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Electrical
- Apr 25, 2008
- 265
Hello all. I am new on this forum. I am an electrical engineer living in Australia. I am a young engineer and eager to learn.
I am designing LV power distribution to replace existing systems approx. 50 years old.
Existing LV T/F on site is 200 kVA with FLA of 270A 3-ph. delta-wye 3.3kV to 415V. The TF supplies a site MSB. The TF is protected from overcurrent by the main switch MCCB set 270A. The mains cabling is unprotected from short circuit.
The MSB contains 2-off MCCBs each supplying a submain to a building. Each submain is protected by a 200 Ampere C/B.
The problem is, when the systems were originally built, discrimination was not a requirement.
Now that we are doing it again, discrimination is required, and the upstream and downstream protection (270 and 200 Ampere respectively as stated above) are too close to achieve it.
Reducing the building demand is not possible. There are 2-off buildings requiring 200 Ampere 3-ph each.
My design achieved total discrimination through providing 2-off 160A MCCBs to each building, essentially feeding two busbars (at each buildinf there is 1-off 120A load (feed from 1-off 160A CB) and the rest is several small loads(feed from the second 160A CB)).This made total discrimination possible at any point in the installation.
The design that has been chosen discriminates between the downstream CBs in each building and that building's submain breaker (the 200 Amp).
This design does not provide discrimination between the site main switch and the CB for each of the 2 building submains. It is a cheaper option, and also slightly more regular than 2-off CBs to each building (unregular and dangerous without labelling).
If a fault occurs on the MSB busbar, or on the submain to each building, the site main switch will trip with the submain breaker and power will be lost to both buildings.
The faults I have described above are very unlikely would you agree? the building MDB escutcheon would have to be open with a live chassis, and the fault would have to occur upstream of the CB. Alternatively someone would have to dig up the submain to cause a fault there.
This design basically provides discrimination where it is more likely to be required, and does not discriminate between CBs where the cables etc. are installed in such a way as to make a fault very unlkely.
In your opinion does this design represent a good engineering compromise?
I am designing LV power distribution to replace existing systems approx. 50 years old.
Existing LV T/F on site is 200 kVA with FLA of 270A 3-ph. delta-wye 3.3kV to 415V. The TF supplies a site MSB. The TF is protected from overcurrent by the main switch MCCB set 270A. The mains cabling is unprotected from short circuit.
The MSB contains 2-off MCCBs each supplying a submain to a building. Each submain is protected by a 200 Ampere C/B.
The problem is, when the systems were originally built, discrimination was not a requirement.
Now that we are doing it again, discrimination is required, and the upstream and downstream protection (270 and 200 Ampere respectively as stated above) are too close to achieve it.
Reducing the building demand is not possible. There are 2-off buildings requiring 200 Ampere 3-ph each.
My design achieved total discrimination through providing 2-off 160A MCCBs to each building, essentially feeding two busbars (at each buildinf there is 1-off 120A load (feed from 1-off 160A CB) and the rest is several small loads(feed from the second 160A CB)).This made total discrimination possible at any point in the installation.
The design that has been chosen discriminates between the downstream CBs in each building and that building's submain breaker (the 200 Amp).
This design does not provide discrimination between the site main switch and the CB for each of the 2 building submains. It is a cheaper option, and also slightly more regular than 2-off CBs to each building (unregular and dangerous without labelling).
If a fault occurs on the MSB busbar, or on the submain to each building, the site main switch will trip with the submain breaker and power will be lost to both buildings.
The faults I have described above are very unlikely would you agree? the building MDB escutcheon would have to be open with a live chassis, and the fault would have to occur upstream of the CB. Alternatively someone would have to dig up the submain to cause a fault there.
This design basically provides discrimination where it is more likely to be required, and does not discriminate between CBs where the cables etc. are installed in such a way as to make a fault very unlkely.
In your opinion does this design represent a good engineering compromise?