Amtech MSDB modelling question

[ibra123]

You might want to ask Schneider support for selectivity guidance. It might be you can change the 160A supply MCCB for a different model (e.g. 250A set to 160A 'In', or maybe going to one with 50kA or 70kA break capacity) or different settings to get enough for your MPB to LLPB requirement of over 5kA selectivity (on the computed PSCC current, more probably it would be L-E fault not 3 phase bolted fault so around half that PFC and OK).

I have spoken to Schneider and they just explained me how to use their selectivity tool on their website. I have tried most combinations and the maximum selectivity limit I can get between a 160A MCCB and 100A MCCB is 2.4kA.

If I were to change the 160A MCCB to a 250A, I would get a pre-arc error between the 250A MCCB and the 315A fuses at the cut-out which is why I am limited to a 160A MCCB downstream of the 315A fuses. Note that the pre-arc energy of the 315A fuses is 300kA2.

What options are there to reduce the energy let-through of a protective device? The energy let-through of the 100A MCCB is 327kA2 of the 100MCCB for the DB which is higher than the pre-arc energy of the 315A fuses at the cut-out:

 

[pc1966]

I have spoken to Schneider and they just explained me how to use their selectivity tool on their website. I have tried most combinations and the maximum selectivity limit I can get between a 160A MCCB and 100A MCCB is 2.4kA.

OK.

If I were to change the 160A MCCB to a 250A, I would get a pre-arc error between the 250A MCCB and the 315A fuses at the cut-out which is why I am limited to a 160A MCCB downstream of the 315A fuses. Note that the pre-arc energy of the 315A fuses is 300kA2.

Do you also have the 250A fused-switch in your model?

What options are there to reduce the energy let-through of a protective device? The energy let-through of the 100A MCCB is 327kA2 of the 100MCCB for the DB which is higher than the pre-arc energy of the 315A fuses at the cut-out:

Up to 63A then MCB are commonly used for final circuits and they have less let-through, but also less breaking capacity (6kA domestic, 10kA commercial is common).

Above 63A or so you are in to a choice of fuse or MCCB. Fuses are simple, reliable and well-behaved here, but not good for systems without skilled support due to the issues of replacing one that has failed (and other potential aspects like coping with 2 out of of 3 phases remaining live, etc).

MCCB usually have a higher let-through than MCB but that is part of the design and why they usually give acceptably good selectivity with downstream MCB.

I guess no option to have two EV charge feeds and split to chargers from those, thus bringing down both to ~50A MCCB in that panel?

Looking at this data sheet on page 9 for the pre-arc values:

https://docs.rs-online.com/c77e/0900766b8167459e.pdf

It seems 315A is listed there even lower at 270k (compared to 300k in the software model), but going to 400A pushes that to 505k so one option is to look at a larger supply to increase the tolerance to down-stream let-through.

However I don't know the practicalities of what the DNO will offer / charge so I can't really say what is the best case here. I had hoped someone with more experience might have added to this discussion but we recently lost an exceptional guy and others might be on holiday.

 

[ibra123]

Do you also have the 250A fused-switch in your model?

Morning buddy. No, I don't have a 250A fuse. I have 315A fuses at the cut-out, then I have a Main Panelboard after cut-out that feeds resi apartments via 160A MCCBs and one feed for Landlords Panelboard via 160A MCCB. From the Landlords Panelboard the EV feeder pillar and Landlords DB are the loads that are giving me errors.

I guess no option to have two EV charge feeds and split to chargers from those, thus bringing down both to ~50A MCCB in that panel?

That is an option even though that was not the initial design intent (at Stage 3) but wouldn't I need to double on the Distribution board/meter which would make it a lot more expensive?

 

[pc1966]

Morning buddy. No, I don't have a 250A fuse. I have 315A fuses at the cut-out, then I have a Main Panelboard after cut-out that feeds resi apartments via 160A MCCBs and one feed for Landlords Panelboard via 160A MCCB. From the Landlords Panelboard the EV feeder pillar and Landlords DB are the loads that are giving me errors.

The LV schematic has "250A TPN FUSE SWITCH DISCONNECTOR" shown between the MPB and the Landlor's CT stuff on LV212 feed.

That is an option even though that was not the initial design intent (at Stage 3) but wouldn't I need to double on the Distribution board/meter which would make it a lot more expensive?

The LV schematic has 400A cut-out and 315A fuses, it is not clear where billing meters are (if one for main supply) but MPB is already 630A rated.

So change is DNO fuses but more significantly if that alters the DNO cost, cable limits, etc.

 

[pc1966]

Just looked again, no incoming meter as LL has meter for feed, and all flats own meters.

 

[pc1966]

That is an option even though that was not the initial design intent (at Stage 3) but wouldn't I need to double on the Distribution board/meter which would make it a lot more expensive?

Doh! Sorry too early, didn't realise this was the EV feed.

If the feed pillar has its own RCBO for each charger then I would be inclined not to worry too much about the LLPB/MPB selectivity.

But I would worry about the MCCB to incomer fuse selectivity. The loss of a fuse is a major inconvenience to 1/3 of the flats per fuse, as well as everything else, and not as easy to sort a flipping the LLPB breaker back on.

 

[ibra123]

The LV schematic has "250A TPN FUSE SWITCH DISCONNECTOR" shown between the MPB and the Landlor's CT stuff on LV212 feed.

Sorry that is just meant to be a switch disconnector for isolation purposes.

Just looked again, no incoming meter as LL has meter for feed, and all flats own meters.

Correct, each apartment is metered separately as well as a whole current meter at the Landlords Panelboard for Landlords supplies.

Doh! Sorry too early, didn't realise this was the EV feed.

If the feed pillar has its own RCBO for each charger then I would be inclined not to worry too much about the LLPB/MPB selectivity.

But I would worry about the MCCB to incomer fuse selectivity. The loss of a fuse is a major inconvenience to 1/3 of the flats per fuse, as well as everything else, and not as easy to sort a flipping the LLPB breaker back on.

Ok, I have actually managed to clear the error by changing the length of the cable from 20m to 10m for the landlords DB with a 100A MCCB. For the feeder pillar I have used a 63A MCCB.

 

[pc1966]

Ok, I have actually managed to clear the error by changing the length of the cable from 20m to 10m for the landlords DB with a 100A MCCB. For the feeder pillar I have used a 63A MCCB.

Good to hear you have a fix.

Shortening cables as a fix is odd, usually a longer cable drops the PFC/PSCC and so lowers let-through.

 

[ibra123]

Good to hear you have a fix.

Shortening cables as a fix is odd, usually a longer cable drops the PFC/PSCC and so lowers let-through.

Yep, I had the same reaction!