GarageYears,
Your 'Breakers & Ground Wires' is very interesting reading, esp:
So, to the tiny meter voltage and current it exhibits < 1 Ohm but rather more resistance to the phase line?

Westlakes definitely know what he is talking about:ok:



he's an electrickerical injunir:}

Edit; did you know that delta/wye connected transformers are being phased out:}

:ouch:

Principally the issue is impedance versus resistance - a meter measures resistance using a DC voltage as a source (typically a 9V battery in a digital meter like a Fluke), but the mains voltage is AC, so the current is switching back and forth at 60Hz (or 50Hz in the UK). The impedance is a measure of opposition to the AC current flow. Since we only need a marginal increase in "opposition" to prevent our breaker tripping, it is not hard to see that a small increase in the impedance at 60Hz to around 4-5Ohms or greater would prevent the breaker tripping (assuming say a 30Amp breaker).

- GY

Aircraft tyres tend to be conductive to ensure any static potential is discharged on touchdown. An old British type I used to fly actually had a copper mesh moulded into the nosewheels and you could see bit's of copper glinting in the sunlight sometimes. These days they put something in the rubber which makes the earth.

Helicopters can generate massive static potentials. If you're in the unfortunate position of having to be winched in a rescue, you may notice the winchman has an earthing lead danging from his harness. He will touch that on the surface before touching either the sea/ground or you. It's very important to let that happen - otherwise there may be a big flash and a yelp! If you're in water, it's you who will take the shock so don't be too enthusiastic to be rescued! With other rescue systems where they use either a strop or a basket, the crew will dip it in the water before it comes near you.

I know this is a bit of thread drift but:
If 'Earth' was strapped to Neutral, say, just before the domestic meter, would a Neutral short to Earth still trip the RCCB? i.e. like Mig15's system.

Don't worry, I have no intention of carrying out above modification :)

A bit like this video :}

YouTube - High Voltage Cable Inspection

Yes - that would cause the RCCB to trip. The way the RCCB works is to compare the current flowing in both the live and neutral wires (remember: mains is AC, or alternating current, so it flows in both directions, in both wires). When the circuit is working correctly current should ONLY flow in the live and neutral wires and be exactly in-balance. Any current flow outside of those wires will cause the RCCB to trip, since the current flow will no longer be equal, which is exactly the point of the RCCB! A short between the Earth and Neutral will allow the current to flow in both Earth and Neutral lines (split as a function of resistance), and will pop the RCCB. The RCCB has no idea where that current has gone, but it will see the imbalance - for all the RCCB 'knows' that missing current could be flowing through YOU!

Typically the trip current imbalance will be 5mA, so any mismatch greater than this will pop the breaker. For humans as little as 50mA @ 120V can be fatal, depending exactly where the current path flows. The worst possible path is across the chest for obvious reasons, so grabbing the live and neutral lines with left and right hands is a VERY bad idea. Many electricians I know will purposefully slip their left hand into their back pocket or belt strap to ensure they don't complete a chest path if they are forced to work on a live circuit.... not something recommended ever but occasionally unavoidable.

- GY

who, alot of complicated ideas flying about here, so i thought i would just simplify things;

a conventional earth is not a stick into the ground, consider it a 0v reference point with a very low impedance (IE lower impedance than through your body) therefore an attractive route to compete a circuit for any stray current (IE from a loose wire or whatever) if it were to touch a metal case, and somone was in contact with that case- should the case be earthed the current would go through the earth line (AC AND DC ALIKE) instead of through you.

In an AC it is for exactly the same reason, remembering that for a current to flow there has to be a potential difference (if you and the aircraft are all at 10,000v and you touch a 10,000v wire it doesn't matter because there is no difference - hence how birds sit on power lines). Therefore an ACs earth can merely be a mass connection of everything metal that stops a difference from forming between different parts - IE connection to the neutral of the battery too. A loose wire would therefore short through the low impedance mass of metal and not through the higher impedance human flesh. On the ground, the AC ground can be connected to the real world ground so there is no difference between the AC and the rest of the world - remembering it is a difference that causes a current. you do not want this difference to equalize through the fuel hose - a grounding reel is much more preferable.

source: electronic engineering degree

I would like to bet the risk of injury from falling of a ladder because they have their hand in their pocket is greater than that of completing a chest path! (at least for a competent electrician)

Go with the second answer