Compare equipment nameplate information, including CT and PT ratio, fuse sizes and communication links with the latest one-line diagram and report discrepancies. If the nameplate is damaged/missing, install the properly labeled engraved plastic nameplate, using rivets or screws.
A Circuit Breaker (C/B) is a mechanical device that will involve a magnetic solenoid and bimetallic strip inside. When the strip heats up due to current, it will bend and activate the solenoid to trip the circuit breaker and stop the flow of power. A Fuse is a type of insulating material with a filament in the middle of it. When the fuse starts to heat up due to current, the filament will break, breaking the flow of electricity in the circuit.
Over the years major manufacturers have removed internal semiconductor fuses to cut costs. Most are recommending standard circuit breakers or fuses recommended by NEC for protection, again to minimize installed costs. While this will protect the circuit and the motor, the internal components of the VFD are unprotected. Most VFD failures occur from overvoltage and transient voltage conditions. Left unprotected, this will result in a shorted IGBT. IGBT failures usually lead to rupture and melting of bonding wires, also damaging other components, such as capacitors. Only a high speed fuse can prevent this.
what makes IGBTs fail and so violently rupture? Many VFDs are not protected as well as they could be. Drives run successfully for years but then endure a serious fault that circuit breakers are not fast enough to respond to. High speed VFD fuses can be critical to reliability.
Many drive manufacturers have removed semiconductor fuses from VFDs and provide protection guidelines per UL standards. There are multiple levels of short circuit testing and protection; and the manufacturer makes that choice. The question is could you get even better? In many cases, there are options to improve short circuit protection beyond that of the manufacturer recommendation.
A Short Circuit Study is an analysis of the protective devices of one's electrical system, such as fuses, circuit breakers, fused switches, or any other devices intended to protect the equipment or isolate short circuits. An SCS is needed to ensure that during a fault or short circuit, that the protective devices are not subjected to mechanical or current levels that they are not designed to withstand. If this happens, they might not open, explode, or some other undesirable result. It helps ensure that you are operating within the manufacturer’s tolerance for the devices.
Traditional fuses are designed to limit threats to people and property by protecting your electrical system from overheating, fire and arc flash. But increasing safety requirements have created an immediate need to reduce the potential for arc flash even further. OSHA, for example, uses the new requirements of NFPA 70E to judge whether an employer “acted reasonably” in protecting its workers from these arc flash hazards.