Agreed - taken from personal experience! Wood, fibreglass - it makes no odds sometimes!

"My question is if disconnecting times don’t comply what is the dangers that arise"

MrJack96 


I think this thread could be helpful.  It shows curves of touch-voltage versus time, that are supposed to be pretty safe (my sloppy choice of words) in 'normal' dry conditions or 'particular' conditions with more wetness of the skin.  It's my plot (so not a copyright problem), but the curves are defined in an IEC document (1200-413) that describes some of the background of the requirements in the IEC standards that form the wiring regulations.  In turn, the input data to these curves are from IEC 60479-1: the current-versus-time plots (often shown in brochures about RCDs etc) that show effects such as 5% or 95% probability of the heart stopping working effectively, and the body impedance plots for various contact types. 


So, you could say that the dangers from going beyond the specificed times is that the risk of death from body currents changes from very unlikely to increasingly likely ... not a sudden step, of course, and it's based on quite a few assumptions.


The disconnection times are a simplification: it's relatively easy to say "it's a 230 V to earth system", "my loop tester tells me 0.2 ohm", (perhaps do a step or two more here with voltage factors etc), "the resulting fault current makes this fuse operate in less than the required 0.4 s for a 230 V Uo)", "therefore: it's OK".  It would be more demanding to require that the touch voltages and durations don't go above the appropriate green curve" (in the above link): then you'd have to think more about the touch voltage as well as the time .. what are the sizes of PE conductor and L conductor, where is the bonding. 

The 0.4 time is based on an assumption that the touch voltage is ~40% of the system voltage to earth. It could be  a good deal less (e.g. within a bonded area in a TT installation with low fault current).  Or it could be a good deal higher, such as in a TN* system where most of the loop impedance is the internal wiring and it's done in classic UK flat-twin-and-earth cable with a PE considerably smaller than the live conductors. 

 

So what determines touch voltage at the point of the fault body resistance and the size of the CPC? Thanks for your detailed answers guys just something you never get taught in detail at college.

That depends on the earthing resistance of the supply. If this is low (20 Ohms usually, see BS 7430), then the body resistance should be able to be generally ignored as it's much higher ... at least in dry conditions.

So what determines touch voltage at the point of the fault body resistance and the size of the CPC?

For a TN system the voltage at the point of the fault is the result of a potential divider starting with the supply voltage (nominally 230V) with all the line conductors from the source to the fault one one side and all the protective conductors from the fault back to the source on the other. So if everything is equal (which is rarely is precisely but is a reasonable first guess) then it would be half the line voltage (115V say).


For TT systems, it's the same principle but the earth side of the loop includes the resistance of the soil around the consumer's and source's electrodes - which typically adds tens if not hundreds of Ohms to the bottom half of the divider - so the voltage will be much higher - likely much closer to 230V.


The touch voltage (i.e. what's experienced by the victim) is also depends on the 'other' voltage the victim is exposed to (you need a voltage difference across someone to get a shock) - if they're stood on open ground (at "true earth" potential) they'll feel the full voltage at the fault (as above), if however they're inside a building with main bonding, and touching a bonded extraneous-conductive-part (or some other exposed-conductive-part) the 'other' voltage will closer to that on the main earthing terminal (MET) of the installation - which itself will be at a higher voltage than true earth as it's part of the earth fault loop - so the victim will be exposed to a smaller voltage difference.


Body resistance is comparatively large (usually over 1000 Ohms) compared with a TN earth fault loop impedance of an Ohm or two, or a TT loop impedance of usually less than a couple of hundred Ohms, so the parallel path the victim provides does little to change the voltages.


C.s.a. of c.p.c.s is calculated to suit a number of requirements - often smaller than the corresponding line conductor, but should be co-ordinated so that the final Zs is acceptable.


   - Andy.

943D4B35-6D9B-4DD5-8A22-BC76BD43EDE8.png


is this equation correct for touch voltage from this I can see that the higher ZE the less touch voltage between the the appliance and the earth. The longer the circuit the greater the touch voltage. The smaller the resistance of r2 the smaller the touch voltage. I think I’m getting somewhere. ?

yes that  is the right idea - but of course your victim may not be at the same voltage as the MET and or may not be touching the faulty equipment directly, there may be other bits of pipework or whatever connected part way along that R2, so there are a few extra options for a nasty surprise that are not mentioned here, and/or are not the full voltage.

What would cause the MET to be at a different potential to a victim? Why do we uses 50v maximum touch voltage however the touch voltage may become much higher, is this due to the 0.4 second disconnection times? And one last question TT touch voltage according to that equation should be fairly low due to the ZE being high this doesn’t seem correct. Thanks again guys

What would cause the MET to be at a different potential to a victim?

The voltage on the MET will naturally rise during an earth fault - due to the current flowing and the resistance of the protective conductor back to the source of supply's earth electrode. If the victim is within a building with main bonding installed then their feet (in this example) are likely to be at a similar potential as the MET. If however they're outside standing on real Earth (using a class I appliance outdoors) then they'll be subject to the full voltage at the fault.

 

Why do we uses 50v maximum touch voltage however the touch voltage may become much higher, is this due to the 0.4 second disconnection times?

50V is considered safe (in normal conditions) for any duration (all day, all week, whetever). As the voltage rises however the length of time a victim can survive gets shorter - typically around 0.4s for 115V and 0.2s for 230V.

 

And one last question TT touch voltage according to that equation should be fairly low due to the ZE being high this doesn’t seem correct.

If you're inside the equipotential zone, then yes the victim might be exposed to quite a small voltage - if outside (or the equipotential zone is less than perfect - damp floors for instance) then the voltage difference can be much higher.


   - Andy.