ungrounded delta primary ?

Fred B

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
I know some of you deal more with the higher voltage systems so thought I'd ask here as my experience has been limited to up to 480 and under 600A.
We seem to have a lot of ungrounded primary in our more rural areas around here. What safety systems are at work for the primaries when you get a short to ground on the system like a tree or say a car accident that breaks off the pole? A lot of the one line, no neutral, lines feeding down for several miles sometimes to one or more transformer(s) that then derives it own neutral for a 120/240. Will the fuse break blow with such a high resistance ground potential?
Any additional considerations or caution needed with this type of system vs a wye system such as potentially being "worse" with only a high resistance earth only grounding, that might be different when dealing with the 120/240?
 
Branch line fusing would be the method I would choose with a single line primary traveling a rural area. So as the line falls and the current rises, it wouldn't need to rise to the level of a true fault value to trip and you could isolate that section without loosing the whole feeder from the substation.

But I can't tell you how they do it.

It is usually dependent on the feeder length, size, normal work practices, engineering considerations, and configuration (delta system vs wye).
 
wwhitney said:
What does this mean? With an ungrounded system, there's no way for a single line to form a circuit to supply power.

Cheers, Wayne
Click to expand...
Don't know, that is why asking. Power pole only has the one conductor, dropped or tapped onto the transformer primary, it then has a ground wire running from the secondary side to the earth that, along with the neutral the L1 and L2.
Are they using the earth as the return? Only other wire on pole besides the secondary is the CATV cable.

very similar to this:
1738096858249.png
 
Fred B said:
still doesn't explain the only one wire on the pole.
Click to expand...
Is there actually a stretch of poles with only one wire on the pole? Or is it just that there is only one wire on the top of the pole, and lower down you have say 3 secondary wires?

If the latter, then one of the those 3 secondary wires is a grounded conductor common to both the primary and secondary side of any transformers.

Cheers, Wayne
 
Seen some suggestion on google of a SWER system to use the earth as the return.
wwhitney said:
Is there actually a stretch of poles with only one wire on the pole? Or is it just that there is only one wire on the top of the pole, and lower down you have say 3 secondary wires?

If the latter, then one of the those 3 secondary wires is a grounded conductor common to both the primary and secondary side of any transformers.

Cheers, Wayne
Click to expand...
The picture was just illustration of what I saw related to the transformer not the actual wiring, one upper bushing. There is a whole string of poles and only one wire other than a definitely CATV cable one. A second transformer back about another 8 poles going to some more homes.
 
Fred B said:
Seen some suggestion on google of a SWER system to use the earth as the return.

The picture was just illustration of what I saw related to the transformer not the actual wiring, one upper bushing. There is a whole string of poles and only one wire other than a definitely CATV cable one. A second transformer back about another 8 poles going to some more homes.
Click to expand...
That would be a multi grounded neutral system. They often use the messenger of the secondary triplex for the primary natural for some ways, then it will jumper back over to the MGN neutral at some point
 
Fred B said:
The picture was just illustration of what I saw related to the transformer not the actual wiring, one upper bushing.
Click to expand...
Dual-bushing transformers are rare around here, even in three-phase banks.

They can be used in either delta or wye, while single-bushing only in wye.
 
There are two wires in that photo. They are line to neutral. I have seen a transformer that was phase to earthed neutral and I don't think there was a neutral run back to the substation.

Neutral doesn't always make it back to the substation.

ground fault detection is relative on a single phase circuit. Think 1ph current on a 3ph feeder. When one line faults it might not be enough for the settings at the sub. Though you can use other methods like voltage differential protection to read a under voltage event or a phase / frequency event.

Most likely they utilize branch line fusing. Sometimes referred to as inline fusing. Where the single phase line has a small fuse rating for the loads on that line.

You will see them where the single phase line is derived or at fused disconnect or line isolators.

electrical-engineering-portal.com

Fuses as the oldest and simplest method of protecting rural distribution systems | EEP

Fuses are the oldest and simplest methods of protecting rural networks. There is no method simple as fuses, but don't be tricked, fuse is a very reliable
electrical-engineering-portal.com electrical-engineering-portal.com

1738099320177.png
 
wwhitney said:
Is there actually a stretch of poles with only one wire on the pole? Or is it just that there is only one wire on the top of the pole, and lower down you have say 3 secondary wires?
Click to expand...
In rural areas of Texas I have seen miles of poles that appear (from a car) to have only a single wire strung pole to pole.
 
electrofelon said:
yup. Happens often when there is a longish run of triplex serving street lights or a whole block of houses. They wont bother running a primary neutral, they'll just tag onto the secondary neutral.
Click to expand...
Was one thought I had, but this after 3 poles no secondary and gap to the next pole then it picks up from a transformer back another 3 poles from there with secondaries running off for another set of houses in both directions from that transformer.

This system is sounding like what I saw referenced on google as a SWER (Single Wire Earth Return) when I was trying to figure out what I was looking at. Any Experience with any additional safety concerns beyond the normal considerations if this is what I am seeing? Seems to me it could have a real impact for pool bonding safety if not done correctly. Seems too that this type of system would have a higher incidence of stray voltage or NEV complications in a house.
 
Fred B said:
Was one thought I had, but this after 3 poles no secondary and gap to the next pole then it picks up from a transformer back another 3 poles from there with secondaries running off for another set of houses in both directions from that transformer.

This system is sounding like what I saw referenced on google as a SWER (Single Wire Earth Return) when I was trying to figure out what I was looking at. Any Experience with any additional safety concerns beyond the normal considerations if this is what I am seeing? Seems to me it could have a real impact for pool bonding safety if not done correctly. Seems too that this type of system would have a higher incidence of stray voltage or NEV complications in a house.
Click to expand...
I highly doubt its a SWER. I think the only places you will find SWER in the USA is maybe Alaska. Theres a primary neutral somewhere, you are just not seeing it.
 
You must log in or register to reply here.
Top