UPS and PSCC?

[ericmark]

If using a 13 amp fuse I would think ELI 2.42 Ω or 95 amp is needed to rupture it within the time allowed, or it would need RCD protection or some protection within the inverter but then there would be some thing to reset or a fuse to replace this

does not show any reset button, and I can't see an inverter giving out 95 amp to blow 13 amp fuse. And no built in RCD again there would need to be some way to reset if there was.

Am I going daft?

 

[JohnW2]

If using a 13 amp fuse I would think ELI 2.42 Ω or 95 amp is needed to rupture it within the time allowed, or it would need RCD protection or some protection within the inverter ...

Glancing at the curves for a 13A BS1362 fuse (which I presume is what you are talking about), it looks as if a bit less than 95A (maybe about 80A) might be adequate to achieve required disconnection times, but what you say is certainly in the right sort of ballpark.

You are talking about a UPS. The question then arises as to whether or not the UPS is capable of supplying 95A (or 80A, or whatever). If not, then fault protection would presumably have to be provided by an RCD (with, of course, the output of the UPS earth-referenced).

Kind Regards, John

 

[plugwash]

I recall in older editions of BS7671 that the 0.4s disconnection time only applied if the touch voltage was over 50V, with far more generous disconnection times if the touch voltage was lower. I don't know if that is still the case.

 

[JohnW2]

I recall in older editions of BS7671 that the 0.4s disconnection time only applied if the touch voltage was over 50V, with far more generous disconnection times if the touch voltage was lower. I don't know if that is still the case.

I don't really understand that - can you possibly expand a bit. A lot depends upon what you mean by 'touch voltage'- between what and what?

Whatever, I don't recall having seen anything such as you suggest in current or recent editions of the regs.

Kind Regards, John

 

[plugwash]

I don't really understand that - can you possibly expand a bit. A lot depends upon what you mean by 'touch voltage'- between what and what?

Between the CPC and true earth and/or things bonded to the main earth terminal during the fault.

As a worst case scenario we can consider the voltage between the point of the neutral to earth bond, and the point of the fault.

When a short circuit happens, a voltage will develop across the earthing system, said voltage will persist until the fault is disconnected. How hazardous this voltage is will depend on the relative characteristics of different parts of the circuit.

In a typical mains supplied scenario, the supply impedance is very low, and the impedance of the earth path is usually similar or higher than the impedance of the phase path. So the resulting "touch voltage" between different parts of the earthing system and/or between the earthing system and true earth is likely to be dangerouly high.

On the other hand with a small inverter there may simply not be enough current available to create a hazardous voltage across the earthing system, even under short circuit conditions.

 

[ericmark]

There is an earth rod, so there is an earth even when no connection to DNO supply, there is a 3.5 kWh battery and solar panels so between them there could be quite a high current, but the instructions for the inverter are not clear as to how much power could be drawn.

Noted some odd displays where although battery not fully charged still exporting, so there may be some limits.

I expected one RCBO feeding all UPS outlets, this would also mean easy to isolate outlets, but this is not the case, so trying to arm myself with reasons why the central heating supply should be RCD protected.

 

[JohnW2]

Between the CPC and true earth and/or things bonded to the main earth terminal during the fault.

OK -so you're talking about the VD in the CPC as a result of the fault current. For that to be less than 50V would require that that R2 was less than about 27% of the Ze - which I would imagine would be unlikley to be the case in any real-world final circuit. Hence, if a relaxation (in disconnection time for 'touch voltages <50V) such as you mention did exist in the past, I doubt that the conditions for it to apply would 'ever' have been satisfied.

However, what I've said relies on the supply voltage remaining at something like the nominal (230V) during the fault but, as we've both said, in the case of an inverter-derived supply, it's doubtful whether there would be an appreciable (if any) 'supply voltage' in the face of an L-E fault 'of negligible impedance' - so the 'touch voltage' could then be very low, quite possibly zero!

Kind Regards, John

 

[JohnW2]

There is an earth rod, so there is an earth even when no connection to DNO supply, there is a 3.5 kWh battery and solar panels so between them there could be quite a high current, but the instructions for the inverter are not clear as to how much power could be drawn.

Regardless of the capacity of the battery, do you really think that the inverter would be capable of delivering 80A+ at around 230V (i.e. enough to rapidly blow a 13A fuse)?

I suspect that, under near-short-circuit conditions, the inverter would just stop, either because of built-in 'short-circuit protection' or, even in the absence of such protection, just because it was 'overwhelmed' by the load.

Kind Regards, John

 

[JohnW2]

.... For that to be less than 50V would require that that R2 was less than about 27% of the Ze - which I would imagine would be unlikley to be the case in any real-world final circuit.

I should perhaps have added, for completeness ... what I say above obviously assumes a TN earth. If it's a 'TT' one (including the situation in which a TN-C-S CNE is 'lost'), it would obviously be very probable that R2 would be (probably considerably) less than Ze, hence the 'touch voltage' well under 50V.

Kind Regards, John