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.
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.
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.
