Metrosils are externally mounted and take the form of annular discs. Their operating characteristics follow the
expression:
V = CI
0.25
wher
e:
● V = Instantaneous voltage applied to the Metrosil
● C = Constant of the Metrosil
● I = Instantaneous current through the Metrosil
With a sinusoidal voltage applied across the Metrosil, the RMS current would be approximately 0.52 x the peak
current. This current value can be calculated as follows:
I
V
C
RMS
S RMS
=
0 52
2
4
.
( )
where:
● V
S(RMS)
= RMS v
alue 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 Metrosil units ar
e normally supplied without mounting brackets unless otherwise specified by the customer.
Chapter 9 - Current Transformer Requirements P14x
204 P14xEd1-TM-EN-1
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