How to measure leakage current?
I have an AC powered device, and a customer asks for exact leakage current.
Of your device? I believe you are referring to current to ground that should be on the neutral. If you want to find how much current is leaking to ground in a device, electrically isolate it and measure current to ground through a known path.
Current that can trigger a leakage current circuit breaker, so it's obviously something flowing through Y capacitors to the ground wire.
Hmm in retrospect there may have been something wrong with that first sentence and maybe it was supposed to be part of this one. Anyway I think you're talking about an AC GFCI circuit breaker. Most GFCI devices work by passing both live and neutral wire through an induction loop. If all of the current flowing out on the line wire flows back on the neutral, the magnetic fields will cancel out and no current flows in the induction loop. If any of the current flows to ground instead, it won't return through the loop on the neutral and a portion of the magnetic field won't cancel out, causing a current to flow in the sense loop and breaking the circuit. This article goes into detail about what's legally defined as a class A, C, D, E device and why trip level requirements are what they are.
The question is, what is usually measured and what actually matters? Peak to peak? RMS? Pulse width?
The circuit I'm aware of mentioned above is basically measuring the absolute value of the total lost current, meaning both that it is measuring quantity and that it measures current regardless of direction of flow. The legal requirements for NEC specifications are clearly based on both current and exposure time, but because faster is always better in this case it matters more that it measures up to the minimums set by the government and performance may be significantly better in reality.
At any rate, Class A GFCIs must trip at 5mA and class B GFCIs at 20 mA, class B GFCIs are for old underwater fixtures in swimming pools where leakage current exceeds 5 mA and can cause nuisance tripping.