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Breakers in series

Breakers in series

(OP)
How does one go about controlling two breakers in series? Do I just wire the 52A contacts in series and the trip / close contacts in parallel and program the relaying as with a single breaker or am I overly simplifying it?

RE: Breakers in series

It would help to know why you have two breakers in series and what you hope to accomplish in doing so.

RE: Breakers in series

(OP)
Being done in order to mitigate the failure of another breaker. Ie, if one breaker becomes stuck (no trip), the other one will take care of it.


RE: Breakers in series

Is this for stability? You can't wait for breaker failure to time out?

RE: Breakers in series

(OP)
Reliability. Breaker-and-a-half where in one bay I'd like to eliminate the simultaneous loss of two circuits for a center breaker contingency (see example pic).

Also common place for series breakers are split bus substation where two bus breakers are used in series so a bus fault does not clear the entire substation via stuck bus tie breaker.

RE: Breakers in series

My transmission planners have been asking about this type of arrangement.

That might be one way to set up the tripping. Another might be by using the re-trip to trip the second breaker.

How does the impact of operating three breakers at the same time affect your battery life?

RE: Breakers in series

@Mbrooke, we take the single pole status of both breakers and then coordinate 1-pole, 3-pole tripping and BF between the two as well as teleprotection for remote breaker with PUTT/DTT

RE: Breakers in series

(OP)
What type of relay do you use? Never thought about using the 1 pole feature to cover two breakers... can you elaborate on this?

RE: Breakers in series

I've only seen single pole tripping for 240kV and higher. We use GE-C60. takes in the 52a (or b) status of your breakers. then it takes the inputs from your other protections (Line, TX, etc). for line protection we use sel-421 and siemenes 7SA522 to trip the C60 and the breaker control module with 3-pole tripping. we also designate another contact from the line protection to do 3-pole tripping independently. this can be done for 79 and BF. Sometimes, for whatever reason they just parallel the 3 contacts from the line protection to send a single signal to the C60 to do 3-pole tripping. I think this depends on how much you want to spend on settings because the C60 can do both. I am not a settings guy though, so my knowledge in that dept is limited.

RE: Breakers in series

Mbrooke, I may be making a poor assumption but basically you want a double breaker double bus station?

I don't see why any special considerations need to be made for breaker control outside of a breaker and a half station.

If this is so, the only thing I really see you gaining is cost. Unless there's a very special customer connected I wouldn't go down this road. It seem like one of the consequences you have for trying to minimize the impact of a failed breaker is adding more breakers, which can then fail. In my opinion the decreased security doesn't justify the cost of an incremental increase of reliability.

RE: Breakers in series

(OP)

Quote (Marks1080)

Mbrooke, I may be making a poor assumption but basically you want a double breaker double bus station?

Technically in this cases you would indeed place the element on its own bay and treat it as double breaker double bus, but for an existing station its not always possible to expand the substation, especially if that also involves moving a line outside the substation all the way down to a new bay. Having to go over or under other circuits becomes a tricky mess.


Also, as mentioned by another member here (David Beach) breaker-and-a-half is more reliable than double breaker double bus in that you can not clear an entire station without multiple breaker failures in each bay (technically zone two might also open in that case, but you get the point). Id further add in BAAH you can loose both bus bars and still be able to pass power via the center breakers, which is of major benefit especially in a bulk flow gate application that is just cutting over to supply load. Plus BAAH, even with two series breakers tends to be more compact (less land) than DBDB. So even if you took two series in new construction, there are still benefits over DBDB.




Quote:

I don't see why any special considerations need to be made for breaker control outside of a breaker and a half station.

If this is so, the only thing I really see you gaining is cost. Unless there's a very special customer connected I wouldn't go down this road. It seem like one of the consequences you have for trying to minimize the impact of a failed breaker is adding more breakers, which can then fail.

Not so much as a special customer- but rather a special set of lines feeding many customers- 345kv 1,500MW rated lines that is.


Quote:

In my opinion the decreased security doesn't justify the cost of an incremental increase of reliability.


Why would security become decreased? I can't see that- but then again I don't know.

RE: Breakers in series

So, from what I've gathered Double Bus Double Breaker is the most reliable if you never have more than one breaker failure for any one event. I now know of two Double Bus Double Breaker installations that tripped everything for cascading breaker failures that might have been contained in a 1.5 breaker station. Pick your poison, no extra outages for a single breaker failure or a better hope at containing cascading breaker failure. Breaker and a third might be even better at stopping cascading failures.

