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The maximum permissible disconnection time is 0.4 s in TN system. Why and from where this value (0.4 s) is obtained?
Former Community Member
The maximum permissible disconnection time in in the event of a short circuit between a phase conductor and a body or protective conductor or a protective-neutral conductor is 0.4 s in TN system. Does anybody know this value (0.4 s) obtains from where? How this value is obtained?
And the reason it is that shape, and all the action is in the region of a few hundred milliseconds, is to do with the human heartbeat timing, and the chances of a current passing through the body putting the heart muscles into an irreversible fibrillation, so that it no longer beats properly.
A current surge that is very short compared to a heart beat is can be much higher before it has the same effect as a lower current of long duration.
In slang, you need to get the fault current interrupted in half a heartbeat to give the victim a good chance of survival.
Hidden behind this are various assumptions about hand to foot shocks, and how good the contact really is, most of the time a glancing contact is a lot higher resistance than a kilohm, so a lot of shocks that look like they ought to kill just cause some bad language and a bit of a flinch, these figures assume good contact.
The limit of 50V for ELV has another assumption that the current is low enough that it needs no ADS, but beware - being cut on live metal so you lose the skin protection, or even just being wet with seawater can lower your resistance, and means this is not true, and a lower 'safe' voltage, like 25, for example, may have been a better choice.
Once upon a time some confined working regs acknowledged this, and recommended a lower voltage for tools for crawling into wet pipes etc.
As a final note, the volts and currents in a defibrillator are so large, the idea is to clench the muscles up, so the heart is actually stopped in a known state, and the idea is that when the current stops the heart re-starts in correct rhythm, a sort or reset. Works often enough to be worth trying anyway.
And the reason it is that shape, and all the action is in the region of a few hundred milliseconds, is to do with the human heartbeat timing, and the chances of a current passing through the body putting the heart muscles into an irreversible fibrillation, so that it no longer beats properly.
A current surge that is very short compared to a heart beat is can be much higher before it has the same effect as a lower current of long duration.
In slang, you need to get the fault current interrupted in half a heartbeat to give the victim a good chance of survival.
Hidden behind this are various assumptions about hand to foot shocks, and how good the contact really is, most of the time a glancing contact is a lot higher resistance than a kilohm, so a lot of shocks that look like they ought to kill just cause some bad language and a bit of a flinch, these figures assume good contact.
The limit of 50V for ELV has another assumption that the current is low enough that it needs no ADS, but beware - being cut on live metal so you lose the skin protection, or even just being wet with seawater can lower your resistance, and means this is not true, and a lower 'safe' voltage, like 25, for example, may have been a better choice.
Once upon a time some confined working regs acknowledged this, and recommended a lower voltage for tools for crawling into wet pipes etc.
As a final note, the volts and currents in a defibrillator are so large, the idea is to clench the muscles up, so the heart is actually stopped in a known state, and the idea is that when the current stops the heart re-starts in correct rhythm, a sort or reset. Works often enough to be worth trying anyway.
