Had to do a quick bit of welding yesterday. Well, I say quick- welding took 5 minutes, disconnecting every ECU and electronic item in the car took about 3 hours!
Being Mr Ultra Cautious and not used to welding on any car that actually has electronic modules, I followed the instruction in RAVE to:

1. Disconnect all ECUs.
2. Disconnect airbags and sensor system.
3. Remove airbag modules. See
   SUPPLEMENTARY RESTRAINT SYSTEM,
   Repair.
4. Disconnect alternator.
   OK, I didn't remove the airbag modules and carry them in a bomb box to the middle of a field, but could see the logic in the rest of the instructions.

My question- has anyone ever blown up anything electronic (or heard of it) on a P38 when just following the time honoured method of disconnecting the +ve and -ve battery leads then hitting it with the MIG?

I've never blown up anything when there had to be done some welding after I took the following measurements:
Take off both batt. terminals, switch lights on (when HID or xenon)
connect both terminal leads to eachother for a few seconds to discharge all electrickery and disconnect
You could disconnect the connectors at the ecu on the bulkhead to be sure but that's about it I think.
Success, Tony.

You also could switch ignition key in pos II when both batt. terminals are connected to discharge airbag systems.
Just do not weld with terminals connected.
What else can you do?
Tony.

I blew my diesel engine ECU several years ago when one of the crocodile clips from a battery charger slipped off the battery when I was charging it and it touched the case of the ECU. The cover was off the ECU cover at the time. It spiked the electronics and it was kaput.
The insurance company paid for it except for £100 excess.
I have pretty much every spare ECU now and full diagnostics so less concerned about damage.
I have welded on the car. The manual says disconnect all ECUs but that is a BIG job.
I disconnect both battery terminals and I use an earth clamp either side of where I am welding to stop stray currents. Never had a problem so far.

When I got a bit stuck with some snapped exhaust studs last month we tried welding nuts to the exposed stubs.

Just disconnected the battery and went for it.

Nothing bad happened and the car fired right up once it was all done.

Thanks for the info chaps!