7.13.5Disconnect times#
Disconnect times are specified to minimise the time a person may be in contact with a dangerous potential. The harmful physiological effects of current through the body can thus be limited. For a touch potential of 50 V ac or less, a five-second disconnect time may be used. Where voltages are higher, a 400 ms disconnect time should be used.
The disconnect times are integrally related to the touch voltage limits as indicated in the following graph from IEC 61200 413 Figure C.2.
The L curve indicates the maximum time a human may be in contact with various voltages under normal situations. The Lp curve is similar but is used where it is more likely that a low impedance path will be present.
The L curve touches the 50 V ac line at approximately five seconds and stays at 50 V ac for time durations above five seconds. This indicates that the human body can withstand voltages up to 50 V ac almost indefinitely without sustaining injury.
The 0.4 seconds time corresponds to factors that typically exist in final circuits. This time is confirmed by long experience in many countries as providing satisfactory protection against injury from electric shock. The prospective touch voltage corresponding to this time is approximately 100 V (IEC 61200 413 p33).
In summary:
- Touch voltage 50 V ac Maximum duration 5 s
- Touch voltage 100 V ac Maximum duration 400 ms
Transport and Main Roads requires circuit protection devices protecting cables (including consumers’ mains cables) directly connected to metal enclosed electrical equipment to clear active-to-earth faults within 400 ms, unless the touch potential at the switchboard can be demonstrated to be less than 50 V ac. Circuits consisting of underground cables enclosed within a pits and conduit system may be designed with protection having a maximum clearance time of five seconds.
For road lighting circuits, traffic signals field circuits, and ITS equipment automatic disconnection is provided by fuses.
