Assuming this is a general question, then my answer is you are trading off:
- Reduced risk of a PME-fault making bonded (Earthed to most folk) equipment live.
- Increased dependency on the RCD action to clear any fault (which is less reliable then the MCB trip action).
Now I don't really have any numerical values of the probability of either fault leading to death/injury so I can't honestly advise either way. But if going TT then I
personally would spend the extra to have a 100mA S-type incomer and separate 30mA RCBOs on the final circuits to avoid any single point of failure in earth fault protection.
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With TT you have two things to test as both are needed for fault clearing:
- Earth rod impedance
- RCD trip action
Testing the RCD is standard for any MFT, the issue really is the earth rod and making sure it is reliably below 200 ohms (for most systems at 100mA/30mA RCD action).
Testing the rod without AC power is more fundamental, but needs suitable test equipment and many MFT lack that capability, and usually involves a bit of poking rods in the ground at a fair distance to see if you get the plateau for the "fall of potential" result. But there are various other test approaches (mostly more expensive in equipment to carry out).
If you have AC power then simply using your MFT to do a Zs measurement with the rod as the earth is easiest, but needs more care and may lead to the unfortunate case of finding out it is not good enough and having to isolate it again before getting another rod (assuming the first was not driven down far enough).
That is another risk with going TT - you need a earth rod (or equivalent reliable connection to Earth) and some folk just hammer it in without checking if there are buried pipes or cables in the area!