GM1851

LINEAR INTEGRATED CIRCUIT

UNISONIC TECHNOLOGIES CO., LTD

7 of 10

www.unisonic.com.tw

QW-R122-007,B

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

1 × T

V

(5)

Where T=integration time

V=threshold voltage

I =

×

heavy fault

current generated

portion of

fault current

shunted

around GFI

(6)

1 turn

1000 turns

1

2

0.91

××

current

division of

input sense

transformer

Ct charging

on half-

cycles only

rms to

average

conversion

×

therefore:

×

120

500

0.4

1.6+0.4

1

1000

××

1

2

0.91

×

× 0.0008

17.5

(7)

In practice, the actual value of C1 will have to be modified to include the effects of the neutral loop upon the net

charging current. The effect of neutral loop induced currents is difficult to quartile, but typically they sum with normal

fault currents, thus allowing a larger value of C1.

For UL943 requirements, 0.015μF has been found to be the best compromise between timing and noise.

For those GFI standards not requiring grounded neutral detection, a still larger value capacitor can be used and

better noise immunity obtained. The larger capacitor can be accommodated because RN and RG are not present,

allowing the full fault current, I, to enter the GFI.

In Figure 2, grounded neutral detection is accomplished by feeding the neutral coil with 120 Hz energy

continuously and allowing some of the energy to couple into the sense transformer during conditions of neutral fault.