MCCBs with Icw

[NickParker]

Why the MCCBs (with Icw rating) do not have the values same as the Ics value? What is the use of providing the very low Icw value? why does IEC permits the use of Icw less than Ics by manufacturers? Shouldn't it be around Icu or Ics values?

For example, I have attached Terasaki model H400 - NE in which you can see,

Icu = 125kA
Ics = 85kA
Icw = 5kA for 0.3s

The Icw value is low in relation to the Icu & Ics values. Will this 5kA @ 0.3s be enough for co-ordination purpose?

 

[FreddyNurk]

I can only speculate, however, it would appear that the withstand rating with the contacts closed is 5kA for 0.3 seconds. This gives the condition of a through-fault beyond the breaker, but the breaker trip unit doesn't pick up.

One of the often advertised differences between an Air Circuit Breaker (ACB) and a Moulded Case (MCCB) is the inherent fault current limiting due to magnetic forces forcing the contacts apart (Schneider appears to have the best description, but I would expect other manufacturers are the same). If this is the case then a much lower withstand would be expected as the breaker contacts will be (at least partly) forced open for a through-fault beyond the breaker.

I've not really ever had to look as all of the higher fault current applications I've dealt with didn't really exist beyond 0.3 seconds. I'll have to make a note of checking the ACB ratings for comparison next time.

I'd just add that 5kA for a 250A frame is likely well within the trip curve for any protection unit that's likely to be used with this breaker, so expecting 35kA withstand for a second is unlikely.

 

[RRaghunath]

With Ii=13xIn as max setting available, 5kA / 0.3s withstand rating seems OK.
13xIn corresponds to 5200A for Instt trip. So, beyond 5kA, no current based grading is expected.
It is known that due to compact dimensions, the MCCBs do not offer much of withstand rating.

 

[che12345]

Dear Mr NickParker

Q1. Why the MCCBs (with Icw rating) do not have the values same as the Ics value? What is the use of providing the very low Icw value? why does IEC permits the use of Icw less than Ics by manufacturers? Shouldn't it be around Icu or Ics values?
A1. In general irrespective of manufacturer, all MCCBs are manufactured with moulded/plastic casing equipped with [internal instantaneous trip] to protect itself from (destruction) within Icu. It is [not intended/designed] to have Icw rating of 0.5s or 1s. Therefore, the Icw 0.5s rating is <<< Icu .
International standards e.g. IEC, individual national standards e.g. ANSI/IEEE, CSA and individual organisation standards e.g. UL, NEMA etc. publish their own rating/testing requirements. These requirements are not detected by the manufacturer.
FYI: Most ACBs which are of much heavier built (usually with metallic frame) may have
Icw 1s = Ics or 50-75% Ics.

Q2. The Icw value is low in relation to the Icu & Ics values. Will this 5kA @ 0.3s be enough for co-ordination purpose?
A2. In most cases it is difficult to achieve good co-ordination(selectivity) between two MCCBs. Manufacturers do publish selectivity table between two MCCBs, usually up to certain current level; after which selectivity fails.

Che Kuan Yau (Singapore)

 

[NickParker]

One of the often advertised differences between an Air Circuit Breaker (ACB) and a Moulded Case (MCCB) is the inherent fault current limiting due to magnetic forces forcing the contacts apart (Schneider appears to have the best description, but I would expect other manufacturers are the same). If this is the case then a much lower withstand would be expected as the breaker contacts will be (at least partly) forced open for a through-fault beyond the breaker.

Are you implying that this is a current limiting MCCB. Do current limiting MCCBs have short time delay?

 

[NickParker]

With Ii=13xIn as max setting available, 5kA / 0.3s withstand rating seems OK.
13xIn corresponds to 5200A for Instt trip. So, beyond 5kA, no current based grading is expected.

Say the aforementioned MCCB is an Incoming device and for the fault on the downstream of an outgoing MCCB is higher than the Max Inst setting on the Incoming MCCB, How the co-ordination could be achieved?

I think the Icw rating on MCCB is only for the sake of co-ordination purpose and the Ics rating is within the available short circuit current.

Is the Max. Inst setting is intentionally kept below the Icw rating?

 

[RRaghunath]

For grading of MCCBs, we are supposed to follow the tables / software tool provided by OEM.
The grading is not just current/time based but also energy based.

 

[che12345]

Dear Sir,

1. Mr FreddyNurk
" .... I'd just add that 5kA for a 250A frame is likely well within the trip curve for any protection unit that's likely to be used with this breaker, so expecting 35kA withstand for a second is unlikely".
A1. In practical case, unless the 250A CB is fed from a very low kVA transformer or located some 200-300m away, the short-circuit level is very likely to exceed 5kA. Therefore, a CB with Icu =5kA would likely be [not] suitable.

