Accident and Failure Analysis
Often the process of trying to figure out what went wrong and what can be done to avoid the same thing in the future require in depth engineering analysis without the preconceptions that may have led to the incident in the first place. Often, the additions of circuits and systems to protect people and systems can turn on us and become key to the hazard. Here are a real of examples of incidents we have investigated:
A security gate installation uses an external telephone line to allow visitors to call the landowner and gain access regardless of where the owner is. This telephone line has surge protection devices to a ground connection solely for the purpose of protection of the system from surges on the telephone line. Also at this sub assembly are connections to the gate actuator and a power supply. The installation does not have the ground properly installed since it does not seem to effect operation. A fault in the power supply fires surge devices on the supply lines linking mains voltage to the unconnected ground. This fires the telephone line surge devices and connects the mains voltage to the telephone line through the signal common since the ground does not provide adequate load to shunt the voltage to ground. This situation then destroys the gate actuators and leaves a hazard to anyone trying to actuate or repair the gate or telephone line.
A dump truck leaves a gravel pit before its bed is fully retracted. It clips and breaks the bottom wire on the power poles supplying rock crushers back at the pit. This causes an immediate call to the power company to restore power to the facility on Saturday. The phone company does not take action until Monday from non working phones and no linkage is made since the phone service is buried. What happened is that the bottom wire on the poles is the grounded return for power going to the facility and high voltage was applied to the common ground at the facility. Since this was rocky soil, the ground rods provided little ability to carry the power. The next possible path was the grounded telephone cable sheath. This was not designed to carry this kind of load and the voltage on this rose enough to fire the voltage telephone surge protectors and permanently cause them to connect the ground to the telephone lines. The power ground was fixed but due to the load of rock crushers at the facility and the remote distance, it had a substantial voltage drop. The telephone lines now carried this voltage the distance back toward the telephone office. On Monday, the telephone repairman opened a pedestal some distance from the facility and made contact with the telephone wires in the process of diagnosing the problem with them. With his knees on the damp grass and hands in contact with 180 volts, he died.
A telephone switching system in a hospital has a battery backup power system. This system also supplies power to cooling fans in the system through a DC to AC inverter. There are redundant fans to ensure cooling even if one has failed or frozen up. What was not anticipated was the extra load from a frozen fan. During a power outage at the hospital, the inverter transformer supplying the cooling fans overheated and spit fire. The inverter kept powering the fans for some time. Inside the cabinet, a plastic safety shield covered the back of the mother boards to prevent contact to ring voltage. The flame coming from the transformer ignited the shield and the fans turned the entire cabinet into a blow torch. The last part of the system to be destroyed by the fire was the inverter and cooling fans fanning the fire. The fire systems in the hospital prevented spread beyond the telephone room.