Distribution Neutral

[timm333]

The star point of 120/208V distribution system is grounded so the voltage between one phase and star point is 120V. If the ground connection at the start point breaks: then will the voltage between one phase and star point still be 120V, or will it become 208V (or 240V). Thanks.

 

[kwired]

The star point of 120/208V distribution system is grounded so the voltage between one phase and star point is 120V. If the ground connection at the start point breaks: then will the voltage between one phase and star point still be 120V, or will it become 208V (or 240V). Thanks.

It is still 120 volts to the other side of that particular coil, you just lost the ground reference. Because of capacitive effects, when you don't have a ground reference the voltage to ground could be about anything, but yet any such voltage is normally not capable of delivering any significant current either.

 

[timm333]

Yes that makes sense. How about 120/240 V single phase system, here the neutral (center point of transformer secondary) is grounded; I have heard that if the connection of neutral to ground breaks at the pole, the 120 V in the house will become 240 V and will damage the equipment.

 

[kwired]

Yes that makes sense. How about 120/240 V single phase system, here the neutral (center point of transformer secondary) is grounded; I have heard that if the connection of neutral to ground breaks at the pole, the 120 V in the house will become 240 V and will damage the equipment.

single vs three phase doesn't really matter. If you lose the conductor that runs to the grounding reference (AKA a grounding electrode and grounding electrode conductor) all you lose is a reference to earth. If you lose your grounded conductor connection back to the source (transformer) then you effectively have made 120 volt loads on line A in series through what neutral conductor is still there and back through loads on the other 120 volt line B. If resistance of each load is identical then half the voltage will drop and each load will still see 120 volts, but that seldom happens and usually one line will have a higher resistance which means more voltage will drop across that particular load - remember we start out with 240, so if we drop 170 volts across the load connected to A then the remaining 70 volts has to appear across the line B load.

Maybe someone will post a link to some graphics that explain this, there has to be something on Mike Holt's general site somewhere that will show this pretty well, I don't seem to find such specific things as fast as some do but I may look a bit.

Add: Ok not on Mike's site but still Mike's content on EC&M, maybe take a look at this.

 

[GoldDigger]

Yes that makes sense. How about 120/240 V single phase system, here the neutral (center point of transformer secondary) is grounded; I have heard that if the connection of neutral to ground breaks at the pole, the 120 V in the house will become 240 V and will damage the equipment.

That would make no difference at all as long as the POCO neutral wire still gets from the transformer center tap to the neutral bar on your main panel.
And there would still be a connection to ground at the main panel, for what it is worth, which is almost nothing.

If you lose the POCO neutral wire (a break in it or open connection) between the transformer secondary and your panel the only path left for unbalanced current would be though your GEC to your ground electrode, into the earth across a resistance that could easily be more than 25 ohms, through the earth (zero ohms) into the POCO ground rod (maybe another 25 ohms) and up their ground wire to the transformer secondary.

If you are lucky it will instead go to your water pipe ground, through the main to your neighbor on the same transformer and back through his neutral to the POCO transformer. Only a few ohms that way.

In the first case, you are not quite putting the loads on opposite sides of the service (L1 vs. L2) in series, but the high resistance of the alternate neutral path will still let the voltage go wild enough to burn out light bulbs and destroy electronics that is not rated for 240.

 

[JFletcher]

Yes that makes sense. How about 120/240 V single phase system, here the neutral (center point of transformer secondary) is grounded; I have heard that if the connection of neutral to ground breaks at the pole, the 120 V in the house will become 240 V and will damage the equipment.

Consider that 120/240V single phase is the same as a multi-wire branch circuit (MWBC). Two ungrounded conductors paired with a grounded conductor. Normally Line->Neutral loads see 120V or close to it. However, if you lose the neutral between the panel and the xfmr*, the connected 120V loads now operate in series instead of parallel. Instead of 120V on each leg, you could have anywhere from 10-230V (theoretically 0-240V however it is ultra rare to have most 120V loads connected to one leg). Obviously your refrigerator motor isnt going to like 10 or 230V, or fluctuating voltage that isnt +/-10% of 120V. Electronics can catch fire.

If you'd like to see the effects of a lost neutral, use 2 shop lights with different wattage bulbs on a MWBC and lift the neutral in the panel (make sure nothing else is on this circuit, and do it in your own home!). If you have 2 60W bulbs plugged in, you will not see a change (or an incredibly small one brightness wise) as each one has an equal voltage drop across it. Change them to a 20W and 100W bulb, and one will glow brightly for a second before it blows the filament, then the other, not having a complete circuit at that point, will go off. This is assuming you arent using a GFCI outlet which will probably be a $20 paperweight after such a test.

240V loads like water heaters are not affected by a lost neutral since they lack one anyway and are already connected L-L. It doesnt matter if L-L is 0-240, 120-120, 240-0, 175-65V, etc

*If the home has metal water pipes and proper bonding, the loss of the POCO neutral will not insta-blow electronics. Current can flow thru the pipes, thru your neighbors pipes, and thru his service neutral back to the xmfr. The cable co's coax connection can also serve as a neutral, tho not for very long. If you ever see fried coax, better check it before replacing it.

eta: Golddigger covered much of this far less wordily.

 

[timm333]

If the home has metal water pipes and proper bonding, the loss of the POCO neutral will not insta-blow electronics. Current can flow thru the pipes, thru your neighbors pipes, and thru his service neutral back to the xmfr.

So if POCO service neutral is broken, the current will come back to the neutral terminal of the panelboard of the house; and as the neutral terminal is connected to the ground terminal in the panelboard, so the current will make is way back to the transformer via earth/water-pipe/neighbor. Is it correct?

Also in the above example of 20W and 100W bulbs when the neutral is broken: will the voltage be 40V across 20W bulb, and 200V across 100W bulb?

 

[GoldDigger]

So if POCO service neutral is broken, the current will come back to the neutral terminal of the panelboard of the house; and as the neutral terminal is connected to the ground terminal in the panelboard, so the current will make is way back to the transformer via earth/water-pipe/neighbor. Is it correct?

Also in the above example of 20W and 100W bulbs when the neutral is broken: will the voltage be 40V across 20W bulb, and 200V across 100W bulb?

If there were no alternate path (via the GES), then the voltages would divide as you stated. Probably worse, in fact, since the cooler than normal 100W filament will have a lower than normal resistance and that will decrease the voltage drop across that bulb. Meanwhile the overheated filament in the 20W bulb will have a higher than normal resistance while it lasts.

 

[ActionDave]

Helpful graphic

 

[JFletcher]

So if POCO service neutral is broken, the current will come back to the neutral terminal of the panelboard of the house; and as the neutral terminal is connected to the ground terminal in the panelboard, so the current will make is way back to the transformer via earth/water-pipe/neighbor. Is it correct?

Yes. Also, if the cable TV coax is properly bonded to the panel or water pipes, its grounded portion, the foil and metal braids, will try to carry the imbalance of current between the two legs. If they are balanced, it is fine, but try to put 5 or 15A across a CATV cable, and *pffff*, out comes the smoke.

Most CATV here is not properly installed by today's code; Continental Cablevision put in the CATV systems here some 30 years ago, and drove a 5' ground rod at the cable demarc and ran an insulated #14 wire from the first splitter/lugged barrel connector to it. No connection with the buildings GES, which, ironically, makes it safer if you do lose the service neutral.

Also in the above example of 20W and 100W bulbs when the neutral is broken: will the voltage be 40V across 20W bulb, and 200V across 100W bulb?

See GoldDigger's reply.