I've always wondered why BS7671 states that a disconnection time of 0.4s is required for circuits NOT exceeding 32A and for circuits exceeding 32A it is 5s
Surely you would want the higher current to trip quicker?
Can someone clarify this?
It was explained to me like this:-
First you need to understand that the 5 second and 1 second times (TT) are not based on the touch voltage curve.
They represent achievable values in that such disconnection times can be met.
Consider a TN system - we are addicted to the use of overcurrent protective devices for fault protection. This requires a substantial current flow to operate the device in a reasonable time. If we consider high current circuits, such as distribution circuits rated at say, 100OA, we need a very low impedance to allow sufficient current to flow to operate the overcurrent device. If we want to operate it quickly this impedance must be as low as possible.
We start to reach the limits with these high current circuits and all we can do is extend the disconnection time to try and make things fit.
5 seconds is a reasonably short time to allow a dangerous voltage to exist on a circuit that is unlikely to actually be in a fault state. i.e. A high current distribution or final circuit is usually fairly robust and faults are not very likely. If a fault should occur there is only a small probability that some one would actually be touching the circuit. Contrast this to a small final circuit that might be supplying something that we actually hold on to such as a power tool.
We could get round this by using RCD or similar technology for fault protection but we usually don't http://www.------.org/forums/forum/i/expressions/face-icon-small-shocked.gif.
TT is a different problem - we are already using RCD's or similar but we need a reasonable time span to achieve discrimination over a large installation. This can mean the use of instant, fixed time delay and adjustable time delay devices in series. 1 Second gives us the required time span.