RE: Breakers in series

(OP)
Not only cascading failures, but picture one bus out for maintenance and the other clears (faults). I've read documented cases in India and South America where this has happened, and it was prized as an advantage because generation and lines continued to work via the center breakers preventing a blackout. In many ways I'd argue breaker and a half is the ideal substation when all is factored together. Technically two center breakers could be called breaker and a third, but nothing connects between them in my case.

RE: Breakers in series

Agreed, I wouldn't call yours as breaker and a third since there isn't a third position in there. Breaker and a half doesn't require bus protection in the bays, nor would breaker and a third; but you'd have to have dedicated bus protection in each bay where you have two breakers with nothing between them.

To the original question, I think I'd trip both of them simultaneously but hope that I never have to turn that from a theoretical answer to a practical answer. To date I've not had to deal with critical clearing times that would cause one to entertain such outlandish schemes; but I've heard of conditions that would make something of the sort a reasonable reaction. I think, though, if I ever got there I'd try to have the extra breaker on the position. The bay, from bus to bus, would still have only three breakers for two positions, but then I'd add a single breaker (possibly with a bypass switch) on the line or transformer. Then I'd have a 4 or 5 cycle breaker failure trip of that breaker followed by the 10 cycle (our standard, others might be 7 or 8 cycles) breaker failure trip of the adjacent bus and the far end of the bay. I'd like to keep that as a pencil and paper exercise.

RE: Breakers in series

(OP)

Quote (David Beach)

Agreed, I wouldn't call yours as breaker and a third since there isn't a third position in there. Breaker and a half doesn't require bus protection in the bays, nor would breaker and a third; but you'd have to have dedicated bus protection in each bay where you have two breakers with nothing between them.

Correct and agreed.

Quote:

To the original question, I think I'd trip both of them simultaneously but hope that I never have to turn that from a theoretical answer to a practical answer.

Hate to say it, but my bets are you will one day encounter such a setup. :P To be honest I'm surprised you have not seen this at least once in the wild- but then again every utility has vastly different standards and conditions that it must work with.


Quote:

To date I've not had to deal with critical clearing times that would cause one to entertain such outlandish schemes;

In this case its not critical clearing time forcing it, but rather the removal of the line directly across from it should the center breaker fail to clear; otherwise the breaker on the other side connecting to the bus would need to be duplicated as well if that was the case.

The system has a rough critical clearing time of about 12 cycles for a 3 phase fault and about 20 for a single phase fault. When a fault occurs the line protection relay will send a trip signal to each line breaker consisting of 3 single pole breakers which should clear in about 2 to 4 cycles, after which the 3 phase fault is down graded to a single phase fault if one of the breakers stick (it is assumed that only one of the 3 single pole breakers will fail to open, and 3 single pole breakers are deliberately chosen at higher voltages over 3 pole units to increase the critical clearing time, ie a single phase fault can be tolerated much longer than a 3 phase fault). If current is still present on any breaker CTs 6 to 8 cycles latter, a 3 phase trip is sent to all breakers attached to that bus for a side breaker failure which should clear in about 2-4 cycles. For a stuck middle breaker its about the same, after 6-8 cycles a DTT is sent to the remote terminal plus tripping of the other side breaker; in 5 cycles both the remote end and the side breaker should clear. In total in about 16 cycles a fault is removed involving a stuck breaker, about 5 cycles with working breakers- both below the critical clearing time.

However, once two 345kv lines are removed (possibly an auto transformer if the remote end has one attached directly to that line), the remaining 345kv lines can overload during peak periods with reduced local generation, hence the need make sure a failed breaker does not remove other elements.


Quote:

but I've heard of conditions that would make something of the sort a reasonable reaction. I think, though, if I ever got there I'd try to have the extra breaker on the position. The bay, from bus to bus, would still have only three breakers for two positions, but then I'd add a single breaker (possibly with a bypass switch) on the line or transformer.

If critical clearing time was the factor (ie fault could not persist for more than 8 cycles) I would certainly do that. FWIW there have been cases involving very large generators (over 1,500MW) where faults in front of it had greatly reduced clearing times due to the lower inertia as compared to smaller paralleled generators. I've also heard of increased renawbles increasing the CCT.


