Can't say I agree. I deal with plenty of stuff that is sufficiently old that I have to deal with a rebuilder. I do all I can to pick good ones. No generally not with 15A qob.
Well if you know the history of the equipment its generally a none issue, but I was thinking more along the lines of buying second hand or reusing something that has been pulled out of service. My view is use new as much as possible, the less its been handled the less chance of the unforeseen lurking by.
Don't agree. If coordination is an issue - CBs, electronic trips are likely mandatory. Mains, feeders, ansi damage points (xfm), cable damage curves, overload curves, motor starting curves. Get them pounded on to one T-C coordination curve and sometimes the contortions to get coordination is severely interesting. I recall one noted authority saying, "You can coordinate anything - It's just a matter of money."
Your right more than me:happyyes:
In complex scenarios micro processor electronics is the only way. With electronics almost any type of trip curve is possible, interlocking relaying, SCADA, along with directional relying if needed. With enough computer logic, and of course money coordinating any setup is possible.
Although, am I correct to say, where none adjustable or minimally adjustable breakers are present in 600 volt and under radial circuit applications: fuses tend to be more accurate, and easier to coordinate? Yes microprocessors will always win, but some applications can fair without them. Fuses over a standard thermal magnetic breaker can be made easily to be more accurate?
By far, price wise fuses are the cheaper option when electronics can be made without.
Just for kicks, kind of going along with the top kind of not but just thinking of a known example where fuses win for economy reasons.
In small to medium sub transmission to distribution and small transmission to distribution substations protecting step down transformers with fuses is more attractive then with relayed breakers for economy reasons.
A common example is a 34.5kv to 13.8kv step down substation with a 10/12.5 MVA transformer. The economy reasoning holds true up to 40MVA units though usually less so.
The transformer can be protected by a breaker in the substation or a pole mounted 35kv rated recloser with a pole mounted micro processor. In the breaker scenario cts are placed on the transformer bushings along with a control hut house containing batteries with protective relays. They protect the transformer from damage, the physical substation and sub transmission line just from monitoring the transformer. Differential logic, over pressure logic (if added), temperature (if added), low oil (if added) time over current and definite over current logic is the most common used. Differential logic disconnects the transformer when turn to turn or turn to core faults are sensed, oil and temperature same scenario. Over current logic is used for 2 scenarios. One being the rare event the transformer fails from an internal bolted fault the other being a secondary buss fault or a distribution line fault where the secondary 13.8kv substation feeder breaker/recloser fails to open. Both these scenarios must be disconnected since they can ultimately overheat the transformer to violent failure eventually taking out the 34.5kv sub transmission line with it. The breaker feeding the sub transmission line at its source substation 138kv to 34.5kv will in nearly in all cases see the bolted fault on the secondary of the 10/12.5mva transformer as simple heavy load current and will trip well after the transformer's damage curve is exceeded or not at all if this line feeds other substations which is usually the status quo. Because of this its also a good idea to have back up relaying in case the primary breaker protecting the 10/12.5 MVA becomes stuck. Ie, if protective relaying senses a transformer or buss fault but the protective breaker fails to open within a certain amount of cycles a transfer trip signal is sent to the source substation to trip the 34.5kv breaker feeding the sub transmission line. The 35kv recloser option can be configured to do similar things (minus the oil, temperature and differential monitoring) via ANSI and IEC trip curves based around the transformer damage curve. If the transformer is a delta primary some level of primary turn to ground fault protection within the transformer can be used through deferential (GFI) logic. Both recloser and breaker can easily be configured with SCADA for remote trip and monitoring.
However both have a major downfall. Both the recloser or breaker require batteries, cabling, enclosures, auxiliary power, poles/station hardware. Both have the weak point if the auxiliary power source fails (such as the 10kva station service pole pig fails) the batteries can run out unnoticed if there is no SCADA, so expensive SCADA begins to look more attractive. Both can fail from a number of electrical or mechanical contingencies. Both require maintenance, and because of this both raise the cost of a new substations dramatically as well as the in service life.
Its for this reason many pocos find it economical to protect transformers below 20MVA that feed distribution or large customers via power fuses. While a power fuse may not give differential protection or remote SCADA tripping it will provide exceptional over current protection that is fail safe. No auxiliary equipment or external power is needed. Fuses are sized to protect the transformer from both overload and bushing/buss faults and prevent a hard faulted transformer from taking out a line. If the fuse is appropriately chosen to handle secondary feeder trip curves along with auto reclosing attempts if an MV overhead system is in use the fuses will never nuisance blow. Key is however not to come close to the melting point because if you do the first few faults will not blow it but the next one will since the fuse has already been severally weakened.
Just though you would find it interesting where fuses do win in economy :lol:
Yes,
Yes,
and Yes
Yes, as long as one sticks to standard stuff, GEthqb, SQDqo, molded case
Some curves are really strange:
Inverse (normal)
very inverse
extremely inverse (might be wrong on this one)
i^2t
i^4t
LSI
LS (no I)
LSIG
And sometimes these are what it takes to get bent around so one can get coordination.
However, my point was about things like unlimited outside taps. The OCPD at the end of the tap should protect against overload - all the electrons going in from the xfm end have to come out the ocpd end.
Invariabily some one will ask so what happens if the xfm secondary overload in the middle and it doesn't show up at the ocpd. That is a backhoe attack. The protection against that is concrete and steel - not an ocpd.
Buss protection differential logic where the currents compared at the xfm primary and the sum of all the secondary ocps is compared. Relaying looks for the extra energy. Ok I know not always used.
Motor feeders, overload is set to 140%. CB is a T-M set to 250% fla. Cable protection looks pretty marginal. If this was any size, I'd be lookin for the cable damage curve.
The rest is design philosophy. I consider the motors and conductors are protected from overload by design. One protects the cables with concrete and steel. The CB is to put out the fire and save the structure - and generally can also protect a cable
Just some thoughts - I'm not stuck
ice
