Anyone who goes and measures the ESR of mains side caps in a SMPS without first discharging them manually needs their head examined!
The Peak 70 is robust, but it doesn't have idiot protection built in.
McBryce.
If you want to use your ESR meter for finding bad caps in circuits, it must be protected at least to the voltage you can find accidentally in the circuit your are testing.
It is not an "idiot protection"...as it is not idiot to have a multimeter protected up to 5KV when you only measure mains voltage...
SMPS with PFC has voltage up to 400V on capacitors...they can stay charged with 200V or more.
To forget to discharge a capacitor is only a mistake that can happen to everybody, no need to have "their head examined !"
That's for this that protections exists...in case of mistake or distraction.
In practice, if you have to discharge every capacitor before testing, you loose a lot of time and it is not rentable nor efficient.
For this reason, an ESR meter must be able to measure ESR on loaded capacitors and be protected at least to +- 200V.
Another important application of ESR meters is to measure the internal resistance of batteries.
For this application, it's also needed to be be protected against dc voltages, but not for such a high voltage.