I wanted to test how much power a GFCI outlet uses while idle, so I hooked up my multi-meter to test the current flow. I put my test lead in the amp socket, and confirmed the current was on the order of a milli-amp, so I was safe to plug my lead into the milli-amp socket.
After doing so I measured a current of a few milli-amps. But then suddenly through testing I got a sudden current spike, and blew my 400mA fuse. I'm not sure what caused the spike… maybe tripping the GFCI, etc, but some GFCIs seem to operate this way.
Is there some way I can even this out, and absorb whatever the spike current is for that sub-second that it spikes protecting my fuse and multi-meter, but still be on the milli-amp scale? I know conceptually this is what a capacitor does, but I also know you can't simply hook a capacitor up in series and expect to get any current flow.
I'm thinking maybe there's some simple circuit, or inexpensive device I can hook up between the GFCI and my multi-meter that'll prevent the fuse from blowing, but I don't know enough about electronics to know if this is possible, or where to even begin.
EDIT:
Another possibility is some sort of solid-state, fast trip 300ma circuit breaker that I put in series, before the multi-meter that'll trip before I blow the fuse? Does such a thing exist?
EDIT2:
How about something like a Inrush Current Limiter? https://en.wikipedia.org/wiki/Inrush_current_limiter
On the face of it, it sounds like the solution to exactly the problem I'm having.
Best Answer
I think you're barking up the wrong tree.
You are attempting to view a momentary surge with a common DVM. They're not made for that.
You would be better off installing an ammeter shunt, and then looking for millivolt rises across the shunt. Further, you should use some sort of data recorder that will sample at a very high rate, so you can observe the transient.
Most likely this was the GFCI device's internal electronics initially charging their capacitors. It could also be some part of the startup testing that some GFCIs do. For instance some are able to detect neutral-ground faults on power-up; they can only do that by attempting to flow some current on neutral. Whatever it is has to fit inside a recep form-factor.