general dead times of 3 - 10 seconds are normally satisfactory, but there may be special
cases for which additional time is required to permit the resetting of manual controls and
safety devices.
Loss of supply to lighting circuits, such as street lighting may be important for safety reasons
as intervals of 10 seconds or more may be dangerous for traffic. The main considerations of
supply interruptions for domestic customers are those of inconvenience.
An important measurement criterion for many power utilities is the number of minutes lost
per year to customers, which will be reduced on feeders using autoreclose and will also be
affected by the dead time settings used.
For high-speed autoreclose the minimum dead time of the power system will depend on the
minimum time delays imposed by the circuit breaker during a tripping and re-closing
operation.
Since a circuit breaker is a mechanical device, it will have an inherent contact separation
time. The operating time for a modern circuit breaker is usually within the range of 50 - 100
ms, but could be longer with older designs.
After tripping, time must be allowed for the mechanism to reset before applying a closing
pulse. This resetting time will vary depending on the circuit breaker, but is typically 0.1
seconds.
Once the circuit breaker has reset, it can begin to close. The time interval between the
energizing of the closing mechanism and the making of the contacts is termed the closing
time. Owing to the time constant of a solenoid closing mechanism and the inertia of the
plunger, a solenoid closing mechanism may take 0.3 s. A spring-operated breaker, on the
other hand, can close in less than 0.2 seconds.
Where high-speed re-closing is required, which is true for the majority of medium voltage
applications, the circuit breaker mechanism itself dictates the minimum dead time. However,
the fault de-ionizing time may also have to be considered. High-speed autoreclose may be
required to maintain stability on a network with two or more power sources. For high-speed
autoreclose the system disturbance time should be minimized by using fast protection, <50
ms, such as distance or feeder differential protection and fast circuit breakers <100 ms. Fast
fault clearances can reduce the required fault arc de-ionizing time.
For stability between two sources a dead time of <300 ms may typically be required. When
only considering the CB the minimum system dead time is given by the mechanism reset
time plus the CB closing time. Thus, a solenoid mechanism will not be suitable for highspeed autoreclose as the closing time is generally too long.
For high-speed autoreclose the fault de-ionizing time may be the most important factor when
considering the dead time. This is the time required for ionized air to disperse around the
fault position so that the insulation level of the air is restored. This can be approximated from
the following formula:
De-ionizing time = frequency
1
5.34
5.10 Vsys⎟ ×
⎠ ⎜ ⎞
⎝
⎛ + [s] (Vsys = System voltage in kV)
For 66 kV = 0.25 s (50Hz)
For 132 kV = 0.29 s (50 Hz)
It is essential that the protection fully resets during the dead time, so that correct time
discrimination will be maintained after re-closing on to a fault. For high-speed autoreclose
instantaneous reset of protection is required.
Typical 11/33kV dead time settings in the UK are as follows:
1st dead time = 5 - 10 seconds
2nd dead time = 30 seconds
3rd dead time = 60 - 100 seconds
4th dead time (uncommon in the UK, however used in South Africa) = 60 - 100 seconds