Quote:

Then I'd have a 4 or 5 cycle breaker failure trip of that breaker followed by the 10 cycle (our standard, others might be 7 or 8 cycles) breaker failure trip of the adjacent bus and the far end of the bay. I'd like to keep that as a pencil and paper exercise.

At this point since I have CCT coverage, I am debating tripping both breakers, or just tripping one and then the other 10 cycles latter.

RE: Breakers in series

(OP)
But- to keep the overly complicated simple- view it like this. Lets say you had a straight bus and had two line breakers in series. How would you control those two breakers?

RE: Breakers in series

Mbrooke: Security is reduced because you've added 'stuff.' One of the balancing acts between reliability and security.

So I agree with you when you say the double bus double breaker is more reliable. But my point is that the very small amount of reliability you gain doesn't justify the cost and decreased security. Security is decreased just by adding more elements which could represent a failure point. Generally, "A station with more breakers than another station will experience more breaker fails." is a true statement.

You want to consider real life scenarios where you gain anything by having a double bus double breaker. It's easy to imagine those scenarios. Now try to find real life situations where they happened and how double bus double breaker would have helped. If you can actually find these scenarios ask how adding the extra breaker would have helped. After finding that out ask yourself if it would have been worth the cost to get that extra reliability. I think you'll find the answer to that is 'No' because there simply haven't been enough of those operations to justify catering for above a breaker and a half configuration. Add with the security you lose by adding the extra breakers, the answer should most definitely be "No". This is why you see breaker and a half used so much. Like I said originally, unless you have a very special customer connected, where a loss of power equals a large law suit, or it's some large generating station critical to maintaining system stability, generally the best solution is breaker and a half.

This discussion actually reminds me of a book I read called "Confessions of an Economic Hitman." At least I think that's what it's called. It's about an ex-CIA operative who worked with American utilities who went over seas to build power systems for third world countries. One of his CIA duties was to get these countries to over build their power systems to a point where it was just indebting them to American companies for no good reason. This guy would have built plenty of double bus double breaker stations I'm sure.

RE: Breakers in series

(OP)

Quote (Marks1080)

Mbrooke: Security is reduced because you've added 'stuff.' One of the balancing acts between reliability and security.

So I agree with you when you say the double bus double breaker is more reliable. But my point is that the very small amount of reliability you gain doesn't justify the cost and decreased security. Security is decreased just by adding more elements which could represent a failure point. Generally, "A station with more breakers than another station will experience more breaker fails." is a true statement.


I agree with what you are saying in general, but ask yourself this question: why do we choose breaker and a half over straight bus or single breaker double bus? Adding all those extra breakers only increases the odds of a stuck breaker, makes relaying far more complicated (yes easier with modern relays but settings get more complex and thus error), adds a lot of cost, and there is more equipment to maintain. Bus faults are rare, and in single breaker double bus you can do maintenance on any bus without shutting down the station.


Quote:

You want to consider real life scenarios where you gain anything by having a double bus double breaker. It's easy to imagine those scenarios. Now try to find real life situations where they happened and how double bus double breaker would have helped.

In this case double breaker double bus would be the "standard" solution, but said station would need to be expanded, and said line would need to be moved to a new bay.

Quote:

If you can actually find these scenarios ask how adding the extra breaker would have helped. After finding that out ask yourself if it would have been worth the cost to get that extra reliability. I think you'll find the answer to that is 'No' because there simply haven't been enough of those operations to justify catering for above a breaker and a half configuration.

What if I told you I can list as least 20 substations in the US the have the above configuration, often in more than one bay? I think this is more of not being seen in the wild than actually being a poor idea. There are two other solutions to this problem, one costs 5x as much the other +45x as much.


Quote:

Add with the security you lose by adding the extra breakers, the answer should most definitely be "No". This is why you see breaker and a half used so much. Like I said originally, unless you have a very special customer connected, where a loss of power equals a large law suit, or it's some large generating station critical to maintaining system stability, generally the best solution is breaker and a half.

Am I more likely to inadvertently loose a single element? Sure. But I am also more likely not to loose a second element when another one fails.


Quote:

This discussion actually reminds me of a book I read called "Confessions of an Economic Hitman." At least I think that's what it's called. It's about an ex-CIA operative who worked with American utilities who went over seas to build power systems for third world countries. One of his CIA duties was to get these countries to over build their power systems to a point where it was just indebting them to American companies for no good reason. This guy would have built plenty of double bus double breaker stations I'm sure.

