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.