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Chapter 8 - Restricted Earth Fault Protection
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:
2
I
0 52
.
=
RMS
where:
●
V
= RMS value of the sinusoidal voltage applied across the metrosil.
S(RMS)
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:
Device Voltage Setting
Up to 125 V RMS
125 to 300 V RMS
Note:
Single pole Metrosil units are normally supplied without mounting brackets unless otherwise specified by the customer.
Metrosils for devices with a 5 Amp CT
These Metrosil 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
Amps RMS
188
4
V
S RMS
(
)
C
Nominal Characteristic
b
C
450
0.25
900
0.25
Recommended Metrosil types for various voltage settings
Up to 200 V RMS 250 V RMS
Recommended Metrosil Type
Single Pole Relay
600A/S1/S256
600A/S1/S1088
275 V RMS
Triple Pole Relay
600A/S3/1/S802
600A/S3/1/S1195
300 V RMS
P64x-TM-EN-1.3
P64x
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Troubleshooting
