I do speak and read English but often the regs (such as the 411.5.1) really confuse me how they are written.

You're certainly not alone there! The regs have been around in some form since 1882 - so we're having to contend with about 140 years worth of 'amendement by committee' as well as attempts to merge in requirements from other standards (e.g. IEC ones) that might have been originally written from a different standpoint or using a different style of language. Often definitions don't keep up with changes in technology or accepted practice so we end up with an increasing amount of 'interpretation' to make sense of things.

Take the definition of an exposed-conductive-part for instance - on the face of it it would seem only to apply to things than can be easily touched - yet we normally consider things like flush steel back boxes or the steel armour of SWA cables to be exposed-conductive-parts as their connection to Earth is fundamental to providing protection against electric shock by automatic disconnection of supply. With SWA for instance, an armoured cable is normally selected because it's in a situation where damage to the cable is foreseeable - mechanically the armour alone isn't sufficient to keep things safe from all damage - nails or digging tools can easily displace the individual wires and penetrate through to the live conductors - so we earth the armour and arrange the protective devices so that if something conductive does penetrate the cable then the supply is cut off. Even then the armour itself isn't often directly exposed but the nail or garden fork that's in contact with it will be - so from a shock prevention point of view the result is the same as if the armour could be touched directly. Likewise with flush back boxes - if the box was allowed to remain live after a fault, then the (possibly slightly damp) plastered wall a few mm away will be at a similar hazardous voltage (often referred to as "live wall") - so pose a risk of shock in the same way as if the box itself could be touched.

But, perhaps paradoxically, we do rely on the outer plastic sheath of SWA cables and the boots over glands to keep those earthed parts out of reach in normal conditions - e.g. when we're running a TN cable into a TT zone - where otherwise simultaneously accessible parts would be connected to different earthing systems. So perhaps we should have some sympathy for the difficulty in coming up with a simple definition that covers all situations nicely.

As for a practical solution, I'd agree with Zoomup - adding an extra RCD as the transition point from TN to TT avoids any doubt, they're not particularly expensive these days and the duplication has the benefit of reducing the risk from a sticky/faulty RCD considerably. There's no loss of discrimination since nothing would loose power as a result of the new RCD tripping that wouldn't have lost power anyway when the original RCD tripped, if anything the situation might be slightly improved since there is a small possibility of the new RCD tripping first, leaving the upstream RCD still on.

The transition from TN to TT does require some care however - a fault from a pre-RCD line conductor to the TT earth won't usually result in disconnection but will raise the TT earthing system to 230V and that dangerous situation can persist indefinitely. So where there's no upstream RCD, or you prefer not to reply on it, then all the live wiring upstream of the RCD within the TT area needs to be protected by double or reinforced insulation (or equivalent).

   - Andy.

I do speak and read English but often the regs (such as the 411.5.1) really confuse me how they are written.

You're certainly not alone there! The regs have been around in some form since 1882 - so we're having to contend with about 140 years worth of 'amendement by committee' as well as attempts to merge in requirements from other standards (e.g. IEC ones) that might have been originally written from a different standpoint or using a different style of language. Often definitions don't keep up with changes in technology or accepted practice so we end up with an increasing amount of 'interpretation' to make sense of things.

Take the definition of an exposed-conductive-part for instance - on the face of it it would seem only to apply to things than can be easily touched - yet we normally consider things like flush steel back boxes or the steel armour of SWA cables to be exposed-conductive-parts as their connection to Earth is fundamental to providing protection against electric shock by automatic disconnection of supply. With SWA for instance, an armoured cable is normally selected because it's in a situation where damage to the cable is foreseeable - mechanically the armour alone isn't sufficient to keep things safe from all damage - nails or digging tools can easily displace the individual wires and penetrate through to the live conductors - so we earth the armour and arrange the protective devices so that if something conductive does penetrate the cable then the supply is cut off. Even then the armour itself isn't often directly exposed but the nail or garden fork that's in contact with it will be - so from a shock prevention point of view the result is the same as if the armour could be touched directly. Likewise with flush back boxes - if the box was allowed to remain live after a fault, then the (possibly slightly damp) plastered wall a few mm away will be at a similar hazardous voltage (often referred to as "live wall") - so pose a risk of shock in the same way as if the box itself could be touched.

But, perhaps paradoxically, we do rely on the outer plastic sheath of SWA cables and the boots over glands to keep those earthed parts out of reach in normal conditions - e.g. when we're running a TN cable into a TT zone - where otherwise simultaneously accessible parts would be connected to different earthing systems. So perhaps we should have some sympathy for the difficulty in coming up with a simple definition that covers all situations nicely.

As for a practical solution, I'd agree with Zoomup - adding an extra RCD as the transition point from TN to TT avoids any doubt, they're not particularly expensive these days and the duplication has the benefit of reducing the risk from a sticky/faulty RCD considerably. There's no loss of discrimination since nothing would loose power as a result of the new RCD tripping that wouldn't have lost power anyway when the original RCD tripped, if anything the situation might be slightly improved since there is a small possibility of the new RCD tripping first, leaving the upstream RCD still on.

The transition from TN to TT does require some care however - a fault from a pre-RCD line conductor to the TT earth won't usually result in disconnection but will raise the TT earthing system to 230V and that dangerous situation can persist indefinitely. So where there's no upstream RCD, or you prefer not to reply on it, then all the live wiring upstream of the RCD within the TT area needs to be protected by double or reinforced insulation (or equivalent).

   - Andy.