Solar Earth Value

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JamieP
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Solar Earth Value

Post by JamieP »

I always ensure continuity of my earthing when completed a PV Array as we are required too and my values have always been very low so it's never been a concern but what value should I be looking for? Protective earthing is usually based on the protective device but I'm unsure what that would be in this case

If this was just for bonding I assume I could just use the 0.5 ohm value but it's not, it's earthing
pluto
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Re: Solar Earth Value

Post by pluto »

Lightning protection is another factor that needs consideration
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Re: Solar Earth Value

Post by pluto »

Lightning protection is another factor that needs consideration
AlecK
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Re: Solar Earth Value

Post by AlecK »

true , pluto, but that doesn't actually answer the question.

Normal protective earthing (fault protection by automatic disconnection of supply" sets
- PEC size by reference to size of largest relevant active ; and
- PEC continuity by reference to the fault current (ie the fault current that will cause the protective device to operate within the time limit)
For normal 230V circuits, that's easy, and we can simply use Table 8.2.
For earthing structure of building,we use the largest circuit that could fault to the structure - typically the mains.

The same principles apply to earthing of a PV array; but applying them is more complex because we first have to determine what the prospective fault current could be and what - if any - protective device will operate to limit the time that fault current flows for.
PFC (DC) from the array is inherently current-limited by the nature of the source.
PFC (AC) from the LV side depends on the type of PCE installed.
PFC (DC again) from lightning is huge, but inherently time-limited.
JamieP
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Re: Solar Earth Value

Post by JamieP »

No lightning protection needed in these situations

And I understand the value needs to be low enough to operate the device is a suitable time but what I don't know is I have no protective device on th DC side, or is it the inverter? And also I assume the max disconnection time is 5s as per usual? Or is it different in this scenario?
PeteRig
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Re: Solar Earth Value

Post by PeteRig »

The inverter should have MPPT fusing for the DC side
AlecK
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Re: Solar Earth Value

Post by AlecK »

If the source of the fault current is the inverter and / or the rest of the installation, then the protective device(s) in question are those on the output circuit of the inverter supplying LV to the installation.
Noting that inverters with transformer can't pass fault current from a.c. back to the d.c.
JamieP
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Re: Solar Earth Value

Post by JamieP »

Apologies but I still don't understand how to work out what value is acceptable for the module and rail earthing
AlecK
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Re: Solar Earth Value

Post by AlecK »

As per my earlier, there are several possible sources of earth fault current; including lightning, faults in the array wiring & modules, and faults in the inverter which may allow a.c. to be applied to the array wiring.
where there a battery in the arrangement, that also needs to be considered as a source - and unlike a PV array batteries are a source with very high PFC.
Applying the principles of reducing the time fault current flows for, and installing a PEC that's capable of carrying that fault current for that time; the "acceptable" value will be different for each source.
How many of these types of fault current need to be dealt with depends on the exact nature of the installation, and (for lightning on the location.
In some cases, no earthing is required at all. In other cases one or more types need to be dealt with; and each needs to be dealt with separately.
Overall, having determined how many & which types need to be dealt with, we install one PEC capable of satisfying all requirements.

For lightning, because the current is high we need a large, ie low impedance, conductor. There's no protective device, but the current is only ever there for a very short time. No calculation based on operating any protective device is necessary (or possible).

For d.c array faults in modules / array cabling (as explained in 5033; 2.1.7 ) a PV array source is inherently limited current so the PFC is low.
That's why special types of fuse are used for overcurrent protection. For these types of fault the PEC only has to carry the I-max, but it has to carry it for a longer time. In terms of fault protection, the max operating time is 5 s. There are no nice easy Tables of acceptable PEC value for this, as Table 8.2 only covers 3 types of mcb (for 0.4 s operating time) and one type of fuse (HRC) for 5 s operating time.
PV fuses operate at only a small margin higher than their nominal rating.
Earthing is only required for LV arrays [5033; 3.4.1], because if it's ELV then that fact alone is deemed to provide both basic and fault protection [3000; 1.5.7].
Appendix B of 5033 provides further info for various configurations - but no simple statement as to PEC size / resistance deemed to comply.
MIs - required by 5033; 3.4.1 - should provide adequate guidance.

But the same wiring / module fault fed by a battery is a high-current fault, and the array fusing can't protect the downlead from the battery source - so the battery output overcurrent protection is the relevant protective device for that scenario (5033; 3.3.3 refers). Again, if the battery is ELV, no earthing required.

For PCE faults, the worst case is livening of the array & array wiring at LV a.c.; in which case we need to remove both sources PCE output and back-feed of "mains" through the faulted inverter onto the array.
Note that while currently-cited 2005 edition of "4777.1" only requires an overcurrent device for the interconnecting cable in some cases [5.3.5]; 2016 edition requires it in every case [3.4.2]. This protection may be built into the inverter output, or may be a separate device - typically using an mcb as the "Main Switch: Inverter" at the feed-in point. Either way, that's the device that counts when sizing the array PEC.



While there are no nice easy Tables of "deemed to comply" resistance values; "3008" will provide a value in ohms per km which will tell you whether you have a bad connection.
JamieP
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Re: Solar Earth Value

Post by JamieP »

Thanks Alec, I'll have to have a good read of that to break it down

Appreciate the knowledge as always
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