© 2003 Fairchild Semiconductor Corporation

www.fairchildsemi.com

RV4141A • Rev. 1.0.8

6

The SCR anode is directly connected to a solenoid or

relay coil. It can be tripped only when its anode is more

positive than its cathode. It must have a high dV/dt

rating to ensure that line noise (generated by electrically

noisy appliances) does not falsely trigger it. Also the

SCR must have a gate drive requirement less than

200μA. C3 is a noise filter that prevents high-frequency

line pulses from triggering the SCR. The relay solenoid

should have a response time of 3ms or less to meet the

UL 943 timing requirement.

Sense Transformers and Cores

The sense and ground/neutral transformer cores are

usually fabricated using high-permeability laminated

steel rings. Their single-turn primary is created by

passing the line and neutral wires through the center of

its core. The secondary is usually from 200 to 1500

turns. Transformers may be obtained from Magnetic

Determine

the

nominal

ground-fault

trip-current

requirement. This is typically 5mA in North America

Determine the minimum delay time required to prevent

nuisance tripping, typically 1 to 2ms. The value of C2

required to provide the desired delay time is:

t

C



 6

2

(2)

where:

C2 is in Nf and t is the desired delay time in ms.

current specification is:

180(t/P)

COS

I

N

4.6

R

FAULT

SET







(3)

where:

t is the time delay in ms;

P is the period of the line frequency in ms;

N is the number of sense transformer secondary turns.

Note:

2.

This formula assumes an ideal sense transformer is

changed up to 30% when using a non-ideal

transformer.

Figure 3. GFI Application Circuit