Lightning Aresstors

[KyLineman]

You are correct that placement of arrestors does not cause ferroresonnace but, if you lose a phase and try to reclose the fuse's you will have more problems with arrestors blowing because of there location on a line. If the arrestor is mounted before the cutout it will not fail on you but if they are mounted on the load side of cutout then ferroresonnace does come into play if you have a wye-delta transformer connected on the underground line. Ex. We had an underground line have two fuses blown, and one phase the stinger was burnt off, there was one wye-wye transformer and one wye-delta transformer on this underground line. They pulled off the line, made repairs to stinger and refused the doors, as they started to energized the line after the second dooor was closed a arrestor in the wye-delta pad (cable ended there so it had arrestor's there) blowed, blowing both fuse doors. They removed the arrestors and replaced them with higher rated voltage arrestors and reclosed the line. The arrestor's in the pad acts just like a arrestor mounted at the cutout on load side, as soon as you energized one phase the transformer will start causing ferro and it will increase the voltage across the arrestor. If you elimate the arrestor or have higher voltage arrestors it will not blow it.

[PA BEN]

Why would it make a difference if the aresstor is on high side or bottom load side. All that is between is a fuse. How can this fused cutout cause problems?

[loadbreak55]

Hopefully this will answer your question.The best place(according to our engineers) for an arrester on an underground riser is between the fuse and the underground line.The reason is to protect the ug cable in case of a lightning strike.According to our engineering staff,when a ug cable is hit with a lightning strike, the the theary is that the arrester will take the"brunt" of the surge.We also install elbow arresters at the end of an underground tap.This prevents the surge from returning back down the cable.We are told that when this occurs,the voltage will double,thus causing the "treeing"affect that you see in old xlp cable.

[Hemingray Insulators]

well i'm not in the trade, but this is my educated opinion after hearing both standpoints from a few companies i have talke to.

Personally i would put it on the load side of the cutout, and like KyLineman said, it would have to be a higher rated arrestor, and also would have to be disconected before hi-poting the URD. the reason for it to be on the load side is because it is the underground you want to protect. its a lot easier to replace an OH tap than a whole URD line. also if the fuse blows, or the cutout is in the open position the URD line is no longer protected, unless you have arrestors located elsewhere on the URD. if you put it on the OH line side, as soon as you open the cutout or switch you are removeing the protection for the underground, and also i do not think you really need an arrestor on the tap, because it doesn't need much protecting, because of multiple other arrestors in the general area on the OH system. for the best protection i'd put it on the load side for the above reasons. I have also read somewhere (if you need i can try to find it) where whenever you hook up an arrestor it should be BEFORE the load, not to go to cutout or pothead or whatever and loop to the arrestor. if you go to the arrestor first and then to the cutout or pothead etc, it suposedly has a better chance of protecting it.
just my $0.02

[shocker]

there seems to be two trains of thought here but i can tell you every arrestor in our system is connected on the high side of the cutout for downfeeds and we use surge arrestors rated 15kv.(7200-12kv).on transformers anything 25kva and larger is a tank mount dist. arrestor to the load side of the switch.when protecting for lightning stikes the odds of it hitting the cable before the line are probably minimal.but either way top or bottom is better than no arrestor at all.

[dbrown20]

Arresters do not have to be at the exact site of the device they are protecting. It is supposed to be better though for them to be as near as possible. Kind of an oxymoron maybe? I have seen plenty of companies mount the cut out and arrester on the same bracket,as we all have. Back to back but with a lead for the arrester going one way and the cutout the other way. As long as the arrester is close to the transformer etc. it is supposed to do it's job. As we all know lightning does it's own thing though.

Since they have started using so many tank mounted arresters on pots it was soon found out that with pots mounted in a bank of delta secondaries that sometimes arresters would blow when closing in the bank. Therefore it was found prudent to leave the tank mounted arresters as is on single phase pots and on Wye connected banks, but remove them and mount them on the arms on the delta connected banks. The overvoltage experienced was not reckoned to be ferroresonance but simply an overvoltage from back feeding one pot into the other via the delta connection. This seemed to be true for 3 pot banks but not 2 pot banks in a Wye system. In a true delta system or unigrounded wye you would have to remove the arresters from the pot tanks also on delta secondaries. dbrown20

[Hemingray Insulators]

thought id'd add, that while i don not know if it is spec, i do know that from what i have seen, every new riser Consumers builds seems to have the arrestors on the metal risor bracket where the cables are terminated.

[Stick-it]

Correct me if I am wrong. Ferroresonance is when your inductive reactance equals your capacitive reactance therefore they cancel eachother out and your circuit sees ZERO resistance. So with Zero resistance it pumps ALL available current. Tens of thousands of amps. Your underground cables would provide your capacitance and your transformer your inductance.

I do not know if an arrester produces any type of reactance but i would not think that it could contribute to ferroresonance.

[CenterPointEX]

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[Swollen Tongue]

I know one thing. A bad arrestor on the line side of a switch can be a bitch to find when its causing a section to be out...I seen it lock out a breaker at one of our little 4kv unit subs before.