Calculation of VD within/as well the adiabatic?

[Rockingit]

Here’s something I’ve never thought about until now….

So, we use the adiabatic to return an S based on the usual parameters and for arguments sake we end up with a 35mm conductor. But what if it’s also 300m+ length end to end before the earthing conductor gets to the MET? Do we then also calculate for vd, and if so based on what current (pefc?).

 

[Pretty Mouth]

Not sure I've 100% understood the question but here are my thoughts/assumptions anyway:

The current that would flow under fault would be based on the impedance of the entire fault loop, so it would be the voltage across that entire loop (ie the supply voltage) that would be used to calculate it.

A longer circuit would mean higher loop impedance, and for some devices (fuses rather than MCBs) this would mean a thicker CPC is needed (I2t increasing as fault current decreases, so adiabatic calculation is made at the far end of the circuit). In these cases you'd need to calculate the adiabatic several times, for various thickness CPC and the corresponding loop impedance.

Or have I totally misunderstood the question?

 

[DefyG]

VD under fault conditions is irrelevant. As long as the csa calculated using adabatic equation is adequate to carry the fault current for the duration it will be ok.
If calculated its likely to be much higher than the tabulated VD but only for a split second or time it takes fault to clear! but as supply will be interrupted in any case, trying maintaining a VD of x% to supply equipment is pointless.

 

[pc1966]

In designing the cable size there are, as we all know, these key requirements:

• Current carrying capacity suitable for max load (fixed) or OCPD (sockets, etc)
• Fault impedance L-E Zs low enough to achieve disconnection times (from high enough PFC)
• Supply impedance L-N low enough to meet load VD
• Conductors able to survive the adiabatic heating during fault clearance

They are all interrelated of course but usually I would start with CCC for the minimum size for a short cable, and then VD to get any size increase, then check if Zs & adiabatic is met and possibly increase again (or size CPC up to live). To specifically answer your question, the answer is almost certainly "yes" that you need to iterate through the calculations again, just as @Pretty Mouth said since failing on VD might mean your Zs is too high to meet disconnection times and I2t of the OCPD could be far higher than the usual assumptions.

Meeting 5% VD overall implies a PSCC of 20x operating rating, that is usually enough for the OCPD disconnection. But that 5% spec is on the circuit and not necessarily including the supply impedance (though if it is a small load off the CU then it probably is close enough), and also meeting VD is based on L & N sizes, not L & E as E is often smaller (hence the adiabatic calculation) or higher Z (if SWA armour used).

Tables B1-B5 in the OSG are handy cheat-sheets for this when dealing with fuses, they give you the Zs requirements for a given CPC size so are solving both disconnection time (TN systems) & adiabatic for you.

 

[pc1966]

Just to add that disconnection on the RCD rather than OCPD might meet time for safe ADS, but you have to allow that the 't' in your I2t could be 40ms for instant RCDs or much more for any delay versions. Depending on the PFC you might not meet the adiabatic as you would normally expect for MCB disconnection on the magnetic trip which is way faster.

 

[Rockingit]

Thanks all, between you you've clarified my thoughts as to what I need to do in this particular case - GN8 wasn't much help!! This application is where there are multiple separate generating sets working in tandem with a 2000A DNO source (not adding into it, obviously!) but distributing across a common earthing zone, so everything needs tieing back to the MET in a sort of Multiple Main Earth / Bonding mash-up where nothing is a short walk.

 

[pc1966]

If it is the TN-C-S bond size for extraneous conductive systems then length is not really an issue, as the size is more down to the long-term open PEN current that might be seen, but for a 2000A supply I would imagine it is a local transformer so not that case? That would be a bit outside of Table 54.8 range!

 

[Rockingit]

If it is the TN-C-S bond size for extraneous conductive systems then length is not really an issue, as the size is more down to the long-term open PEN current that might be seen, but for a 2000A supply I would imagine it is a local transformer so not that case? That would be a bit outside of Table 54.8 range!

Yeah, hence my post! I'll temporarily have a 2000/3A DNO source, a 500kVA setup, a 300kVA setup and a baby 160kVA all working separately but needing bonding/earthing as it's the same location and a sh*t tonne of common structural steel, concrete and steel railings designed to be touched with temporary power and signal looms running randomly all over the place! Theoretically, a momentary fault on the DNO PEN would imply that all the interconnections might be required to carry the entire fault current (although Kirschoff applies and we're talking milliseconds before the sets would all go into protect mode anyway having upset the software).