Rapitest Power Detector

[Brian]

Anybody got any experience with a Rapitest Power Detector? It's one of those "voltsticks" designed to find out if a wire is live or not (but it's NOT designed to find wires in walls - that's a different device; this one is designed to be used on wires you have access to so you can test for breaks).

I got one to test my wiring and noticed a curious thing. When held in front of a socket, on some sockets it beeps, but on others it doesn't. Yet those sockets are definitely live and working. On some sockets the L beeps but the N doesn't! Again, the sockets are normally work fine (I've opened them up and the wires are correct inside).

Could this be indicative of a break in the ring main at some point? What do these things actually detect, anyway?

[antgel]

I'd like to know as well. I just found out the same on a socket that I just fitted. I plugged in a socket tester and all looks ok. The only difference is that this new socket I refer to has a metal front plate which is earthed.

Antony

[John Rumm]

It is indicative of what happens when you use an unreliable method of detection ;-)

[Mike Tomlinson]

In article , Brian writes

I have a Rapitest socket tester and was so impressed with it [1] that I prefixed the model name with a "C" using tippex.

[1] i.e. not

[Peter Andrews]

Your first paragraph says it all - it's useful to see if a circuit is live and then dead when you remove the fuse/trip, etc. but can be misleading in some circumstances. Don't rely on it if you are sticking your fingers in and it's no substitute for proper test equipment.

Peter

[Brian]

That's probably the answer - that the earth plate somehow affects the reading.

[John Stumbles]

Electric field: they're basically an amplifier with a very high input impedance (> tens of megohms) and probably bandwidth of c. 100Hz driving a rectifier and LED and/or buzzer. Or something like that. Thus if you hold it near a conductor at around 240V 50Hz AC (i.e. mains) it'll pick up enough voltage from the electrical field for the amplifier to drive the indicator. You'd get much the same effect connecting a wire to the high-impedance input of an old-fashioned amplifier for a crystal mic or record pickup and waving it around, giving a bloody great mains hum from the speakers when the wire is near a mains conductor.

Limitations are:

it won't pick up so much if there are earthed conductors nearby, especially between the live conductor and the voltstick (false negative)

it can pick up induced voltages from adjacent conductors so that a conductor that is not connected to anything but is near a live conductor somewhere along its length can appear to be live when tested with the voltstick (false +ve)