Shower with no RCD or supplementary bonding

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

My attempt…

Supplementary bonding satisfying the R<50/Ia rule won't guarantee the touch voltage will never ever exceed 50V - clearly if the earth fault current exceeds Ia (which it usually will in TN systems if you're aiming for 0.4s disconnection times) then 50V could quite plausibly be exceeded.

What it does do though is ensure that 50V can't be exceeded if disconnection time takes 5s or more - and given the characteristics of most overcurrent protective devices(*) - that puts you on a curve whereby if the situation meets 50V and 5s, it's going to meet (if the fault current is higher) 0.4s at 115V (or higher again) 0.2s at 230V - i.e. below the ‘safe’ line for electric shock.

So in an installation where 5s disconnection times might be present (and thus on a TN system people might be exposed to 115V or more for up to 5s) - either from large/old circuits within the bathroom itself, or distribution circuits etc outside the bathroom but touch voltages imported through extraneous-conductive-parts or shared c.p.c.s - supplementary bonding within the bathroom gives protection similar to 0.4s disconnection times on everything.

(*) Clearly the idea originated with fuses so that there was a definite relationship between increased current and decreased disconnection time - so it might seem at first glance that MCBs with their instantaneous operation above a certain fault current wouldn't support this approach, but for (B and C types at least) it does sort of come out in the wash, since if they meet the requirement for 5s (which they'd do in <0.1) then they'd also meet the 0.4s requirement etc (if still <0.1).

The big gotcha is making sure you choose Ia for the worst case device/circuit that could impose a touch voltage in the bathroom - which could be a circuit almost anywhere in the installation - and not just consider the bathroom circuits alone.

   - Andy.

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

My attempt…

Supplementary bonding satisfying the R<50/Ia rule won't guarantee the touch voltage will never ever exceed 50V - clearly if the earth fault current exceeds Ia (which it usually will in TN systems if you're aiming for 0.4s disconnection times) then 50V could quite plausibly be exceeded.

What it does do though is ensure that 50V can't be exceeded if disconnection time takes 5s or more - and given the characteristics of most overcurrent protective devices(*) - that puts you on a curve whereby if the situation meets 50V and 5s, it's going to meet (if the fault current is higher) 0.4s at 115V (or higher again) 0.2s at 230V - i.e. below the ‘safe’ line for electric shock.

So in an installation where 5s disconnection times might be present (and thus on a TN system people might be exposed to 115V or more for up to 5s) - either from large/old circuits within the bathroom itself, or distribution circuits etc outside the bathroom but touch voltages imported through extraneous-conductive-parts or shared c.p.c.s - supplementary bonding within the bathroom gives protection similar to 0.4s disconnection times on everything.

(*) Clearly the idea originated with fuses so that there was a definite relationship between increased current and decreased disconnection time - so it might seem at first glance that MCBs with their instantaneous operation above a certain fault current wouldn't support this approach, but for (B and C types at least) it does sort of come out in the wash, since if they meet the requirement for 5s (which they'd do in <0.1) then they'd also meet the 0.4s requirement etc (if still <0.1).

The big gotcha is making sure you choose Ia for the worst case device/circuit that could impose a touch voltage in the bathroom - which could be a circuit almost anywhere in the installation - and not just consider the bathroom circuits alone.

   - Andy.