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.
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)
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)
