I recently bought a Fluke 1664FC. It's great but recently I chose to do a loop test on the no trip setting. Plugged the 3 wires in but noticed the screen says it's set for a L-N test with only Live and Neutral plugged in. I pressed F1 button to change this to L-PE, the tester beeps but it doesn't change. Anyone know why that might be? Cheers