Fuses vs Circuit breakers in switch gear

[zog]

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?

Bolted pressure switch uses an operating mech (Springs, etc..) , QMQB is just a handle attached to the contact arm.

 

[Ingenieur]

fuses may not be cheaper
13.8 kv 400 A

disconnect switch, fuse rack, another dss if on a bus
the fuses alone might be 3-4k each

cb
as mentioned basically a vaccum contactor with shunt trip coil
relays and ct's

one cycle of fuses likely wipes out any cost advantage
might be close even in initial cost

 

[mbrooke]

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

Thats obvious- and something you must take into account when doing coordination studies.

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

If there is no overhead portion. An underground to overhead setup without a recloser would require auto-reclosing the cable- that is unless the cable is first in line and you have MHO impedance elements which will drive the relay to lockout should it pickup for the cable zone- at least the the first 90% of the cable anyway.

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

Thats true- but is such speed and accuracy really needed all the time?

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

Very valid- which is why you want to have single phasing protection at the MCC / 480 volt portion of the systems. How many POCOs feed large motor customers with fuse protected risers and transformers?

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

Substation breaker is always a breaker- but down stream there is lots of room to chose.

 

[mbrooke]

Bolted pressure switch uses an operating mech (Springs, etc..) , QMQB is just a handle attached to the contact arm.

This mech is manual, right? Or does some relay kick it?

 

[Ingenieur]

Thats obvious- and something you must take into account when doing coordination studies.

If there is no overhead portion. An underground to overhead setup without a recloser would require auto-reclosing the cable- that is unless the cable is first in line and you have MHO impedance elements which will drive the relay to lockout should it pickup for the cable zone- at least the the first 90% of the cable anyway.

Thats true- but is such speed and accuracy really needed all the time?

Very valid- which is why you want to have single phasing protection at the MCC / 480 volt portion of the systems. How many POCOs feed large motor customers with fuse protected risers and transformers?

Substation breaker is always a breaker- but down stream there is lots of room to chose.

obvious to some
you are stating the obvious
and you will still not match a relay with a fuse

????

yes
unless you can tolerate damage

but up stream is a cb
and for large industrial users they get a util sub...w/cb

there are numerous reasons it is not done
you are asking open ended questions that need specifics to be answered
and those have been answered in dozens of texts and decades of practice
do you design T&D grids?

hi Z gnd fault
a fuse will sit there all day
a cb with ct's configured for 0 seq will trip and clear
the most common fault, as 90% of all util dist faults

 

[Ingenieur]

This mech is manual, right? Or does some relay kick it?

manual
like a cam over mechanism
crank it to a break point then snaps over with spring assist, very fast

 

[mbrooke]

obvious to some
you are stating the obvious
and you will still not match a relay with a fuse

Yes- only because you stated the obvious and I was basically agreeing.

????

You have never used MHO elements at MV? It can be done.

yes
unless you can tolerate damage

In most cases you can.

but up stream is a cb
and for large industrial users they get a util sub...w/cb

For smaller guys it varies- as does by POCO.

there are numerous reasons it is not done
you are asking open ended questions that need specifics to be answered
and those have been answered in dozens of texts and decades of practice
do you design T&D grids?

And numerous reasons that it is done- otherwise major manufacturers (like S&C for example) would not be offering enter lineups based on fused protection.

But my question is more around over all 40-60 year costs when fuses are compared with breakers. This is my #1 intrigue.

hi Z gnd fault
a fuse will sit there all day
a cb with ct's configured for 0 seq will trip and clear
the most common fault, as 90% of all util dist faults

This is a very valid point and I would like to know more regarding how often High Z faults occur on XLPE/EPR cables- but answer me this: even though high impedance faults are common on over head networks, why are taps almost always fused with cutouts in many POCOs?

 

[mbrooke]

manual
like a cam over mechanism
crank it to a break point then snaps over with spring assist, very fast

Makes sense- fast breaker to quickly break the load (increased interrupting rating) I am assuming.

 

[Sahib]

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.

Generally fuses are faster than breakers. A short time or momentary dip in voltage, when a breaker operates, would not happen when a fuse operates for a fault.

 

[ggunn]

Generally fuses are faster than breakers. A short time or momentary dip in voltage, when a breaker operates, would not happen when a fuse operates for a fault.

There are published current vs trip time curves you can look up for a lot of breakers and fuses.

 

[mbrooke]

There are published current vs trip time curves you can look up for a lot of breakers and fuses.

Maybe Sahib is thinking current limiting fuses?


Anyways- to get the trade back on track- does anyone have a detail cost break down?

