When is disconnect required to be fused?

[JATorres]

There is this industrial building where non-fused disconnect switches were specified and installed within sight of electric motors (pumps). Each pump is fed from motor control centers and protected with either MCBB or MCP.

The purpose of the disconnect is to service the equipment (pumps). Obviously, the disconnect will not open under short circuit conditions. Per NEC-110-9, as long as the switch is rated for the motor FLA or HP the installation is safe. As you can see, NEC does not require the non-fuse disconnect to have a fault current listing (which is the case, because a disconnect with no fuse has no kAIC rating, unless is installed in combination with a disconnect fuse upstream).

Now, a consultant for the owner says that the disconnect switches shall be fused where the short circuit exceeds 10kAIC. I don't agree. Because the disconnect is not intended to interrupt fault currents!

Do you agree?


Thanks.

 

[rbulsara]

I would agree with you. Make sure your MCCB or MCP are rated properly for the SCC.

 

[jraef]

I agree with you. If fact, adding more fuses in series to the system may create more problems that it will solve.

The solution may be to stop referring to them as "disconnect switches" and start calling them "Safety Lockout Devices". Then the consultant can get paid for his "work" and you can get on with yours without wasting a bunch of money replacing perfectly good and properly applied devices.

Sometimes its just the semantics....



Quando Omni Flunkus Moritati

 

[dpc]

In general I agree with you - the disconnect switch does not need to be fused and things are better if they are not fused. But....

The disconnect switch must be able to withstand a downstream fault without destroying itself. Unless protected by an upstream fuse of appropriate rating, the non-fused disconnect does have a very low withstand rating - typically 10,000 A. So....

A case can be made that a typical non-fused disconnect switch would be a problem when protected upstream by a standard molded-case breaker or MCP. Adding fuses does solve this problem.

 

[skiier]

9m or out of sight. That is the rule in Ontario. Fuses are not required unless your over engineering customer specs them. Then you have to keep them happy if you want eat.
Politics, I hate em. I once worked on a mschine that required about 30 isolating disconnects- or so the customer thought. Turns out all the motors in the system were out of commission if any one motor was off for maintance or whatever. I argued that we only had to lock off the main control enclosure. Saved a ton of money and it was all common sense. 3 engineers at daimler chrysler couldnt decide on common sense.geez. is your system similiar?
Anyways, if it's going to cost you fight against it politely and have them show you some rule. If its an extra charge them and be happy. You want to work for them again?

 

[skiier]

I'm sorry I missed the part at the bottom about short circuit capacity. I see what the consultant is trying to do. He is trying to limit the fault at the point of fault. A wise thing to do in a plant that has a high available short circuit current. This one is tricky but u can have a simple solution. Seach Yahoo for "short circuit calculations" and find a generic current formula for your situation. Real simple stuff-not complicated. Using rough estimates of lengths, conductor types, and available short circuit capacitys find what the fault value is at all your disconnects. Less than 10K ok. Above 10k use a fuse that falls within the range of your fault- say 50K.Have the customer specify the available fault current(this is important!)by some outside engineering source - dont let it blow up in your face.

 

[skiier]

Oh yes. A disconnect that will hold a fuse will also safely pass the short circuit rating of that fuse. Is this not true if the fuseholder is in or out? Wouldn't the enclosure that holds the fuse also be rated to mechanically withstand the forces imposed on it same as the fuse?
Wouldn't make sense if it couldn't.
Because the disconnect is not intended to interrupt fault currents! No it is not, But it must safely pass the fault current imposed on it. Hehehheh try and get this spec out of a manufacturer! Good friggin luck.
IE; chokes are used on drives to limit fault currents right?
Well try and get a spec out of a choke manufacturer. Ask them what value of fault current their equipment can safely pass without "blowing up" and they will tell you too fuse it according to their specs. Boy thats some help when a customer is asking for a documented solid number. It seems alot of manufacturers dont do destructive testing on their equipment and this leaves you in the middle of a predicament. No documentation so you can't copy the customer and get them off your back.
Allen Bradley and their drives are the same as well. Ask them what fault current level blows up their stuff and they tell you to fuse it so it won't. Again no number.
The problem here is that in high available fault current situations some damaging current values may pass thru equipment even though one thought they were protected properly. IE; a 2OOK HRC can safely limit a peak 200ka fault. But some peak let thru current value will flow that could exceed the value of some components short circuit rating in the circuit. So Allen Bradley says use an HRC fuse rated 200KA but what if the short cct capacity was such that 20KA was allowed thru before the fuse opened? Would there drive withstand 20KA? Again no answer.
Good luck getting information. You'll need it.