Or rather plenty of straight bus... I'm not kidding. :)





RE: Breakers in series

Hi Mbrooke... I think you need to go back and review reliability vs secruity philosophy. Obviously straight bus is very secure, but it's reliability is just one notch above having a black-out. Therefore the balance between security and reliability hasn't been properly achieved. Considering how large the US is I absolutely believe there are at least 20 stations in the whole country that don't use breaker and a half. I bet you will find a lot more than 20 if you look hard enough. I am personally working on a job now rebuilding a double ring bus station. Am I upgrading it to breaker and a half? Hell no. Way too expensive. But if I was building a green-field site you can bet it would be breaker and a half.

Basically your question boils down to this: What station design gives you the best balance of Security and Reliability at the best cost. The answer, in general, is breaker and a half. The industry has known this for some time now... but not always, as you can see from your old stations that have weird configurations. And before everyone replies with their one off situations that go against this I am speaking generally. In general, breaker and a half is the best choice. If you can get the funds approved to over build, by all means, overbuild :)

RE: Breakers in series

Sorry Mbrooke: I also should have added my two cents on how to handle a double breaker double bus system. If I were going to implement that I wouldn't trip both of the middle breakers simultaneously. I would still zone out my station using breakers and I would still only trip adjacent breakers from any zone protection. Keep in my you will have to either overlap breaker CTs or you will have to find a way to protect the chunk of bus in between them. There's nothing complicated about that, just that you need to be aware of it. You don't want to have a chunk of bus between two breakers that isn't protected.

RE: Breakers in series

(OP)

Quote (Marks1080)

Hi Mbrooke... I think you need to go back and review reliability vs secruity philosophy. Obviously straight bus is very secure, but it's reliability is just one notch above having a black-out.

Sure, that is true if you put straight bus directly in the place of where BAAH, DBDB and ring bus is used today. You can have a very, very reliable system with straight bus that takes breaker failure and bus faults into account, provided you design the system for N12 and up. This is certainly doable, but you need more lines, more stations, and possibly higher voltages. The magnitude of which costs many, many times more than if you were to have the most expensive substation, say triple breaker triple bus or ring bus with a breaker on each line as well.

An example would be places like France, single breaker double bus is used almost exclusively at the 380kv levels, and there lies the rise of clearing the whole station under the right conditions. As a result a 380kv ring is built around Paris. That means more capacity/lines and more sub-stations for the same level of reliability that breaker redundant substations give, ie DB,DB.


Quote:

Therefore the balance between security and reliability hasn't been properly achieved. Considering how large the US is I absolutely believe there are at least 20 stations in the whole country that don't use breaker and a half.

Correct, these which I have in mind having breakers in series.


Quote:

I bet you will find a lot more than 20 if you look hard enough. I am personally working on a job now rebuilding a double ring bus station. Am I upgrading it to breaker and a half? Hell no. Way too expensive. But if I was building a green-field site you can bet it would be breaker and a half.

But you would be if transmission capacity was starting to dwindle beyond N-1 and you wanted to keep reliability, correct?

RE: Breakers in series

Mbrooke: To your very last statement - There are many many many reasons why brown field work is done in various ways. A lot of these reasons are way beyond the power system. If the economy was roaring and the province I'm in wasn't in massive debt, perhaps we would bulldoze the site I'm working at and start a new. Although the connected generators may have an issue with being down long term. Or maybe they won't because our government will just pay them anyway (hence the debt lol) Or perhaps not. Either way I'm just replacing EOL equipment and I'm ok with not re configuring the entire site. So I really don't know how to answer that question practically.

My only comment towards your initial question is that I think the cost doesn't justify changing station configuration to double bus double breaker. I don't think the extra reliability will justify the decreased security. This is a subjective area, and my thoughts may be different than others. If your company is ok with funding a double bus double breaker rebuild and you have the real-estate and clearances to do so, along with a healthy purse, than go for it. But it IS a mistake to think that the reliability gained doesn't come at a cost to security. I will confidently argue that the reliability gained in your case doesn't justify the security lost. I would be happy to back that up statistically if I had time and access to analyze the stats. Other (better) people have already done this. I've read their books and am ok accepting breaker and a half as the best solution in most situations. I am aware there are one off's where I would prefer a different solution, but in general I wouldn't.

