Amtech MSDB modelling question

ibra123 · Jun 29, 2023

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 · Jun 29, 2023

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 · Jun 29, 2023

pc1966 said:
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.

[ElectriciansForums.net] Amtech MSDB modelling question

pc1966 · Jun 29, 2023

ibra123 said:
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 · Jun 29, 2023

pc1966 said:
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 · Jun 29, 2023

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 · Jun 29, 2023

ibra123 said:
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 · Jun 29, 2023

pc1966 said:
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 · Jun 29, 2023

ibra123 said:
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 · Jun 29, 2023

ibra123 said:
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.

ibra123 · Jun 29, 2023

pc1966 said:
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.

Those are 3No. 125A fuses (TPN), which will supply 13No. flats. Should I consider dropping from 80A to 63A fuses (even though the calc works with 80A fuses)?

What is the difference between a fuse and a link that you are referring to?

pc1966 · Jun 29, 2023

ibra123 said:
Those are 3No. 125A fuses (TPN), which will supply 13No. flats. Should I consider dropping from 80A to 63A fuses (even though the calc works with 80A fuses)?

Why was 125A chosen, is it a limit of the MSDB or someone's choice?

Dropping to 63A fuses would help of course, and also buy a bit more selectivity with the 160A MCCB, but really there should be a design aspect as to why 63A or 80A was chosen. Who has designed the flats and assessed the expected demands? If they are small and not using multiple electric showers, etc, then 63A might be perfectly adequate.

ibra123 said:
What is the difference between a fuse and a link that you are referring to?

You can get metal bars, etc, designed to replace a fuse if the protection is not needed, say to turn a fused-switch in to a simple isolator as other OCPD exists. Check with the MSDB manufacturer if they offer one without incoming fuses (or linked out) for a situation like this where the up-stream protection is adequate.

Having the 125A fuse where it is not needed is just another thing to blow if many flats max out demand at the same time and cause problems for restoring it, more so than someone who is able to flip the MCCB closed to reset it.

ibra123 · Jun 30, 2023

pc1966 said:
Why was 125A chosen, is it a limit of the MSDB or someone's choice?

Dropping to 63A fuses would help of course, and also buy a bit more selectivity with the 160A MCCB, but really there should be a design aspect as to why 63A or 80A was chosen. Who has designed the flats and assessed the expected demands? If they are small and not using multiple electric showers, etc, then 63A might be perfectly adequate.

You can get metal bars, etc, designed to replace a fuse if the protection is not needed, say to turn a fused-switch in to a simple isolator as other OCPD exists. Check with the MSDB manufacturer if they offer one without incoming fuses (or linked out) for a situation like this where the up-stream protection is adequate.

Having the 125A fuse where it is not needed is just another thing to blow if many flats max out demand at the same time and cause problems for restoring it, more so than someone who is able to flip the MCCB closed to reset it.

If I'm honest, I selected 125A as the previous design did not show a fuse rating so you might be right that I should use solid links as I am already protecting that cable upstream with an MCCB.

I also selected 80A as the stage 3 design did not say what rating that fuse is. See below attached image of the loads for each apartment. I applied a manual diversity that is based on a ADMD calculation method from an energy supplier. The ADMD load for each apartment is calculated to be 22.87A, so it looks like I should go for 63A fuses.

Do you know if I can feed the EV charging feeder pillar from the MPB instead of the LLPB because it is difficult to discriminate the upstream and downstream MCCBs? Note that the whole current meter is at the LLPB and I have to think about how I would meter the EV feeder pillar.

pc1966 · Jun 30, 2023

ibra123 said:
If I'm honest, I selected 125A as the previous design did not show a fuse rating so you might be right that I should use solid links as I am already protecting that cable upstream with an MCCB.

Those incoming fuses are needed when that sort of panel is used by a DNO and they form the cut-out for protecting the feed cable for overload, as well as a means of isolation.

Here you don't need that as the supply is already protected and an easy means of isolation by the MCCB.

ibra123 said:
I also selected 80A as the stage 3 design did not say what rating that fuse is. See below attached image of the loads for each apartment. I applied a manual diversity that is based on a ADMD calculation method from an energy supplier. The ADMD load for each apartment is calculated to be 22.87A, so it looks like I should go for 63A fuses.

That makes sense.

ibra123 said:
Do you know if I can feed the EV charging feeder pillar from the MPB instead of the LLPB because it is difficult to discriminate the upstream and downstream MCCBs? Note that the whole current meter is at the LLPB and I have to think about how I would meter the EV feeder pillar.

Normally MCCB - MCCB gives good selectivity, at least if the settings are sensibly chosen. What sort of limit does the software give for the selectivity limit? If it is above the expected fault currents then it is not really something to worry about.

More fundamentally though I would expect the EV charge points will need RCD protection You can do that with fancy MCCB add-ons, but does the EV pillar have RCBOs, etc, for protection of the charge outlets? If so the RCBO should be selective with the MCCB feeding them so your only real worry would be physical damage to the SWA cable.

ibra123 · Jun 30, 2023

pc1966 said:
Normally MCCB - MCCB gives good selectivity, at least if the settings are sensibly chosen. What sort of limit does the software give for the selectivity limit? If it is above the expected fault currents then it is not really something to worry about.

More fundamentally though I would expect the EV charge points will need RCD protection You can do that with fancy MCCB add-ons, but does the EV pillar have RCBOs, etc, for protection of the charge outlets? If so the RCBO should be selective with the MCCB feeding them so your only real worry would be physical damage to the SWA cable.

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.