rcd
RCD-Testers can be used to check if the residual current device (RCD) of the installation is properly working. How to they do this?
I want to build my own RCD-tester. Which circuit can be used to generate a certain fault current? (e.g. 30mA AC or 30mA DC)?
As Martin hinted in his comment, an RCD is most usefull in a situation where a fault is likely to cause a current flow to ground/earth. This is actually a very probable situation for a fault: touching one wire (while being somewhat ground-connected) is much more likely than touching two wires simultaneously. And it doesn't need to be you: the compromised live wire could touch a grounded part of the appliance.
In my country (230V, both wires live) there are generally two classes of appliances: metal casing earthed, and insluated casing 'double isolated'. In this situation a RCD is very usefull for detecting a compromised wire touching a metal part of an earthed appliance.
What kind of self-test circuits or method do they used in the GFCI outlets generally?
An temporary imbalance (leak) between the hot and neutral is created, and the GFCI circuit should detect it.
Is it just firmware or is there a physical addition.
There is a physical addition: a resistor for creating a leak, some kind of switch for connecting and disconnecting this resistor. (By the way, most GFCI deices are hard-wired, and they don't have microcontrollers with firmware.)
I've designed an GFCI (RCD) circuit once, and it has a self-test. R9 provides the controlled leakage. U2 is the solid state relay for connecting the R9 during self-test and disconnecting it the rest of the time.
Best Answer
They contain a current source that is able to generate a current of any waveform and polarity between neutral an earth.
Cheaper ones may contain an array of resistors, those can only test AC and rectified AC fault currents.
If you have access to both sides of the RCD, the equipment can become a bit less complicated. Since then you don't need earth.