Good luck getting information. You'll need it. I agree with dpc. Many people forget that interrupting rating is only one type of fault current rating for electrical equipment. The typical unfused disconnect switch is rated at 10kA Momentary, per just about every manufacturer I have inquired about. The solution was to install fused instead of unfused disconnect switches. Suggestion: Molded case switches can be used as disconnect switches.
Therefore, molded case switch interrupting ratings are the same as for the MCCB of the same type or category. Fused and non-fused disconnects have their interrupting ratings and other applicable ratings, e. I don't believe molded case switches are available with UL labels. Fused and non-fused disconnects do not have the same interrupting ratings. The interrupting rating of a fused switch is based on the fuse interrupting the fault. The interrupting rating of a non-fused switch is based on the switch contacts, and is usually the same as the continuous rating there is almost no chance of a disconnect switch opening at the exact time of a fault, since it has no fault protection.
Again, this is the difference between 'interrupting' ratings and 'momentary' ratings. A fused switch with current-limiting fuses has a high momentary and interrupting rating since it will interrupt a fault. A non-fused switch does not have high fault ratings because it cannot interrupt a fault. If the fused disconnect has low or non-rated fuses, its interrupting rating will be low, also.
The fused or non-fused switch has its interrupting rating. If the fused switch closes, it may have to withstand A rms to allow the fuse to clear that current. The switch cannot collapse because of electromechanical forces during the short or closing into short circuit.
Larger, MV switches will have several ratings, and if they have an interrupting rating, it will usually be the same as the continuous rating. Just because a fuse is rated for kA does not mean that anywhere near that amount of current will flow in a fault since the fuse is current limiting.
One thing is the common sense and another thing is the professional engineers responsibility. Interrupting ratings are reserved for Low Voltage circuit breakers. Except, there is a very short segment in the switch between the upstream switch and fuse line terminal where the short circuit current may materialize. If there is not current limiting fuse upstream, the switch may experience kA for that fault location. Some brush up on LV and MV voltage interrupters would do a lot of good.
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Join Eng-Tips Forums! Join Us! Since a disconnect is almost always required, this is a very convenient place for the fuses to be placed. Since fuses are not always required at the disconnect, the disconnect and fuse makers often sell solid metal blanks to use in place of fuses. Fuses have two advantages over circuit breakers. Fusing and conductor size Fusing and conductor size OK, Say you have an air conditioning unit with a dataplate showing Min. It is a dedicated circuit, presently with a 40 amp breaker and 10 wire.
If I install a fused disconnect at the unit with 35amp fuses, can I leave the 40amp breaker and 10 wire? Location Louisville, Kentucky. OK, Say you have an air conditioning unit with a dataplate showing Min.
Click to expand Jim W in Tampa Senior Member. Location Tampa Florida. Often it may be part of the specs even thow its not needed. Often you will not know what the equipment will say till it arrives. In residential i have never seen it needed but commercial yes. DaveBowden Senior Member. Of course, the Bussman rep will have a strong, heart-felt opinion that a fuse can do just about anything. Quote: i was told by the Bussman fuse rep that this is can be used as secondary protection in case the overloads fail.
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RE: when to use a fused disconnect If the circuit is protected upstream by a circuit breaker or another fuse, then the general rules of the NEC would not require another fuse at a local disconnect. RE: when to use a fused disconnect Thanks. RE: when to use a fused disconnect Probably not, but you need to look at the recommended installation information from the fan manufacturer. RE: when to use a fused disconnect Watch the ratings of non-fused disconnects.
RE: when to use a fused disconnect jraef First of all I am a EE and second of all if the overloads fail in the motor there is a chance for a short circuit due to overheating and deterioration of insulation which can result in short circuit.
RE: when to use a fused disconnect Since you are a EE, check with your buss fuse manufacture representative for a book that they hand out free explaining the use of the fuse and non fused disconnect usage and code compliance.
RE: when to use a fused disconnect i was told by the bussman fuse rep that this is can be used as secondary protection in case the overloads fail RE: when to use a fused disconnect My understanding is that fuses are acceptable motor overload protection per NEC if sized appropriately. RE: when to use a fused disconnect Quote: i was told by the Bussman fuse rep that this is can be used as secondary protection in case the overloads fail In my opinion, that is an irresponsible statement on the part of that Bussman rep.
I have never heard or seen Bussman promote that concept, it would be opening them up to a lot of liability. The restrictions on fuse sizing along with the necessity for allowing motor starting without nuisance clearing precludes the use of using fuses as OL protection in all but the smallest of motor loads.
Just because the NEC says you are not restricted from trying something doesn't mean that it will work! The fuse recommended for providing SC protection is a A dual element time delay. A Class 10 I 2 t overload relay should have dropped out the starter in approx.
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