Fused switchboards

[mull982]

I have seen a few project recently that have used switchboards and panelboards with fused switches instead of breakers in the distribution system. An example would be Cuttler Hammers FDPB fusible panelboard switches.

I was wondering if anyone could explain to me the reason or benefit of using this type of panelboard or switchboard in a distribution system?

I know these units usually have higher interruping ratings or 100kA or 200kA because of fuses, but you can achieve these same ratings with using currentl limiting breakers in a standard panelbaord. I have also seen these panels used in systems where there was not much fault current avaliable such as generator systems etc... So I'm wondering if higher interrupting rating is the reason?

My second thought was cost. I'm guessing that maybe because fuses are being used instead of molded case breakers that the cost of these panels would be cheaper. I'm not sure if the reduced cost would be that much of a benefit, considering when a fuse blows you have to replace the fuses rather than simply reset the breaker.

Was curious to hear what others had to say.

Thanks

 

[augie47]

Normally when I see such installations it is due to higher fault currents and the benefits of using current limiting fuses. I'm told, in certain instances, this method is more cost efficient than current limiting or series rated breakers.

 

[petersonra]

IME, CBs are almost always less expensive > 400A then a fused system.

A lot depends on brands and how much of a discount to list you can get.

A lot of our customers do not want fuses because then they have to stock spares. Is less of an issue with commonly seen fuses like J or CC, especially in smaller sizes.

Most of our customers do not care what brand of fuses, CBs, or fuse holders we use. They think of them as interchangeable so we are free to use whatever costs less and is suitable for the application at hand.

Most of our customers would have issues with us using class L or T fuses. They are just not that easy to come by in any size. Neither are the >100A class J fuses. But, even a lot of hard ware stores have smaller CC and J fuses, so there is often less resistance there.

We sometimes do have to use fuses because of the SCCR needed, but it is usually for a specific device. Might be more of an issue with switchboards though.

Some people are just cheap too, so they get mostly fuses in smaller sizes and CBs in larger sizes. Sometimes a mixture.

 

[Jraef]

IME, CBs are almost always less expensive > 400A then a fused system.

A lot depends on brands and how much of a discount to list you can get.

A lot of our customers do not want fuses because then they have to stock spares. Is less of an issue with commonly seen fuses like J or CC, especially in smaller sizes.

Most of our customers do not care what brand of fuses, CBs, or fuse holders we use. They think of them as interchangeable so we are free to use whatever costs less and is suitable for the application at hand.

Most of our customers would have issues with us using class L or T fuses. They are just not that easy to come by in any size. Neither are the >100A class J fuses. But, even a lot of hard ware stores have smaller CC and J fuses, so there is often less resistance there.

We sometimes do have to use fuses because of the SCCR needed, but it is usually for a specific device. Might be more of an issue with switchboards though.

Some people are just cheap too, so they get mostly fuses in smaller sizes and CBs in larger sizes. Sometimes a mixture.

All of the above, plus, Bussman and some of the other fuse mfrs used to run around and show horrible videos to plant engineers that depicted circuit breakers exploding and fires and death and mayhem, blah blah blah. They scared a lot of people with that for a number of years. Of course they never showed the videos of the fuses not clearing on low level arcing faults, or the damaged caused by single phasing motors when only one fuse blows, blah blah blah.

Sometimes fuses vs CBs is more like religion than engineering principals. People tend to believe either what their predecessors believed, or what they want to believe.

 

[templdl]

Sometimes fuses vs CBs is more like religion than engineering principals. People tend to believe either what their predecessors believed, or what they want to believe.

The breakers Vs fuses is so true.
KAIC is a very good reason for fused panels but a fuse can not be tested for calibration if it nuisance blew or not nor is there a way that I know of to see if it's within specs like you can with a breaker. You have to believe the rating that is printed on the label.
And how many wished that they had breakers while scrambling to find that spare fuse that they may of may not have. And it is "Darned, I forgot or simply put off buying a replacement fuse to replace the spare that I had previously used."

 

[steve66]

If you are seeing fuses clear down at the panelboard level (i.e: 20 amp branch circuit fuses), there is a good chance fuses were used to achieve coordination with upstream fuses. Especially if there is a generator.

Is this healthcare by any chance??

 

[templdl]

If you are seeing fuses clear down at the panelboard level (i.e: 20 amp branch circuit fuses), there is a good chance fuses were used to achieve coordination with upstream fuses. Especially if there is a generator.

Is this healthcare by any chance??

Interesting. Generators usually are not commonly known for their available fault current. So other than fault current coordination what would be their benefit?

 

[don_resqcapt19]

Interesting. Generators usually are not commonly known for their available fault current. So other than fault current coordination what would be their benefit?

Coordination would be the purpose because of the rules in 700.27 and 701.27.

 

[jim dungar]

Coordination would be the purpose because of the rules in 700.27 and 701.27.

IMHO fuses are one of the 'hardest' devices to coordinate with. The generator fault current is so low that the fuses never enter their current limiting region meaning they are in the 'delay ' portion of their curves. Then there is the issue of branch devices downstream from the fuses, which are often close to the same ampacity. And of course the loss of coordination if the 'specified' fuse is ever replaced with a non-identical one.

 

[templdl]

IMHO fuses are one of the 'hardest' devices to coordinate with. The generator fault current is so low that the fuses never enter their current limiting region meaning they are in the 'delay ' portion of their curves. Then there is the issue of branch devices downstream from the fuses, which are often close to the same ampacity. And of course the loss of coordination if the 'specified' fuse is ever replaced with a non-identical one.

