Overview
Power Testing Ltd (PTL) has been engaged by a large industrial user to upgrade their 11 kV main distribution system. The site consists of various 11 kV and 0.4 kV switchboards, large motors and significant on-site combined heat and power (CHP) generation. The site decided that as part of this overall they would also review their safety systems and requested that an arc flash risk hazard assessment be carried out. PSC undertook this study in line with the NFPA 70E and IEEE 1584 requirements and provided guidance to the site.
Challenge
The arc flash energy presented as the result of a low-impedance connection through the air to the ground can cause serious harm to personnel and equipment in its vicinity. Health and safety legislation imposes numerous duties on employers in relation to arc flash and electrical safety. The arc flash assessment determines the incident energy, appropriate personal protective equipment (PPE) and the recommended arc flash warning labels for each location supporting a safer workplace.
The industrial site wished to carry out a complete review of its 11 kV and LV distribution systems to identify and risks and safety requirements for its personnel. The site is large and complex. The main challenge is identifying the thermal hazards associated with the arcing fault energy at numerous locations. This requires detailed knowledge of the characteristics of the overcurrent protective devices used as well as their protection settings and fault clearing time.
Solution
PSC developed a system model for the industrial site and carried out fault level studies, load flow analysis, and protection grading studies. The model has sufficient detail to also carry out arc flash studies on the 11 kV and 0.4 kV switchboards within the system.
The arc flash studies were performed following industry best practice and in accordance with IEEE-1584. Further calculations to determine approach boundaries and PPE requirements were then identified following the requirements of NFPA 70E. For the fault clearing time, actual relay tripping times have been used which take into consideration the protection settings, relay pick-up time delay, and breaker contact separation.
Results
PSC provided to the customer for each 11 kV and 0.4 kV feeder the calculated incident energy at the specified working distances, the recommended arc flash protection boundaries and the recommended PPE category as per NFPA – 70E. These details were also provided in a summary form as recommended arc flash warning labels to be installed on each item of equipment.