"Is there any way that I can test the earth leakage to each radial circuit, and also over the installation as an whole?"
Yes, there is! And this refers to both parts of the question i.e. radials and whole installation. What's more, most times the residual current (the more correct name for "earth leakage") can be measured live. This proves a huge boon to IT 'experts' as they do not like having their stuff shut down, even if they are dual power fed!
Before covering the methods, there is one basic requirement that must be possible before embarking on this task.
You must be able to separate out the individual Phase(s) and Neutral of the circuit being tested. I have seen people try to measure residual current by putting the (correct) current clamp around the whole cable. Unfortunately, some don't quite understand the residual current may well be flowing down the earth within the cable and thus cancelling out what they were trying to measure (in this game, I have seen pretty much everything!).
This point highlights the importance of having a well designed distribution board. The measurement method used will depend on the wiring of the distribution board. If the board has been wired too closely, and things are a little 'tight', then things do get a little tricky and an alternative plan is called for. We'll cover the various methods in detail.
Unfortunately, this is the only option open for measuring the residual current of the whole installation as one requires access to the main incoming supply cable Phases and Neutral (as a group).
There are some tricks I can pull to measure residual current should the cables be too far apart for one clamp - but that is a trade secret!
If the circuits can be disconnected (unplugged) for short periods such as a few seconds, then the slightly better option (should the distribution board not be ideal) is to use a test block which consists of a "blue" plug and socket connected to each other but the wire in the middle is separated out so as to only have the Phase and Neutral so as to measure the residual current. It is, sadly, not just a case of separating out the wires as the pair (Phase and Neutral) must be shielded so as to ensure it is only the current measured, not any coupled AC voltage.
The introduction highlighted the importance of having a well designed distribution board. However, there is a way of measuring residual current on individual circuits without having to do so within the distribution board. The only requirement is the circuit must be available for measuring (in the form of a cable or similar).
If it is a cable (such as a radial feed from a Power Distribution Unit), then it is imperative that the Earth to the circuit must be able to, and be disconnected (only during the measurement, of course!).
Before the main Earth is disconnected, a secondary (temporary) Earth is supplied to the device (radial) under test. The temporary is put in place should any fault currents occur during test; These then flow down the alternative Earth.
Do not be tempted to rely on other earthing that may be in place unless 100% certain this earthing can carry the full whack of a Live-Earth fault (including keeping touch potentials within limits) should this unlikely event occur during testing.
With the temporary Earth in place, the clamp is placed around the cable and the real Earth disconnected. As this temporary Earth is not within the current clamp, any residual currents are not cancelled and can now be measured.
As can be envisaged, all work can be performed LIVE, which is a huge boon as IT 'experts' do not like having their stuff shut down - even if they are dual power fed!