There is some guidance in Traffic signs manual chapter 8 Part 2:
O3.21.19 When traffic signals are not operating (removed from use), for any significant period of time, the signal head should be bagged over. Alternatively, for periods of less than one week, signs to diagram 7019 may be erected to indicate that the signals are not in use. The signs should be fixed to the traffic signal posts, and may also be erected in advance. However they should never be used if portable signals are in use at the same location. Signs to diagram 7019 must be reflectorised if not directly illuminated by specially provided luminaires.
O3.21.20 A risk assessment should be carried out and contingency plan prepared in anticipation of signal failure at signalled intersections. The sign “Light signals not in use”, diagram 7019, is prescribed for use where signals fail. White retroreflective borders on the backing boards of the signals, to enhance conspicuity, are also very useful if signals fail. However, if the absence of signal control is predictable and long term, signs to diagram 7019 may be necessary, whether or not the backing boards have a white retroreflective border. The risk assessment and contingency plan need to take into account predictability and the duration of the problem. For example, a power failure is neither predictable in occurrence nor duration. If a sign to diagram 7019 is used as part of a contingency plan, it could be confusing and potentially dangerous if the signs remain in place when the signals are working again. Switching off the traffic signals should be considered to avoid the risk of confusion and a potential dangerous situation until the signals are ready to be operated again. Maintenance of traffic signals is dealt with in Part 1: Design, Section D3.25.
Hope this helps