Shower with no RCD or supplementary bonding

geoffsd: 
 

Could someone please explain how the R<50/Ia actually affects anything and makes a difference?

It's intended to keep the voltage between simultaneously-accessible exposed-conductive-parts at less than 50 V for faults that last up to 5 seconds (with Type B and Type C mcb's and RCBO's however, it's 50 V for faults above 0.1 seconds), so that if you are unfortunate enough to be touching the two parts when a fault occurs it is, to all intents and purposes less of a shock (pun certainly intended).

It won't make a difference to someone in Zone 0 of a bath, but in general it provides a reasonable level of protection, effectively limiting the touch voltage for long-duration faults to ELV level (higher current, quicker operating time, but still well below the 230 V level so reasonable).

In medical locations of Group 1 or Group 2, the resistance is modified to  0.2 ohms - but see also Regulation 710.411.3.2.5 where the voltage available between simultaneously accessible exposed- and/or extraneous-conductive-parts from a fault in a Group 1 or Group 2 medical location must be limited for ALL faults to 25 V AC or 60 V DC.

geoffsd: 
 

Could someone please explain how the R<50/Ia actually affects anything and makes a difference?

It's intended to keep the voltage between simultaneously-accessible exposed-conductive-parts at less than 50 V for faults that last up to 5 seconds (with Type B and Type C mcb's and RCBO's however, it's 50 V for faults above 0.1 seconds), so that if you are unfortunate enough to be touching the two parts when a fault occurs it is, to all intents and purposes less of a shock (pun certainly intended).

It won't make a difference to someone in Zone 0 of a bath, but in general it provides a reasonable level of protection, effectively limiting the touch voltage for long-duration faults to ELV level (higher current, quicker operating time, but still well below the 230 V level so reasonable).

In medical locations of Group 1 or Group 2, the resistance is modified to  0.2 ohms - but see also Regulation 710.411.3.2.5 where the voltage available between simultaneously accessible exposed- and/or extraneous-conductive-parts from a fault in a Group 1 or Group 2 medical location must be limited for ALL faults to 25 V AC or 60 V DC.