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 diffi-

cult to quantize but typically they sum with normal fault cur-

rents thus allowing a larger value of C1

For UL943 requirements 0015 mF has been found to be

the best compromise between timing and noise

For those GFI standards not requiring grounded neutral de-

tection a still larger value capacitor can be used and better

noise immunity obtained The larger capacitor can be ac-

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 trans-

former during conditions of neutral fault

Typical Application

TLH5177 – 6

FIGURE 2 120 Hz Neutral Transformer Approach

6