When you tally your costs don't just look at the cost of an extra breaker and hardware. There's an increase in administration costs as well as routine maintenance (which also impacts work force).

As an anecdote: Do you know when you can possibly have a power system with reliability and security ratings that are both a 1 (or 100%)? This is only partly a trick question....

Answer: When the system is offline. A dead power system is both 100% reliable and 100% secure. Please don't pitch this idea to your management, because in my experience they don't always know when you're joking or not.

RE: Breakers in series

(OP)

Quote:

Mbrooke: To your very last statement - There are many many many reasons why brown field work is done in various ways. A lot of these reasons are way beyond the power system. If the economy was roaring and the province I'm in wasn't in massive debt, perhaps we would bulldoze the site I'm working at and start a new. Although the connected generators may have an issue with being down long term. Or maybe they won't because our government will just pay them anyway (hence the debt lol) Or perhaps not. Either way I'm just replacing EOL equipment and I'm ok with not re configuring the entire site. So I really don't know how to answer that question practically.

Its ok- but question. What is Brown Field work?


Quote:

My only comment towards your initial question is that I think the cost doesn't justify changing station configuration to double bus double breaker. I don't think the extra reliability will justify the decreased security. This is a subjective area, and my thoughts may be different than others. If your company is ok with funding a double bus double breaker rebuild and you have the real-estate and clearances to do so, along with a healthy purse, than go for it. But it IS a mistake to think that the reliability gained doesn't come at a cost to security. I will confidently argue that the reliability gained in your case doesn't justify the security lost. I would be happy to back that up statistically if I had time and access to analyze the stats. Other (better) people have already done this. I've read their books and am ok accepting breaker and a half as the best solution in most situations. I am aware there are one off's where I would prefer a different solution, but in general I wouldn't.

When you tally your costs don't just look at the cost of an extra breaker and hardware. There's an increase in administration costs as well as routine maintenance (which also impacts work force).

As an anecdote: Do you know when you can possibly have a power system with reliability and security ratings that are both a 1 (or 100%)? This is only partly a trick question....

Ok, I guess I have to apologize. I mis read. I thought you were advocating for not having a series breaker and choosing other options instead. My mistake on that.

Quote:

Answer: When the system is offline. A dead power system is both 100% reliable and 100% secure. Please don't pitch this idea to your management, because in my experience they don't always know when you're joking or not.

Or when there are no clowns working on it LOL (joking) :)

RE: Breakers in series

No apologies necessary.

Green field refers to work on a brand new site. Brown field refers to work at an existing site.

I'm not sure how common that terminology is around the world, but pretty common in North America.

I believe your project would be a brown field. Brown field is typically more difficult in this industry as sites tend to get really old before major upgrade work. Usually that means at best you're working with poor drawings. Worst case, there are no drawings. Most large utilities today can probably put up a brand new green field station with 90% of the engineering already done on templates.... at least I think that's the case.

RE: Breakers in series

If your building 5 or 6 or more greenfield substations a year then a template is the way to go. But if your building one every few years, the technology is changing too fast to maintain the templates.

On brownfield substations the issue is people in the past usually did not leave room for new equipment.

RE: Breakers in series

(OP)
Thanks and thanks everyone on the terminology :) And yes, old stations not only lack new addtion space, but also working space LOL.


In so far I came up with something more simple and straight forward: Have the left side SEL411L control only one center breaker while having the right SEL411L controlling the other center breaker. CTs overlap to form an overlapping zone of protection. If the normally called to trip center breaker sticks, BF then opens the other center breaker after 8 cycles. In my eyes there is no programming logic change, and the relays for both lines simply believe this is a classic breaker-and-a-half station.

RE: Breakers in series

That will do just fine until that very rare fault between the two center breakers; instead of just opening those two breakers to clear the bus fault you'll open all four and interrupt both lines.

RE: Breakers in series

(OP)
Very true and good point. I may add a differential zone between the two breakers since I have no bus protection to connect the remaining CTs to- but will see. Its rare "enough" so I will weigh my options.

RE: Breakers in series

I may have missed it, but why would you delay the second breaker? Tripping both center breakers at the same time seems much more dependable.

