jebidieh
Member
- Location
- New Florence, PA
Hi Folks!
I need some education. While balancing 3-phase loads in a large high school, a testing company discovered that the main electrical service earthing conductors were carrying 32 amperes of current. The service is a 4,000 ampere, 480/277 volt, 3-phase, 4-wire service. The bare copper earthing conductors run from the main switchboard ground bus to a 3 ground rod grounding arrangement. The current was measured using a clamp-type ammeter. The current measured on the EGC bonded to the switchboard ground bus and running to the water pipe was 0. The current measured on the EGC bonded to the switchboard ground bus and running to the building steel was 10 amperes. To isolate the cause of the current, different portions of the electrical service were de-energized until they discovered that most of the current measured was coming from electrical distribution equipment serving HVAC equipment. The EGC current was reduced by 20 amperes when 2 large rooftop air handlers serving the pool area were de-energized, 10 amperes for each unit. Almost all of the remaining current stopped incrementally when each remaining piece of HVAC equipment was de-energized. The testing company was satisfied that the current was ?normal? leakage current. I have not heard of this before. First question, is it normal for equipment to ?leak? current such that it causes current on the bonding conductors? Second, could the current measured simply be the result of how a clamp-type ammeter measures current by measuring a magnetic field? Since the bonding conductor is routed in close proximity to current carrying conductors, and therefore have a magnetic field around them, could that current measured simply be a result of the implied magnetic field? Is the current even there? Third, should there be further investigation?
Thanks for helping me understand!
Jebidieh
I need some education. While balancing 3-phase loads in a large high school, a testing company discovered that the main electrical service earthing conductors were carrying 32 amperes of current. The service is a 4,000 ampere, 480/277 volt, 3-phase, 4-wire service. The bare copper earthing conductors run from the main switchboard ground bus to a 3 ground rod grounding arrangement. The current was measured using a clamp-type ammeter. The current measured on the EGC bonded to the switchboard ground bus and running to the water pipe was 0. The current measured on the EGC bonded to the switchboard ground bus and running to the building steel was 10 amperes. To isolate the cause of the current, different portions of the electrical service were de-energized until they discovered that most of the current measured was coming from electrical distribution equipment serving HVAC equipment. The EGC current was reduced by 20 amperes when 2 large rooftop air handlers serving the pool area were de-energized, 10 amperes for each unit. Almost all of the remaining current stopped incrementally when each remaining piece of HVAC equipment was de-energized. The testing company was satisfied that the current was ?normal? leakage current. I have not heard of this before. First question, is it normal for equipment to ?leak? current such that it causes current on the bonding conductors? Second, could the current measured simply be the result of how a clamp-type ammeter measures current by measuring a magnetic field? Since the bonding conductor is routed in close proximity to current carrying conductors, and therefore have a magnetic field around them, could that current measured simply be a result of the implied magnetic field? Is the current even there? Third, should there be further investigation?
Thanks for helping me understand!
Jebidieh