Fuses vs Circuit breakers in switch gear

[mbrooke]

What are the advantages and disadvantages of using fuses over circuit breakers in 13.8kv switch gear? How much maintenance, and what type of maintenance do fuses require? In so far I see many compelling reasons to use fuses in place of breakers where maintenance and inspection is infrequent. Here is one reason as an example:



https://www.sandc.com/globalassets/...s---all-documents/article-reprint-621-r69.pdf


Cost is also less. Is it to good to be true? Most stuff I have evaluated have used breakers so I am not to familiar with fuses outside of fused cutouts on poles.

 

[zog]

MV Fuses are expensive and have to be replaced every time they interrupt a fault, breaker can just be reset. Breakers can be operated remotely and can tripped for other reasons than just overcurrent (Freq, UV, etc...). I really depends on the application as to which is the better choice.

 

[mbrooke]

MV Fuses are expensive and have to be replaced every time they interrupt a fault, breaker can just be reset. Breakers can be operated remotely and can tripped for other reasons than just overcurrent (Freq, UV, etc...). I really depends on the application as to which is the better choice.

How much do fuses cost as compared to that saved on having them? My (thin) understanding are that breakers cost much more upfront.

 

[petersonra]

How much do fuses cost as compared to that saved on having them? My (thin) understanding are that breakers cost much more upfront.

I don't know much about MV costs but I have found that for 600V and under fuses are usually only cost effective <200A. Depending on AIC requirements MCCB are often cost effective at ratings as low as 100 A.

 

[mbrooke]

I don't know much about MV costs but I have found that for 600V and under fuses are usually only cost effective <200A. Depending on AIC requirements MCCB are often cost effective at ratings as low as 100 A.

Cost in equipment of cost of the fuses themselves?

 

[petersonra]

Cost in equipment of cost of the fuses themselves?

total cost of one option versus another.

I only design industrial control panels.

The cost of a 400A fuseblock, 3 400A fuses, and the extra space they take up over a 400A MCCB adds up.

There is also the issue of being able to use the MCCB as a disconnecting means which is not possible with a fuse block. It requires a seperate switch which are quite pricey over 200A and eat up a lot of space.

Also, all the major MCCB manufacturers have distribution lugs so you can get rid of distribution blocks if it is a feeder. Bussman has some fuse blocks that have the same feature but they are only recognized so we had to pay UL to get them added to our procedure.

Incidentally, I have found it is often more cost effective to buy an extra MCCB with distribution lugs in lieu of a distribution block because most distribution blocks have issues with SCCR ratings and require fuses to get much of a rating.

 

[mbrooke]

total cost of one option versus another.

I only design industrial control panels.

The cost of a 400A fuseblock, 3 400A fuses, and the extra space they take up over a 400A MCCB adds up.

There is also the issue of being able to use the MCCB as a disconnecting means which is not possible with a fuse block. It requires a seperate switch which are quite pricey over 200A and eat up a lot of space.

Also, all the major MCCB manufacturers have distribution lugs so you can get rid of distribution blocks if it is a feeder. Bussman has some fuse blocks that have the same feature but they are only recognized so we had to pay UL to get them added to our procedure.

Incidentally, I have found it is often more cost effective to buy an extra MCCB with distribution lugs in lieu of a distribution block because most distribution blocks have issues with SCCR ratings and require fuses to get much of a rating.

Noted and makes sense.


By any chance have you ever worked with QMQB bolted pressure switches?

 

[zog]

How much do fuses cost as compared to that saved on having them? My (thin) understanding are that breakers cost much more upfront.

True, but say you spend $20k on a MV breaker, that's it for 20 years or so until you have to refurb it and/or replace VI's, etc..

MV fuses can be a few grand each, plus as mentioned you will need a switch to house them in ($10k). A phase to phase fault will blow 2 of those fuses and that costs a few grand everytime it happens. Google 400E fuse and look at the prices, not cheap.

 

[Ingenieur]

many faults on the utility side are self clearing
trees, wind, birds, critters, etc.
every time you have one of these and blow the fuse you are down for hours, while with a CB you can reclose when cleared

a cb with relaying can be coordinated much better than any fuse set-up

 

[petersonra]

By any chance have you ever worked with QMQB bolted pressure switches?

no

 

[zog]

Noted and makes sense.


By any chance have you ever worked with QMQB bolted pressure switches?

QMQB's are not bolted pressure switches, they are just fused switches.

 

[mbrooke]

True, but say you spend $20k on a MV breaker, that's it for 20 years or so until you have to refurb it and/or replace VI's, etc..

