Some questions regarding Earth connection

Cork boy 55

Non-professional here just some wondering

Just taking a look at some broken appliance here and wondering

How come some appliance only have a live and a neutral connection i.e no earth wire?

Does the Earth wire only carry a load (at all levels) if there is a fault?

What happens in these appliances(no earth wire) if they become live and some one touches them?

2011

Have a read about double insulated appliances. That will answer most of your questions.

CountyHurler

Cork boy 55 wrote: »

Does the Earth wire only carry a load (at all levels) if there is a fault? What happens in these appliances(no earth wire) if they become live and some one touches them?

The best way that I think about it is this... Take a scenario where you have an appliance that has a body that is made of a conducting metal... that could be the metal case on a light / case of your PC / metal chassis of your washing machine...

Now say something pulls the power cable extremely hard and the live wire dislodges out of it terminal and touches the conductive body of your appliance, making the entire body of the appliance live... If you touch this when it is live, you will provide a path to ground and receive a serious shock at the same time....

To avoid the situation of EVER having the body of the appliance live, the earth wire is connected to the body of the appliance which provides an alternative path to ground... So if the live wire becomes dislodged and touches the metal body of the appliance, it will short to ground through the earthwire and blow the breaker or RCD, instead of killing you...

Bruthal

CountyHurler wrote: »

The best way that I think about it is this... Take a scenario where you have an appliance that has a body that is made of a conducting metal... that could be the metal case on a light / case of your PC / metal chassis of your washing machine...

Now say something pulls the power cable extremely hard and the live wire dislodges out of it terminal and touches the conductive body of your appliance, making the entire body of the appliance live... If you touch this when it is live, you will provide a path to ground and receive a serious shock at the same time....

To avoid the situation of EVER having the body of the appliance live, the earth wire is connected to the body of the appliance which provides an alternative path to ground... So if the live wire becomes dislodged and touches the metal body of the appliance, it will short to ground through the earthwire and blow the breaker or RCD, instead of killing you...

Only if either touching another earthed item or neutral at the same time, or standing knee deep in wet cement (etc). Other than that, a serious shock is unlikely, contrary to popular belief even amongst many electricians. Not saying its not possible, but its very far from certain.

Standing in dry conditions in shoes and nothing will be felt if only the live item is contacted. (Don't try this at home):)

For the op, as said, some appliances are double insulated or non metallic, so require no earthing.

BrianDug

Bruthal wrote: »

Only if either touching another earthed item or neutral at the same time, or standing knee deep in wet cement (etc). Other than that, a serious shock is unlikely, contrary to popular belief even amongst many electricians. Not saying its not possible, but its very far from certain.

Standing in dry conditions in shoes and nothing will be felt if only the live item is contacted. (Don't try this at home):)

For the op, as said, some appliances are double insulated or non metallic, so require no earthing.

The system itself is earthed - I have gotten a few shocks over the years and two of the times it was direct contact with a low power phase conductor in dry conditions with shoes on (of course there are many variables to consider).

But you are right, the chances of a serious shock are slim and the main danger present I would see is falling over/off ladder and banging your head.

I find earthing/grouding a really interesting area to study (large power system earthing, RoEP studies etc).

Bruthal

BrianDug wrote: »

The system itself is earthed - I have gotten a few shocks over the years and two of the times it was direct contact with a low power phase conductor in dry conditions with shoes on (of course there are many variables to consider).

There will likely be another point of contact not noticed. I've gotten a fair few myself. Never perceived anything in cases of phase only contact in dry conditions.

Doing some work on farms was in damp conditions, get a fair tingle then, but still nothing like phase to earth/neutral between hands. Bare feet in the mud would be nasty though.

A large power transformer or 6 amp lighting circuit would present the same shock level, except that the large transformer scenario will have larger areas to make contact with, or far more dangerous if shorted with a spanner etc. But only contact the phase and its the same 230v.

But you are right, the chances of a serious shock are slim and the main danger present I would see is falling over/off ladder and banging your head.

Yea I think people receiving shocks and saying they were blown across the room, were in fact terrorised by the shock and went stumbling after losing balance.

I find earthing/grouding a really interesting area to study (large power system earthing, RoEP studies etc).

Its probably like a separate discipline in itself the earthing.

monseiur

Hi,
While on the subject of 'Earths' etc. - when wiring a garage/shed with supply coming from house, why is it advised not to have a second earth rod at garage/shed

M.

Bruthal

monseiur wrote: »

Hi,
While on the subject of 'Earths' etc. - when wiring a garage/shed with supply coming from house, why is it advised not to have a second earth rod at garage/shed

M.

On a neutralised system, there is no need of another earth rod on a separate part of the installation, since the fault path is very low impedance via earth conductors to the earth bar, and to the neutral via the main protective conductor. And on failed/broken neutral faults upstream of the neutralised point, the neutral current flows down the earth rod (although its much lower than normal neutral current due to earth rod to earth connection relatively high impedance compared to intact neutral, and the installation will not be working at all properly).

We wouldnt really want that happening at separate points of the ground around the premises, as it might possibly cause voltage differences along the ground between the earth rods within the area of the premises, and it has no real benefit to the intact TN Cs system having extra earth rods.

It may also lead people to think that an extra earth rod connected to a shed at the end of the garden means no earth conductor needed to the shed. But an earth fault down this earth rod will be too low to trip breakers unless they are RCDs, unlike the fault path back on an earth conductor to the house DB earth bar which will be easily low enough to trip MCBs, assuming its properly installed.

On TT systems, only actual fault currents on appliances etc flow through the earthing wiring. They are usually low fault currents depending on earth rod conditions, and so there is a main RCD protecting the installation, so it will work on the RCD protected TT system.

Other posters may have other ideas on it.