With a sinusoidal voltage applied across the Metrosil, the RMS current would be approximately 0.52 x the peak
curr
ent. This current value can be calculated as follows:
I
V
C
RMS
S RMS
=
0 52
2
4
.
( )
where:
● V
S(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.
The Metrosil characteristic should be such that it complies with the following requirements:
● The Metrosil current should be as low as possible, and no greater than 30 mA RMS for 1 A current
transformers or 100 mA RMS for 5 A current transformers.
● At the maximum secondary current, the Metrosil should limit the voltage to 1500 V RMS or 2120 V peak for
0.25 second. At higher device voltages it is not always possible to limit the fault voltage to 1500 V rms, so
higher fault voltages may have to be tolerated.
The following tables show the typical Metrosil types that will be required, depending on relay current rating, REF
voltage setting etc.
Metrosils for devices with a 1 Amp CT
The Metrosil units with 1 Amp CTs have been designed to comply with the following restrictions:
● The Metrosil current should be less than 30 mA rms.
● At the maximum secondary internal fault current the Metrosil should limit the voltage to 1500 V rms if
possible.
The Metrosil units normally recommended for use with 1Amp CTs are as shown in the following table:
Nominal Characteristic Recommended Metrosil Type
Device Voltage Setting C
b
Single Pole Relay Triple Pole Relay
Up to 125 V RMS 450 0.25 600A/S1/S256 600A/S3/1/S802
125 to 300 V RMS 900 0.25 600A/S1/S1088 600A/S3/1/S1195
Note:
Single pole Metr
osil units are normally supplied without mounting brackets unless otherwise specified by the customer.
Metrosils for devices with a 5 Amp CT
These Metr
osil units have been designed to comply with the following requirements:
● The Metrosil current should be less than 100 mA rms (the actual maximum currents passed by the devices
shown below their type description.
● At the maximum secondary internal fault current the Metrosil should limit the voltage to 1500 V rms for
0.25secs. At the higher relay settings, it is not possible to limit the fault voltage to 1500 V rms so higher fault
voltages have to be tolerated.
The Metrosil units normally recommended for use with 5 Amp CTs and single pole relays are as shown in the
following table:
Secondary Internal Fault Current Recommended Metrosil types for various voltage settings
Amps RMS Up to 200 V RMS 250 V RMS 275 V RMS 300 V RMS
Chapter 9 - Current Transformer Requirements P24xM
198 P24xM-TM-EN-2.1
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