Prolong time is a time that the emergency device will stay on after the resumption of the normal supply.
If the mains lighting does not come back on immediately after an interruption (for example HID lamps), you need to keep the emergency lights energised throughout this period.
This time is important when HID or other special lighting is used that may not re-light when the normal supply is resumed.
In Australia the emergency standard AS2293 defines the requirement of this condition when HID lighting is used.
The standard prolong time is 2 minutes but this can be increased to a period of 90 minutes.
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