Earthing neutral

Oh yes, like all UK ADS systems, the permissible exposed voltage and how long it may be exposed for are intimately linked, by the IEC shock curves, and an assumption that a healthy dry human is always more than 1000 ohms. ( link  to that curve)

So the AC3 region upper boundary is the danger line. With a bit of a margin for safety we end up deciding that we can stand 50v for ages, 100V or so for 0.4 seconds, and 250V for about 0.2 seconds and anything much more than 500V needs to be disconnected very smartly indeed.

That shape of that time kink is really down to the period of a human heartbeat - if you nudge the heart electrically for a very small fraction of the beat time, it takes more current to throw it off rhythm than a longer pulse duration - so the maximum safe exposure can be higher for short duration events. (and a really low current can pass forever and you scarcely feel  it)

So the prospective fault current times the resistance to ground (by a combination of armour and electrodes if it is that sort of cable ) tells us the 'rise of earth potential' or ROERP that will occur. Breaker designs at the HV end tell us the breaking time = fault duration, and then someone paid to say so goes ho-hum and decides to bond or not.


It was much simpler in the ' if it is 1 ohm treat as hot site, less then treat as cold ' days ! (that is 'hot' in the UK sense of being badly earthed, not in the USA sense of 'live'  though there is some overlap in what it may feel like, at least while a fault is on.)

Oh yes, like all UK ADS systems, the permissible exposed voltage and how long it may be exposed for are intimately linked, by the IEC shock curves, and an assumption that a healthy dry human is always more than 1000 ohms. ( link  to that curve)

So the AC3 region upper boundary is the danger line. With a bit of a margin for safety we end up deciding that we can stand 50v for ages, 100V or so for 0.4 seconds, and 250V for about 0.2 seconds and anything much more than 500V needs to be disconnected very smartly indeed.

That shape of that time kink is really down to the period of a human heartbeat - if you nudge the heart electrically for a very small fraction of the beat time, it takes more current to throw it off rhythm than a longer pulse duration - so the maximum safe exposure can be higher for short duration events. (and a really low current can pass forever and you scarcely feel  it)

So the prospective fault current times the resistance to ground (by a combination of armour and electrodes if it is that sort of cable ) tells us the 'rise of earth potential' or ROERP that will occur. Breaker designs at the HV end tell us the breaking time = fault duration, and then someone paid to say so goes ho-hum and decides to bond or not.


It was much simpler in the ' if it is 1 ohm treat as hot site, less then treat as cold ' days ! (that is 'hot' in the UK sense of being badly earthed, not in the USA sense of 'live'  though there is some overlap in what it may feel like, at least while a fault is on.)