MV design guide - Schneider Electric
MV design guide - Schneider Electric
MV design guide - Schneider Electric
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Switchgear<br />
definition<br />
Current transformer<br />
c An ammeter is generally guaranteed to withstand a short-time current of<br />
10 Ir, i.e. 50 A for a 5 A device.<br />
To be sure that this device will not be destoyed in the case of a primary<br />
fault, the current transformer must be saturated before 10 Ir in the<br />
secondary. A safety factory of 5 is suitable.<br />
c In accordance with the standards, <strong>Schneider</strong> <strong>Electric</strong> CT's have a safety<br />
factor of 10. However, according to the current consumer characteristic a<br />
lower safety factor can be requested.<br />
Accuracy limit factor (ALF)<br />
In protection applications, we have two constraints: having an accuracy<br />
limit factor and an accuracy class suited to the application.<br />
We will determine the required ALF in the following manner:<br />
Definite time overcurrent protection.<br />
c The relay will function perfectly if:<br />
ALF real of CT > 2 •<br />
Ire<br />
Isr<br />
Ire : relay threshold setting<br />
Isr : rated secondary current of the CT<br />
c For a relay with two setting thresholds, we will use the highest<br />
threshold,<br />
v For a transformer feeder, we will generally have an instantaneous high<br />
threshold set at 14 Ir max., giving the real ALF required > 28<br />
v for a motor feeder, we will generally have a high threshold set to 8 Ir<br />
max., giving a real ALF required > 16.<br />
Inverse definite time overcurrent protection<br />
c In all cases, refer to the relay manufacturer's technical datasheet.<br />
For these protection devices, the CT must guarantee accuracy across the<br />
whole trip curve for the relay up to 10 times the setting current.<br />
ALF real > 20 • Ire<br />
c Special cases:<br />
v if the maximum short-circuit current is greater than or equal to 10 Ire:<br />
ALF real > 20 • Ire<br />
Ire : relay setting threshold<br />
v if the maximum short-circuit current is less than 10 Ire:<br />
Isr<br />
ALF real > 2 •<br />
Isc secondary<br />
Isr<br />
v if the protection device has an instantaneous high threshold that is<br />
used, (never true for feeders to other switchboards or for incomers):<br />
ALF real > 2 • Ir2<br />
Isr<br />
Ir2 : instantaneous high setting threshold for the module<br />
58 Merlin Gerin <strong>MV</strong> <strong>design</strong> <strong>guide</strong> <strong>Schneider</strong> <strong>Electric</strong>