Strange Meter Reading

additional info this was a radial circuit

There was probably a capacitive and inductive load somewhere downstream, or upstream, and you were reading the result of a series-resonant circuit.

Remember, when the neutral is broken all loads are effectively in series, not parallel. so current will be constant, and voltage will vary at different points in the circuit.

HTRN

I assume you used a DVM (?).

A DVM set to measure voltage has a very high input impedance - which is good as it presents a negligible load to the thing you're measuring, but can give misleading results.

The problem is that you can get capacitative and inductive pickup on a 'floating' wire (it acts as an aerial) - particularly if that wire is in the same cable or conduit as 'live' wires. Only a minute current would flow into the meter, so it could measure a considerable voltage, even though there would barely be any power there (power = voltage x current).

I suspect that you'd have measured little if anything using an "old-fashioned" meter such as an AVO.

Hi. Thanks for those answers they make perfect sense. I was using a DVM and I was aware of the issues you've metioned, but unfortunatly I found myself questioning my readings only because I was explaining all this to an apparent expert earlier who tried to convince me I was wrong and the readings were impossible!

Many thanks

When you say "radial circuit" are you describing a 3-phase 415/220 delta/star system? If so, there would be some ways you could see 415V.

Hi,

This is a common event in euro / africa /asian systems where grounding and neutrals are not as closely monitored as in the US and UK and you are getting feed back thru a device, over a common neutral, from another phase,

Soy Cowboy

That is why test lamps are used to confirm voltages on lines where high impeadence meters can be misleading.

If you had an appliance plugged in up line it should have made no difference unless you had left the power supply to the circuit on.

If you have a 3phase distribution circuiy, you have 415V between phase and 220V phase to neutral...OK?

Then if the Neutral wire coming to the socket in question is the one looping to other sockets or points with different pahses, you have had the neutral broken before reaching the particular socket! In this case, the part of the Neutral wire that is looping to other circuits will be at the voltage of the other phase where some appliance was connected to it: hence the 415V reading!

If any other appliance was working or ON, it must have exhibited some anomaly like low voltage or sudden surge or a lamp coming on when you switched something else...The isolated Neutral (from the supply neutral will become FLOATING in voltage sense...

When troubleshooting, never use a test screw driver, use a moving coil tester it will give real voltage readings.

The reason why you are getting this 415v reading is because the original wiring most likely used the same neutral for another load with a different line supply which is against good wiring. I think this is against code.

So, you'll read between twoi line supplies without the neutral.

Test screw driver is an instrument just for testing the presence of voltage, never for determining how to fix circuits.

Live and learn.