I've seen many lighting circuits protected by 100mA ELCB instead of 30mA, why is that? isn't 30mA safer than 100mA?
Yes, 30mA is safer than 100mA. 10mA, the lowest standard size, is safer still.
The actual device used depends on what is being protected, what it's being protected against and the regulations applying to the location where it's being used (both in terms of country, state etc. and type of building, i.e. domestic premises, commercial, school, hospital etc.)
For protecting people against electric shock in domestic premises in the UK the maximum RCD current is 30mA. In hospitals it would be more likely to be 10mA.
However, in other situations, it's not necessarily personal shock protection you're looking for. In commercial premises, you would expect to have trained electricians changing light bulbs rather than normal untrained employees, and the RCD is there for protection against other things like fire, water leakages and so on. You would, however, still expect ordinary employees to plug things like lamps, phone supplies, computers, kettles and vacuum cleaners into wall sockets so the requirements on those would be for personal protection.
So why would you ever use a larger RCD at all? It's to prevent "nuisance tripping". This can occur when a large number of devices are connected to the same circuit, especially when the devices have electronic controls or surge protection fitted, which may cause them to have a constant small leakage current to earth. Even if the continuous leakage current were not sufficient to trip the RCD, spikes and surges on the power line can cause peaks in earth current that are enough to push the total leakage current over the trip limit and shut off the circuit. The same happens when circuits are spread over a large area, as there is more cabling to pick up spikes caused by interference from other equipment.
For instance, thirty light fittings with a leakage of 1mA each, perfectly possible in a large office, would trip a 30mA RCD despite being perfectly safe. Using a 100mA RCD may be permitted in such circumstances to overcome this; if regulations don't allow it, the circuits would have to be split up into smaller ones.
This is a particular issue on circuits supplying power to a lot of IT equipment such as PCs and printers, where 30mA must be used for personal protection and the general rule of thumb is that no more than ten PCs or other devices can be connected to each RCD.