Strange question but I suggest a panel sparky be called in the do the job correctly. I assume (and would suggest) that fixed connectors be used - there may be spares in the panel that would serve the purpose.
As I always advise - if you’re not qualified - keep your hands in your pockets
I work in maintenance and have been told so many different things in the past for example any spare cores being used should be earthed. I usually just disconnect both ends and terminate into a connector block and tape up but just wanted clarification.
Providing all elements of the installation is earthed in accordance with the wiring regs any spare cores should terminated into either spare terminals or connector blocks - just leave enough length in case they are needed in future.
I accept that you’re in maintenance but are you qualified to be working on control panels?
Apologies if I seemed a little curt with my responses. With 50years from Sparky to Principal Electrical Engineer in design and construction management, your initial question seemed strange.
If you’re working to a Specification the designer should have specified what happens to spare cores - if not ask for clarification - they are responsible for their design.
As an occasional Design Authority for projects that more often than not involve wires ...
The answer will depend on the type of cable, and if there is a possible requirement to re-energise it in the future.
Obviously it needs to be kept out of the way of the working bits of the design, so cutting it off and leaving it dangling is not an option. (And the 1988 Clapham junction crash should be held up as an example of what can happen to anyone who thinks cut and drop is OK.) So take it right out of the box, or make sure it is tied back or put under a spare terminal.
If the cable may be re-used it should not be cut so short it cannot be spliced, and once tied back it should be sealed against moisture ingress. Maybe adhesive lined heatshrink is your friend. Mineral insulated cables need a different approach!
If there is any chance at all of it picking up an unwanted voltage by capacitance to neighbouring conductors, and long runs of multicore mains cable, and the 'mains logic' of some heating systems come into that category, then the unused cores should be connected at least at one end, maybe both, to a know and safe potential, ideally earth.
In some areas (Avionics, telecoms and railways come to mind, there will be others) there will be industry standard methods to be applied. Elsewhere there will probably be house styles.
