How do you achieve shock risk protection in TT

[King84]

Hi everyone

I posed a question to my teacher when I was in Level 3 about if we do not use 30mA RCD for additional protection in TT system and use 100mA for fault protection then how do we stay protected from electrical shocks to which he said RA*IA>=50V something like this and when I said could you elaborate to which he just changed the subject.
I know Ra is resistance of earthelectrode and cpc and IA is nominal tripping current but how does this fit into any situation to ensure that voltage stays below 50V thus protection is ensured?
I tried to look further into this but still do not get grip on this concept if someone could throw some light.

Furthermore, there is another scenario which maybe not related to TT but just a general question about a hypothetical situation.
If there is a proterty which does not have an RCD or even if have an RCD but lets say the Main bonding is done on the entry of water and gas meters but after a minimum length plastic couplers have been used by plumbers then furtherdown after these plastic pushfittings a damaged wire touches the metal pipes and obviously pipes would become live so how would someone accidently touches the pipies would stay protected as plastic pushfits makes the main protective bonding pretty useless ? I have attached picture as I have seen this situation in one group so just got me thinking as I got confused. Anything with knowledge if could throw some light thanks

 

[baldelectrician]

Water has plastic connections and pipes, gas has (after the meter) only copper connections

Now we enter the debate of if there is a gas plastic incomer then it may not need to be bonded.

In an ideal world we should have straight RCBO's on installs but a hybrid is cheaper (RCBO on lights and heating with spilt load on the rest).

In the picture above the RCD may well have tripped