Ferro. Resonance

[Trbl639]

I know this was just discussed, not long ago, and I understand what it is and how we dealt with it here, but need someone to jog my memory..........

What's the deal with the 4 fused switches on UG.........saw that the other day with the 4th switch on the neutral..........heard it talked about, but never worked one hooked up that way, so if someone can........

jog this old farts memory........

[Pootnaigle]

We never used a 4th cutout that was fused They were always deadblades the other 3 were always fused.

[topgroove]

I know this was just discussed, not long ago, and I understand what it is and how we dealt with it here, but need someone to jog my memory..........

What's the deal with the 4 fused switches on UG.........saw that the other day with the 4th switch on the neutral..........heard it talked about, but never worked one hooked up that way, so if someone can........

jog this old farts memory........the fourth switch on the floating primary nuetral gives you the ability to ground the float before you do any switching on the circuit or bank. When the float is energized if its not grounded and there is little or no load on it dangerous circulating voltage develops.

[Trbl639]

Thanks Poot and Top!!

I got ya now.....like I said, never dealt with a hook-up like that............we just always dumped the customer load.....opened the pot, then opened the UG..then reversed it going back in.........very seldom did we have Ferro problems, but other times we had plenty.....

[thickthumb38]

i gotta question we dont hang the fourth switch and we float the grounded high side bushing.... my question is to keep ferro occuring can you close two or three of the fuses in at the same time and keep it from occuring????:confused:

[thickthumb38]

my question is regarding our 3 phase banks

[wtdoor67]

to be a problem on Wye/Delta banks until you get in some of the higher distribution voltages. Somewhere around 15 KV or higher I've been told by the literature I have. Can get to be a problem on long runs of 3 phase URD also.

Here's what the stuff I have says. This is the possibility on Wye/Delta and Delta/Delta banks.

For 2.4/4.16 at transformers-No, Through 200' Open Wire- No, through 200' of Cable-yes.

7.2/12.5 at transformers-No, Through 200' Open Wire-No, through 200' of Cable-yes.

For 12 KV thru 34.5 all responses are Yes.

Just keep in mind these responses just mean that Ferro is possible under the aforementioned conditions. It is not guaranteed it will ocurr, just that the conditions are right for it.

[thickthumb38]

our volt is 14400, 7620, and 7200 (all phase to ground) so we shouldnt have much ferro problem other than on our 144?????

[wtdoor67]

On banks that's my understanding. Except where a lot of cable is involved. The grounding (4th) cutout is what you see the REC's putting on their 14.4/24.9. That way you can ground the "floater" when you're opening or closing it in. Afterwards the ground cutout is left open.

[Pootnaigle]

I have heard that a gang operated switch will prevent prollems with ferro since they all open and close at the same time, Its not really practical or cost efective on transformers. and fuses still hafta be somewhere ahead of the pots.

[wtdoor67]

The only one swift enought to do that is Gregg. He's so fast he can ketch it between cycles.

[Special ED]

I have heard that a gang operated switch will prevent prollems with ferro since they all open and close at the same time, Its not really practical or cost efective on transformers. and fuses still hafta be somewhere ahead of the pots.

Gang switches are very effective in preventing ferroresonance on URD cable. Usually the switches are in switchgear on all my commercial jobs. Utility feed comes in one cabinet hits the bus work inside and then goes to various other cabinets and or a dry transformer for the customer.

At the airport they have 4 compartment switch gears. Feed in one and gang switches it the other 3 operated by a lever outside. They do have some though that have an attachment to mount on your shot gun inside of them. This attachment allows you to close or open all three switches in the compartment at the same time. It was kinda interesting to see the first time I needed to use it.

[Line_Man.]

Ran across this today.....

Opened a three phase dip that fed a padmount bank to a theme park all went fine...

After work was complete we went to shut the switches back in I shut the field phase then had to boom around to get the middle and road phase (approx 30 seconds move time)

I went to shut the middle in a it flashed over on top of the switch knocking the line out but never blew the fuse...

Could this be paid off to ferro? First time this has happened to me

[Pootnaigle]

What type of bank? and what Primary voltage?

[Line_Man.]

The Bank: Not sure, not a common practice of ours. I would assume 120/240 bank


Voltage: 7620/13200

And the URD run was about 500+ feet

[topgroove]

If you had some load on the bank it would've been fine. A three phase dummy load of 2000 watts would be fine

[Line_Man.]

It still had cold load on it because we never told them to drop their breakers, switch might have just decided to go bad? I figured ferro stepped up voltage and bucked phases when I threw it in because we have to LAs off due to laying out of wire

[topgroove]

thats very possible... what kinda switch was it? was it a potted porceline cutout? made by:rolleyes: chance perhaps?

[Line_Man.]

yeah its was a 25 kv porcelian switch made by chance :)

[Pootnaigle]

Ummmmmm my guess is it wuznt Ferro but ummmmmmm a crummy cheap cutout ????????? Could be! Ferro aint never to my knowledge been a prollem on 7.6/13.2. The fact that the fuse didnt blow further makes the case for a crummy cutout. Means no fault current passed thru the door, and since it took out the line that kinda only leaves a prollem at the line side of the switch.

[Lineman North Florida]

We have an overhead delta bank that is fed by URD and it will blow 1 of the arrestors on the bank every time if you don't ground down the floating neutral with the fourth cutout and the primary voltage is 7.6/13.2, but on a system with around 100,000 customers that is the only place I have seen it on 7.6.