Total Selectivity between small breakers
Total Selectivity between small breakers
(OP)
Hello,
Merlin Gerin says that if the input breaker is an MCCB of frame 100 equiped with a magnetic of trip unit 100, then if the downstream circuit breaker is an MCB Multi9 C60 32A, the selectivity is as follows:
If the fault current is less than 800A, total selectivity is acheived. If the fault current is more, then both breakers may trip.
My question is: what can be done to achieve total selectivity? I know that it depends on how critical the application is and if it is really worth the extra cost of equipment, but what are these equipment in the first place? should each circuit be accompanied by a special relay and all circuit breakers must be equipped with a tripping mechanisms?
Merlin Gerin says that if the input breaker is an MCCB of frame 100 equiped with a magnetic of trip unit 100, then if the downstream circuit breaker is an MCB Multi9 C60 32A, the selectivity is as follows:
If the fault current is less than 800A, total selectivity is acheived. If the fault current is more, then both breakers may trip.
My question is: what can be done to achieve total selectivity? I know that it depends on how critical the application is and if it is really worth the extra cost of equipment, but what are these equipment in the first place? should each circuit be accompanied by a special relay and all circuit breakers must be equipped with a tripping mechanisms?






RE: Total Selectivity between small breakers
You could try to find an upstream breaker with higher (or no) magnetic pickup.
Or separate the breakers with enough impedance that the downstream breaker doesn't see more than 800A of fault current.
RE: Total Selectivity between small breakers
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If we learn from our mistakes I'm getting a great education!
RE: Total Selectivity between small breakers
So i think that in a small 3 phase panel board, a short circuit on one branch circuit may eventually trip the whole panel board.
The available Icc is 10 kA that the level of that panel.
I will look up the solution using fuses, but since breakers cost more i thought they "should" have better performance as well :)
RE: Total Selectivity between small breakers
ScottyUK is correct about fuses, both the coordination and the downside.
Look at the fault current at the end of the 32A circuit. If the circuit goes very far it may be that the fault current at the end is less than the 800A limit. Throw a transformer in ahead of the 100A breaker such that the available fault current out is 800A or less.
RE: Total Selectivity between small breakers
To add some real-world experience with the equipment you're working with, we have a large installed base of Merlin Gerin's NS series MCCBs and the earlier Compact design in the 160A - 400A range and our standard MCB is the Multi 9 with literally thousands of them on site. I have never seen a fault on an outgoing circuit take out the board incoming supply except when a feeder was fitted with a Vigi earth fault relay specc'd by an over-cautiuous designer which did not coordinate with any of the MCBs. The Vigi unit was removed - it was a bad idea from the outset in that application. Fault level on our site is typically very high, >30kA at the panelboard in quite a few locations and >50kA at the main distribution boards. I doubt you will ever have a problem unless you manage to get a bolted fault close in to the board - possible but unlikely.
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If we learn from our mistakes I'm getting a great education!
RE: Total Selectivity between small breakers
RE: Total Selectivity between small breakers
I agree about Vigi, it should not be placed at the main breaker in a banch circuit panel board, rather individual or group of circuits shall be protected by separate RCDs/GFCIs. The Vigi should be used as a larger RCD but maintaining the same concept, i.e. when supplying a motor or other large load.
Thanks for the insight but if it is possible i have this one final doubt about this issue, concerning the "hassle" of demonstrating selectivity to the consultant. There are 2 approaches:
1. Put the panel board (and the whole project) in a calculation software, in the case of MGE it is Ecodial, and have it calculate the actual current in each branch and show selectivity based on calculated currents, Or:
2. Select the breakers "more quickly by experience" from the catalog (Multi9s and MCCBs) and then fill in a spreadsheet showing: type of chosen main breaker, and the type of the branch circuit breakers, and right next to each the value of selectivity, either T for total or x kA, depening on the manufacturer's sheet. This approach, if valid, saves a lot of time.
Will i be missing something critical if i use option 2?