Distribution Dead End Stub 1

[oldagpe]

I have run across a dead end distribution line, with an extension running about 200 feet further from the last transformers. Supposedly, this is for lightning surges; the reflection at the end of the line subtracts at the transformers.

Has anyone else seen this? Does it really work?

 

[jghrist]

It's not that the reflection subtracts at the transformers, but that the reflection at the end doubles the surge. If the transformer isn't at the end, the theory is that it won't experience the doubling. In reality, the reflected wave will double all the way back until it attenuates (not much in 200') or gets diverted by an arrester. The surge at the transformer will be pretty much the same as at the deadend. That's why you put arresters at the transformer.

IMHO, the extension is a waste of money. It does allow you to put the transformer on a tangent pole where there is more space. It might be worth it if you needed the extra span anyway for secondary.

 

[oldagpe]

jghrist, thanks for the response. I pretty much share your thinking on this.

But, maybe I am missing something. Maybe an arrestor at the end where the reflection doubles up means less surge at the transformers (and they would still have their own arrestors).

 

[jghrist]

If both the arresters operate, they will share the surge current and have lower discharge voltage. This will increase the safety margin of the transformer insulation. Having one arrester at the end will not be as effective as having both at the transformer location, however.

When there is an arrester that is conducting surge current, there is a negative reflection. The voltage at the arrester is minimum (equal to the arrester I²R discharge voltage), but the negative reflection reduces the voltage at other points on the line as it propogates down the line.

 

[oldagpe]

That's good analysis.

It's not a new trick; just something messed up!

Thanks for the reality check.