Which costs how much more (to refurbish)? My understanding is that a fuse and switch does not need a refurbish after 20 years. There is also the cost of maintaining the relays and batteries with breakers.

MV fuses can be a few grand each, plus as mentioned you will need a switch to house them in ($10k). A phase to phase fault will blow 2 of those fuses and that costs a few grand everytime it happens. Google 400E fuse and look at the prices, not cheap.

What about "K" links? I'll have to check, but I think you can get S&C gear that takes standard fuse tubes found in overhead cutouts.

many faults on the utility side are self clearing
trees, wind, birds, critters, etc.
every time you have one of these and blow the fuse you are down for hours, while with a CB you can reclose when cleared

a cb with relaying can be coordinated much better than any fuse set-up

Thats true for overhead wire- but not for underground cables. You don't want to reclose on those.

 

[mbrooke]

QMQB's are not bolted pressure switches, they are just fused switches.

Ok- my mistake them.

Not to derail the thread- but just a quick question: whats the difference between a bolted pressure switch and a fused switch?

 

[Ingenieur]

Thats true for overhead wire- but not for underground cables. You don't want to reclose on those.

??? no one said you would never reclose on UG, although they often do
the fault may be reflected from a customer and on premisis protection may clear it so reclosing on the util side may be desirable
many more OH than UG lines anyways
lost revenue, labor and material cost, possible damage due to clearing times, ineffective/unpredictable/imprecise coordination
will a fuse react the same at -10 F vs 90 F?
a cb opens all 3 phases, a fuse on a gnd fault may lead to single phasing
dangerous: replace a fuse and closing it on a load vs a remote operated enclosed cb

if it was a good idea everybody would be using fuses

 

[mbrooke]

??? no one said you would never reclose on UG, although they often do
the fault may be reflected from a customer and on premisis protection may clear it so reclosing on the util side may be desirable

That would be a miss-cordination issue. And whats to say that they downstream OCPD will be open when the breaker recloses back in?

many more OH than UG lines anyways
lost revenue, labor and material cost, possible damage due to clearing times, ineffective/unpredictable/imprecise coordination

When you reclose an underground cable, you are increasing the incident energy at the failed point.

will a fuse react the same at -10 F vs 90 F?

Of course not- thats why you take ambient temps and even preloading into account during fault simulation and coordination studies.

a cb opens all 3 phases, a fuse on a gnd fault may lead to single phasing

if it was a good idea everybody would be using fuses

Which keeps the lights on for none effected parts of the system while protecting healthy cable portions from thermal damage.

 

[drktmplr12]

that article makes me think plants need to do a better job checking batteries instead of applying fuses all over the place.

your switchgear batteries are just as important, if not more, as doing vibration tests on large vertical pumps.

wait, they don't check that either?

the vibration switch was bypassed because it kept triggering?

they calibrate by hitting it with the back of their screwdriver???

that said, it's not unusual to see fused switches in lieu of "circuit breakers" when feeding from a MV switchgear lineup. i use quotes because at MV they are really glorified contactors. in many cases the downstream breakers will stop a fault before the fuse pops.

 

[mbrooke]

that said, it's not unusual to see fused switches in lieu of "circuit breakers" when feeding from a MV switchgear lineup. i use quotes because at MV they are really glorified contactors. in many cases the downstream breakers will stop a fault before the fuse pops.

Is this because they breaker is faster on clearing a short circuit or because faults first leak "low" levels of current to ground picked up by differential/ground elements?

 

[drktmplr12]

Is this because they breaker is faster on clearing a short circuit or because faults first leak "low" levels of current to ground picked up by differential/ground elements?

trip ratings being equal.. I can't say if a fuse or breaker is faster without know the fault current and the exact fuse and breaker we are comparing.

 

[mbrooke]

trip ratings being equal.. I can't say if a fuse or breaker is faster without know the fault current and the exact fuse and breaker we are comparing.

Including the ground element?

 

[Ingenieur]

That would be a miss-cordination issue. And whats to say that they downstream OCPD will be open when the breaker recloses back in?

When you reclose an underground cable, you are increasing the incident energy at the failed point.

Of course not- thats why you take ambient temps and even preloading into account during fault simulation and coordination studies.

Which keeps the lights on for none effected parts of the system while protecting healthy cable portions from thermal damage.

no it isn't
fuses do not have the same characteristics as relaying

IF the fault is in the cable
a UG cable fault is far less likely than an OH one for obviously reasons

a relay will respond the same
better flexibility, accuracy, repeatability

not so good for 3 ph motors
or the lines carrying the extra load to supply them

that is why no one does it on util T&D
it would be a very unstable, unreliable system with only fusing