Why is there an Earth Wire Connected to My understairs Sink???

johnwest288

Just noticed there is an earth wire connected to the tap pipe on my under stairs sink. Ok so commen sense would suggest this is in case it somehow comes into contact with a live wire. But there are no electrical sockets or switches in the room so just wondering why is it there?

Bruthal

johnwest288 wrote: »

Just noticed there is an earth wire connected to the tap pipe on my under stairs sink. Ok so commen sense would suggest this is in case it somehow comes into contact with a live wire. But there are no electrical sockets or switches in the room so just wondering why is it there?

Everything being earthed or bonded to earth in a house is more to do with having all metal at the same potential in a neutralised installation, should the neutral fail outside the house. It will of course, or should, also operate devices like MCBs or RCDs should a live conductor contact the earthed metal like you said.

If the neutral fails, the neutralising connection means that the broken neutral potential/voltage will be present on all earthing and earthed metal. So having everything in the house bonded will at least have all metal at the same voltage, reducing shock risks.

The single earth rod, now isolated from all the other earth rods on the supply system , would in theory keep the voltage down at earth potential, but in practice, this wont happen, unless the load is extremely small, and it behaves like a broken neutral.

In the case of the sink pipes under your stairs, they would not really need an earth if the pipes are qualpex, but would if they are copper, for the reasons outlined above.

stoneill

It's called Equipotential bonding - it is to prevent you getting a shock if there is potential difference between metal conductors (hot tap, cold tap)
Have a look inside your hot press - you will see all the pipes there are also bonded.

Equipotential bonding - commonly referred to as bonding - is a very important measure in reducing the risk of equipment damage and personal injury. Bonding involves joining together all metalwork and conductive items that are or may be earthed so that it is at the same potential (voltage) everywhere. If a component failure occurs, all circuits and conductors in a bonded area will have the same electrical potential, so that an occupant of the area cannot touch two objects with significantly different potentials. Even if the connection to a distant earth ground is lost, the occupant will be protected from dangerous potential differences resulting in injury or death from electric shock.

Bruthal

Looks like a wiki quote or something.

stoneill

robbie7730 wrote: »

Looks like a wiki quote or something.

Institution of Electrical Engineers hand book

Bruthal

stoneill wrote: »

Institution of Electrical Engineers hand book

stoneill wrote: »

Even if the connection to a distant earth ground is lost, the occupant will be protected from dangerous potential differences resulting in injury or death from electric shock.

If there is an earth rod, whether or not the connection to it has failed wouldnt have much bearing on the possibility of potential differences on seperate pipes etc.

The above sentence about the connection to a distant earth rod being lost, is obviously indicating that an earth rod will keep the earthed metalwork at ground potential in a fault.

In reality, with a single earth rod setup, this wouldnt really happen, because earth rods are a relatively high impedence path. In a neutralised setup, there are many earth rods on the neutral in parallel with the installation earth rod, and the neutral is the main fault path anyway. In neutral failure outside the meter position, there is now only a single earth rod through which the installation load is now connected, due to being neutralised. But the installation wont operate, due to the earth rod impedence, and all earthed items will experience near phase voltage with anything but the smallest of loads on in the house.

Earthing the neutral from the supply transformers is really to give reference and fault indication between phases and ground, rather than to be a fault path. Transformer fuses for example, wont operate if a phase comes in contact with the earth. But neutralising the earth in a house gives a very low impedence path for earth faults, to operate protective devices quickly.

Thats why installations are either neutralised, or for TT systems, there is a main RCD for the installation, as a live to earth fault on a TT system would have a low fault current, due to its possible earth rod(s) impedence being relatively high, which would mean a good voltage on earthed items if the supply stayed on.