Arc Flash Study

[KRG9729]

We have an upcoming project that involves replacing utility pole mounted street lighting. The lights are fed from overhead secondary conductors in residential neighborhoods. The feed is fed from the secondary side (single phase, 240 volts) of a pole mounted transformer that also feeds overhead residential services. There is not an accessible means of disconnect (other then involving the utility company) and as such we are preparing to change the lights while working live. We will be installing inline fuses for the new lights. I would like to calculate the appropriate arc fault protection PPE for performing the work live. Can someone please give an example of a typical calculation for this type of application. Thank you.

 

[jim dungar]

I would like to calculate the appropriate arc fault protection PPE for performing the work live. Can someone please give an example of a typical calculation for this type of application. Thank you.

There is no single industry accepted methodology for calculating single phase arc flash incident energy, especially at only 120V to ground. Do you have a software package, or are you planning to get one?
So without a standard calculation, you are left with using some other type of analysis method. Some people simply treat the system as if it was three three phase and live with the overly conservative results. Others try to use the 'task tables' in NFPA70E, but they don't really work well on the secondary of transformers, as the clearing time requirement is hard to meet.

 

[wbdvt]

The present standard for incident energy calculations is IEEE 1584-2002 which states the scope is 3 phase circuits. In that study, they were only able to sustain a 208V arc in one test case. What I have seen people doing is to use <1.2 cal/cm2 for single phase 120/240V. This is essentially non melting clothing (which any electrician should already be wearing), long sleeve shirt, gloves, safety glasses.