Wiring garden Shed

[Spark123]

What is the easiest way to transport an earth to an outside building?
To go down the SWA route or transfer it direct from the earth cable in the house?

As every installation is different, it is impossible to say. I don't follow what you mean by SWA route or direct from earth cable in the house?

 

[Hightowermark]

Well what i've read here means i can either go from the cu with 6mm and then transport the earth, or install SWA 3 core and not transport the earth. I think i need to do more reading

 

[Spark123]

Well what i've read here means i can either go from the cu with 6mm and then transport the earth, or install SWA 3 core and not transport the earth. I think i need to do more reading

I think you are getting the terms mixed up, when you say not transporting the earth I take it you mean to go for a TT installation which utilises a buried earth electrode for the supply earth. As with any TT system it must be tested with the correct instruments to confirm good contact with earth which is why it is not generally considered a DIY job.
If transporting an earth to an outbuilding you may use a 3core SWA (steel wire armoured) cable doubling up the armour with the black core (sleeved yellow/green) for the earth which automatically gives compliance with table 54G (csa of protective conductor vs phase conductor). The requirement to main bond extraneous conductive parts also requires attention.
Further info:
http://www.iee.org/Publish/WireRegs...matters_electrical_installations_outdoors.pdf

 

[danlightbulb]

Does anyone mind if I add a comment about the RCD workings thing earlier:

The rcd doesn't compare live and neutral, it compares earth and neutral. There is always a current imbalance between live and neutral because the neutral is at almost 0 volts so no current can pass through it.

If live is touched with neutral then a current will pass through neutral which, when compared with earth current in th rcd will trip the device.

A live-earth shock is protected by the fuse mainly wheras the rcd can protect against live-neutral shock as well.

So yes, an rcd does need an earth connection to work!


Please ignore me - this info is wrong - see why below

 

[Steve]

Does anyone mind if I add a comment about the RCD workings thing earlier:

yes, i do, because what you just posted is nonsense. What is your source for this?

The rcd doesn't compare live and neutral, it compares earth and neutral. There is always a current imbalance between live and neutral because the neutral is at almost 0 volts so no current can pass through it.

current is equal and opposite on live and neutral full stop, under normal conditions

If live is touched with neutral then a current will pass through neutral which, when compared with earth current in th rcd will trip the device.

EH????

A live-earth shock is protected by the fuse mainly wheras the rcd can protect against live-neutral shock as well.

NO! an electric shock can be 50mA, HOW WILL THIS EVER BLOW A 32A FUSE? THE MAIN PURPOSE OF RCDS IS TO PREVENT ELECTRIC SHOCKS!

live-neutral shorts, yes, these are dealt with by a fuse due to huge current surge.

live-earth short will trip an RCD and/or the fuse. live-earth LEAK (slight imbalance between live and neutral caused by motors and heaters) will trip RCD too.

LIVE-NEUTRAL SHORT WILL NOT TRIP AN RCD

to sum up,

So yes, an rcd does need an earth connection to work!

then why does the RCD in my consumer unit not have an earth tail? <yawn>

 

[danlightbulb]

I totaly apologise - you are correct. I must have got my wires crossed somewhere- good job i'm not an electrician!

There is one thing I hope you can tell me though and its about the neutral.

The neutral carries no voltage and no current - so how can the rcd compare them?

 

[Steve]

neutral only carries current when an appliance is connected between live and neutral.

live is always live at 230v AC, but doesn't always carry any current.

When you connect an appliance across live and neutral, the neutral carries current back.

The RCD compares CURRENT, not VOLTAGE.

Dont quote me on this, but I think:

The potential difference (voltage) between phase and neutral is 230V. Due to alternating current (and voltage), the 230V is an RMS value on the LIVE - it peaks at something like 400V, and troughs at -400V. Because neutral is earthed at the substation, the voltage is 0v all the time. However, when an appliance is connected, neutral also carries 230V AC.

So if live carries current, but neutral doesn't, RCD trips. It is not possible for the neutral to carry current without a live conductor also carrying current (AFAIK)

Live-earth SHORT - MCB/fuse trips AND RCD trips
live-earth leak - RCD trips
neutral-earth fault - RCD trips when current flows.
live-neutral SHORT - MCB trips.
Any electric shock - RCD trips. Unlikely to trip MCB due to lack of sufficient current!

i apologise if i sounded a trifle harsh earlier - i didnt want DIYers thinking what you had said was correct, and looking for earth tails on their RCDs!!! Some do have earth tails, but your reasoning was wrong im afraid!

 

[danlightbulb]

You're absolutely right I am glad you corrected me because now i am a little wiser also.

