It’s 10 a.m. on a weekday. The lights in your building start flickering and then suddenly go out. You smell smoke and immediately call the fire department. After the firemen put out the fire in your service equipment room, you ask yourself, “What just happened?”
The clues offered in this gallery of photos reveal the root cause of this catastrophic electrical equipment failure and the resulting fire that ensued. Each image helps reinforce the need to properly inspect, operate, and maintain the equipment under your control.
Upon initial investigation, you see the 400A main breaker (on left) is fried. The service conductors are melted right through the steel breaker enclosure and front cover. All of the service conductor insulation is melted away, and only bare and burned 500kcmil copper remains. As you look at the charred ¾-in. plywood behind the main breaker you realize how lucky you were that this incident happened during normal business hours so you could quickly react to the fire.
Damage Could Have Been Much Worse
The ¾-in. plywood caught on fire from the intense heat produced by the short‐circuited internal breaker components. Additional heat was generated by the phase‐to-ground and phase‐to‐phase faults that ensued after the conductor insulation melted. With no conductor insulation, the copper conductors pressed against the enclosure top and front cover. Note the other wood framing members in this electrical room. Black soot from the fire can be seen on the 2x4 framing members and plywood ceiling above the steel enclosure.
The intense heat in the breaker enclosure transferred to the adjacent panelboard enclosure. You can see the scorched paint on the left interior wall of this enclosure. An unused red conductor in the panelboard was in contact with the side wall and melted on the steel. If you look closely, you can see the mark left on the enclosure after the conductor with melted insulation was pulled away.
Melted Cable Insulation in Adjacent Switch Panel
Upon further investigation, you examine the 500kcmil conductors in the through‐the‐wall pipe nipple that feed a 400A switch, which serves as a second service entrance for the building. The service conductor insulation in the nipple is also melted together. So how did all of this happen?
Pole-Top Source of Power
We start our investigation by reviewing the service and service equipment layout. The building is fed by a 208Y/120V, 3-phase pole‐mounted utility transformer arrangement. Each transformer has its own pole‐mounted fuse for each of the three utility phases. The service lateral consists of three 500kcmil phase conductors and a reduced‐size neutral. Most of the service equipment is 65 years old.
Ground Fault Current Flowed Through This Main Breaker Panel
The service conductors enter the bottom of this enclosure (lower left) and are connected to the three terminals at the top of the switch. A second set of phase conductors is tapped from these same terminals and feeds the line‐side of the back‐to‐back 400A fused switch on the back side of this wall. The intense heat from the shorted and grounded phase conductors transferred enough heat to the phase conductors in the through‐the-wall pipe nipple to melt all of the insulation together in the nipple (see previous slide). Although no fault current was actually flowing in the tapped phase conductors in the nipple, ground fault current was flowing through the steel enclosures as it tried to find a circuit path back to the electric utility source through the bonded neutral.
Energized Bare Conductor Burns through Top of Steel Enclosure
When the line‐side service conductors overheated from the short circuit within the main breaker, the conductor insulation melted. The residual bending stress in the conductors forced the conductors tight against the top of the enclosure and the front cover. The C‐phase conductor ground faulted to the enclosure top while the B‐phase conductor ground faulted to the enclosure front cover and melted completely open. When both conductors contacted the enclosure steel, a phase‐to‐ground‐to‐phase fault was established.
Failed Arc Chute Plates Lead to Phase-to-Phase Faults
It is clear in this photo that one or more of the arc chute plates fell across the movable contact of the B‐phase. The B-phase arc chute steel plate is severely arc‐damaged from making contact with the A phase. It looks like the cross‐connection occurred through the notch in the insulated housing. This notch is where the operating shaft connects all three movable contacts to the breaker trip mechanism. The A‐phase movable contact assembly is severely melted away. A similar shorting occurred between the B-phase and C-phase.
Severe Arc Flash Damage on C-Phase Contacts
This is a close up of the C‐phase movable contact for the breaker. The left main contact appears to be missing its precious metal contact pad. The spring for this same contact appears to be broken. It is unclear if the contact pad unsoldered itself due to the intense heat created by the arcing. It is possible that the missing contact pad and broken spring pre‐existed and were contributory to initiating the breaker meltdown. The triangular contact is the C‐phase arcing contact. It “makes” before the main contacts close and “breaks” after the main contacts open. The arcing that occurs during opening and closing of the breaker is designed to occur on the sacrificial arcing contacts and not the main contacts of each phase.
Molten Metal Drips Down from Above
Molten copper and brass dripped down within the breaker housing. The solidified metals are seen just above the three thermal‐magnetic trip assemblies. The arc chute plates shorted the breaker internally near the main contacts. All the fault current flowed upstream of these current sensing elements. The breaker was not able to sense the overcurrent conditions from the shorting. The only remaining overcurrent protective devices for this service were located on the electric utility pole. The utility’s fuses provide primary fuse protection for its three pole‐mounted transformers. These fuses allowed the maximum transformer capacity to feed into this fault before they melted opened.
Load Side of Breaker Enclosure Suffers Less Damage
The bottom area of the breaker enclosure was away from the zone of intense heating. The intense heating occurred near the breaker contacts and the grounded phase conductors at the top of the enclosure. You can see that the conductor insulation is intact on the breaker load‐side conductors. The grounded‐conductor insulation in the forefront is intact (and not melted) until it passes to the left side of the breaker housing where arcing occurred within the breaker housing.
Two for the Price of One is Not Always a Good Deal
Here’s a close up shot of the line‐side terminals on the failed main breaker. You can see on the left-most terminal, some of the conductor strands have been cut to allow them to fit in the terminal clamp hole. It is not likely that this terminal was designed or rated for two 500kcmil conductors. If it was rated for two conductors, there would be no need to cut the conductor strands to make them fit. You can see severe discoloration of the copper due to the overcurrent and overheating.
Improper Use of 400A Switch
This photo shows the lower section of the 400A switch that is tapped from the load side of the 400A main breaker that melted. This switch is not a service disconnect and is not classified as service equipment. It is a feeder switch. The neutral of this feeder switch should not be bonded to the steel enclosure. Since this switch neutral was incorrectly bonded to the enclosure, when ground‐fault current flowed on the main switch enclosure, it also formed a parallel fault‐current path through the feeder supply conduit, switch enclosure, back to the improperly bonded neutral conductor, and then back to the utility source. The fault current flowing in the enclosure heated the steel up enough to melt the sealing compound in the screw holes of the black insulation board. The melted, dripping sealant is visible in several places in the photo.
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