Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations LittleInch on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Total Selectivity between small breakers 1

Status
Not open for further replies.

AusLee

Electrical
Sep 22, 2004
259
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?
 
Replies continue below

Recommended for you

Not much can be done using those specific 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.
 
If you can deal with the negative aspects of fuses, you could well find that series fuses offer coordination at all your available fault levels. I don't have my reference books at home - I'm on holiday, yippee! - so I can't say for sure but instinctively it would be ok in most circumstances with the sizes you are working with. Fuses bring some pain in the ass characteristics though so I would think long and hard before going that route. An alternative on bigger circuits than the 100A one you're working with is to use an ACB with a definite minimum time characteristic instead of an MCCB with an instantaneous trip.


----------------------------------
image.php

If we learn from our mistakes I'm getting a great education!
 
Yes i know about the ACB solution. ACB is around 6,000 euros, MCCB is in the few hundreds and Multi9 are 20 euros :)

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 :)
 
AusLee said:
ACB is around 6,000 euros, MCCB is in the few hundreds and Multi9 are 20 euros
You get what you pay for.

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.
 
Auslee,

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.


----------------------------------
image.php

If we learn from our mistakes I'm getting a great education!
 
Not to slight fuse engineers too much, but I'm not convinced they are guaranteed to coordinate at high fault current unless there is a large separation between the t-c curves. I don't think the manufacturers say any of them coordinate for less than 2:1 ratio.
 
Thanks for the tip about using a transformer, i think it is an affordable solution, even if, as ScottyUK says, NS and Multi9 have a great selectivity proven on site.

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?








 
Status
Not open for further replies.

Part and Inventory Search

Sponsor