Can someone let me know the science behind this please. I’ve been told that if there is a fault on a circuit the ZS values in the regs are there to give ADS in a time that is not going to cause considerable harm. My question is if disconnecting times don’t comply what is the dangers that arise 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. Thank you for you help in advance guys just want to clear up some unanswered questions.
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.
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.
