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High PFC issues

Former Community Member
Former Community Member

Hi all,

First post coming in…

I have a installation that we have been asked to Test, I conducted the usual tests for the Ze & Pfc.  I got 0.4 ohms & 11.7kA respectively.  Its a commercial block and the transformer is behind a locked door about 20m away, so probably reasonable to assume it’s accurate.  So here is my question given the absence of 15kA RCBO’s and the rarity 15kA MCB’s, what are my options?

Thanks in advance.

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    seconded, unless there LN impedance and the L_E impedances are very different, suggesting another problem if it is TNCS.

    0.4 ohms is more like 600A, not really enough to clear anything more than a 60A C type promptly

    Or 11kA fault current would need a loop impedance of more like 0.02 ohms !!.

    You could perhaps have an LN impedance of 0.02 and an LE of 0.4, but I;d expect them to be closer than that, and I'd be looking out for corroded bolts or loose lugs in the earth path.

    But look at the BS88 let through energy - for a dead short close to the MCBs, the fuse may blow first, but it will almost certainly still protect the breakers from damage, even if they are 6kA types.

    In addition real faults are very rarely zero impedance if they did, they would be silent, but all that flash crackle and pop takes real energy, and therefore a real resistance, so the fuses will not get popped that often.

    Only the cable upstream of the fuse will see the full fault current.

    Mike.

     

Reply
  • 0

    seconded, unless there LN impedance and the L_E impedances are very different, suggesting another problem if it is TNCS.

    0.4 ohms is more like 600A, not really enough to clear anything more than a 60A C type promptly

    Or 11kA fault current would need a loop impedance of more like 0.02 ohms !!.

    You could perhaps have an LN impedance of 0.02 and an LE of 0.4, but I;d expect them to be closer than that, and I'd be looking out for corroded bolts or loose lugs in the earth path.

    But look at the BS88 let through energy - for a dead short close to the MCBs, the fuse may blow first, but it will almost certainly still protect the breakers from damage, even if they are 6kA types.

    In addition real faults are very rarely zero impedance if they did, they would be silent, but all that flash crackle and pop takes real energy, and therefore a real resistance, so the fuses will not get popped that often.

    Only the cable upstream of the fuse will see the full fault current.

    Mike.

     

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