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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
    But that factor of 2 in the denominator of your equation 8 should be more like 1.5 in the case of twin and earth. Experience in the rest of Euroland is not so relevant here - in the UK our final circuits do not have full sized CPCs,  (nor do quite a few in-building sub-mains either) and in built up areas we may have larger lower impedance transformers, indeed in parts of London 1MVA transformers are meshed and the LV network in the streets is not even fused (AKA "the solid system"). Then there are a great many tower blocks with a megawatt transformer in the basement, and then bigger office and mixed use buildings with HV going up to a transformer on every 5th floor or so (look at Canary Wharf for an example of that if you like)

    In such cases your assumption that the supply droop has a dominant effect is not appropriate - it is mostly in the cables.

    Note that just because something is written by a committee does not make it infallible - look at the number of times the regs get updated for proof of that. ( I've sat on telecoms standards meetings, I know how it works, and it has perhaps made me slightly cynical. )

    I agree the incoming voltage will droop very significantly during fault for those rural sites fed by smaller (100kVA and down) pole-pig transformers, but they are more commonly earthed as TT anyway.

    I'm not sure we should underestimate the touch voltages to city dwellers by assuming that all installations are like that.

    There is an additional complication in a PME system, as the live voltage goes down the neutral comes up to meet it, so you have to be clear if you mean touch voltag relative to the CPC of the system, or to terra-firma earth voltage either at ground level as on incoming telephone cables etc.

    M.
Reply
  • 0
    But that factor of 2 in the denominator of your equation 8 should be more like 1.5 in the case of twin and earth. Experience in the rest of Euroland is not so relevant here - in the UK our final circuits do not have full sized CPCs,  (nor do quite a few in-building sub-mains either) and in built up areas we may have larger lower impedance transformers, indeed in parts of London 1MVA transformers are meshed and the LV network in the streets is not even fused (AKA "the solid system"). Then there are a great many tower blocks with a megawatt transformer in the basement, and then bigger office and mixed use buildings with HV going up to a transformer on every 5th floor or so (look at Canary Wharf for an example of that if you like)

    In such cases your assumption that the supply droop has a dominant effect is not appropriate - it is mostly in the cables.

    Note that just because something is written by a committee does not make it infallible - look at the number of times the regs get updated for proof of that. ( I've sat on telecoms standards meetings, I know how it works, and it has perhaps made me slightly cynical. )

    I agree the incoming voltage will droop very significantly during fault for those rural sites fed by smaller (100kVA and down) pole-pig transformers, but they are more commonly earthed as TT anyway.

    I'm not sure we should underestimate the touch voltages to city dwellers by assuming that all installations are like that.

    There is an additional complication in a PME system, as the live voltage goes down the neutral comes up to meet it, so you have to be clear if you mean touch voltag relative to the CPC of the system, or to terra-firma earth voltage either at ground level as on incoming telephone cables etc.

    M.
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