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
The difference between genius and stupidity is that genius has its limits.
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.IF the fault is in the cable
a UG cable fault is far less likely than an OH one for obviously reasons
Thats true- but is such speed and accuracy really needed all the time?a relay will respond the same
better flexibility, accuracy, repeatability
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?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
Substation breaker is always a breaker- but down stream there is lots of room to chose.
I'm in over my head...
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
The difference between genius and stupidity is that genius has its limits.
Yes- only because you stated the obvious and I was basically agreeing.
You have never used MHO elements at MV? It can be done.????
In most cases you can.yes
unless you can tolerate damage
For smaller guys it varies- as does by POCO.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?
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?
I'm in over my head...
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