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Reliance Upon Mechanical and Electro-mechanical Devices and Machines.

Zoomup
There have been concerns shown about the reliability of R.C.D.s in the past, and the cruise liner Viking Sky's engines and also this......

https://www.youtube.com/watch?v=aPKJRikunJw


Z.
  • 0
    I agree that both society in general and individuals place far too much reliance in machines etc. working. In most cases DESIGN of installations  needs to be improved so as to mitigate the consequences of any single failure.


    The engines of cruise ships need to be at least duplicated in order that the ship may proceed at reduced speed if one engine fails. The engine installation needs careful design so as to avoid any common point of failure in fuel supply, cooling or electrical systems disabling both engines.


    RCDs should be duplicated in particularly high risk situations, and in more normal situations should be tested regularly.


    And as regards the apparent failure of the hose reel pump fitted to a fire engine, firemen should be aware of other ways of fighting the fire. One option would be to connect full sized hose to the nearest fire hydrant and use this water directly for fire fighting without a pump. Another possibility would be to fill buckets with water from the appliance tank and apply this water manually to the fire. Or use buckets of water and a stirrup pump.
  • 0
    Yes, Broadgage,

                                 the cruise ship recently lost all 4 engines in a storm and was near to being blown onto the shore. The low lubricating oil detection system cut in as the ship rocked from side to side and the computer controls automatically stopped all 4 engines, presumably to prevent an engine seizure. Passengers had to be air lifted off the ship in terrible conditions by helicopters.


    I am a firm believer in two series R.C.D.s in installations. If one fails or is a bit sticky the other should operate and clear a fault. I have personally found several R.C.D.s that were faulty in installations. Relying on just one R.C.D. is taking a bit of a gamble.


    Bye,


    Z.
  • Former Community Member
    0 Former Community Member

    Zoomup:

    I am a firm believer in two series R.C.D.s in installations. If one fails or is a bit sticky the other should operate and clear a fault. I have personally found several R.C.D.s that were faulty in installations. Relying on just one R.C.D. is taking a bit of a gamble.


     



    You would probably be better off focusing on the bonding and then design for an increased effective CPC size - that way you have a combination of both passive and active safety. RCD's in series are effective from some faults, but not all faults - if you are thinking along these lines then it's important to ascertain the common mode failures


    That said, twice the expenditure on RCD's to save a very small number of people hardly seems worth it  - from a safety perspective


    Regards


    OMS
     

  • 0
    Better bonding even for TT installations O.M.S? (Where perfect operation is imperative for R.C.D.s.)


    Anyway R.C.D.s are as cheap as chips these days. 


    Z.


  • 0
    Well clearly for TT for whole installations, or islands, the RCD is the main safety feature, as without it there is no ADS for earth faults - bonding just means that it all goes live together - which may still be a good thing, just that if the RCD jams, it  stays live for ever and never clears.

  • Former Community Member
    0 Former Community Member

    Zoomup:

    Better bonding even for TT installations O.M.S? (Where perfect operation is imperative for R.C.D.s.)


    Anyway R.C.D.s are as cheap as chips these days. 


    Z.


     



    Bonding and CPC sizes works for any installation actually - touch voltages are where I was heading


    For TT, yes, you need the RCD for ADS - I was thinking more along the path of "additional protection"


    Regards 


    OMS
     

  • 0
    I would only duplicate RCDs in situations that I judged to be high risk.

    One simple way of achieving this is to fit RCD socket outlets to sub circuits that are also RCD protected. The two devices probably wont co-ordinate, but having to reset either/both is IMHO an acceptable price to pay for increased safety.


    Returning to the cruise ship on which all the engines were stopped by low oil level alarms, that was in my view a defective design. IMHO such engines should have a means of bypassing such alarms. If the ship is in danger, it is arguably preferable to allow the engine to run until destruction, rather than to risk loss of the vessel and the many lives therein by running onto rocks.

    Nothing complex is needed, a simple switch behind a sealed glass panel would serve.

    "IN CASE OF SERIOUS EMERGENCY, BREAK GLASS AND OPERATE SWITCH. THIS WILL ALLOW THE ENGINE TO RUN WHEN SAFETY SYSTEMS WOULD OTHERWISE PREVENT THIS. Use only if vessel is at risk. Serious damage may be caused."
  • 0

    Better bonding even for TT installations O.M.S? (Where perfect operation is imperative for R.C.D.s.)



    Actually, TT is one of the situations where equipotential bonding can work best. Where you have comparatively low earth fault currents you end up with a low voltage difference (e.g. between exposed- and extraneous-conductive-parts) - so a much reduced shock risk, even if ADS doesn't operate promptly (either by design - e.g. large or distribution circuits, or due to faulty protective device). The problem with TN systems with relatively low Ze is that most of the 230V supply voltage is divided across the faulty circuit's conductors within the installation during an earth fault - typically over 100V between MET and exposed-conductive-part of faulty equipment - and main bonding can't really reduce that.


      - Andy.
  • 0

    OMS:




    Zoomup:

    Better bonding even for TT installations O.M.S? (Where perfect operation is imperative for R.C.D.s.)


    Anyway R.C.D.s are as cheap as chips these days. 


    Z.


     



    Bonding and CPC sizes works for any installation actually - touch voltages are where I was heading


    For TT, yes, you need the RCD for ADS - I was thinking more along the path of "additional protection"


    Regards 


    OMS
     


     




    Oh, I see O.M.S. Thanks for that.


    Z.

  • 0

    Actually, TT is one of the situations where equipotential bonding can work best. Where you have comparatively low earth fault currents you end up with a low voltage difference (e.g. between exposed- and extraneous-conductive-parts) - so a much reduced shock risk, even if ADS doesn't operate promptly (either by design - e.g. large or distribution circuits, or due to faulty protective device).







    I'd qualify that, for an indoor TT installation - for one with lots of earthed metal outside - petrol stations, lampposts etc the situation is more like the outdoor EV charging case that gets so complicated - if there is a fault and the RCD fails to trip, the situation becomes un-avoidably dangerous.  Even with the rebar grids earthed under concrete floors becoming live grids under concrete, while it may help reduce the risk of shock in a warehouse, does not really make it safer if this is outdoors. In such a  case less bonding may be safer than more, in the very rare  event of fault and an RCD failure.


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