Neutral System

Dear All,

Thanks for your valuble suggestions

Hanu

Hanu, There are many items to consider before you can receive an answer of any value to you. A few of the problems might be overcome if complete information is supplied. Some of the other respondents have touched on these matters.

I am only discussing this as a qualified electrician working on a supply using the MEN System of earthing. If you are not familiar with this it simply means that the neutral conductor is solidly earthed. Normally at one point at your main switchboard, but there can be under certain conditions other points where the neutral is connected to earth, but this can not be a random condition. Some one mentioned 25 ohms, but under the AS 3000 rules followed in Australia and New Zealand, that sort of value would be excessively high resistance. A resistance of 25 ohms at a voltage of 240 volts to Earth would only have a current of 9.6 amps flowing when a short circuit occurred, however at an earth to neutral resistance of 0.5 ohm, a current of 480 amps would flow and effectively cause any safety device to become open circuit quite quickly.

Leaving that however a couple of items that need to be known are:

The supply voltage available and the voltage needed for each of the machines / heaters / other devices that make up the whole installation as well as any control circuits or other devices to be connected

As someone suggested earlier in other words, single phase may indicate phase to neutral, or phase to phase. It can still be correctly considered as single phase, when connected across two of the three phases. The applied voltage is the critical factor.

Items that should be noted are that no earthing or bonding conductor should ever be connected so as to supply the return path for a circuit. Or more simply the earthing conductors must never be used as active conductors. Earthing conductors are not intended to carry load current for any period of time and are normally single insulated. They are only calculated to carry short circuit current for very short periods calculated in milli-seconds and the system is not well designed if the earthing conductors carry load current for even the briefest periods. because of this they are often smaller than active conductors, but must never introduce excessive resistance resistance into the protection circuit.

Understand the term "active" to include both the phase and neutral.

Keep in mind that under open circuit or out of balance conditions the neutral conductor if contacted can be as deadly as the phase conductor and easily allow many times the 30 or 40 milliamps to flow through your body, causing your heart to cease beating almost instantly. A solidly connected neutral conductor of sufficient size to avoid any significent voltage drop is however perfectly safe if not severed. The insulation of the neutral conductor must always be the same as any other active conductor and the neutral is always run in parallel with the other active conductors, not seperately.

It is a requirement (under AS 3000) that any neutral conductor used, should originate at the same source of supply as the other active conductors. This assists in identifying neutrals and avoiding accidental disconnection, as well as for other reasons.

I hope this will assist you in either seeking additional assistance or at least removing some of the ambiguity and increase understanding the complexity of the situation. I am quite willing to try to help further, as I believe most of the people on this Forum would also be willing to assist, providing sufficient information was available. To advise without sufficient information is downright dangerous and apart from the danger to your people from false information, there is always the moral problem for any person giving advice which leads to another's injury or death, because of this misleading information and advice

Regards and good luck, but do not become afraid to ask because of what I have said. The neglect of trying to understand is far worse than not understanding.

Bill Pilgrim, Queensland, Australia.

Dear Mr. MS & All,

one more doubt. The power line(415 V) neutral also grounded in sub-station at transformer side from that only dustrubuting lighting neutral also. why the difference solidly grounding?

If lighting neutral seperatly solidly grounded, why we can't use earthing terminal as a neutral?

what is the difference between solidly grounded neutral and through Transformer neutral grounding.

Please clarify

I am very thankful to all for my clarifications.

My friend, There is a world of difference, between the Earth and Neutral. Again, I do not know what system of earthing is used where your enquiry originates and there can be several. I believe that the one in use in a number of countries, the MEN system, is by far the better option, for safety and reliability.

I have stated previously that the MEN system is basically that the Earth and Neutral are solidly bonded together with a very low resistance, between all earthed portions of an installation, including the neutral, for the system to function correctly.

Having said this, the earth system is not designed to be an active conductor. In low current circuits the earth is usually about the same size as the active conductors, but often in heavy load circuits it is as small as half the size of the active conductors. If you were to rely on that earth for the return path for your circuits then it could be drastically overloaded at times. It is also single insulated, which would automatically bar its use as an unprotected active conduuctor.

It is normal also to use an earth electrode driven into the ground under this system. Also each Transformer neutral is bonded to earth and along the path taken by the over head or underground lines, the neutral is again bonded to electrodes driven or buried in the ground. In fact the only difference between active and neutral is the fact that one of the wires is bonded to earth throughout the entire distribution system. They are both active conductors, but one is bonded to earth to avoid dangerous combinations of short circuits, which could make two seperate machines sitting side by side each have different potentials and cause a person touching both machines to receive an electric shock.

