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.
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.
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.