 

[romex jockey]

Not @ 13KV Mbrooke, sorry/i]

@ 480V and under i know i can get a fused service rated main disco cheaper and w/more AIC 'bang for the $$'

~RJ~

 

[mbrooke]

Not @ 13KV Mbrooke, sorry/i]

@ 480V and under i know i can get a fused service rated main disco cheaper and w/more AIC 'bang for the $$'

~RJ~

AIC- you have a good point there. I am seeing fuses into the 63ka range. Sounds like moderate value- and it is for LV- but massive for MV. Most MV faults are several thousand amps max, 12,500 worst case at the busbars in most large MV substations. 63ka is some monster interrupting capability for an MV fuse.


http://www.cooperindustries.com/con.../bus-ele-ds-10353-mv-fuses-e-rated-15-5kv.pdf

 

[Sahib]

mbrook:There are advantages in using fuse. One I already mentioned. The other is avoiding explosion risk of a transformer/OCB due to any internal arcing fault by fast acting upstream fuse. Such considerations should figure in the cost benefit analysis of providing fuse or breaker.

 

[mivey]

Maybe Sahib is thinking current limiting fuses?


Anyways- to get the trade back on track- does anyone have a detail cost break down?

Detailed costs require detailed design.

Suffice it to say fuses are cheaper when they are not expected to blow a lot and cheaper for smaller loads or low priority loads.

I would not recommend fuses alone for a substation. Highside fuses for smaller subs can be ok. Fuses on large feeders is a bad idea. Fuses on taps is economical if you use a fuse saving scheme and coordinate with upline breakers since the majority of faults are temporary.

Fuses tell you when something extreme happens but have no intelligence to warn you before something happens or additional info to help you diagnose what happened. They are not flexible or adjustable and play by only one set of rules.

 

[Sahib]

mbrook:There are advantages in using fuse. One I already mentioned. The other is avoiding explosion risk of a transformer/OCB due to any internal arcing fault by fast acting upstream fuse. Such considerations should figure in the cost benefit analysis of providing fuse or breaker.

One interesting question is how much contribution by fast acting fuses in avoiding/reducing arc flash hazards in MV circuits while MV circuit breakers themselves are faster in action compared with LV breakers. Perhaps by acting as upstream protective devices to MV breakers, the fuses avoid/ reduce arc flash hazards in the MV breakers themselves.

 

[mbrooke]

Detailed costs require detailed design.

Suffice it to say fuses are cheaper when they are not expected to blow a lot and cheaper for smaller loads or low priority loads.

I would not recommend fuses alone for a substation. Highside fuses for smaller subs can be ok. Fuses on large feeders is a bad idea. Fuses on taps is economical if you use a fuse saving scheme and coordinate with upline breakers since the majority of faults are temporary.

Fuses tell you when something extreme happens but have no intelligence to warn you before something happens or additional info to help you diagnose what happened. They are not flexible or adjustable and play by only one set of rules.

I would never fuse an overhead line at the source substation for that reason- most faults are transient. I have entertained the idea on smaller underground feeders however, but in utility applications breaker is generally the way to go for many reasons.


One concern I have and I am hoping if you could shed any info: what are the odds of developing ferroresoance on a multi load feeder cable? Everything I know about ferresonance involves single phasing at the transformer primary itself, but I have never heard of it on a single phased (blown fuse) feeder.

 

[mbrooke]

One interesting question is how much contribution by fast acting fuses in avoiding/reducing arc flash hazards in MV circuits while MV circuit breakers themselves are faster in action compared with LV breakers. Perhaps by acting as upstream protective devices to MV breakers, the fuses avoid/ reduce arc flash hazards in the MV breakers themselves.

I honestly do not know

Though I doubt I will exceed 10,000amps.

 

[Sahib]

One concern I have and I am hoping if you could shed any info: what are the odds of developing ferroresoance on a multi load feeder cable? Everything I know about ferresonance involves single phasing at the transformer primary itself, but I have never heard of it on a single phased (blown fuse) feeder.

A light load on the 3 phase transformer is an important condition for ferroresonance to occur, not just blown feeder fuse. Another condition is long primary feeder. A case where a 1000-kVA transformer was replaced three times in five years before the utility realized a low-level resonant circuit – yes, without an open phase – was killing the transformer every night when the commercial building load dropped to about 6 percent of the transformer rating during kWh saving night lights and low AC.

 

[mivey]

One concern I have and I am hoping if you could shed any info: what are the odds of developing ferroresoance on a multi load feeder cable? Everything I know about ferresonance involves single phasing at the transformer primary itself, but I have never heard of it on a single phased (blown fuse) feeder.

None that I know of but I think about it with lightly loaded transformers and higher voltages.