Datasheet

Theory of operation STPMC1
30/77 Doc ID 15728 Rev 7
The value I
MAX
corresponds to the maximum current value hold by each RMS current
register (internal value FFFF). It is a function of the sensor type, sensitivity and of the
current channel gain. Let us suppose that
I
MAX
= 180 A
The tamper condition is evaluated only if
This means that the sum of the RMS value of currents is not negligible with respect to
I
MAX
(the threshold corresponds to about 0.4% of I
MAX
).
In this case this is true since:
I
R
+ I
S
+I
T
+I
N
= 14.4 A > 0,703125 A = I
MAX
/ 256
The criterion for tamper detection is
This can also be expressed as
which means the sIRMS value must not exceed 3.13% of (I
R
+ I
S
+I
T
+I
N
).
In this example:
sIRMS = 0,149967 < 0.45 = (I
R
+ I
S
+I
T
+I
N
) / 32
Then BCS = 0.
Figure 11. Currents of the three phase system in example
∑
>+++=
256
I
IIIII
MAX
NTSRX
()
8
I
i
X
RMS
X
∑
∑
>
()
32
I
4
i
4
i
sIRMS
X
RMS
X
RMS
X
∑
∑
∑
>=
⎟
⎠
⎞
⎜
⎝
⎛
=