Yet Another Earthy Question

[David006]

Just my musings with my Chang numbed brain perhaps:

What is the point/advantage of a MEN system?

Seems to me that if you put the PEA neutral ( and all your outlet earths?) at the same potential of a super dooper ground system it negates the logic for the latter...one may as well not worry about adding an earth at all just use the neutral and GFI....?

If for any reason the PEA neutral goes open will your ground not become the local neutral /return for everyone on the same transformer?

.....just asking our bright sparks..lol

Think I read someplace that NZ used to have a system without neutral returns ie single conductor network ....like an old WW field telephone?

[Crossy]

MEN (a variant of TN-C-S) saves on cable costs over TN-S whilst giving all the advantages of a low impedance path for L-E faults. Look here for details of the various earthing systems http://en.wikipedia....Earthing_system .

The hazard of an open neutral is one of the reasons MEN (we call it PME) was not introduced in the UK until relatively recently. Even so it was only last year that a row of cottages blew up due to an open neutral (return current flowed via the gas pipe bonding with predictable results).

Having a LOT of earthing points on the neutral does mitigate this issue somewhat but you are correct, there is a possibility that your ground stake could start carrying significant currents, that's why it needs to be a nice fat wire.

TN-C where there is no earth (metalwork is connected to the neutral) is not used inside buildings, the main reason being that all the metalwork becomes live in the event of a polarity reversal.

SWER (Single Wire Earth Return) is still used in many parts of the world, it is only effective for the high voltage distribution system where the currents are relatively small and the volt drop across the return path is insignificant. So you can run a single 25kV feed to your transformer and save 50% of the cable cost if you have a long run from the main transmission lines, the LV side is still two (3) wires.

[David006]

MEN (a variant of TN-C-S) saves on cable costs over TN-S whilst giving all the advantages of a low impedance path for L-E faults. Look here for details of the various earthing systems http://en.wikipedia....Earthing_system .

The hazard of an open neutral is one of the reasons MEN (we call it PME) was not introduced in the UK until relatively recently. Even so it was only last year that a row of cottages blew up due to an open neutral (return current flowed via the gas pipe bonding with predictable results).

Having a LOT of earthing points on the neutral does mitigate this issue somewhat but you are correct, there is a possibility that your ground stake could start carrying significant currents, that's why it needs to be a nice fat wire.

TN-C where there is no earth (metalwork is connected to the neutral) is not used inside buildings, the main reason being that all the metalwork becomes live in the event of a polarity reversal.

SWER (Single Wire Earth Return) is still used in many parts of the world, it is only effective for the high voltage distribution system where the currents are relatively small and the volt drop across the return path is insignificant. So you can run a single 25kV feed to your transformer and save 50% of the cable cost if you have a long run from the main transmission lines, the LV side is still two (3) wires.

Thanks Crossy....good stuff..

It's a wonder why a neutral is fed into a residence at all? Have often thought I could have saved a few bob by using a single live conductor for my 1km feed and a big fat earth at my end. Or atleast used open wire ( is it steel or aluminum??) as per network pole neutrals...

I was also thinking that if Mr Somchai down the road has a live to earth leaky appliance does that not potentially feed back onto the ( my earth) network neutral and raise it (dangerously?) above earth. Would that fire my GFI?

No wonder the Thais say we think too much.....

[electau]

As Crossy states in his post, the PME system was introduced in the UK in 1974. It was to suppliment the existing TN-S system for underground LV distribution. Aerial LV distribution in the UK is TT. With the PME the bond is not at the main switchboard and the conductors from the supply authority are TN-S, The L,N, and E are separate.

Australia and in NZ the MEN system is mandatory. The MEN system has the advantage of the fault curent returning through the main neutral to the transformer and thus has a fast fault clearance time. Fault currents are high. It has the advantage over the TT system in this regard where the earth fault current flows to earth by way of the main earth. It is a low fault current system. Fault current on the TT system may not be sufficient to operate a protective device within the required clearance times, 0.4secs. RCDs must be used.

It Is essential that polarity tests are carried out before connection to supply.

The main earth and electrode maintain the voltage to earth in the event of a broken neutral to less than 50VAC. The distribution neutral is earthed at regular intervals along the to maintain the voltage between the neutral and earth to 0 volts.

In the US it is known as the grounded neutral. AS/NZ and US bond the neutral at the switchboard.

After the MEN bond at a switchboard the wiring is TN-S, the L.N.and E are separate wires.

SWER systems are supplied from 2 phases of an HV through a transformer giving an isolated single phase supply, one side is connected to earth electrodes and the transformer tank and the other is run to individual transformers and are earthed.

The LV may be 3 wire single phase or 2 wire single phase. They are limited to about 10kVA per installation and are used to supply farms in rural areas.

[David006]

As Crossy states in his post, the PME system was introduced in the UK in 1974. It was to suppliment the existing TN-S system for underground LV distribution. Aerial LV distribution in the UK is TT. With the PME the bond is not at the main switchboard and the conductors from the supply authority are TN-S, The L,N, and E are separate.

Australia and in NZ the MEN system is mandatory. The MEN system has the advantage of the fault curent returning through the main neutral to the transformer and thus has a fast fault clearance time. Fault currents are high. It has the advantage over the TT system in this regard where the earth fault current flows to earth by way of the main earth. It is a low fault current system. Fault current on the TT system may not be sufficient to operate a protective device within the required clearance times, 0.4secs. RCDs must be used.

It Is essential that polarity tests are carried out before connection to supply.

The main earth and electrode maintain the voltage to earth in the event of a broken neutral to less than 50VAC. The distribution neutral is earthed at regular intervals along the to maintain the voltage between the neutral and earth to 0 volts.

In the US it is known as the grounded neutral. AS/NZ and US bond the neutral at the switchboard.

After the MEN bond at a switchboard the wiring is TN-S, the L.N.and E are separate wires.

SWER systems are supplied from 2 phases of an HV through a transformer giving an isolated single phase supply, one side is connected to earth electrodes and the transformer tank and the other is run to individual transformers and are earthed.

The LV may be 3 wire single phase or 2 wire single phase. They are limited to about 10kVA per installation and are used to supply farms in rural areas.

whew!!