When testing for volts why doesn't earth become live?

[sensible]

Hi,

I am thinking of doing the 2365 course but in the mean time I am learning a bit myself and I have a question which I hope someone can help me with please:

When I test between line and earth to see if something is live (230V), do I make the earth live? I kind of know you don't as I left my MFT on the socket connected between line and earth showing 230V and tested another socket between earth and neutral (with another tester) to see if the earth was live and it wasn't. But why isn't it?

I also attached one probe of the MFT to a known 230V line, the other probe I connected to the probe of my voltage tester, and then the other probe of the voltage tester to the earth of the socket and the voltage tester showed 230V, which kind of threw me?

Basically if I am testing for voltage and have one probe on the line, will the other probe be live at 230V?

Thankyou,

ps.. I would have put in the DIY section but it wont let me post?

 

[Marvo]

Hi,

I am thinking of doing the 2365 course but in the mean time I am learning a bit myself and I have a question which I hope someone can help me with please:

I'm assuming you're not a qualified electrician so I've put this in the DIY forum.

When I test between line and earth to see if something is live (230V), do I make the earth live? I kind of know you don't as I left my MFT on the socket connected between line and earth showing 230V and tested another socket between earth and neutral (with another tester) to see if the earth was live and it wasn't. But why isn't it?

The earth or the earth wire of a circuit will always remain at zero volts (earth potential). When you put your test leads across the live and the earth the internal circuit of the tester is a very high resistance so the live lead of the tester will be at 230v and the lead of the tester that's on the earth will be at 0 volts. The tester 'sees' the voltage difference between the two leads and displays it as a number on the screen.

I also attached one probe of the MFT to a known 230V line, the other probe I connected to the probe of my voltage tester, and then the other probe of the voltage tester to the earth of the socket and the voltage tester showed 230V, which kind of threw me?

Basically if I am testing for voltage and have one probe on the line, will the other probe be live at 230V?

Because digital testers have such a high internal resistance they're extremely sensitive and in fact for some tests they're over sensitive. Without getting into the technicalities if you repeat your experiment using an old fashioned analogue tester (the type with a moving needle on a scale) you won't get any reading. Similarly if you have a cable that isn't connected to anything at both ends and you run it in close proximity alongside a live cable for several meters you get a slight voltage induced into the disconnected cable. This induced voltage can't produce any substantial current flow so you can't get a shock from it but it's not uncommon for a digital tester to pick it up and give you a reading of around 90v. If you use an analogu tester you'll get zero volts showing for the same test.

 

[sensible]

Thanks Marvo,

I found that very informative and interesting.

 

[SirKit Breaker]

Sensible,

Firstly i like your name, but be sensible by name and sensible by nature.
I have no gripes with people having a go, and don't wish to dampen your enthusiasm as you appear to be trying to work things out. This electric lark isn't rocket science as you will find out in due course, but i would like to advise you to get someone to show you what you are actually doing when you connect your tester to something.

I advise this for 2 reasons,

1. You will better understand what you are doing, and the implications of doing it wrong.
2. You are less likely to hurt yourself or someone else.

In the wrong hands, and in the wrong situation, an Electrical test instrument can cause serious injury or worse, so be very careful about what you connect it to, and the tests you do, especially whilst you are learning.

Cheers……………..Howard