I'm surprised this question is still lurking around. To definitely avoid tripping the building's earth leakage breaker use a local isolation transformer with a centre-tap on the secondary.
Also use a residual current trip device from the transformer secondary to the SMPSU you are testing. An RCD detects differences in voltages down the two live wires (aka Live and neutral) so you need to wire your SMPSU earth connection to the transformer secondary centre tap.
Should either of the two AC wires have an excessive current to earth the RCD will trip.
If both ac wires have equally excessive fault currents to earth (a less likely scenario) the RCD won't detect anything but neither would the ELCB because the currents balance.
CAUTION - working on live equipment can be dangerous and you need to ensure that the transformer centre-tap (pseudo earth) is grounded locally to protect yourself. However, this may still cause your distribution board's ELCB to trip - however, if you replace that ELCB with an RCD you should be OK.
In a nutshell
Electricity is not supposed to flow through ground stakes in normal conditions. It doesn't mean its resistance is high, it's actually surprisingly small. That branch of the circuit is simply not closed normally.
In details
A ground is a reference point. You could litterally take any net in your circuit which is supposed to stay at a steady voltage and call it ground. After all voltage sources create a difference of potential (called a voltage) between two nets, regardless of what their potentials are - if they're both fixed externally, there will be a conflict and bad things, but if one of them is fixed the other potential will change accordingly. Generally the ground is taken such that we work with positive supplies predominantly, e.g. ground on the - terminal of a rectifier bridge. It doesn't mean all the current flow through that, it's only a reference.
The Earth has mainly a person protection role. No current is supposed to flow in the Earth because the actual supply circuit is isolated from the Earth, however what if this isolation is compromised (wires eaten by rabbits, children shoving their fingers in sockets...)? Everyone is indirectly connected to Earth (no isolation is perfect), which means that that circuit will now be closed and the only thing that will limit the current going through whatever is closing the circuit (e.g. people) is its internal resistance. Depending on the environment, that resistance can be sufficiently low to kill someone; refer to this thread about what voltages are considered safe. To prevent that, every enclosure is connected to Earth (a Earth-R-Earth circuit has a near-0A current), and the electric supply has a residual current device that compares the current going in and out, and cuts off the supply if there is a leak (through Earth).
The Earth is used for an equi[reference]potential supply The electricity provider needs to protect its people too, so the upstream supply is also referenced to Earth. Just like everywhere else. So what happens if the Earth is not a good conductor and its potential is not homogeneous? Users could be in contact with 2 different Earths, which can be a high difference of potential (=voltage). Thankfully, moist in dirt and water patches are good conductors, but above all the equivalent cross section of this fictive conductor is massive. Except during short upsets such as lightning, it has an excellent homogeneity in potential. Why use another conductor for ground which will use more copper and actually be less effective if we can use what's under our feet?
The Earth is also useful as a protection against lightning: lightning is just like any dielectric/isolator breakdown, it occurs where the resistance between the charged cloud and the Earth is minimal (see this amazing GIF). High trees, towers etc., and we can't risk relying on luck alone so highly conductive spikes are used to attract lightning, and the Earth is used to dissipate that energy. Loosely said. Normally lightning has enough current flowing to create through Earth and across human legs a voltage high enough to kill them, so it is spread out more evenly.
As usual, I'll warmly welcome anyone correcting me if not accurate.
Best Answer
Put it this way: there is a case on record of ball lightning rolling out of a fireplace, across a room and out the door. Brian Drummond's comment is spot on. The enormous current transients involved, with their associated electric and magnetic fields, can produce completely bizarre effects. For the most part, lightning strike effects can be handled effectively. I suspect that your PC has a good power supply, and there's a spark gap protector in there somewhere. If this is so, then you're seeing and hearing a successful protection incident.
Nonetheless, if this is a common occurrence, I strongly advise that you invest in an external surge/lightning protection unit, and not a cheap one. I well remember, twenty years ago, when I discovered that my phone was dead. After the line was repaired, I discovered that my modem was dead. Opening the case found it full of confetti, which was the innards of several capacitors. The capacitors had not handled the surge produced when lightning hit my phone line very well (which is why my phone line went dead, of course), but the modem did successfully protect my PC.