The maximum permissible disconnection time is 0.4 s in TN system. Why and from where this value (0.4 s) is obtained?

Charlie the aged fitter/turner standing on a wet concrete floor whilst operating his lathe grasping a chrome operating handle of his lathe which is supplied from a 40A final circuit? He is wearing his worn out old leather soled army boots. 

Playing devil's advocate a little, when Charlie was in training the machine room supply would have been what we now call TNS (or TT), (I'm assuming he is not so old that they had line shafting like this when he started - though perhaps if he started working as an apprentice for UKPN .. )  In such a case the impressed fault voltages and duration are limited, at least if the building wiring has been kept up with modern regs .

A modern building with a TNCS supply should have the steels of the building and any rebar of the concrete floor bonded in any case.

Perhaps he too should have a rubber mat.  swarf mat  for the modern factory.   He should also be provided with modern PPE, including toe protector boots that do not leak.


For what  it is worth, I have an island of wooden floor in front of mine for exactly this scenario, but in the manner of the cobbler's barefoot children, and 'do as I say not as I do' , the earthing on my lathe is not quite right anyway, as the centrifugal switch that disconnects the starter winding occasionally arcs out to the case, so the RCD is, ahem, absent.

Charlie the aged fitter/turner standing on a wet concrete floor whilst operating his lathe grasping a chrome operating handle of his lathe which is supplied from a 40A final circuit? He is wearing his worn out old leather soled army boots. 

Playing devil's advocate a little, when Charlie was in training the machine room supply would have been what we now call TNS (or TT), (I'm assuming he is not so old that they had line shafting like this when he started - though perhaps if he started working as an apprentice for UKPN .. )  In such a case the impressed fault voltages and duration are limited, at least if the building wiring has been kept up with modern regs .

A modern building with a TNCS supply should have the steels of the building and any rebar of the concrete floor bonded in any case.

Perhaps he too should have a rubber mat.  swarf mat  for the modern factory.   He should also be provided with modern PPE, including toe protector boots that do not leak.


For what  it is worth, I have an island of wooden floor in front of mine for exactly this scenario, but in the manner of the cobbler's barefoot children, and 'do as I say not as I do' , the earthing on my lathe is not quite right anyway, as the centrifugal switch that disconnects the starter winding occasionally arcs out to the case, so the RCD is, ahem, absent.