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I am replacing a luminaire (light fitting). The new fitting is double insulated and the instructions include the requirement 'This light fitting is Double Insulated [[]] and must not be earthed' It has a plastic outer case but internally there is a metal plate (presumably to reflect the light from the bulb). My question is: Why is the requirement stated as a 'must not be earthed'? I understand why it need not be earthed, but what would be the issue with earthing this metal plate?

Why would you want to do such a thing?

Yes it does seem odd, we have to provide an earth to fixed equipment even if not required, the rules say "A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point." so common to find items like thermostats have a provision for "Parking" the earth wire, and lamps often have provision to take the earth so it is connected for the next lamp.

Thanks for your reply. Here's a bit more info and a rough hazard assessment:

The luminaire is a bulkhead fitting with concealed cabling (rear entry), thus it can't be unplugged as you would for, say, a desktop lamp.
In normal operation it is indeed double insulated, safe and doesn't need a protective conductor.
However, consider the situation where the Live supply cable becomes disconnected inside the unit and touches the internal metal plate(reflector).
With no protective conductor the light no longer works, the plate is live but this has no other effect at this point. With a protective conductor the circuit RCD should trip.
If the light no longer works the user suspects a failed bulb, so removes the outer cover, in this place a glass lens,to gain access to the bulb.
There is now a real hazard - a potentially live bit of metal exposed to the touch, only isolated (in this case) by the luminaire switch. If this switch has been left ON and the user attempts to change the bulb then electric shock will follow. Sure it should be caught by the RCD on the ECU, but it remains a hazard. Note: it virtually impossible to change the bulb without touching the metal plate. This is a recently purchased luminaire from well known retail supplier. Yes, the cables should all be terminated securely, in this case screw terminals in the back of an ES socket, but if a cable is loose there is no barrier between it and the metal plate previously discussed.
Therefore the [revision of a EPC will guard against the hazard/failure mode.

The regulations refer to a tool or key, and if it needs a tool or key to change the bulb, then the person using the tool or key should isolate first, this means turn off all live feeds, which include both line and neutral, so the light switch only switches line so does not isolate.

Yes I know that seems daft, but it is what the regulations say.

As to the regulations, the requirement for the earth arrived in 1966 it said "D.6 At every lighting point an earthing terminal shall be provided and connected to the earth-continuity conductor of the final sub-circuit." before it said "403 The application of Regulation 401 (iv) which exempts from the requirement of earthing metal which is so isolated that it is not liable to come into contact with live parts or with earthed metal, shall be restricted to the apparatus detailed in the following \clauses (i)-(ix):
(i) Short isoated lenghts of conduit used for mechanical protection of tough-rubber- or p.v.c. -sheathed wiring.
(ii)Short unexposed isolated lengths of metal conduit used for the mechanical protection of cleated wiring where it passes through floors, walls, partitions or ceilings.
(iii)Short isolated lengths of catenary wire used with tough-rubber-sheathed or p.v.c.-sheather wiring systems, if screened by non-conducting material.
(vi) Lamp-caps.
(vii) Small parts such as screws or nameplates isolated by insulating material.
(viii) Metal chains for the suspension of lighting fittings, not in contact with flexible cord of the twisted type.
(ix) Lighting fittings using filament lamps installed in a room having a non-conducting floor, mounted at such a height that they cannot readily be touched and are out of reach of earthed metal." and today "A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point.".

So wall lamps have needed an earth before 1966 and today if the lamp is not suspended then needs an earth at the accessory, so not to fit an earth would not comply. But it also says "134.1.1 Good workmanship by competent persons or persons under their supervision and proper materials shall be used in the erection of the electrical installation. Electrical equipment shall be installed in accordance with the instructions provided by the manufacturer of the equipment." so one assumes the type testing has identified some risk when fitting an earth.

However I think it is more likely that it has been translated from Chinese and a mistake has been made in the translation. Many thermostats have a parking terminal for the earth, and the same with down lights, so I would be asking the question at the "well known retail supplier" I am always weary of ES bulbs, the screw should be neutral, but so many use same colour for line and neutral often hard to comply without getting the meter out.