This discussion could be rather long. if you do not fully understand the problems that could occur in an electrical installation because of trying to use the earth as a neutral, perhaps it weould be better to write to my email address and I would be pleased to try to assist, by giving a more lengthy explanation, without going off subject and annoying our other friends on line. There is a little more to explain and I will find it easier to type if I can use a larger type than is available here.

Regards Bill

brtini. install a single phase control transformer with the rated phase to phase voltage

on the primary side and the secondary for the control voltage, fused. perry

In the interests of safety of machinery and personel, I should say definitely not. PLEASE IF YOU DO NOT UNDERSTAND WHY, ASK FOR LOCAL QUALIFIED HELP, or respond and ask for more answers and explain with more detail about your system details

Bill Pilgrim

I totally agree with MSAMAD re "outside" neutral.

BUT- do any of the single phase loads in the machine require a neutral? Just being single phase- like the heater- does not require a neutral. Since you stated that the design is for a 4-wire connection, I assume that some parts of the system are true single phase with a power and a neutral.

If some part of the machine needs a neutral (maybe the control circuit) then install a 2-wire single phase by power and neutral 1-phase transformer and connect its neutral to the ground circuit of the machine. Use only one of the phases used by the heater (assuming that the heater is only a 2-wire single phase load- if it is 3-phase, no problem).

Dear Sir,

As I have been taught in engg. college the neutral is to carry the out of

balance current in a 3 ph system 4 wire (at the star point the summation of

currents will be zero ) for a perfectly balanced load. If your heater load is made

many heating elements (1) Please see that all phases are equally loaded so that

Star point has zero potential.(2) If some heaters fail still it will work but the load

current will be different so choose the heater supply cable size for an additional

20 % load.(3) Try for Delta connection if possible with 20 % load deviation if some

elements fail. (4) If you need some phase power for control circuits etc go for a

Scott connected Transformer 220-0-220 ( Input 415 - 3 phase) from the same

voltage source. (5) Go in for Delta / Star transformer of lesser capacity (connected

to the same source) & use the star point for single phase loads.

Note: Without neutral it is better to provide some current sensing

devices to cut off the power in case the heavy currents are drawn in some phases.

I will be happy if my observations are useful to you.

Dear friend, How many heaters are there, if three/ six/nine, then you can make it star connection. Regarding control circuit how many cont actors/ relay Indication lamps if possible replace to according two phase supply voltage other wise use control transformer two phase to single phase [ 415N/230V]

There are other considerations, much work is to be done, but ground (earth) connection at every box and machine must be in place read not more than (25) ohms to ground but insure that the ohms are zerO ohms if YOU are responsible for the safety of others. lock all switch boxes out, make new power connections, turn on switch boxes ONE AT A TIME, Use face shields, gloves, body armor, DONOT look at switchs at moment of closure a flash can absolutely blind you, with 3rd degree burns on exposed skin. 3 ph motors may run backwards, with power off, switch any two leads at motor CHECK AMPS

What is the constant voltage transformer? Is it the ferroresonant transformer ?

hi friend,

i think it be possible to do what you want. but first check the fellow:

1- voltage between line and neutral of this panel

2- the cross section of wire

i do this many times in my work when need phase voltage and have only 3-phase supply some time i use the earth wire of panel {the earth and neutral at my site are connected together.

I am afraid that if you were caught doing this in Australia, you would be prosecuted, by the Electrical Safety Office of the State in which you were working, for unsafe workmanship and possibly be fined and lose your right to do electrical work. The very best outcome would be that you would be compelled to do a revision course and be tested to show your understanding of the theory and practical knowledge that is required in this country.

I have been trying to explain to others in this forum why this is an unsafe practice, but have not finished as yet. The earthing system is not part of any active circuitry and using it as a neutral could cause a fatality for a number of reasons. In the event of a poor connection through rough usage or corrosion any person handling a bare earth or a earth that has had the insulation tremoved could be electrocuted through using an earth as a neutral. I do not know what is acceptable in your country, or to swhat standards you must work, but if what you are suggesting is acceptable, then they hsave a long way to go in developing safe standards. I can not speak of any system of operation other than the tried and poroven pone used in Australia and New Zealand

Are there any other qualified electricians out there who work to similar standards to those used by Australia & New Zealand?

Bill Pilgrim

There are drawbacks for using CVT(ferro-resonant Transformer)

1- Low efficiency specially at light load.

2- Too heat generated and in many cases the transformer burns.

So be careful when using CVT.