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]

I'm not familiar with Amtech's design software but do they have options for setting the MCCB parameters? Do they offer a current-time plot of the various OCPD overlayed?

Depending on the fault current you could well find the 80A fuse fails to prevent the 160A MCCB for tripping with both going, but to know if that is a realistic risk you need to factor in the PFC / PSCC at the installation.

Finally, the red Henley cutout "fuse" is normally a solid link simply for local isolation when changing meter, etc. They are fitted in situations like this when there is already a 60A/80A fuse in the MSDB, if you are modelling two 80A fuses in series than clearly it will complain about a lack of selectivity!

 

[ibra123]

I'm not familiar with Amtech's design software but do they have options for setting the MCCB parameters? Do they offer a current-time plot of the various OCPD overlayed?

Depending on the fault current you could well find the 80A fuse fails to prevent the 160A MCCB for tripping with both going, but to know if that is a realistic risk you need to factor in the PFC / PSCC at the installation.

Finally, the red Henley cutout "fuse" is normally a solid link simply for local isolation when changing meter, etc. They are fitted in situations like this when there is already a 60A/80A fuse in the MSDB, if you are modelling two 80A fuses in series than clearly it will complain about a lack of selectivity!

See below current-time graph of the protective devices. The red henley cut-out fuse is not modelled in Amtech so just the 80A fuse in the MSDB is used in the calculation.

 

[pc1966]

See below current-time graph of the protective devices. The red henley cut-out fuse is not modelled in Amtech so just the 80A fuse in the MSDB is used in the calculation.

You can see at about 1kA fault current the MCCB will trip as the 80A fuse curve intersects with the MCCB curve. You might be able to fiddle with MCCB short-term delay to around 0.4s and maybe put it in 'I2t' mode to push that crossover point high enough so you have selectivity at the expected PFC/PSCC levels, but I think that needs the fancy electronic trip module.

Does the software model the supply characteristics and cable lengths to know what those current values are expected to be?

MCCB are good in they are adjustable, user-resettable, etc, but for simple selectivity goals having a 160A fuse for the MSDB feed would meet all goals (except non-skilled replacement).

 

[ibra123]

You can see at about 1kA fault current the MCCB will trip as the 80A fuse curve intersects with the MCCB curve. You might be able to fiddle with MCCB short-term delay to around 0.4s and maybe put it in 'I2t' mode to push that crossover point high enough so you have selectivity at the expected PFC/PSCC levels, but I think that needs the fancy electronic trip module.

Does the software model the supply characteristics and cable lengths to know what those current values are expected to be?

MCCB are good in they are adjustable, user-resettable, etc, but for simple selectivity goals having a 160A fuse for the MSDB feed would meet all goals (except non-skilled replacement).

I have adjusted the selective short circuit (Isd) from 10.0 to 8.0 and all the errors have gone, thank you so much:

 

[pc1966]

Do you have a few MSDB so need a MCCB panel to feed them?

If just one then a 160A fused-switch such as Schneider SQB1603K would do for approx same cost as MCCB & enclosure.

If a couple you might consider a bus-bar chamber and those fused-switches off it, but any significant number then MCCB panel is far better solution I suspect, even if you have to go to the fancy trip module.

 

[pc1966]

I have adjusted the selective short circuit (Isd) from 10.0 to 8.0 and all the errors have gone, thank you so much:

Good to hear!

My last post just happened before yours became visible.

 

[ibra123]

Do you have a few MSDB so need a MCCB panel to feed them?

If just one then a 160A fused-switch such as Schneider SQB1603K would do for approx same cost as MCCB & enclosure.

If a couple you might consider a bus-bar chamber and those fused-switches off it, but any significant number then MCCB panel is far better solution I suspect, even if you have to go to the fancy trip module.

I have 2No. MSDBS fed from separate MCCBS from the main panelboard, see attached PDF of the LV schematic that shows this.

 

[pc1966]

I have adjusted the selective short circuit (Isd) from 10.0 to 8.0 and all the errors have gone, thank you so much:

Actually looking at your plot it seems it has just moved the MCCB curve a bit to the right, but the reason it is no longer complaining is it only plots the fuse characteristics down to 0.1s, which is not terribly clever (but not unique). There is still some risk that at 1.2kA or so the MCCB would be tripped.

However, it is unlikely that would happen on final circuit faults as then the MCB should trip below the MCCB's short-term delay, at least up to a few kA and that might well be above the PFC/PSCC at that point.

 

[pc1966]

I have 2No. MSDBS fed from separate MCCBS from the main panelboard, see attached PDF of the LV schematic that shows this.

The diagram show a 125A fuse on the incoming side of the MSDB boards. That seems a bit low for having 4 * 80A flats on each 125A fuse but maybe that is considered acceptable diversity.

Given you already have the 160A MCCB protecting the board you might want to consider putting links in for those fuses.