A bit of biology as well as physics. The human body (most specifically the heart) can survive very short duration electric shocks - longer ones tend to cause the heart to fibrillate (the various bits of the heart muscle go out of synchronisation so it doesn't pump properly) and death typically follows quite quickly. The actual time allowed depends on the current through the body, which in turn depends to a large extent on the voltage the body is exposed to. In short we end up with a requirement to disconnect in 0.4s where the voltage across the body is likely to be around half mains voltage (TN systems) and 0.2s where it could be the full mains voltage (TT systems). For large and distribution circuits we have 5s or 1s - which is a compromise to avoid blacking out the entire installation for the smallest fault and faults on such circuits being less likely.


Where ADS is provided by fuses or MCBs generally the larger the earth fault current, the quicker the device will disconnect. Low Zs (earth fault loop impedance) the larger the fault current so the quicker the disconnection time.


   - Andy.

Interesting question about being exposed to current with no shock MrJack96.



Whilst we are told, quite correctly, that it's the current passing through major organs (like the brain and heart) that kills, we need to use our Engineering knowledge to fill in the gaps.

Ohm's Law is the answer to this - the body has an impedance, or resistance. This resistance is different for everyone, and changes, so for example it will be lower in wet conditions, and depending on the area of contact with electrical conductors (finger vs whole hand for example). On average in dry conditions we use somewhere between 1,000 and 1,500 ohms - perhaps with a 230 V supply using 1,150 ohms makes the sums easier !

Of course, Ohms Law tells us that, for a given resistance, the more voltage we have, the more current passes through that resistance:

 U = IR


So, the lower the voltage, the lower the current.


That's how equipotential bonding can work, and it's why it's possible to touch main bonding bars with many amperes passing through them, and not feel much at all - provided the bonding is designed properly, the touch voltage will be low enough to prevent too much touch current  passing through the body - in BS 7671, we use 50 V AC as the performance requirement for touch voltage when we are designing supplementary local equipotential bonding - except in medical locations (Section 710 of BS 7671) and filling stations (using the APEA/EI publication Guidance for Design, Construction, Modification, Maintenance and Decommissioning of Filling Stations), both of which lower the voltage limit to 25 V AC.


In final circuits, the voltage at the end of the circuit, however, may be anything up to the full supply voltage until the protective device operates - meaning the touch voltage might exceed 50 V AC for. The disconnection times in Chapter 41 are based on the assumption that in TT systems, the touch voltage at exposed-conductive-parts at the point of fault are at approximately the supply voltage (230 V AC), and in TN systems they are roughly just over half the supply voltage (115 V AC)

Remember you only get a shock if you touch two things at different voltages simultaneously so that current flows through you - the live line working chaps have this off to a tee! (as do birds sitting on the overhead lines)

It is also the reason older engineers put  one hand behind the back or in a pocket when poking something that may just be live -  in the days of maintenance on valved TVs with a few hundred volts of HT, this simple habit was literally a life saver. (the 20 odd kV EHT was less dangerous, as it was incapable of supplying high current for a long time, and therefore while really painful for the short time the strored charge was dumped, was not so risky as the mains derived HT that could supply a good fraction of an amp all day)


In the case of your earthy bus-bars, assuming you are also earthed more or less, then when you touch them there is no voltage difference across your body to drive a shock current.

Touch the earthy bus bar and a live one with the other hand however, and its all over....

So in theory under fault if we was to touch a faulty appliance with an earth fault and we are standing on something earthy we shouldn’t receive any shock due to being the same potential as the earthed appliance? My confusion is why disconnection times are so important if the above is the case thanks for your detailed explanation everyone

That is the idea of earthing appliances and if the earthing could be perfect, then indeed it would be fine - however, if the cable doing the earthing (the CPC - circuit protective conductor) has a real if low resistance comparable to that of  the cable supplying the live, then during a fault in the appliance the case is no longer at earth voltage - rather there is a voltage division as these two low value resistors are in series between the live voltage and the reference earth voltage at some far-away point.


The resistor mid point (the appliance chassis) will be at half mains voltage if the two resistances are equal and the fault has no voltage drop at all.

Neither of these conditions is true, and you may be on more or less the same local earth as the equipment ,or connected to earth via something else to  a point  that is 'further back up the slope', and nearer the origin so the actual shock voltage is a bit indeterminate.

Is this not ass backwards.  If you touch a faulty appliance, so the case is live due to a fault, in fact it would have to be 2 faults.  Broken earth and metal case live. Then you stand on something earthy you create an earth path and get a belt.  Where as if you are on a wooden floor there is no earth path and you might not know it was faulty.



Gary

To be live for a long time, yes 2 faults.


But fault from live to metal case is one fault, and in the hopefully  short period where the correctly sized ADS operates, the case is  bit live, but how live rather depends on the Zs and the fault current.

If Zs is way  too high, that is moving in the direction of broken earth...

If you touch a faulty appliance, so the case is live due to a fault, in fact it would have to be 2 faults. Broken earth and metal case live.

Not entirely - even with a perfectly intact earth conductor (c.p.c.) the voltage on the appliance case will rise up to a hazardous voltage during the time it takes the protective device to operate - which hopefully is quick enough to prevent death, but isn't always (especially if Zs is too high).


   - Andy.

" I know it’s physics but have seen marshalling bars carrying current through earthing conductors all day and we don’t get a shock of them."

As long as the CPC/earthing conductors are properly connected then if you touch the conductors you will become a parallel resistance at the same voltage with a body resistance anywhere up to 1000+ times higher than the conductor path and will not experience a shock current.

If however, the earthing conductors  are at a different voltage to the ground you are standing on then when you make contact you may experience a tingle. PME earthing systems present this problem.

Legh