IT system - how do RCD's ever work

[Julie.]

That is not really the case though as even a domestic TT system can have currents to tens of amps on each final circuit, but a 200 ohm rod so PFC below 1A. I guess the defining point would be not the FLC but the operating point of the designed protection system (so in such a TT case the RCD would be tripping typically for faults below 7.6k ohm at most).

Which kind of returns to the original discussion about IT systems and that they are normally designed to continue under single fault conditions, and ideally safely (as far as practical) so you might be looking at earthing impedances of tens of k ohm if trying to keep the 2nd fault touch shock current below, say, 10mA (assuming that is even possible depending on filter caps, etc).

Yeah, I am not sure how to word it though, TT systems still have a decent amount of current in the order of amps (minimum should be above 1A ) whilst IT systems are usually 2 or more orders of magnitude smaller than this - usually 10mA or less.

What I am trying to say, is that an earth fault on a TT OR TN system is a reasonable level to detect - you could use a 100mA rcd and still have fault current in excess of 10x the trip setting, these are easy and almost foolproof systems to be able to detect an earth fault.

This is a vastly different situation to an IT system where the current, on first fault will be in the order of mA or less.

There isn't really a cross over where you might have an IT system having a first fault current of 100mA, or a TT system of also 100mA (200 ohm is the expected minimum).

Detection of first faults is likely to have to be much less than 1mA unless you are forcing a first trip situation by upping this to 10mA or so.

 

[Simon47]

Where are these dangerous touch voltages coming drom

... if trying to keep the 2nd fault touch shock current below, say, 10mA

That implies a situation where at least some items are neither Class II nor earthed/bonded - which is not generally permitted for obvious reasons.
Yes, there will some cases where the IT system is there to directly control touch currents - shaver sockets come to mind. But in the context of the OP's IT supply to a distribution panel, all the normal protective measures (principally bonding via CPCs) are (or should be) there to control touch voltages/currents.

 

[pc1966]

My thoust was as much a "if you need to chose a single-fault threshold" sort of point than seeing big IT supplies as touch-safe.