MCB - MCB Coordination - European Miniature Circuit Breakers 1

[NickParker]

Can reliable Coordination/selectivity between Upstream and downstream MCBs possible both in the overload and in the instantaneous regions? (or) is it better to go for Upstream MCCB to downstream MCB?

The Upstream system is of 400/230V AC (Phase-Phase / Neutral) and the downstream power consumer is 230V AC (Phase to Neutral).

 

[Kiribanda]

It may be possible to coordinate in the O/L range. But instantaneous range will be difficult unless
you use Type D mcbs.

 

[dpc]

Even MCCB to MCB will be questionable unless the fault current is pretty low. If coordination is absolutely required, fuses will be the best choice.

 

[che12345]

Dear Mr. NickParker (Electrical)(OP)21 Oct 21 14:49
[highlight #3465A4]" 1)Can reliable Coordination/selectivity between Upstream and downstream MCBs possible both in the overload and in the instantaneous regions? (or) 2)is it better to go for Upstream MCCB to downstream MCB? "[/highlight].
1)reliable Coordination/selectivity between [Upstream and downstream MCBs] is i) no issue on over-load region. ii) not achievable=failed in instantaneous region.
2) [Upstream MCCB downstream MCB]:
i) no issue on over-load region.
ii) there can be three possibilities a) failed, b) partial=up to certain kA only and c) total=up to the MCB's kA rating. Attention: NOT the MCCB's kA rating.
3) Check up the information from the manufacturer's publication. Attention: the information is applicable to his product only. Not intended to be used by other brands or a mixture.
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr. Kiribanda (Electrical)21 Oct 21 17:17
"...1) It may be possible to coordinate in the O/L range. But instantaneous range will be difficult unless ...2)you use Type D mcbs."
1) I agree. The thermal (over-load)curves of all MCCBs including MCBs looks alike on a log-log scale. That is, the curves do not intersect. Until the current reaches the instantaneous (magnetic) region where the MCCB and MCB curves drop a straight line down, with the tripping time <=0.1s. This is the region where the selectivity failed.
2) I agree with condition. In MCB, the (magnetic trip current levels) are designed [ B(3-5Ir), C(5-10Ir), D(10-20Ir), Z , K ]. When the fault current is say 15Ir, B, C would trip while D remains closed. However, when fault current >20 Ir; all breakers B,C and D would trip = selectivity failed.
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr. dpc (Electrical)21 Oct 21 18:20
"...1) Even MCCB to MCB will be questionable unless the fault current is pretty low... 2)If coordination is absolutely required, fuses will be the best choice."
1) Agree, even Type D MCB would failed with short-circuit current say >20Ir.
2) Agree with condition. Fuse generally respond (selectivity) when [time-current characteristic do not intersect]. This requirement is usually met when grading the fuse current ratings in the ratio of 1:1.6 or higher. e.g. i) F1=20A, F2=25A ; where F1:F2 = 20:25 = 1:1.25 i.e. < 1:1.6; selectivity failed.
ii) F1=40A, F2=63A ; where F1:F2 = 40:63 = 1:1.575 i.e. = 1:1.6 approx.; selectivity would be achieved.
Che Kuan Yau (Singapore)

 

[dpc]

This requirement is usually met when grading the fuse current ratings in the ratio of 1:1.6 or higher. e.g. i) F1=20A, F2=25A ; where F1:F2 = 20:25 = 1:1.25 i.e. < 1:1.6; selectivity failed.

Completely agree. In the US, a 2:1 ratio upstream to downstream is the normal requirement.

 

[che12345]

Dear Mr. NickParker (Electrical)(OP)21 Oct 21 14:49
"..1)Can reliable Coordination/selectivity between Upstream and downstream MCBs possible both in the overload and in the instantaneous regions? 2) (or) is it better to go for Upstream MCCB to downstream MCB?..."
1. For selective between (MCCB-MCCB), (MCCB-MCB) and (MCB-MCB); there is a (possibility that selectivity may? failed) even in the [over-load region]. This failure is not common, but may? happen:
Take note that usually irrespective of (MCCB-MCCB), (MCCB-MCB), (MCB-MCB) their thermal curves do not intersect. However, consider that the upstream (which had been on before the breakdown) is in "hot" state; but the downstream (which is a replacement) is in "cold" state. There is a possibility that the downstream "cold" curve is above or intersects with the upstream "hot" curve. It result to unexpected selectivity failure in the over-load region. This unforeseen phenomenon may happen when especially with narrow current difference.
Che Kuan Yau (Singapore)

 

[NickParker]

Hi Che,

Thank you!

Now I have a question of whether to use MCCB or MCB for the breaker Q1 in the below picture. I prefer to use MCB because of the cost over MCCB. Does using MCB has any disadvantage/ code violation?

 

[che12345]

Dear Mr. NickParker
Hi NickParker, if you would preferred.
"... Now I have a question of whether to use MCCB or MCB for the breaker Q1 in the below picture. I prefer to use MCB because of the cost over MCCB. Does using MCB has any disadvantage/ code violation? "
1. I take it that the Q1 (25A) is sufficiently [short-circuit kA rated] at the 400A board. Then, it does NOT matter whether it is a MCCB or a MCB. The important criterial is that Q1 shall be sufficiently kA rated. MCB is usually lower in cost and smaller in physical size. Attention: usually the (400A board) would be of higher kA than that of the downstream (incoming disconnect switch) board.
2. Whether MCCB or MCB; they do not offer selectivity at high short-circuit current, as we had looked into it earlier.
3. BTW: The (conductor) shall be sized that is [protected by the 25A breaker]. Therefore, the (final board incoming) may be an [disconnect isolating switch].
Che Kuan Yau (Singapore)









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