 

[DanDel]

I agree with dpc. Many people forget that interrupting rating is only one type of fault current rating for electrical equipment. 'Momentary', 'Fault Close', and/or 'Close & Latch' ratings are also needed to be sure that the equipment can allow the fault current to pass through, or close into a fault, without damage.
The typical unfused disconnect switch is rated at 10kA Momentary, per just about every manufacturer I have inquired about.
I know this because I ran into the very same problem that JATorres speaks about while performing a Short Circuit & Coordination Study for an industrial system. The solution was to install fused instead of unfused disconnect switches.

 

[jbartos]

Suggestion: Molded case switches can be used as disconnect switches. Essentially, the molded case switch is the Bolded Case Circuit Breaker (MCCB) without protective features of MCCB. 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.g. close and latch for voltages higher than low voltage level) the same since the switching part is the same for both.

 

[dpc]

I don't believe molded case switches are available with UL labels.

 

[DanDel]

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.

 

[jbartos]

Suggestion: Visit

http://www.uafacilities.ua.edu/const-admin/design-standards/16115.html

http://ecatalog.squared.com/pubs/Ci...itches/Door Closing Mechanisms/9420CT9701.pdf

http://ecatalog.squared.com/catalog/173/html/sections/07/17307012.html

http://ulstandardsinfonet.ul.com/scopes/1053.html

http://www.pioneerpowersystems.com/products/MetalEnc.pdf

(for non-fused switch interrupting rating)

http://www.trane.com/commercial/partsandsupplies/pdf/05Controls/RSPSLB056_042302.pdf

(for fusible and non-fusible, 10000A rms interrupting rating)
etc. for more info.
The fused or non-fused switch has its interrupting rating.
If the fused switch closes, it may have to withstand 200000A 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.

 

[DanDel]

jbartos, common sense will tell you that disconnect switches do not interrupt faults, unless you happen to open the switch at the instant of the fault. Larger, MV switches will have several ratings, and if they have an interrupting rating, it will usually be the same as the continuous rating.
Closing a switch into a fault will not require an 'Interrupting', but rather a 'Withstand'(or 'Momentary') and 'Fault-Close' or 'Close & Latch' rating.
Just because a fuse is rated for 200kA does not mean that anywhere near that amount of current will flow in a fault since the fuse is current limiting. A current limiting fuse will open before the peak current at 1/2 cycle, well before the 1.5 - 4 cycle fault value.

 

[jbartos]

Suggestion to the previous posting marked ///\\jbartos, common sense will tell you that disconnect switches do not interrupt faults, unless you happen to open the switch at the instant of the fault.
///This may actually happen if there is a short and motor contributions to it. One thing is the common sense and another thing is the professional engineers responsibility.\\ Larger, MV switches will have several ratings, and if they have an interrupting rating,
///According to ANSI C37 they have interrupting capacity. Interrupting ratings are reserved for Low Voltage circuit breakers.\\ it will usually be the same as the continuous rating.
///I see continuous rating as the steady state circuit current rating, e.g. 1200A, 2000A, 3000A, 4000A.\\Closing a switch into a fault will not require an 'Interrupting', but rather a 'Withstand'(or 'Momentary') and 'Fault-Close' or 'Close & Latch' rating.
///True.\\Just because a fuse is rated for 200kA does not mean that anywhere near that amount of current will flow in a fault since the fuse is current limiting.
///Yes, for current limiting fuse. 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 200kA for that fault location.\\ A current limiting fuse will open before the peak current at 1/2 cycle, well before the 1.5 - 4 cycle fault value.
///True. Some brush up on LV and MV voltage interrupters would do a lot of good.
Please, see for example reference:
Eaton Cutler-Hammer 2003 13th Edition, Consulting Application Guide, Solutions Book, Distribution and Control Products,
page 20.2-2
Table 20.0-1 Safety Switch Application Guide - See Catalog Selection Table for Specific Ratings
Application/Feature Type DG Type DH
Short Circuit Rating for 10,000 rms 10,000rms
Non-Fuse Switches
\\