High protective conductor currents - Deletion of 543.7.1.204

If a circuit has a protective conductor current of 10mA and we lose the conductor, exposed conductive parts do take on a potential but a human being making contact will not have the full 10mA diverted through their body by virtue of their own body impedance.

True, but the difference is small. To get 10mA from 230V the "leakage" would have an impedance of 230V/0.01A = 23kΩ - add say 1kΩ for body resistance and the current only drops to 9.58mA. Or to put it another way the original leakage current need only have been about 10.5mA to be back to square one.

   - Andy.

gkenyon said:

the vast majority of circuits complying with 411.3.3, would be required to have a maximum protective conductor current of 9 mA

Assuming this is from 531.3.2 which 411.3.3 references in Note 2. The 9mA (30% of 30mA) seems to be only one of a number of suggested solutions to the problem of unwanted tripping which should be "considered", so not really a requirement for all circuits with RCDs. And if a requirement, where does this leave circuits with pc currents of between 9 and 10mA?

Is there evidence for this? Are we seeing regular problems attributed to this?

I've certainly found plenty of instances of 1.0/1.5/2.5 wires popping out of screw terminals when front plates are removed. Maybe the problem is more due to front-plate mounted terminals than the conductor size, with the wiring being stressed as the accessory is pushed home after wiring and any movement of the conductors being then invisible, and the majority of plate accessories using the smaller sizes. Maybe if we'd used more 4.0mm² on socket circuits we'd be less happy about that size not needing duplication.

   - Andy.

Indeed so, AJ. What I stated was a comment made by the electrical engineer in control of safety in a global company. Many years ago when Geoffs proposal was being introduced as a regulation, he had joined a tutorial I was taking for company operatives on the new requirements. He gave me a difficult time and despite the general acceptance of Ohm’s law across the whole world, set out as you have, he was most insistent that the touch current would be of no concern. 
Still, as I understand it, changes were made to their electrical installations in their huge Derry plant to accommodate Mr Blackwell (a hero of mine, by the way).

OlympusMons said:

And if a requirement, where does this leave circuits with pc currents of between 9 and 10mA?

> 9 mA and < 10 mA has no special consideration (except for the fact that you can't power the load straight from a circuit protected by 30 mA RCD without nuisance tripping. The following would be recommended for protective conductor currents exceeding 9 mA

• If the protective conductor current is caused by more than one load, more final circuits are necessary, and 30 mA RCDs can be used.
• If the protective conductor current is caused by one load alone, then that load should be installed considering the product standard and manufacturer's instructions (which may include additional earthing, e.g. as described in BS EN 60204-1, or BS EN 62368-1, or by the adoption of BS EN 50310).
  In BS 7671 terms, consideration might be needed for the use of particular wiring systems or routes to avoid the use of 30 mA RCDs ...

Simples.

AJJewsbury said:

Maybe if we'd used more 4.0mm² on socket circuits we'd be less happy about that size not needing duplication.

Single perhaps ... in T&E, 4.0 mm² has 1.5 mm² protective conductors !

Indeed - we have very little experience of using ≥4mm² c.p.c.s with front-plate accessories...

   - Andy.