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5 second disconnection times

Sparkymania
Hi all


Something that I have always wondered about since I started doing electrical work.


The 0.4 and 5 second disconnection times. 0.4 makes sense as it is quick.

However 5 seconds still seems a long time for exposed conductive parts to remain live. When I first started, lighting circuits had a 5s time.

Now it's 0.4 for all circuits feeding socket outlets up to 63A but only for fixed equipment up to 32A. So any equipment over 32A can be 5s.

The reason given in collage was that it was portable equipment that can be picked up and gripped but fixed equipment can be pulled away from.

Previously, in 16th ed regs, the 0.4 was for socket outlets and circuits supplying equipment that can be hand held.

However, 5 seconds still seems a long time for exposed metalwork to be live. I know with a low impedance earth the voltage will be lower, but still.


The other thing is that even a distribution circuit that can have 5s dis time, on an earth fault, say in an armoured cable, all earthed metalwork can be live for the full 5 seconds, even hand held equipment on circuits with a 0.4s dis time. I realise that if the fault was on the actual item of equipment itself the voltage would be higher.


Any equipment, though, above 32A can still have a 5s dis time. I come across fixed equipment all the time that is above 32A. This equipment quite often has parts of it that can actually be gripped. When the body has electricity passing though it the muscles contract so it may be hard to pull away.

I've seen a video of three men pushing a tower hitting an overhead HV line. all three dropped down but their hands still gripped the scaffold poles.

I know were dealing with LV but the muscles still react the same.

Even showers could once have a 5s dis time and the only thing that has changed that is the regs for RCDs in rooms containing a bath or shower. It's still on a circuit that, without the RCD, allows 5s.


The fact that the regs have tightened up of what circuits can have 5s dis times shows that there is still a danger on 5s. Otherwise, why change them to 0.4s.


Any thoughts?



Parents
  • 0

    AJJewsbury:




    230V supply.



    • 1.5 sqmm cable has 1.0 sqmm CPC   touch voltage on fault  becomes 138

    • 2.5 sqmm cable has 1.5 sqmm CPC   touch voltage on fault  becomes  144V

    • 4 sqmm cable has 1.5 sqmm CPC      touch voltage on fault  becomes  167V

    • 6 sqmm cable has 2.5 sqmm CPC  touch voltage on fault  becomes 162V

    • 10 sqmm cable has 4 sqmm CPC    touch voltage on fault  becomes  164V

    • 16 sqmm cable has 6 sqmm CPC  touch voltage on fault  becomes 167V


    But that only holds entirely true if the entire earth fault loop consists of that type of cable - which typically it isn't. For a PME system for example you'd have a full sized PEN making up the 'Ze' part of the circuit - so diluting the increased voltage somewhat. Then you'd likely have main bonding at the intake which should in theory (and to some extent in practice) remove the p.d. along the supply PEN from what someone in the installation is exposed to. Actual numbers will vary considerably from each installation and fault to the next - but the general assumption seems to be that the mitigating factors like these usually balance out the increased voltage differences from the reduced c.p.c.s and so the 0.4s for 120-ish volts is still 'reasonable'.


       - Andy.

     




    What if the circuit supplies equipment outside of what we used to call the equipotential zone ?

Reply
  • 0

    AJJewsbury:




    230V supply.



    • 1.5 sqmm cable has 1.0 sqmm CPC   touch voltage on fault  becomes 138

    • 2.5 sqmm cable has 1.5 sqmm CPC   touch voltage on fault  becomes  144V

    • 4 sqmm cable has 1.5 sqmm CPC      touch voltage on fault  becomes  167V

    • 6 sqmm cable has 2.5 sqmm CPC  touch voltage on fault  becomes 162V

    • 10 sqmm cable has 4 sqmm CPC    touch voltage on fault  becomes  164V

    • 16 sqmm cable has 6 sqmm CPC  touch voltage on fault  becomes 167V


    But that only holds entirely true if the entire earth fault loop consists of that type of cable - which typically it isn't. For a PME system for example you'd have a full sized PEN making up the 'Ze' part of the circuit - so diluting the increased voltage somewhat. Then you'd likely have main bonding at the intake which should in theory (and to some extent in practice) remove the p.d. along the supply PEN from what someone in the installation is exposed to. Actual numbers will vary considerably from each installation and fault to the next - but the general assumption seems to be that the mitigating factors like these usually balance out the increased voltage differences from the reduced c.p.c.s and so the 0.4s for 120-ish volts is still 'reasonable'.


       - Andy.

     




    What if the circuit supplies equipment outside of what we used to call the equipotential zone ?

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