140XBP00600 SCHNEIDER

When the value given by the formula is greater than 3KV peak, it is necessary to use 

Metrosils. They are connected across the relay circuit and they allow to shunt the secondary 

current output of the current transformer from the relay in order to prevent very high 

secondary voltages. 

Metrosils are externally mounted and have annular discs shape. 

Their operating characteristics is according to the formula: 

V = C.I0.25

Where 

− V: Instantaneous voltage applied to the non-linear resistor (Metrosil) 

− C: Constant of the non-linear resistor (Metrosil) 

− I: Instantaneous current through the non-linear resistor (Metrosil) 

With the sinusoidal voltage applied across the Metrosil, the RMS current would be 

approximately 0.25 times the peak current. This current value can be calculated as follows: 

4

52.0 2).(

⎭

⎬

⎫

⎩

⎨

⎧ = C

Irms rmsVs

Where 

− Vs(rms): RMS value of the sinusoidal voltage applied across the Metrosil. 

This is due to the fact that the current waveform through the Metrosil is not sinusoidal but 

appreciably distorted. 

For satisfactory application of the non-linear resistor (Metrosil), its characteristics should 

comply with the following requirements: 

− At the relay voltage setting, the non-linear resistor (Metrosil) current should be as 

low as possible, but no greater than approximately 30mA rms for 1A current 

transformers and approximately 100mA rms for 5A current transformer. 

− At the maximum secondary current, the non-linear resistor (Metrosil) should limit 

the voltage to 1500V rms or 2120V peak for 0.25 second. At higher relay voltage 

settings, it is not always possible to limit the fault voltage to 1500V rms, so higher 

fault voltage may have to be tolerated. 

The following tables show the typical types of Metrosil that will be required, depending on 

relay current rating, REF voltage setting etc.