The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • Locked Locked
  • Replies 27 replies
  • Subscribers 37 subscribers
  • Views 3135 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

High protective conductor currents - Deletion of 543.7.1.204

AJJewsbury

543.7.1.204 - the one about duplicate c.p.c.s needing to be 'terminated independently of each other' - has gone.

Does anyone understand the thinking behind this? It seems a bit odd to me.

Given that (in my experience) more problems occur at terminals rather than along cable runs, if we need the c.p.c.s to be duplicated, it seems like a bit of a flaw that one single loose connection can make them both useless simultaneously. I can see that in some instances (e.g. a terminal on a socket on a ring) that losing a single terminal carries a limited immediate risk (as the leakage current from a single socket should be small and all other sockets are still connected to a c.p.c.) but in other instances - say the connection of both c.p.c.s to the earth bar in a DB - that single fault could be very significant.

   - Andy.

Parents

  • 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.

Reply

  • 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.

Children
  • in reply to AJJewsbury

    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).

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines