Basically that the HSE accept that 'working on or near live conductors' includes testing. They also accept that some testing can only be carried out with when the equipment is live.
However, the assessment and precautions required by Regulation 14 still apply.
So looking at each requirement -
"(a) it is unreasonable in all the circumstances for it to be dead"
Yes because you cannot conduct certain tests when the equipment is dead. These would include the measurement of loop impedance, voltage and load current when required.
"(b) it is reasonable in all the circumstances for him to be at work on or near it while it is live."
Yes provided the person carrying out the work is competent, has proper equipment and follows safe working practices.
"(c) suitable precautions (including where necessary the provision of suitable protective equipment) are taken to prevent injury."
Answered above.
Now this is not a justification for all work on or near live conductors, and it is certainly not a justification for work that involves manipulating live conductors. Testing using proper equipment can be carried out in a safe manner. However, it is most unlikely that moving a live conductor can be carried out safely in all but the the most specialised operations (live line working or similar).
As to the so called safe isolation procedures - there are a number of scenarios that can be put forward to show that they are inadequate. They all involve the unexpected re-energising of a circuit or item of equipment after work on it has begun.
Many of these would be very rare occurrences, but one stands out when working on installations. This is the borrowed neutral between two distribution boards, masked by a neutral to earth fault.
Suppose you are tasked to find a neutral to earth fault on a distribution board that has been 'isolated'. If you are not wary (which basically means that you work as if the board is live at all times) the first you will know of it is the pain you will experience in the last few seconds of your life
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