To my point Jim. That was exactly my oppinion also.

 

[mull982]

The one project that I am working on that has these fused switcboards is a large hotel.

The strange part about this system is that these fuses switchboards are only used on the hotels emergency system fed from generators. The entire rather large normal distribution system has switchboards and panels equipped with molded case breakers however everything downstream of the ATS's fed from generators are fused switches. I find it interesting now that the idea of emergency system coordination was brought up for this may be the case.

I agree with others that the generator is just not capable of supplying high enough fault currents to justify using fused switches to gain a higher interrupting rating. Also if cost was the driving factor I dont know why they wouldn't have used these fused switches in the rest of the system instead of breakers. It is sounding like the reason for using may be for coordination purposes of the emergency system. I was always under the impression that fuses were difficult to coordinate with. I will be performing this coordination study so I'll guess I'll find out soon enough.

 

[steve66]

IMHO fuses are one of the 'hardest' devices to coordinate with. The generator fault current is so low that the fuses never enter their current limiting region meaning they are in the 'delay ' portion of their curves. Then there is the issue of branch devices downstream from the fuses, which are often close to the same ampacity. And of course the loss of coordination if the 'specified' fuse is ever replaced with a non-identical one.

Bussman puts a different spin on it. According to the "slective coordination" link on this page:

http://www.cooperindustries.com/con...ards/quik-spectm_coordinationpanelboard1.html

You just pick your fuse types and make sure their ratings meet the ratio requirements listed. Like 2 to 1. (See page 4 of the selective coordination link.)

It would normally be difficult to get a 20 amp branch breaker to coordinate with a 100 amp panelboard main breaker because their settings are so close. But Bussman makes it look like they can easily coordinate a 20 amp branch fuse with something as small as 30 or 40 amp fuse.

 

[jim dungar]

Bussman puts a different spin on it. According to the "slective coordination" link on this page:
You just pick your fuse types and make sure their ratings meet the ratio requirements listed. Like 2 to 1. (See page 4 of the selective coordination link.)

It would normally be difficult to get a 20 amp branch breaker to coordinate with a 100 amp panelboard main breaker because their settings are so close. But Bussman makes it look like they can easily coordinate a 20 amp branch fuse with something as small as 30 or 40 amp fuse.

Bussmann says that their selectivity tables only work with the specific devices. Replace a fuse with a diferent type or manufacturer and the table is no longer applicable.
How many times have you opened a disconnect to see different types of fuses in it?

Bussmann also seems to forget that just about every major manufacturer of circuit breakers has tables of 'tested' combinations of breakers that can be used to achieve 100% selectivity.

 

[T.M.Haja Sahib]

I agree with others that the generator is just not capable of supplying high enough fault currents to justify using fused switches to gain a higher interrupting rating. Also if cost was the driving factor I dont know why they wouldn't have used these fused switches in the rest of the system instead of breakers. It is sounding like the reason for using may be for coordination purposes of the emergency system. I was always under the impression that fuses were difficult to coordinate with. I will be performing this coordination study so I'll guess I'll find out soon enough.

1) I do not agree that the generator is just not capable of supplying high enough fault currents, because, if it is separately excited, it may be capable of supplying sufficient fault current.

2)One more reason for providing fused disconnects panel instead of breakers panel is the greater ability of fuses than breakers in protecting equipment against fault currents.

 

[don_resqcapt19]

Bussman puts a different spin on it. ...

Of course they do. The rules I cited were proposed by one of their employees.

 

[don_resqcapt19]

..
2)One more reason for providing fused disconnects panel instead of breakers panel is the greater ability of fuses than breakers in protecting equipment against fault currents.

Can you expand on that?

 

[don_resqcapt19]

IMHO fuses are one of the 'hardest' devices to coordinate with. The generator fault current is so low that the fuses never enter their current limiting region meaning they are in the 'delay ' portion of their curves. Then there is the issue of branch devices downstream from the fuses, which are often close to the same ampacity. And of course the loss of coordination if the 'specified' fuse is ever replaced with a non-identical one.

Yes those are all issues, but don't breakers have the same issues?
Is it possible to provide the required selective coordination for emergency systems operating on generator power with either fuses or breakers?

 

[ron]

Bussmann also seems to forget that just about every major manufacturer of circuit breakers has tables of 'tested' combinations of breakers that can be used to achieve 100% selectivity.

Jim,
I use those tables a lot, but the ones I have indciate selectivity over a certain available fault current. Do you know of some that have tested selectivity at all fault current values?

 

[steve66]

Bussmann says that their selectivity tables only work with the specific devices. Replace a fuse with a diferent type or manufacturer and the table is no longer applicable.
How many times have you opened a disconnect to see different types of fuses in it?

Bussmann also seems to forget that just about every major manufacturer of circuit breakers has tables of 'tested' combinations of breakers that can be used to achieve 100% selectivity.

I agree completely.

Of course they do. The rules I cited were proposed by one of their employees.

And I am aware of that.

I'm not saying I think fuses are a good idea for coordination. And I'm not saying I agree with Bussman at all.

I'm just saying I think there is a good chance that the engineer that designed the system Mull982 described did so because of Bussman's propaganda and the Code references Don cited.

 

[jim dungar]

Jim,
I use those tables a lot, but the ones I have indciate selectivity over a certain available fault current. Do you know of some that have tested selectivity at all fault current values?

There is almost always an upper limit on fault current issue with thermal magnetic breakers. Electronic breakers have much more tolerance. UL489 pretty much requires that breakers have some amount of inherent short circuit protection, which means manufacturer testing rather than simple selection tables.