Burnt cable insulation causes damage to store's electrical equipment just before its grand opening. After much effort, its replacement needs a replacement.
After the fire, smoke poured from the electrical switchboard at a large food shop, which was scheduled for its grand opening only two weeks later. The food shop, part of a chain of over 60 units, was one of 20 other stores in a shopping center. The advertising campaign was already in progress, and the shop managers were already training their new employees. The insurance adjuster and others wanted to know what happened, and what was necessary to get the food market ready in time for operation.
An agitated insurance adjuster called us, asking if we could send an electrical engineer to the shopping center: The food market's business interruption insurance policy was adding up.
When I arrived at the site, there was a lot of cleanup activity. The fire department put out the fire the day before. They had sprayed water all over the rear of the store as well as on the mezzanine that extended part way across the back of the store. Water was still running out of the back door. Workman were sweeping up debris and removing water-soaked cartons of food. The affected switchboard was in a rear corner on this mezzanine.
As I approached the switchboard, I found it and the surrounding area covered with soot. The store manager reported only that the rear of the store was engulfed in flames, and there was a lot of smoke. When I opened the panels to the switchboard, the interior was covered with soot. I wiped off the soot from some of the contacts still in the open position. The contacts seemed pitted, while the coverings to the damaged coils were also heavy with soot.
Smoke from electrical fires are caustic, containing acids, carbon particles, and other damaging components. This was the condition in 80% of the switchboard. Before determining the full extent of the damage, we had to learn what caused this event. I walked around the rear exterior of the store and noticed a pad-mounted transformer in the back, installed for feeding the food market's loads. The transformer was sitting on its pad at an angle with a dent in its casing. Possibly, a truck had hit it when backing up, causing the transformer to be out of alignment.
Since the complex was soon to open, the local utility had previously completed its installation around the rear parameter, installing pad-mounted transformers at various locations to serve power at other stores. The food market had the power turned on about two weeks earlier. Installers had not completed the power supply to all the refrigerant systems before the fire. Therefore, they had not yet energized some of the switchboard's contactors.
In southern climates (where this incident happened), the electrical design and supply voltage usually relate to the size of the air conditioning compressors. If they use 50 hp motors and larger, then 277/480V is the usual supply service. This makes the conductors smaller than if a lower voltage was used. Here, the largest compressor motor was 25 hp, and the switchboard was fed using 120/208V, 3-phase, 4-wire conductors, making the conductors somewhat large; so they could handle the additional current.
The feeder ran from the secondary 120/208V side of the food market's transformer (there was no secondary fusing) via underground conduit to a 36-in.-sq26-in.-deep pull box, and vertically to the pull section. Well grounded, the conduit to the building was concrete encased steel, due to the need for high compressibility because of truck traffic in the parking lot. This, with the connection to the switchboard busway, created a total run of about 60 ft long.
We learned a truck had backed up into the pad-mounted transformer, moving the transformer enough to stretch the cables, and damaging the terminals enough to create a short circuit. The short was either intermittent, or just sufficient enough to cause excess current flow, frying the cable insulation. There was a lot of insulation to burn, and more current flow compared to a higher-voltage system. The primary protection ahead of the transformer wasn't affected by the intermittent secondary short circuit.
This was an odd combination of circumstances. When the cables were removed, we found the insulation was about 25% damaged. The smoke from the burning insulation was enough to travel through the conduit, into the terminal box, and up into the switchboard, the latter serving as a chimney.
We were then asked to continue on the job and provide assistance in the repair operation. We accepted this assignment, deciding to do so with the cooperation of the electrical contractor who worked on the job. We decided that replacing the switchboard was probably a less expensive approach for providing electrical service than to try to clean up and replace all the damaged parts. The latter option might have presented future problems.
Fortunately, the manufacturer of the switchboard was 40 miles from the site. We contacted them, explained the situation, and they offered to provide another switchboard. However, due to their workload, delivery would take 4 months. What were we to do?
The contractor had a large operation and he bought a lot of electrical apparatus from this manufacturer. We had also been a large specifier of this manufacturer's equipment. We both knew the same salesman employed with the manufacturer for over 30 years. The contractor persuaded the plant manager to insert our order into his schedule and work two shifts to provide us with the switchboard within 10 days. In the interim, crews removed the old switchboard and prepared the site for installation of the new unit.
When the manufacturer completed the switchboard, they loaded it on a flatbed truck for delivery to the job site. The 40-mile drive was via a major interstate highway, an easy, trouble-free route. Hundreds of trucks use the route every day. However, our driver took a short cut to reduce the distance by about 10 miles. This involved using a hilly road with hairpin turns.
The driver took one of the turns too fast, overturning the truck and its load into a canyon. Fortunately the driver jumped clear and was only slightly hurt. The next day I went to the site of the accident and saw our switchboard lying on the canyon floor in parts.
The adjuster, store management, and now the manufacturer of the switchboard were unhappy. The adjuster had to pay all costs associated with this event. Then he subrogated against the trucking company, and we were involved with the legal aspects.