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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
    ProMbrooke:
    mapj1:
    Am I correct to say that even at maximum allowable EFLIs, breakers 100 amps and under will trip in their magnetic region? 


    Mostly true, and certainly true to say you will get prompt clearance for faults from live to to neutral on circuits that meet the full load voltage drop requirements of Lighting 3% other uses 5% .?


    (and that allows us to get an approx  figure for the live path of half the L-N loop resistance)


    Faults from live to earth need to be considered separately and problems really arise when the earth impedance is significantly higher - for example for a TNS service that is only just meeting the local DNOs spec the earth loop impedance may be as high as 0.8 ohms on a supply with a 100A fuse - so the PSSC to earth is 230/0.8 ~ 285A, but the LN loop may be more like (10V/100A 10 milliohms) 2kA short circuit prospective

    Now that may not fire a C type 32A breaker on an earth fault, but it probably will, and it will certainly fire the more common B32 that we use for most socket circuits, if the fault is at or near the consumer unit end.

    However a shower on a B50Amp breaker might need as much as 250A to instant trip, and if the external earth resistance is the maximum 0.8 ohms permitted it may or may not do so if the fault is at the remote end of more than about 30m of T and  E cable.

    Of course in the last few years that MCB  would have become  a 50A RCBO, but there are a lot  in use out there that pre-date that change. (so remove the bathroom bonding in an old house at your peril.. )

    M.


    In the world of big stuff you may introduce programmed delays of 1sec or 3 sec or 5 sec to give some discrimination with downstream protection, but in practice 5 secs is a long time to be standing in front of something that is growling and smoking because one phase is shorted.




     





    What are the typical real world Ze values being measured in domestics?  




    Ze in my part of the woods (Norfolk U.K.) for TN-C-S average 0.20/0.30 Ohms.


    For TT with an earth rod average 200 Ohms in damp well draining sandy soil.


    Z.


Reply
  • 0
    ProMbrooke:
    mapj1:
    Am I correct to say that even at maximum allowable EFLIs, breakers 100 amps and under will trip in their magnetic region? 


    Mostly true, and certainly true to say you will get prompt clearance for faults from live to to neutral on circuits that meet the full load voltage drop requirements of Lighting 3% other uses 5% .?


    (and that allows us to get an approx  figure for the live path of half the L-N loop resistance)


    Faults from live to earth need to be considered separately and problems really arise when the earth impedance is significantly higher - for example for a TNS service that is only just meeting the local DNOs spec the earth loop impedance may be as high as 0.8 ohms on a supply with a 100A fuse - so the PSSC to earth is 230/0.8 ~ 285A, but the LN loop may be more like (10V/100A 10 milliohms) 2kA short circuit prospective

    Now that may not fire a C type 32A breaker on an earth fault, but it probably will, and it will certainly fire the more common B32 that we use for most socket circuits, if the fault is at or near the consumer unit end.

    However a shower on a B50Amp breaker might need as much as 250A to instant trip, and if the external earth resistance is the maximum 0.8 ohms permitted it may or may not do so if the fault is at the remote end of more than about 30m of T and  E cable.

    Of course in the last few years that MCB  would have become  a 50A RCBO, but there are a lot  in use out there that pre-date that change. (so remove the bathroom bonding in an old house at your peril.. )

    M.


    In the world of big stuff you may introduce programmed delays of 1sec or 3 sec or 5 sec to give some discrimination with downstream protection, but in practice 5 secs is a long time to be standing in front of something that is growling and smoking because one phase is shorted.




     





    What are the typical real world Ze values being measured in domestics?  




    Ze in my part of the woods (Norfolk U.K.) for TN-C-S average 0.20/0.30 Ohms.


    For TT with an earth rod average 200 Ohms in damp well draining sandy soil.


    Z.


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