PRODUCT SPECIFICATION

FAN1851A

REV. 2.0.1 6/17/05

7

where:

T = integration time

V = threshold voltage

I = average fault current into CT

(6)

therefore:

(7)

I

120 V

()

R

B

-------------------------------------

⎝⎠

⎛⎞

RN

RG

RN

+

-----------------------

⎝⎠

⎛⎞

=

heavy fault

current generated

portion of fault

current shunted

around GFI

1 turn

1000 turns

-------------------------

⎝⎠

⎛⎞

1

2

---

⎝⎠

⎛⎞

0.91

()

×

×

×

current

division of

input sense

transformer

CT

charging

on half-

cycles

only

rms to

average

conversion

C

T

120

500

---------

⎝⎠

⎛⎞

0.4

1.6

0.4

+

---------------------

⎝⎠

⎛⎞

×

1

1000

------------

⎝⎠

⎛⎞

×

1

2

---

⎝⎠

⎛⎞

×

0.91

()

×

17.5

------------------------------------------------------------------------------------------------------------------0.008

×

=

C

T

0.01 µF

=

to include the effects of the neutral loop upon the net charg-

ing current. The effect of neutral loop induced currents is dif-

ficult to quantify, but typically they sum with normal fault

requirements, 0.015µF has been found to be the best com-

promise between timing and noise.

For those GFI standards not requiring grounded neutral

detection, a still larger value capacity can be used and better

noise immunity obtained. The larger capacitor can be accom-

full fault current, I, to enter the GFI.

In Figure 10, grounded neutral detection is accomplished by

feeding the neutral coil with 120Hz energy continuously and

allowing some of the energy to couple into the sense trans-

former during conditions of neutral fault.

Transformers may be obtained from Magnetic Metals, Inc.,

(http://www.magmet.com).