Voltage transformers carry full system voltage but no significant current. They have a fine wire of many turns on the primary winding (very high resistance and inductance) connected between the phase conductor and earth, and a secondary winding of only…
Earthing switches are usually installed in combination with an isolator and the two should be interlocked electrically or mechanically to ensure that the isolator is open before the earthing switch is closed, and conversely that the earthing switch is open…
Isolators are simple devices with contacts invariably comprising a moving blade going into two parallel fixed contact faces. The fixed contact faces are usually made up of a number of spring-loaded fingers. The best isolators employ a roll-over action at…
The following basic types are available, typically rated at 350 or 500 MVA at 11 or 22 kV respectively: bulk oil minimum oil air break vacuum SF6 Circuit breakers are designed to deliberately create an arc, and thereafter to control…
The main four different switches used in a power system are: Circuit breakers are designed for controlled closing and controlled or automatic opening of load and fault current. Their duties are to close onto a fault and to automatically open…
The incident energy determined at each assessable location will fall within a defined arc flash/arc blast PPE category, as defined in the table below. Incident Energy PPE Category Cal/cm2 >1.2 to 4 1 >4 to 8 2 >8 to 25…
Arc flash/arc blast hazard labels inform personnel of the arc flash/arc blast hazard and the required PPE at each switchboard or arc flash/arc blast hazard location. The labels should present the following information: electrical hazard and danger symbols; arc flash/arc…
The likelihood of an arc flash/arc blast event is dependent upon the type of activity being undertaken at the time. It is generally accepted that the greatest likelihood occurs when there is direct interaction by an individual on energised equipment…
Arc faults are potentially fatal to any personnel in the vicinity. The intense heat of the arc flash/arc blast can severely burn human skin and ignite the clothing of anyone within several feet of the incident. Treatment for arc flash/arc…
The safe approach distance (SAD) is based on an “exclusion zone” principle. This principle defines an area around an exposed conductor into which no part of the person, mobile plant or object (other than approved insulated objects) may encroach unless…