Even with all sockets loaded in a house, I'm fairly certain that if you were to measure between earth and neutral the voltage would still be 0, and between live and neutral would be 240V ish.

An appliance creates a flow of current by connecting live and neutral through a resistance. The amount of current that flows is exactly enough to be totally reduced to zero by the time it has passed through the load resistance, hence no current on the neutral?

Perhaps someone has a more scientific explanation?

 

[Adam_151]

The potential difference (voltage) between phase and neutral is 230V. Due to alternating current (and voltage), the 230V is an RMS value on the LIVE - it peaks at something like 400V, and troughs at -400V.

Its not as high as that, work it out yourself, its root 2 x phase voltage

Because neutral is earthed at the substation, the voltage is 0v all the time.

... in repect to the physical mass of the earth, in theory, in practice there is usually a few volts difference

However, when an appliance is connected, neutral also carries 230V AC.

I think you are a little confussed here, voltage isn't carried as such, are you trying to say that in a single phase system that the neutral will carry the same current as the phase?

So if live carries current, but neutral doesn't, RCD trips.

Correct

It is not possible for the neutral to carry current without a live conductor also carrying current (AFAIK)

Kinda true, while you have neutral and earth on one end of the transformer winding, you only have phase on the other, so the current has to through through the phase wire, but it might not be going through the phase side of an RCD, such a situation could happen if you borrow neutral from a socket for a light on a split board installation, and yes, the rcd will trip with the imbalence this way around

Live-earth SHORT - MCB/fuse trips AND RCD trips

Depends on your Ze...

Any electric shock - RCD trips. Unlikely to trip MCB due to lack of sufficient current!

Not if the shock is phase-phase, or phase-neutral, as opposed to the more usual phase-earth. and providing enough current flows, to hit the trip point of the rcd, rcds over 30ma do not offer shock protection, simply because if you hit the trip point on them, you're likely to be in quite a bad way

i apologise if i sounded a trifle harsh earlier - i didnt want DIYers thinking what you had said was correct, and looking for earth tails on their RCDs!!! Some do have earth tails, but your reasoning was wrong im afraid!

So who wants to explain ELCBs then

 

[Steve]

An appliance creates a flow of current by connecting live and neutral through a resistance. The amount of current that flows is exactly enough to be totally reduced to zero by the time it has passed through the load resistance, hence no current on the neutral?

no. There is current on the neutral when there is an appliance connected. There is also an AC sine wave - the neutral carries 230V AC from the live, back to the substation. But if you touch neutral, it wont hurt because it is earthed at the substation. There is sometimes a potential between true earth and neutral on a TT system (local earth rod). If you have a PME system, neutral and earth are connected at the supplier head, so are at the same potential - 0V.

perhaps someone else can explain neutral potentials better than i can - im not a spark.

The RCD measures current on neutral and current on live - when under load, they are balanced, the current is the same. When a fault occurs, something takes current away from neutral or live, the RCD trips. Basic principle.

EDIT: adam - just read your post - sorry i may have got this wrong again1!!!!

 

[danlightbulb]

I sort of see what you are saying now - i think.

Because neutral is earthed at the sub, the path of least resistance is through the neutral cable, not through my body to earth. So when i touch a neutral cable I don't get a shock, because the current is taking the path of least resistance, i.e the cable rather than my body.

 

[Adam_151]

An electricial current is a flow of electrons, the voltage is the amount of energy each electron carries, it is the latter which is reduced to zero after flowing through the load, electrons can't just disappear, the current is the same at all points of a circuit, kirchoff's first (IIRC) law.

Neutral is connected to earth at the transformer, this keeps its potential pretty close to physical earth, and without a potential (voltage) you can't have a shock, but because its not exactly at the same potential as the physical earth, there is a possibily of small currents flowing, in a TNC-S * the earth is derived from the neutral at the supply head (as crafty said), all earthed metalwork will therefore be at the potential of the neutral, this sometimes causes problems with animals, for example cows are sensitive to the small difference in potential between the milking systems and the earth on which they stand, it is recommended that in situations like this a metal grid is installed into the floor and connected to the main bonding, or the system is made TT.


The reason the neutral is not at true earth potential is because its got a current flowing through it and the cable has a resisatnce, if you multiply the resistance of the cable between the point you touch the neutral, and the point it is tied down to eath, by the current flowing in the neutral, you will get the volatage drop across that piece of cable, which will be the amount the part you touch is above earth

*a problem with TNC-S is that if the neutral breaks, the part downstream of the break will rise to 230v (assuming to further electrodes in the ground to drag it down), and because your earth is connected to this, it too will rise, and you can have a shock off the earth wire, the supply company is supposed to ensure that this can never happen, but its not unheard of