Current in earth circuit?

Over Clocker · 3 Dec 2006 at 16:13

Hi guys, was doing some lighting wiring in the loft yesterday. Tapped into the upstairs lighting circuit.

Upon testing with a electrical screwdriver, i notices some current going through the earth circuit. I don't have the proper testing equipment for this, but the glow on the screwdriver was much dimmer than when touching the live.

Being the nut that I am, I checked it by touching the earth cable to see if I got a shock, and I didn't

, but was just wondering if this is normal?

Cheers,

Mal

Jotun · 3 Dec 2006 at 16:15

You have to be touching 2 wires to conduct current through you (don't do this!) so you wouldn't have got a shock even if you touched the live wire. I don't think it's normal for the earth wire to be carrying current but I'm not an electrician.

Danny UK · 3 Dec 2006 at 17:11

My guess would be that your screwdriver was picking up the voltage from the other wires, hence the reduced glow.

You can get voltage on the earth cable especially in installations where there is a lot of IT equipment although this is now starting to appear in other situations. A lot of IT equipment has a filter built into it so a few milliamps flows directly to earth. If you've got lots of equipment, this can mount up and prove dangerous so a dual earthing system is employed. See here. You should be a bit careful altering your wiring as Part P of the building regulations is now in force.

By the way, you don't need to touch two wires to get a shock. If you touch any live conductor and provide a path to earth, then the current will pass through you.

Sirrel Squirrel · 3 Dec 2006 at 17:33

Evil-Penguin said:
You have to be touching 2 wires to conduct current through you (don't do this!) so you wouldn't have got a shock even if you touched the live wire. I don't think it's normal for the earth wire to be carrying current but I'm not an electrician.

I thought if you touched the live wire and was standing on the floor you would be earthed meaning you would get an electric shock?

Dr.EM · 3 Dec 2006 at 17:38

You can do yeah, that is the point of earthing equiptment. Though it seems impossible admittably, it can occur. Even upstairs it finds its way through to earth, i tried it with a low current HV source :eek:

Jotun · 3 Dec 2006 at 17:39

Sirrel Squirrel said:
I thought if you touched the live wire and was standing on the floor you would be earthed meaning you would get an electric shock?

Yeah, he was in his loft though so I'd have thought there would be some resistance through his house to the ground. If you're on the ground then you would complete the circuit so current could flow.

Thats why birds are fine sitting on overhead power lines, because they don't offer any path for current to flow.

SiriusB · 3 Dec 2006 at 18:02

It's a good job for the bird they are so high up too, be taking a chance every time they crapped

SiriusB

alexakasloth · 3 Dec 2006 at 18:03

Evil-Penguin said:
Thats why birds are fine sitting on overhead power lines, because they don't offer any path for current to flow.

You learn something every day.

Gavin · 3 Dec 2006 at 18:04

You have also invalidated your home insurance if anything to do with the wiring causes any problems! As you dont have a certificate to prove the work

i would have isolated the circuit first before testing it as well?

For you to be merely asking these questions is a reason why you need qualified electricians!

Thats not to say i dont do my own wiring either :rolleyes:

Eriedor · 3 Dec 2006 at 18:07

Recalling my A level electronics knowledge...surely you wouldn't get a shock because the current can either go through you to earth or through the earth wire (its purpose),and as the earth wire would have less resistance it would go through that, therefore wouldn't shock you.

Also maybe the small current on the earth wire is induced by the electromagnetic field caused by current passing through the live wires in close vicinity.

Over Clocker · 3 Dec 2006 at 18:09

Gavin said:
You have also invalidated your home insurance if anything to do with the wiring causes any problems! As you dont have a certificate to prove the work

i would have isolated the circuit first before testing it as well?

For you to be merely asking these questions is a reason why you need qualified electricians!

Thats not to say i dont do my own wiring either

Not knowing the theory behind earth leakage doesn't really mean I am incapable of doing some wiring tbh!

Have wired up entire ring mains back in ye old days!!

Mal

bmh.01 · 3 Dec 2006 at 18:21

Evil-Penguin said:
Thats why birds are fine sitting on overhead power lines, because they don't offer any path for current to flow.

True, but you offer the earth path when you touch a live conductor even if your feet aren't on the ground outside it'll still find its way. You'd have to be floating in the air or VERY well insulated not to get shocked

Adam_151 · 3 Dec 2006 at 18:26

If the earth was connected to earth then your neon screwdriver shouldn't be glowing when touching it (its possible due to small currents in the earth conductor that it'll be a few volts above true earth, but shouldn't be enough to make the screwdriver glow), its more than possible that its become disconnected and its floating above earth voltage due to capacative coupling to cables around it, but the impedance is such that it can't deliver much measureable current (not even enough to shock you it seems... but remember that when in the loft on wooden boards your impediance to true earth will be very high in itself...)

Really a neon screwdriver is far from adequate test equipment, the integrity of the earth needs to be checked, a cheapo multimeter will give you a quick yes/no answer, but if you want a meaniful resistance value a low ohms meter is needed

electrics forum on www.diynot.com/forums is worth posting in btw for questions like this