ECN Forum Forums Code Related Discussion NEC & other Code issues Hi

Appy,

Thanks for the info!

From the Voltages you described, it appears that the secondary distribution systems are 4 wire Wye's [AKA 4 wire Star].
Also looks like the common conductor is used for a dual voltage system [230Y/400 3 phase 4 wire]. It seems to be bonded to Earth much the same way we do it here.

One thing came up while looking over this type of system is the problems related to arcing ground faults that are inherent with 4 wire Wye systems above 150 VAC to ground. In the US, it becomes a matter when the service's rating is 1000 amps, or more.
The use of Ground Fault Protecting type main breakers [or similar type of main disconnect] is done here. Ground fault limits are very high, as compared to the limits for personnel protection, plus they only monitor Line-to Ground faults, no overcurrent protection is done with these [except that of the main protecting device].
I'll keep the Techno Babble junk low here, but if you are curious about this, let me know.

Don't know if and where this might be used in your situations, so feel free to elaborate if you want to.

BTW: If there is only one Transformer with only two Primary feeder conductors to it, that would be, of course, a Single Phase system. This would be where you would find a center tapped transformer with some connection to that center tap - either a Neutral Conductor, or just a point for grounding.
This was the one I am curious about. If the transformer has the center tap grounded, that lowers the voltage to ground by 1/2 [half of the transformer's winding's potential]. If it's only grounded at the transformer and there is no physical wire coming to a service, which gets bonded to the metal enclosure and possibly grounded once again, that leaves a poor path for ground fault current. The path is in the dirt only.
I am almost completely positive that your systems are not done this way, but if they are please let me know.
The center tap and conductor from it to the service would not be used in any circuits.

If the 1 phase transformer was 230/460 VAC, then the center tap would be used as a typical Neutral Conductor, being part of circuits, along with the typical ground bonding stuff.

So you use 50Hz.. That's about the lowest frequency that is practical when multi use power distribution is concerned.
60 Hz is about the highest [which is what we use].
These Hz values are the best "Happy Medium" for plain old general use power, which is transmitted over a distance more than 25 miles and is to be used in high levels.

The Skin Effect, which greatly effects large conductors, is a little lower for the 50 Hz power. The trade off is a lower Synchronous Speed for the AC. This results in Transformer Cores and Coils that are a little larger. plus a little more lossier than their 60 Hz counterparts, plus general usage type Induction motors will have a max speed of 3,000 RPMs, as compared to 3,600 RPMs using 60 Hz.

25 Hz would be good to overcome Skin Effect, but would require large, lossy transformer cores - and Induction motors would max out at 1,500 RPMs. Not only that, but there would be a noticable flickering of lights when the sine waves crossed the Zero line.

400 Hz would improve the efficency of transformers, plus lights - along with giving a high speed to Induction Motors [24,000 RPMs], but the Skin Effect losses would be dramatically raised at this frequency. That's why 400 Hz is used on low to medium powered systems that are locally derived.

Just my $0.02

Scott SET

Scott,

Years ago I worked for a company that manufactured Avionics Test Equip that went aboard Carriers and also 'Portable' equip. for analyzing Aircraft systems. It used 60Hz and 400Hz.

Bill

You lost me a bit on some of that Scott, I am always keen to learn so if you have more i'd like to read it

The only significant difference I noticed in the distribution systems, of New Zealand, was the absence of dual voltages to a residential service. 230 volts single phase, one conductor earthed, is standard.
A transformer bank, with three separate, single phase units is connected wye, with the mid point connected to a MGN system, the same as common in the US.
The secondary winding has one insulated terminal, the other end of winding is connected to a tank ground pad.
The MGN system is landed on this pad, and then becomes the service entrance grounded conductor.
Secondary earthing is the same as we perform.
A single bank can provide 3 phase 400 volts and single phase at 400 and 230 volts.
The advantages I could see was: only two conductors supplying all residential loads. No loss of neutral mid-point connection, problems. Smaller wire for loads, with less voltage drop.
The 230 volts to ground does not appear to be a problem. Statistics I have seen, actually indicate less risk.

Thanks Bill

[This message has been edited by Bennie R. Palmer (edited 03-24-2001).]

Bennie,

Not related to the conversation here, but I wanted to mention to everyone that there's no reason to ever have 2 Edit messages on the comment because you can edit them out too. Next time you go in just erase any edit messages you see. You will always have 1 but no need for any more.


Bill

Ah..,Yep? No? maybe geez... im lost,this is all pretty new to me. I have been installing security systems for the last 12 years.
I know the max you can get from one phase and neutral is 230v(phase voltage) to get 400v(line voltage) you'd need at least two phases add a neutral and you would have the option of 2 times 230v ..the following is from one of my Text books; Multiple Earth Neutral System, This system of distribution uses the multiple earth neutral or MEN system.the neutral conductor is earthed at the source of the supply and also at one or more other points along the distribution line, and each consumers premises. the resistance between any point of the neutral conductor and earth must not exceed 10 ohms. The neutral is multiple earthed so that, if any live part of the supply system or consumers distribution system comes into contact with earth, current flows back to the neutral by way of parallel paths,and the net resistance of the fault current is low. the lower the resistance of the fault path, the higher the fault current - quick opperation of fuse or circuit breaker.
ADVANTAGES; 1. the greatest voltage to earth at any part of a consumers installation is phase voltage 230 volts
2. parallel low resistance paths are provided for fault current
3. the resulting high value of fault current ensures that protective devices operate effectively.
4. if a customers neutral is broken fault current can still flow back to the point of supply through parallel earth and neutral connections at other premises
5. some protection is provided from lightning and high voltage flashovers
DISADVANTAGES
1. the high fault current drings increased risk of fire or shock if it flows through poorly made connections or joints.
2. A broken neutral is sometimes not noticed because the earthing system carries the neutral current.
3. all exposed metalwork assoicaited with wiring systems must be earthed.

Appy;
we do 25 ohms or less. I see you go for 10, i'll bet that it is harder to achieve. Is there any particular way that you would gauge this???