VFD overcurrent protection device

An overcurrent protection device is required in the AC supply to the VFD. The purpose of this device is twofold:

• To provide thermal protection for the VFD cables etc between the location of the overcurrent protective device (usually at the origin of the relevant supply sub-circuit) and the VFD. This is predominantly a measure to prevent injury and property damage from melting and fire.
• To limit the energy available at the location of a short circuit or near short circuit in the unlikely, but possible, event of a major arcing fault in wiring or within the VFD enclosure. This measure is to control the risk of personal injury and property damage due to arc flash, conductor erosion, explosion and the like.

The first requirement is relatively slow and normally provided by fuses or the timed (traditionally referred to as thermal) characteristic of a circuit breaker. Note that the protection offered by this device on the AC line side of the VFD does not extend to the output (motor) side because the AC line side current may be considerably less than the VFD output current when operating at less than full speed. This is a consequence of the high efficiency of the VFD and the power required by the load being a product of torque and speed, the AC input power (and current) reduce with speed, even if the load torque remains high. The VFD itself provides both timed overcurrent (I²t) and instantaneous overcurrent protection for the output wiring and motor.

The second requirement may be met with either the instantaneous trip function of a circuit breaker or a fuse. The total amount of energy let through in the event of a short circuit or near short circuit event is usually the critical factor in determining the injury risk, extent of physical damage and consequently the time and expense involved in repair. The let through energy may be accessed in terms of the I²t (time integral of current squared) let through the protective device in the process of interrupting the fault current. In order to minimize the I²t let through and the associated risks of injury, property damage and downtime, we recommend the use of appropriately rated current limiting3 type fuses. In some circumstances, the user's protection needs may be met by a suitably selected circuit breaker however, we strongly recommend that any such selection be based on detailed engineering evaluation and not simply a catalogue selection.