58
Merlin Gerin MV design guide
Schneider Electric
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An ammeter is generally guaranteed to withstand a short-time current of
10 I
r
, i.e. 50 A for a 5 A device.
To be sure that this device will not be destoyed in the case of a primary
fault, the current transformer must be saturated before 10 I
r
in the
secondary. A safety factory of 5 is suitable.
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In accordance with the standards, Schneider Electric CT's have a safety
factor of 10. However, according to the current consumer characteristic a
lower safety factor can be requested.
Accuracy limit factor (ALF)
In protection applications, we have two constraints: having an accuracy
limit factor and an accuracy class suited to the application.
We will determine the required ALF in the following manner:
Definite time overcurrent protection.
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The relay will function perfectly if:
I
re
:
relay threshold setting
I
sr
:
rated secondary current of the CT
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For a relay with two setting thresholds, we will use the highest
threshold,
v
For a transformer feeder, we will generally have an instantaneous high
threshold set at 14 I
r
max., giving the real ALF required > 28
v
for a motor feeder, we will generally have a high threshold set to 8 I
r
max., giving a real ALF required > 16.
Inverse definite time overcurrent protection
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In all cases, refer to the relay manufacturer's technical datasheet.
For these protection devices, the CT must guarantee accuracy across the
whole trip curve for the relay up to 10 times the setting current.
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Special cases:
v
if the maximum short-circuit current is greater than or equal to 10 Ire:
I
re
:
relay setting threshold
v
if the maximum short-circuit current is less than 10 Ire:
v
if the protection device has an instantaneous high threshold that is
used, (never true for feeders to other switchboards or for incomers):
I
r2
:
instantaneous high setting threshold for the module
Switchgear
definition
Current transformer
I
re
I
sr
ALF
real of CT
> 2 ·
ALF
real
> 20 · I
re
ALF
real
> 20 ·
I
re
I
sr
ALF
real
> 2 ·
I
sc secondary
I
sr
ALF
real
> 2 · I
r2
I
sr