The Philae Lander may have solar panels on board, but luckily, it does not fall within the scope of the draft best practice guide: IET Code of Practice for Grid-connected Solar Photovoltaic (PV) Systems* (CoP, for short). The CoP mentions bonding to earth quite a lot but completely ignores bonding to comets.
That may be for the future, but one of the interesting technical changes to guidance for earthlings is that all PV array frames should now be electrically connected to earth. This is a departure from previous advice, which suggested that in many cases it was safer not to earth the frame since it would then take a double fault in the DC wiring to cause an electric shock to someone touching the frame.
However, things have moved on. Many modern string inverters now have some clever fault detection circuits built in e.g. earth leakage monitors, and for these to function, the frame must be connected to earth.
Seems simple enough so far: all conductive frames must be connected to earth. Where the complication comes in is in deciding on the type of earth connection. Extraneous conductive parts must have equipotential bonding and exposed conductive parts must have a safety earth. If a lightning risk assessment suggests earthing is required, a suitable conductor must be provided; but if none of the above conditions is true, a ‘functional’ earth must be fitted – this will allow the earth leakage monitors to function.
Any alarms triggered by the monitoring circuits must be of a type that cannot be missed (e.g. big flashing light or SMS) and must continue to operate until the system is switched off or the fault is cleared.
*Can be pre-ordered on the IET web site: www.theiet.org/solar-pv