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 Post subject: Switchboard Incomer
PostPosted: Tue Oct 22, 2019 6:11 am 

Joined: Fri Jun 14, 2019 5:51 am
Posts: 13
i am currently designing a project which is putting in a MCC for use on a petrochemical plant.

the HV works is being done by the sites HV Contractor, we are looking at the LV works.

there are two 2.5MVA transformers that will feed a LV intermediate board which is supplied by the HV Contractor, the outgoing terminals from the LV intermediate board is our cut line of responsibility.

i was proposing to install two MCCs fed from the intermediate board with 4000A incomers.

it is being questioned why i am providing incomers when the intermediate board which is located around 15m away has 4000A feeders. i.e. can the protection be done at the intermediate board.

in my mind i do not see any reason why i cannot use the intermediate board protection and have no incomer protection on the board i am speccing. however, is there anything i should be thinking about / reason why i should.


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 Post subject: Re: Switchboard Incomer
PostPosted: Tue Oct 29, 2019 7:00 am 

Joined: Tue Oct 29, 2019 6:23 am
Posts: 19
- 4000A is very large for a LV MCC! I would suggest splitting into 2000, or 2500A MCCs. Alternatively use a center mounted MLO and split the bus into two 2500A buses with dedicated mains for each bus. If you can segment process to match the bus segmentation even better! But I think I would suggest staying to 3000A or less for an MCC.
- Generally MCCs are not available > 100kA at 480V and in the case of drives and other components getting to 100kA may require large components/fusible switches, etc. If MCCs can be kept at 65kA rating you may be better off.
- If the 4000A OCPD is by others, are you specifying a LVPCB with the ability to not include IOC, or are you getting an ICCB that MUST have IOC on all the time? This may impact your system selectivity and AF protection.
- Would suggest the 4000A device have an ERMS switch! If you are able to add 2000A or 2500A sub-mains and ERMS there may be a good idea as well since the 4000A OCPD may not be able to achieve a low enough ERMS setting for the smaller loads/circuits.
- Generally if the OCPDs in the MCC are 250A or smaller MCCB they will be current limiting and small enough to achieve good downstream AF protection (confirm with AF study). Same with fuses in that size range and maybe slightly larger... but larger fuses and larger CB you need to ensure with known arcing currents to see if they will provide good AF protection.


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 Post subject: Re: Switchboard Incomer
PostPosted: Tue Oct 29, 2019 10:08 am 

Joined: Fri Jun 14, 2019 5:51 am
Posts: 13
Hello, thanks for your response, see below in red

MarceloEValdes wrote:
- 4000A is very large for a LV MCC! I would suggest splitting into 2000, or 2500A MCCs. Alternatively use a center mounted MLO and split the bus into two 2500A buses with dedicated mains for each bus. If you can segment process to match the bus segmentation even better! But I think I would suggest staying to 3000A or less for an MCC. the drives are already split over 3off x MCCs, the 4000A is the rating of the upstream breaker at the main switchboard fed from the 2.5MVA transformers, these are likely to be turned down to around 3000A anyway once we finalise our loading
- Generally MCCs are not available > 100kA at 480V and in the case of drives and other components getting to 100kA may require large components/fusible switches, etc. If MCCs can be kept at 65kA rating you may be better off. Preliminary fault level calculated at approx 71kA based on 2.5MVA Tx and 6% impedance and the associated cabling, switchgear is to be rated 80kA
- If the 4000A OCPD is by others, are you specifying a LVPCB with the ability to not include IOC, or are you getting an ICCB that MUST have IOC on all the time? This may impact your system selectivity and AF protection. this was my main query, i would prefer to specify an incomer circuit protective device, this then mean that we have greater control over the incomer settings and also arc flash calcs, however, my principal engineer is dead against it stating that its just going to add an extra £10k (x3) to the project
- Would suggest the 4000A device have an ERMS switch! If you are able to add 2000A or 2500A sub-mains and ERMS there may be a good idea as well since the 4000A OCPD may not be able to achieve a low enough ERMS setting for the smaller loads/circuits.
- Generally if the OCPDs in the MCC are 250A or smaller MCCB they will be current limiting and small enough to achieve good downstream AF protection (confirm with AF study). Same with fuses in that size range and maybe slightly larger... but larger fuses and larger CB you need to ensure with known arcing currents to see if they will provide good AF protection.


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 Post subject: Re: Switchboard Incomer
PostPosted: Wed Oct 30, 2019 1:37 pm 

Joined: Tue Oct 29, 2019 6:23 am
Posts: 19
Not sure how much will be split off for dedicated drives. However, if the frame and sensor are rated 4000A, then the protected bus should be 4000A, or if less it should be protected by an adequately sized feeder or main. As far as I know MCCs tend to be limited to 3200-3000A, and if that big you may loose vertical stack space to bus structure causing your MCC to be larger. In my experience 2000A is a good size for large MCCs. Your idea of having dedicated feeders or mains for each bus is the better way to do it.

Generally for selectivity purposes you are better off with switchgear type equipment so you have more flexibility on how to set, if at all, the instantaneous. However, to some extent it depends on the size of feeders/circuits in the MCC, if they are all current limiting at ~ 250A and below you probably can use IOC at the main and still have selectivity. I.e. you can use an ICCB. This will vary by brand and other details.

I still recommend an ERMS switch at the main, at the very least.

If the utility fault current is 70K you would need to add something for motor contribution. 85kA rated equipment would be you minimum, but you may want to go to 100kA. My estimating calculator yields about 65kA with motor contribution, so maybe 85kA equipment is enough.

I noticed that you listed an estimated price in British pounds. Are we talking UL or IEC equipment? I am answering from a UL perspective.

Transformer Type 2500-10,000kVA, Liquid filled, 55/65 deg rise, FA
Transformer KVA 2500 kVA
Secondary Voltage 480 Volts
Nominal Impedance 5.75 %
Impedance Tolerance 7 %
Motor load 70% of xfmr kVA
Nominal Secondary Amperes 3,007 Amps Calculated
Maximum SC Current 56,218 Amps Calculated
Max SCA with Motor Contribution 64,638 Amps Calculated
Extended current rating 140 % Calculated
Extended Rating Amperes 4,210 Amps Calculated
Provided Courtesy of ABB
Contact [email protected] if you have any comments [/i]


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