UTC LM1851
LINEAR INTEGRATED CIRCUIT
S1 HOT
HOT
LINE
NEUTRAL
GFI
NEUTRAL
RG (0.8) I
1.6
RN RB
0.4 500
(0.2) I
I
UL943 specifies ≤25 ms average trip time under these conditions. Calculation of Ct based upon charging currents
due to normal fault only is as follows:
≤25 ms Specification
-3 ms GFI turn-on time (15k and 1μF)
-8 ms Potential loss of one half-cycle due to fault current sense of half-cycles only
-4 ms Time required to open a sluggish circuit breaker
≤10 ms Maximum integration time that could be allowed
8 ms Value of integration time that accommodates component tolerances and other variables
Ct=
1× T
V
(5)
Where T=integration time
V=threshold voltage
I=average fault current into Ct
I=
120VAC(rms)
RB
×
RN
RG + RN
heavy fault
current generated
(swamps ITH)
portion of
fault current
shunted
around GFI
×
1 turn
1000 turns
1
× 2×
0.91
(6)
current
division of
input sense
transformer
Ct charging
on half-
cycles only
rms to
average
conversion
therefore:
120
Ct = 500
0.4
× 1.6+0.4 ×
1
1000
×
1
2
×
0.91
Ct = 0.01μF
17.5
× 0.0008
(7)
UTC UNISONIC TECHNOLOGIES CO., LTD. 6
QW-R122-007,A