1 A are the primary and secondary rated currents.

When we’re doing test for Multifunction Relay, current transformer ratio will be the basis for current injection to test relay and breaker coordination.

Medium Voltage Current Transformer

For the first CT, the circuit has to be interrupted in order to connect the CT. For the second one, the conductor whose current one has to measure goes through the CT without any interruption of the circuit.

The primary winding of tores CTs is made up of the line conductor itself. The current transformer is designed to work between 10% of the rated current and its saturation current, i.e. for which the secondary current will not increase proportionally to the primary current anymore, but more slowly.

If the CT is not loaded very much (a few devices connected compared to the number it can supply), the lower limit of operation will no more be of 10% of Ir, but more. Example: if the CT is loaded to 1/20 of its power, the limit will be 40 % of Ir. If the CT is working below this value, the precision is not guaranteed anymore. It is thus necessary to pay great attention to the power requested from the manufacturer and the real load of the circuit.

The manufacturers also give the short length acceptable current (Ith), which is expressed in KA, example : 25kA.

Assigned Dynamic Current

The assigned dynamic current (Idyn) is the ratio of the short-circuit resistance to the primary of the CT. Example: CT with a ratio of 100/5A, Icc of the grid = 12,5kA, Idyn will have to be of 12500/100 = 125Ir.

Insulation Level

Insulation level defined by three voltages: the highest voltage of the grid (Um), the assigned voltage of short length resistance at industrial frequency, and the voltage of lightning shock resistance.

Two types of secondaries used:

Measurement secondary:

Its precision range is narrow. It is generally limited to currents that are equal or lower than the assigned primary current. It is intended for supplying the measurement apparatus. A measurement CT saturates between 3 and 5 Ir when it is loaded at Pr and between 9 and 15 Ir when it is loaded at 25% of Ir. That prevents the measurement apparatus from being damaged in the event of an overcurrent.

Protection secondary:

In this case, the precision range is very broad. It often reaches one to twenty times the rated current. The precision range is wider than the previous one.

In this example, there is measurement CT and one protection CT. Like it is three-phase, three CT are in fact used per phase for each one. Their rated current is 600A in the primary for 5A in the secondary.

The first CT is a measurement CT identified by 5VA Cl 0.5. It can provide 5VA of precision apparent power to its load when it is gone through by the primary rated current. It is of class 0.5, i.e. its ratio error will be of ± 0.5% and the dephasing of 30° (not explained) between 50 and 120% of the rated current.

The second CT is a protection CT identified by 10VA 5P20. It can provide 10 VA. 5P20 corresponds to a maximum error of 5% for a current which can reach 20 times the rated current when its load is between 25% and 100% of the power.

Note: the secondary of a CT should NEVER be opened when a current flows in the primary. It would result from it a very high voltage in the secondary and an excessive heating of the magnetic circuit.

Potential or Voltage Transformer

The voltage transformer makes it possible to bring a high voltage back to a value that can be exploited by the measurement apparatus and the protection relays. It includes a primary circuit submitted to high or medium voltage and a secondary circuit which delivers a voltage of the same kind as the primary, but of lower value.

As in the case of the PT, their role is also to separate the power grid of the measurement, protection and control grid, which use the value of the voltage.

High Voltage PT

Connection to the Measurement Devices

Typical Connection

Here is a PT (in fact, made up of 3 distinct PT for each phase) with a primary winding and two secondary windings: a measurement winding and a residual voltage winding.

Primary Winding

The primary winding is coupled (wye connection) with the neutral point to the ground. That is why 6000/√3 is indicated, because each winding is submitted and thus measures the simple voltage which is equal to the line voltage divided by √3.

Secondary Winding

The secondary measurement winding (made up of 3 coils) is wye connected to the measurement apparatus. The neutral point is connected to the ground. For an assigned voltage of 6000/√3 V at the primary, there is 100/√3 V at the secondary.

The secondary residual stress winding is delta coupled. This is an open triangle in which a resistance has been inserted at the “opened” terminals, which amounts to putting the resistance in series in one of the branch of the triangle. These windings provide 100/3 V when there is 6000/√3 V at the primary.

Resistance and Resonance

The purpose of this resistance is to avoid resonance. Resonance is a phenomenon which appears during transitory periods, atmospheric over-voltages, triggering of transformers or loads, appearance or disappearance of defects and live work. The abnormal harmonic rates, the permanent or transitory over-voltages or over-currents it causes are often dangerous for the electric equipment.

The insertion of this resistance allows the absorption of the energy which maintains the resonance. It does not disturb the precision of the measure and does not consume additional power in normal use.

As for the CT, the power and the precision class are given for the 2 secondaries.
The manufacturer also gives the assigned insulation voltage, the assigned frequency, the assigned voltage factor, and the heating power.

Tores or Residual Current Sensors

If the currents in the three phases are balanced, the resulting magnetic field they create is null and the sensor does not deliver a current at exit. If a defect occurs between phase and ground the currents circulating in the conductors will not be balanced anymore and the tore will deliver a current. The exit of the tore is connected to a specific protection which is relay 64.


In the event of going beyond a fault current threshold and a planned length, relay 64 orders the triggering of the circuit-breaker. Relay 86 will lock the circuit breaker in open position until its order of reclosing is given.

4 thoughts on “Measurements (CT, PT, and Tore)

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