Fused Disconnect Wiring 3

[cslater]

For a fused disconnect at 208V, there are two sets of lugs. One side is labeled "line" and it has the disconnect portion of the switch between it and the fuse. The other is the "load" side.

I was shown an installation where the two were reversed so that the fuse is still connected to "line" one one end, even when the switch is turned off.

While I know that it is best practice to install these properly, I'm wondering - is this a Code violation?

 

[jfpe]

cslater,

Besides being awfully dangerous, it sounds like a pretty clear violation of the National Electric Code. Here is the relevant section.

110.3(B)Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling"

 

[gattie]

It might be that the switch is not rated for the system's prospective fault current, & the fuse is used to protect the switch.
Not too Kosher, unless there are upstream means to facilitate fuse replacement!

 

[waross]

Another point to consider, a knife switch should not be installed upside down. There is a code requirement that vertically mounted knife switches must be mounted so that gravity will tend to open the switch.
If the switch is not rated for the available short circuit current the fuse clips will not be rated for the current either. The fuse clips may not be shown to be tested or rated for fault currents. This is not an issue of interrupting current but a mechanical strength issue. The fuse holder mounting may not be able to withstand the mechanical forces generated by the magnetic field if the current is several times greater than rated current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I've seen 2 types of fusible disconnects.

The first has a "knife" type disconnect with the fuses following. Sometimes the knives are partially visable. The fuses are directly connected to the load terminals.

the second is a molded design with the fuses plugging into the top. These generally disconnect both the line and load terminals from the fuses and they could be rated for being fed either direction.

It's a pretty crappy fusible if it's fault withstand rating doesn't match the fuses it was designed to work with.

 

[jraef]

Disconnect switches have two fault duty "ratings": An "interrupting rating", which is dependent upon the fuse used in it, and a "withstand rating" which is not. The withstand rating is usually much much lower, i.e. 5kA or 10kA if there is no fuse involved, but the interrupt rating is typically the same as the fuse, i.e. 100kAIC or 200kAIC. I don't know of anyone that makes a switch that is not rated for the fuses you would put into it, makes no sense.

I also do not know of any fused disconnect switches that are restricted from being back fed. They all have door interlocking devices that prevent the door from being opened with the switch on, which forces the user to open all poles before replacing a fuse. The only way to have an arc flash when removing a fuse then is if one or more poles of the disconnect were welded closed into a fault, in which case one would hope the fuse had cleared! But it would be an issue when re-installing a new fuse (assuming one were stupid enough not to figure out why it had cleared and fix it first, but that happens ALL the time). I'm pretty sure however that the door interlocks are tied to the operating mechanism, not the handle, so if contacts in the disconnect were welded closed you would not be able to release the door in the first place. I have a unit here, I am going to check that out later today.

But that's not an endorsement of the practice. I personally would never do it. When you pull a cartridge fuse you almost never pull both ends out at once, it's usually one then the other. If you pulled the load end out first and something slipped, you would have a hot ferrule in your face. I don't like it.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[GTstartup]

Very very dangerous. Came across this once on a 480V roof fan fused disconnect. Even though the disconnect was open the fuses still had 480VAC. My multimeter and good work practices saved me. Easily could have assumed tht since the disconnect was open that it was dead.

 

[waross]

Canadian Electrical Code.
Section 14 Protection and Control
Control devices
General
14-402 Disconnecting means required for fused circuits.
Circuits protected by fuses shall be equiped with disconnecting means, integral with or adjacent to the fuse holders, whereby all live parts for mounting fuses can be readily and safely
made dead:.... some exceptions for instrument transformers and some plug fuse applications.
Any similar rules in the NEC?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Yes, NEC 240.40 requires a disconnecting means on the supply side of the fuses.

Rafiq Bulsara

http://www.srengineersct.com