Amtech MSDB modelling question

[ibra123]

Hi guys,

With regards to modelling a multi service distribution board, how would you guys model it in Amtech?

I have tried using a TPN distribution board and each circuit on the DB represents one apartment protected by a BS88 80A fuse (within the MSDB).

The error I am facing is "The disconnection time of the upstream overcurrent protection is lower than that for apartment circuits"

In my example I have a 400A cut-out (315A fuses) that feed my main panelboard, which then feeds my MSDBS with micrologics via 160A MCCB and then 80A fuse to each apartment which also has an 80A fuse at the apartment cut-out (henley red head series fuse).

Thanks in advance!

 

[pc1966]

With a 160A MCCB upstream and 100A MCCB downstream I have a selectivity limit of 2.4kA and my prospective fault current is 5.069 kA (see below error message). I was thinking of supplying the EV feeder pillar via the MPB so that only 1No. 160A MCCB is used but then I would have metering issues I think. I will have RCBOs for each charging point.

You might want to look at having one of the fancy electronic trip modules for the upstream 160A MCCB feeding that board.

At least in the Hager catalogue (I think you are Schneider which I'm not so familiar) the h250 LSI offers total selectivity with the TM version in that sort of combination.

Do you get selectivity errors on any other circuits on the landlord's board?

 

[ibra123]

Do you get selectivity errors on any other circuits on the landlord's board?

Yes, anything I put on that board with a 100A MCCB i get errors. At the moment only the EV charger and the Landlords DB, but for the landlords DB I am getting a different error:

 

[pc1966]

Yes, anything I put on that board with a 100A MCCB i get errors.

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).

At the moment only the EV charger and the Landlords DB, but for the landlords DB I am getting a different error:

View attachment 109194

For info - the "pre-arcing I2t" is the energy needed to cause the fuse to get to the point of blowing. It might just survive but will be seriously weakened, so you want down-stream protection to limit the fault energy to less than that. Once a fuse blows there is arcing as the wire evaporates, and another rating for a fuse is the total let-through I2t, which is what a fuse that rapidly blows will let-through. When cascading fuses if the downstream let-through is below the up-stream per-arcing all is well. For BS88 fuses in the same general series you typically get that at a 1.6: ratio (i.e. 2 steps on the nominal series, for example, a 63A fuse from 100A, from 160A, from 250A, from 400A, etc).

Its not clear how to match the notation there with your LV schematic, but the 300k A2s figure looks like the pre-arcing for a 250A fuse, what it is telling you is a hard fault on the landlord's board it going to take out fuses in the 250A fused-switch feeding it.

As it is fed from a 160A MCCB you could simply put in an isolator switch for that board, or drop it completely as you can isolate all with the landlord's incomer switch, or isolate the feed cable using the supply MCCB (suitably locked off).

But...your 315A DNO fuses might be at some risk as their pre-arc is about 500k. Make sure they are in the model just in case.

 

[pc1966]

Just to add my experience with MCCB is very limited, it might be someone like @Julie. has better suggestions.

 

[pc1966]

Also I would add that sometimes you can't achieve perfection in design at an affordable cost so you need to be pragmatic about what your threshold for acceptance is. Obviously there are some aspects you MUST meet as safety is immediately dependant on them:

• ADS times - to avoid excessive risk of exposure to dangerous voltages.
• Break capacity - so a MCCB/MCB/fuse does not explode in someone's face.
• Adiabatic / CCC limit - so cables (including bonding) are not at risk of fire, or the serious cost and down-time of replacing a damaged sub-main, etc.

Selectivity is a definite design goal and in the regs, but it is not always possible to meet for various reasons and often this is simply accepted as "good enough". A simple and common case is blowing a 13A fuse will normally take out a 20-32A MCB, but as that can be reset with little skill, and often nothing too critical to safety is impacted - we see that as perfectly OK.

So in your design you need to look at cases where the software tells you selectivity is not met and evaluate if that is an acceptable risk or not, and in discussion with the client. There are some points to consider such as:

• What are the implications of selectivity failure? I.e. what other healthy circuits are taken off-line in that situation, and how much risk or inconvenience does that cause.
• How long an outage is tolerable? Based on the impact (above) of such a fault.
• How easy is it to restore upstream power? E.g. a fuse requires electrically skill intervention to replace, but a MCCB/MCB can be reset by a caretaker, etc, with just basic training and written guidance, so time-scales for an authorised person to react looked in to.
• How probable is a fault that would trigger the situation? E.g. is it happening on the more likely L-E fault (PFC) or only on a 3P bolted L-L-L fault (PSCC), and are the related cables or accessories at high-risk of damage (e.g. public socket or flex of some appliance, etc), or low-risk (e.g. sub-main in SWA run in risers, etc, away from DIY drilling regions).
• Is any mitigation present? For example, dual power feeds or UPS/backup generator when failure has serious impact, emergancy lighting, etc. Failure of course might be a wider power outage and not just an installation fault!

So if you get to the point when only a few circuits are vulnerable to low-risk PSCC faults, mitigation is going to be very expensive or impractical, and rectification of the fault manageable by local staff in reasonable time-scales, it might be fine for the client to accept those aspects in writing, and your design is done.

 

[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!