Since faults at the terminals of a transmission breaker are the hardest to interrupt, I have wondered how much series breakers actually reduce the likelihood of a complete bus outage. There are a number of instances of series breakers as bus ties in the northwestern USA.

RE: Breakers in series

Are fuses more reliable than breakers? Do people ever use fuses at transmission level in series with a breaker?

RE: Breakers in series

Fuses are a little slow for transmission work.

RE: Breakers in series

Of insufficient current rupturing capability as well, no?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Breakers in series

Tripping series breakers 'at the same time' might be a recipe for false breaker fail.

RE: Breakers in series

(OP)

Quote (Bacon4life)

I may have missed it, but why would you delay the second breaker? Tripping both center breakers at the same time seems much more dependable.

Since faults at the terminals of a transmission breaker are the hardest to interrupt, I have wondered how much series breakers actually reduce the likelihood of a complete bus outage. There are a number of instances of series breakers as bus ties in the northwestern USA.


The blunt truth is I have no idea how to get both breakers to trip other than tripping both of them and having the 52A contact in series. I have nothing to go by- never seen it done before on paper. Those breakers you have up North, do you have any idea how they are wired or if any special wiring or systems take place to account for the two breakers in series?

RE: Breakers in series

(OP)

Quote (HamburgerHelper)

Are fuses more reliable than breakers? Do people ever use fuses at transmission level in series with a breaker?

Fuses are much more reliable than breakers since they have few if any moving parts. The only place I have seen fuses in series with breakers have been smaller transformers tapping off a bus with lines connected to it (technically in series). In this case its 345kv lines, and they don't make 345kv fuses, and even if they did I doubt they would have a 2000 or 3000amp rating like these breakers.

RE: Breakers in series

Marks1080-what do you mean by false breaker fail?

We used to have a number of 115/12 kV transformers protected with fuses, but the max interrupting rating we could find was 10,000 amps. As fault levels increased over time we had to replace the fuses with circuit breakers/switchers.

RE: Breakers in series

Fuses fail too. Too many other things they can't do, like differential and reclosing.

RE: Breakers in series

(OP)
Or step distance as in the case of transmission lines. tongue But I have to admit when dealing with a tap substation and small transformers (12MVA and under) fuses can be very attractive from cost and maintance standpoint, especially when your 12.47kv breakers are just reclosers and the pole mounted boxes that go along with them. Rural substations tend to be very capitol efficent.

RE: Breakers in series

That seems to be the common wisdom of a few decades ago. Can't wait to be rid of the last transformer fuse. We had the misfortune of loosing two transformers last winter. One had fuses and we had oil to clean up all over the place following an event that was finally, fully, cleared by transmission tripping. A few weeks later when the second one went it had a circuit switcher and a differential relay and cleared without the companion overcurrent relay hardly noticing. No oil outside the tank, no fuss, no muss. Told the Tech that went to test the transformer what it was and he said it was the quickest transformer test ever; straight to the tests that wouldn't pass without taking time to perform tests that fault wouldn't cause to fail. No more fuses, thanks.

RE: Breakers in series

(OP)
I can see that happening unfortunately. Contrary to what might be portrayed, most transformer faults are not hard bolted faults, but rather turn-to-turn faults or low level internal faults. Fuses if anything are just meant to prevent failures from taking out the main line, and a last resort backup to an uncleared secondary fault. The dirty secret in fused protection is one hopes the transformers itself never fails internally, and with proper care it usually does not over its 40-60 years in service- hence why sometimes the risk is taken.

FWIW in defense of fuses there is one major advantage: if you loose the station DC supply and there is no SCADA to catch that, should a fault occur you minimize the risk of catastrophic transformer failure. Rumor was that happened here with a fault occurring latter on:


https://www.youtube.com/watch?v=fzbQjd_Oo4Q

RE: Breakers in series

Years ago, on sub-transmission, and distribution, there was a thing that was called a reclosing fuse. And not that I would recommend one.

They were used on long out of the city feeders, and would handle lighting strikes, by replacing the blown fuse with a fresh one. But after several strikes it would run out of fresh fuses.

But I doubt that was what David intended in his fuse reclosing comment.

RE: Breakers in series

That's just throwing multiple single shot fuses at it. How often would you patrol to see if any of them need to be replaced? I understand that 115kV transformer fuses run several thousands of dollars a piece. Very few operations and the breaker is less expensive.

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