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binky

Unbalanced loads on a 3 phase RCD

Question

ID: 26   Posted

I've been studying some diagrams of 3 phase RCDs, as far as I can see, because all 3 phases pass through the same detection coil, then any imbalances on the phases doesn't affect the RCDs' operation?

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31 answers to this question

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ID: 27   Posted

20 hours ago, Sidewinder said:

I don't think you will find an inverter that is TT only, but you will find them that are TN only.

don't think I've seen TN only inverters ever either, what makes are you thinking of? Only major issue with regards inverters and DNO supplies that I can think of has been micro-inverters needing bonding to frames, and amorphous thin film panels which require earthed -ve on the dc side.

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ID: 28   Posted

Where is @Canoeboy when you need him!

 

I am pretty sure that I have seen instructions for solar inverters that state for use on TN supplies only.

Not doing solar, I only get to check the documentation side when requested for compliance with the regs.

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ID: 29   Posted

I'm fairly sure some SMA are unsuitable for TT, I'll have a look when I get home.

 

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ID: 30   Posted

you get off grid inverters, which are different, but I've been checking a few specs and all they say is connect to earth, nothing about the earthing characteristics. However, a high rod resistance ie over about 500 Ohms can cause inverters to see this as an earth fault.

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19 hours ago, binky said:

the RCd may not see a supply fault

I think I see why. On the mans supply, the neutral will be referenced to earth at the transformer. When the mains DP RCD trips, that reference is lost and the live conductors from the solar are only referenced to earth via the earthing at the inverter. This could be of higher resistance, therefore the current flowing to earth would be less, not enough to trip the PV RCD.

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not quite, the RCD at the inverter will not see an imbalance on the submain - leccy will just be off. But the inverter has a delay before auto-tripping out in the event of a power loss, anything up to 2.5seconds, so the inverter could possibly be still backfeeding the faulty submain and killing anyone connected to it.

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