how did you protect your lab setup?

Good questions Jan.  When I worked on a mains powered transformer with associated parts:

1. covered every potential touch point with kapton tape just in case.
2. spaced everything out so that there was no awkwardness in probing - no nooks and crannies to reach into 
3. Stuck labels/notes etc in a variety of visible places, by parts, stating specifically mains voltage
4. used a RCD between the mains socket and device

Mine was a fairly simple case, mind, and I didn’t need to do much probing on mains parts.

Professional tape job

Cover the back of the PCB with insulating tape first.
Don't count on friction to hold the PCB in place.
Use a power bar and place the OFF/ON switch very near you.  I like scottiebabe's tape job.
Remember that quick connects don't disconnect when you need them to.
When possible, use an in-line fuse.
Learned all of these the hard way.

I would say, the best thing is avoiding mains entirely for the smaller power rating stuff.

I use a floating DC Lab supply to run a pure sine wave inverter (300W) as my synthetic mains supply for testing of smaller items. Fault currents are very limited thanks to the inverter's small power rating and blowing fuses is not likely in case of a hard fault - the inverter usually simply cycles (another benefit).

For higher current mains stuff, I made an isolation transformer out of back to back toroidal transformers. The upside is this breaks the earth connection which means that accidental contact between device and earth won't result in a shock, but also means some modern safety features (e.g. RCDs) won't save you. The jury is usually split on this sort of thing.

I also use a variac with my supplies to bring them up gently if necessary, feeding all power through a Tektronix PA1000 Power Analyzer just to see if anything is wrong as the voltage is coming up.

I would suggest soldering in insulated wire for probe points, maybe screwed down into a terminal block for safer probing. Then you can use a DMM or scope probe and touch the screws or metal parts in a more convenient location. Still prefer screwing the probes down into the terminal block and not changing connections while powered on to avoid surprise smoke events.

Use a remote power switch - I have a radio controlled power switch I can remotely turn on and off. Where nasty results are expected, I'll have my camera observe while I apply power from a distance.

Make sure the item is sitting on a nonconducting surface ...

- Gough

I would have suggested a perspex (acrylic) sandwich (with plastic hex spacers), to protect against accidentally putting fingers in the way, plus it protects against things popping in face, however, the board has no mounting holes, which makes that harder. : ( I keep sheets of perspex handy for making them into bases or covers, and would have been happy to drill a couple for you to the precise holes, but I can't see there's any mounting holes from the datasheet.

I got really close to doing something way too stupid to look at commercial flyback converters' waveforms, but I stopped here (i didn't end up plugging this into mains). and decided not to do anything with mains at all.

For mains work in UK (and Europe) the high (230V) voltage makes it more tricky than 110V areas.

I use a Variac (variable transformer) and wind the voltage up slowly - the variac does not provide isolation.

I have an isolation transformer and often use it on the scope (to avoid earth loops to improve measuring noise) but sometimes on the DUT.

It's worth doing some planning to work out the best approach.

And lots of care (like using proper connectors and HV probes). Everyday scope probes (even expensive ones) are not that good for mains work and the short dangly earth leads are a menace. (and if you connect an earthed scope to the DUT it is no longer isolated.)

Your diff. probe is a good thing.

MK

I'm glad you stepped back - it's always worth the 5 minutes or so it takes to use  a proper mains plug and cable !

MK

I'm no mains expert, the voltage could be reduced with a travel mains transformer (i.e. the ones that go from 220V to 110V), since presumably the board will be fine at that lower voltage. Not necessarily safer though! Ironically, I once purchased a small travel transformer product (maybe 30-50VA) from a local "Maplin" store, but that transformer was later subject to a recall! So that wasn't safe either : )

I now have a larger (200VA) transformer (this one, no longer manufactured) that actually states that it is intended for isolating (separate pri/sec winding locations) with two 50V windings, which suited me fine because I was looking for something that I could use to power up Japanese equipment (100V) at the time.

I applied a few of basic principles.

1. Stand well clear

2. Secured Cover - I like to use clear covers to be able to visually inspect. The one shown is screwed on.

3. Use covered connectors that are suitable for application

4. Label