local/bench earth leakage for testing mains electronics
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.