Old, But Used as Originally Designed
The original Class L fuse blade is bolted to the fuseholder with two ½-in. diameter bolts with washers, which distribute the clamping force. Notice the large thickness and cross-sectional area of the copper bars on the fuse and the switch fuseholder connections. The copper is sized for the 800A rating of this open-type, 3-pole knife switch. The manufacturer’s fuse mounting instructions specify that stainless steel bolts, spring-washers on each side of the bolts, and nuts must be used. The nuts must be torqued to ASTM standards for the bolt size used.
Don’t Look for This in the Big-Box Store Electrical Aisle
Look at the sum of the parts used to adapt a Class H fuse to a Class L, bolted-mount fuseholder. The Class H fuse rating is 600A, 250V. Class H fuses typically have only a 10,000A interrupting rating. The lugs are rated for only 400A at 90°C. The uninsulated, stranded conductor is tinned, 600kcmil copper. The lug-to-fuse-blade bolts are 5/16-in. diameter with washers only on the nut-side of the connections. Compared to all the work it took to assemble this, it would have been much easier to just install the right Class L fuse in the first place.
For business continuity, it is important to always have spare fuses onsite. This is especially true if the fuse types or amp sizes are not common or readily available during an outage.
Close, But No Cigar…
This Class H fuse does not have the same fuse characteristics as the original fuse. The original current-limiting fuse had a time-lag feature designed to allow for high inrush currents without blowing the fuse. The non-current-limiting, Class H fuse typically has only 10,000A interrupting rating compared to the typical Class L with a 200,000A interrupting rating.
NEC Art. 100 defines the interrupting rating as: ”The highest current at rated voltage that a device is identified to interrupt under standard test conditions.” Using a fuse with an interrupting rating less than required can cause the fuse to explode and produce significant fault let-through current. Current-limiting fuses typically open and clear a short-circuit in less than ½ cycle. Non-current-limiting fuses typically open short-circuits in about one cycle.
Wrong Fuse, Wrong Lug, Wrong Bolt, Etc.
Note the black discoloring of the copper lug. Compared to the original bolted fuseholder connection with two ½-in. bolts, can a single 5/16-in. bolt with a washer on one side only be an equivalent current capacity connection?
NFPA 70E, Sec. 225.1 states, “Fuseholders for current-limiting fuses shall not be modified to allow the insertion of fuses that are not current-limiting. Non-current-limiting fuses shall not be modified to allow their insertion into current-limiting fuseholders.”
Section 210.3, states, “Current-carrying conductors (buses, switches, disconnects, joints, and terminations) and bracing shall be maintained to perform as follows: (1) Conduct rated current without overheating; (2) Withstand available fault current.”
Three Strikes You’re Out!
Not only is this set screw clamp misaligned, but it's also cross-threaded. Unqualified persons installing electrical equipment in a non-workmanlike manner further increase the likelihood of electrical arcing, equipment damage, building fire or electrocution. NEC Sec. 110.12, states, “Electrical equipment shall be installed in a neat and workmanlike manner.”
The “Little Details” Matter
Attention to installation details is critical for safe and reliable electrical installations. This improvised fuse installation shows the lug not fully inserted under the set screw clamping point. Had this installation not been discovered and eliminated, this improper assembly could easy have developed into a high-resistance, critical failure point in an already overloaded, makeshift installation.
Overloaded Electrical Equipment Spells Trouble
The improvised fuse installation uses a set screw lug that is only rated for 400A at 90°C. Note the comparatively thinner copper cross-sectional area of the fuse and the lug compared to the Class L fuse. The single, undersized bolt connection does not hold the two copper mating surfaces in close contact with each other.
Improper electrical connections or lack of electrical equipment inspections are a common source of overheating and equipment failure. This is true even on other existing, NEC code-compliant installations.
NFPA 70E, Sec. 225.1 states, “Fuse clips shall be maintained to provide adequate contact with fuses.”
Overheated Electrical Equipment Reduces Lifespan
The lug connections at the Class L fuseholder show obvious signs of copper discoloration and overheating. The 400A at 90°C rated lugs are used with a 600A Class H fuse. The lugs are operating at 200A over the maximum product rating.
The conductors connected to the load side of the switch have old rubber covered insulation that is not rated for 90°C. The excess current and overheating are accelerating the deterioration of the old rubber conductor insulation. The excess heat generated by the improvised fuse installation flows to the connected copper conductors which act as heatsinks.
Remember to Read the Instructions!
NEC Sec. 110.3(B), requires “listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.” This Class H fuse blade was designed and intended to be installed in electrical equipment with compatible Class H fuse clips or clamps. Field modifications, such as drilling a 5/16-in. hole in each blade for a direct cable lug connection, would not comply with the intended installation and use of this fuse. This arbitrary modification and use has contributed to the overheating that is visible at all four of the lug connections.
You Won’t Know If You Don’t Inspect Periodically
This open-type switchboard is already dangerous due to the lack of a modern-day metal enclosure for the switchgear. Adding the perpendicular 600kcmil cable extensions and the “floating” Class H fuse moves the fuse farther away from switchboard. This reduces the safe clearance dimension of energized parts to personnel in the aisle. The projecting fuse makes it more likely that personnel may come in contact with energized parts when in the aisle. NEC Sec. 110.26(A) and Table 110.26(A)(1) require 36 in. minimum of clear distance from energized, 208V, 3-phase system with 120V to ground. This dimension remains the same for all three exposure conditions defined in this section.
It is also clear in this photo that there is an accumulation of dust and dirt on the open switchboard equipment. Dust, dirt, humidity and other airborne contaminant can cause unwanted conductive paths around insulated parts. Arc-tracking and flashover events are possible in extreme contamination situations.