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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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    Hm, some problem there, Ze and PFC don't correspond very well. With the TX close, it is likely to be TN-S, but still, Ze is much higher than the PFC suggests. One point is that an MFT is not very accurate at low Ze or High PFC. Your Ze is a factor of 10 or so out compared to the PFC, and 11.7 kA 20 m from a 500 kVA TX could well be about right. If Ze was really 0.04 Ohms it would be more reasonable, but an MFT would be fairly inaccurate at that resistance. 11.7kA is sufficiently close to 10 kA for those to be OK unless you can get a fully accurate measurement, or a few metres of cable between the Main MCCB  or whatever and the next stage CPDs will get you to a figure which is within RCBO range. Are you sure you need RCD protection everywhere in commercial premises and is this the right place to put it if required?

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  • 0

    Hm, some problem there, Ze and PFC don't correspond very well. With the TX close, it is likely to be TN-S, but still, Ze is much higher than the PFC suggests. One point is that an MFT is not very accurate at low Ze or High PFC. Your Ze is a factor of 10 or so out compared to the PFC, and 11.7 kA 20 m from a 500 kVA TX could well be about right. If Ze was really 0.04 Ohms it would be more reasonable, but an MFT would be fairly inaccurate at that resistance. 11.7kA is sufficiently close to 10 kA for those to be OK unless you can get a fully accurate measurement, or a few metres of cable between the Main MCCB  or whatever and the next stage CPDs will get you to a figure which is within RCBO range. Are you sure you need RCD protection everywhere in commercial premises and is this the right place to put it if required?

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