Ok.. Picture this...
Supply to a factory. 200A three phase DNO cutout with 100A fuses. Tails about six feet long into TWO large switchfuses. [fuse size no idea, as i have not been able to open them yet]
TWO submains, [one from each switchfuse] both with at least 50mm if not 75mm, 4 core SWA about 80 metres long.
DB's at ends of submains [DB's both three phase ones]
One DB contains a load of small SP MCBS to power a small office. ["C" curve"]
Other DB contains a few TP MCB's, a few 32A ones, and ONE 63A one to power various gantry cranes and saws and welders. ["C" curve]
Workers come in on monday morning [works empty all weekend] to find "power off"....
Man next door claims to be an "electrician" and quickly finds TWO cutout fuses blown... [NO other OCPD's tripped, just the cutout fuses]
So, ok, phase to phase fault [my guess] or two phase to earth faults [unlikely??]
DNO replace fuses and everything works perfectly..
Workers go home at night, come in next morning, both cutout fuses blown again. DNO replace them again, everything works again!!!
So, my guess is submain cable damage, inbetween switch fuses at supply origin, and the DB's [so when i can, IR testing is wanted] [perfectly possible [if not extremely likely] given environment that cables have sustained damage though crushing, or being forced round small bends violently]
Only other option is a fault on the 63A circuit as although the 32A fuses would discriminate with the 100A fuses [I would imagine] the 63A i am not so sure. [No let through energy charts for the MCBS here either]
Why though, or how, can you have a phase to phase fault big enough to blow the cutout fuses, with no obvious damage to show?? How can a fault of that magnitude "clear" itself, only to return the next day, and clear itself a second time???
What i cannot understand is, if the fault was of low impedance, surely to god there would be damage evident to the cable, like an open circuit or all conductors welded together, and if the fault was of sufficient impedance to take a few minutes to blow the fuses, the the energy dissipated in the fault [I2r] would have had the cable, or at least the faulty bit glowing red!!
As of this morning, everything perfect, but tonight... Hmmmmm....
john..
Supply to a factory. 200A three phase DNO cutout with 100A fuses. Tails about six feet long into TWO large switchfuses. [fuse size no idea, as i have not been able to open them yet]
TWO submains, [one from each switchfuse] both with at least 50mm if not 75mm, 4 core SWA about 80 metres long.
DB's at ends of submains [DB's both three phase ones]
One DB contains a load of small SP MCBS to power a small office. ["C" curve"]
Other DB contains a few TP MCB's, a few 32A ones, and ONE 63A one to power various gantry cranes and saws and welders. ["C" curve]
Workers come in on monday morning [works empty all weekend] to find "power off"....
Man next door claims to be an "electrician" and quickly finds TWO cutout fuses blown... [NO other OCPD's tripped, just the cutout fuses]
So, ok, phase to phase fault [my guess] or two phase to earth faults [unlikely??]
DNO replace fuses and everything works perfectly..
Workers go home at night, come in next morning, both cutout fuses blown again. DNO replace them again, everything works again!!!
So, my guess is submain cable damage, inbetween switch fuses at supply origin, and the DB's [so when i can, IR testing is wanted] [perfectly possible [if not extremely likely] given environment that cables have sustained damage though crushing, or being forced round small bends violently]
Only other option is a fault on the 63A circuit as although the 32A fuses would discriminate with the 100A fuses [I would imagine] the 63A i am not so sure. [No let through energy charts for the MCBS here either]
Why though, or how, can you have a phase to phase fault big enough to blow the cutout fuses, with no obvious damage to show?? How can a fault of that magnitude "clear" itself, only to return the next day, and clear itself a second time???
What i cannot understand is, if the fault was of low impedance, surely to god there would be damage evident to the cable, like an open circuit or all conductors welded together, and if the fault was of sufficient impedance to take a few minutes to blow the fuses, the the energy dissipated in the fault [I2r] would have had the cable, or at least the faulty bit glowing red!!
As of this morning, everything perfect, but tonight... Hmmmmm....
john..
