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

  • State Not Answered
  • Locked Locked
  • Replies 29 replies
  • Subscribers 32 subscribers
  • Views 17209 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

The maximum permissible disconnection time is 0.4 s in TN system. Why and from where this value (0.4 s) is obtained?

Former Community Member
Former Community Member
The maximum permissible disconnection time in in the event of a short circuit between a phase conductor and a body or protective conductor or a protective-neutral conductor is 0.4 s in TN system. Does anybody know this value (0.4 s) obtains from where? How this value is obtained?
  • 0
    Presumably distribution circuits are allowed to have long disconnect times because they don't expect anyone to actually come into live contact with them very often (apart from the occasional shortly-to-be-deceased sparky fiddling about inside a DB). All the nasty end-user paraphernalia like water heaters, power tools etc are connected further downstream with 0.2/0.4s disconnect times.
  • 0

    Presumably distribution circuits are allowed to have long disconnect times because they don't expect anyone to actually come into live contact with them very often (apart from the occasional shortly-to-be-deceased sparky fiddling about inside a DB). All the nasty end-user paraphernalia like water heaters, power tools etc are connected further downstream with 0.2/0.4s disconnect times.



    Unfortunately the situation isn't quite that good. If there's a fault on a submain, the earth bar on the DB it serves - together with all the downstream exposed-conductive-parts - will likely be held at the fault voltage (e.g. 115V for TN, 230V for TT) until disconnection occurs (e.g. for the full 5s) - so you can still get a 5s shock from metalwork on a 0.4s circuit.


    I suspect the allowance for 5s disconnection is more to do with the practicalities of achieving discrimination (possibly through several stages of sub-mains) and being able to use a range of overcurrent devices (e.g. fuses) within practical loop impedances, together with the lower likelihood of faults on submains etc - rather than any comprehenive plan for shock protection.


    In theory main bonding can help a bit - but in practice the benefit isn't anything like sufficient to be guaranteed to be life-saving.


       - Andy.
  • 0
    And on TT you'd not normally have the upfront earth fault relay or  RCD set to the full 5 seconds, or anything like it, the problems are more likley on PME type supplies, where a fault in thre street can join L and N together,  on the supply side, so your LNE at the consumer side all float up  to some midphase voltage and no consumer side ADS operates.

    There is a good reason for all hand held devices to be double insulated, rather than class I. You are very unlucky to be unloading the washing machine at the key moment, and also in contact with something connected to true ground but not locally bonded,  while power tools hair dryers etc that are held tightly for long periods are all double insulated. Things that go outside, like hedge trimmers and lawnmowers would be especially risky if class I.

    It is for this reason that car chargers are coming in for a lot of scrutiny, as they seem to be the only thing that breaks this rule, being designed so the car metal is at earth potential.
  • 0

    AJJewsbury:

    I suspect the allowance for 5s disconnection is more to do with the practicalities of achieving discrimination (possibly through several stages of sub-mains) and being able to use a range of overcurrent devices (e.g. fuses) within practical loop impedances, together with the lower likelihood of faults on submains etc - rather than any comprehenive plan for shock protection.




    For example, a distribution circuit protected by an 80 A BS 88-3 fuse requires a maximum Zs of 0.27 Ω. That might be difficult to achieve if Ze is anywhere near that or higher. A 5 sec disconnection time allows a Zs of up to 0.51 Ω.

  • 0
    Hmm. What about Charlie the aged fitter/turner standing on a wet concrete floor whilst operating his lathe grasping a chrome operating handle of his lathe which is supplied from a 40A final circuit? He is wearing his worn out old leather soled army boots.
  • 0

    John Peckham:

    . . . I do not know if there is any written requirement to lay rubber matting in front of LV panels . . . 




    It used to be a requirement of the Factories Act. Not sure if it is still in there. Out of curiosity, would anyone trust a piece of rubber matting that had probably not moved since the switchboard was installed? 


    Regards,


    Alan. 

  • 0
    #offtopic


    Congratulations Alan Capon‍! Your post above is the 10,000th post in the new forum! A little something winging its way to you in the post.... ?
  • 0

    Charlie the aged fitter/turner standing on a wet concrete floor whilst operating his lathe grasping a chrome operating handle of his lathe which is supplied from a 40A final circuit? He is wearing his worn out old leather soled army boots. 




    Playing devil's advocate a little, when Charlie was in training the machine room supply would have been what we now call TNS (or TT), (I'm assuming he is not so old that they had line shafting like this when he started - though perhaps if he started working as an apprentice for UKPN .. )  In such a case the impressed fault voltages and duration are limited, at least if the building wiring has been kept up with modern regs .

    A modern building with a TNCS supply should have the steels of the building and any rebar of the concrete floor bonded in any case.

    Perhaps he too should have a rubber mat.  swarf mat  for the modern factory.   He should also be provided with modern PPE, including toe protector boots that do not leak.


    For what  it is worth, I have an island of wooden floor in front of mine for exactly this scenario, but in the manner of the cobbler's barefoot children, and 'do as I say not as I do' , the earthing on my lathe is not quite right anyway, as the centrifugal switch that disconnects the starter winding occasionally arcs out to the case, so the RCD is, ahem, absent.


  • 0
    Mike



    i forgot about the years of swarf impacted in to the soles of his old boots.
  • 0

    John Peckham:

    Hmm. What about Charlie the aged fitter/turner standing on a wet concrete floor whilst operating his lathe grasping a chrome operating handle of his lathe which is supplied from a 40A final circuit? He is wearing his worn out old leather soled army boots. 




    Now look what you have gone and made me do - I couldn't concentrate until I had gone and checked the earth continuity on my own lathe. ?

Community Rules & Guidelines