Possibly changing the 5 Second Disconnection Limit

A long time ago there wasn't any particular disconnection time required at all in the regs - just a a requirement that an earth fault (of negligible impedance) should produce a fault current of at least 3x the rating of the protective device - so a fault couldn't persist indefinitely, but shock protection (as we now look at it) wasn't really a consideration.


Later it was tightened to 5s - rumour has it that 5s was decided upon because one member of a committee did a back of an envelope calculation based on a (factory?) installation he was responsible for and worked out that was the best it could achive without major alteration. So still no significant mathematical basis for shock protection,


As ebee says, we then had 0.4s - but initially only for final circuits supply hand-held equipment (and perhaps bathrooms and outdoors) - more reliance was placed on bonding rather than ADS for everything else - but the benefits of main bonding were never quantifiable (and often quite small where Ze is low - which became increasingly common where the suppliers provided an earth facility).


Gradually the 0.4s (and later 0.2s for TT) spread to encompass most small final circuits, and additional protection by 30mA RCD similarly increased. But we still have the 'heritage' of longer disconnection times on larger and non-final circuits. As others have noted, the actual risks remaining seem relatively small - chances are when a hand-held appliance suffers an earth fault it's being moved or used - so it's quite likely someone is touching a hand-held appliance when that sort of hazardous event occurs, but Earth faults on distribution circuits are far far less frequent, and much less likely that someone will be holding on to a metallic part at that particular moment. Nothing is 100% - ADS fails if the c.p.c. has previously become open circuit, or possibly if the fault isn't of negligible impedance, even PME earthing poses its own risks of shock (broken CNE supply conductors) so striving for mathematic perfection in disconnection times might be a bit pointless if the bigger picture still undermines it.


   - Andy.

A long time ago there wasn't any particular disconnection time required at all in the regs - just a a requirement that an earth fault (of negligible impedance) should produce a fault current of at least 3x the rating of the protective device - so a fault couldn't persist indefinitely, but shock protection (as we now look at it) wasn't really a consideration.


Later it was tightened to 5s - rumour has it that 5s was decided upon because one member of a committee did a back of an envelope calculation based on a (factory?) installation he was responsible for and worked out that was the best it could achive without major alteration. So still no significant mathematical basis for shock protection,


As ebee says, we then had 0.4s - but initially only for final circuits supply hand-held equipment (and perhaps bathrooms and outdoors) - more reliance was placed on bonding rather than ADS for everything else - but the benefits of main bonding were never quantifiable (and often quite small where Ze is low - which became increasingly common where the suppliers provided an earth facility).


Gradually the 0.4s (and later 0.2s for TT) spread to encompass most small final circuits, and additional protection by 30mA RCD similarly increased. But we still have the 'heritage' of longer disconnection times on larger and non-final circuits. As others have noted, the actual risks remaining seem relatively small - chances are when a hand-held appliance suffers an earth fault it's being moved or used - so it's quite likely someone is touching a hand-held appliance when that sort of hazardous event occurs, but Earth faults on distribution circuits are far far less frequent, and much less likely that someone will be holding on to a metallic part at that particular moment. Nothing is 100% - ADS fails if the c.p.c. has previously become open circuit, or possibly if the fault isn't of negligible impedance, even PME earthing poses its own risks of shock (broken CNE supply conductors) so striving for mathematic perfection in disconnection times might be a bit pointless if the bigger picture still undermines it.


   - Andy.