2. Mr RRaghunath
O2.1 " With Ii=13xIn as max setting available, 5kA / 0.3s withstand rating seems OK.
13xIn corresponds to 5200A for Instt trip. So, beyond 5kA, no current based grading is
expected ".
A2.1. Breakers with instantaneous/magnetic trip (adjustable 1-13x In) is [not] relevant to Icw rating. Iinst. is the current value in kA, while Icw is the current value including time duration i.e kA ts , e.g. 35kA 0.5s.
A2.1.1 " So, beyond 5kA, no current based grading is expected ". may [not] be true/correct.
O2.2 "For grading of MCCBs, we are supposed to follow the tables / software tool provided by OEM. The grading is not just current/time based but also energy based".
A2.2 The protection coordination discrimination may be by " difference in current value", or
" difference in tripping time " or " difference in energy i.e. i2t in kA2 s " etc.etc. Each method has their advantage/disadvantage, easy/difficult to apply and their limitations. In any case, Icw plays an important part.

3. Mr NickParker
Q3.1 " .... Do current limiting MCCBs have short time delay? "
A3.1 No. It is specifically designed to open/break (as fast as possible) i.e. without [any] intended delay, besides the (unavoidable) physical/mechanical inertial mechanism constrain.
Q3.2 " I think the Icw rating on MCCB is only for the sake of co-ordination purpose and the Ics rating is within the available short circuit current ".
A3.2 Agreed.
Q3.3 " Is the Max. Inst setting is intentionally kept below the Icw rating? "
A3.3 No. Iinst (1-13x In) has nothing to do with Icw. See above A2.1.

Che Kuan Yau (Singapore)

 

[NickParker]

The grading is not just current/time based but also energy based.

I thought energy based discrimination is only for current limiting MCCBs.

If the MCCB with Icw of 5kA @ 0.3s is an incomer, can the panel board be rated at 85kA @ 1s as the manufacturer has mentioned in the drawings?

I wonder if the breaker can interrupt 85kA (Ics) instantaneously Why can't it withstand 5kA for 0.3s, noting that instantaneos trip also take minimum of half cyle to trip

 

[che12345]

Dear Mr NickParker

Q1. I thought energy based discrimination is only for current limiting MCCBs.
A1. Energy based discrimination is [not necessarily] only for current limiting MCCBs. It is applicable to any two/more CBs (not limited to current limiting CB), where the upstream CB total opening I2t > downstream CB I2t. However, current limiting MCCBs perform very well in this respect.

Q2. If the MCCB with Icw of 5kA @ 0.3s is an incomer, can the panel board be rated at 85kA @ 1s as the manufacturer has mentioned in the drawings?
A2.1 If the incomer is a [current limiting CB] and (verified) that limits the peak current value (Ip kApeck < than 85kA ), then Icw does (not) come into consideration. Attention: verification is required.
A2.2 If the current limiting CB cut off current not exceeding 17kA, verification of Icw is not required.
Note: a) current limiting CBs are (not) intended to have any short-time withstand Icw rating i.e. utilization category A.
b) Reference IEC 61439-x for further detail.

Q3. I wonder if the breaker can interrupt 85kA (Ics) instantaneously Why can't it withstand 5kA for 0.3s, noting that instantaneos trip also take minimum of half cyle to trip.
A3. A current limiting CB with an extremely short break time that opens/breaks the short-circuit current before it reaching the peck value i.e. even though rated Ics=85kA, the CB contact open had completed at lower than say 25-30kA. A current limiting Ics=85kA CB would likely to withstand 5kA 0.3s, but such test is not mandatory. See also above A2.2.

Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr NickParker

Follow up/clarification of reply by che12345 dated 6 May 19 12:48
Q3. I wonder if the breaker can interrupt 85kA (Ics) instantaneously Why can't it withstand 5kA for 0.3s, noting that instantaneos trip also take minimum of half cycle to trip.
A3. For example:
A (non-current limiting) CB X, utilization cat.B, Iu=630A, Icu=100kA, Ics=75kA, Icw=7.6kA 1s,
Ip peck @100kArms=7kA, I2t @100kArms=10.4 kA2 s, opening time=7ms.
A (current limiting) CB Y, utilization cat.A , Iu=630A, Icu=200kA, Ics=100kA, Icw= ----,
Ip peck @100kArms=5.5kA, I2t @100kArms=10.1 kA2 s, opening time=3.5ms.
Both of these CBs are acceptable at location with short circuit current = < 100kA, i.e Isc<Icu

Opinion:
a) Even with these X or Y CB as the incomer, it is [wrong] to claim that the board is rated for 85kA 1s, not even 85kA 0.5s. It may be rated as 85kA 7ms (85kA 3.5ms) respectively. i.e. based on CB opening time.
b) The CB X and (Y) I2t=10.4 kA2 s (10.1 kA2 s ) respectively. Both should be able to withstand the thermal stress of 5kA 0.3s = 7.5kA2 s, i.e. based on I2t=k (a constant) within limits of 3s. See IEC 61439-x .
c) Their Ip peck @100kArms=7kApeck (5.5kApeck) respectively. Therefore, down stream CB may be of low Icu/Ics rating,

Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr NickParker

Follow up/addendum to reply by che12345 dated 7 May 19 00:23
Opinion:
d) If the board had been tested (i.e. verified) with a non-current limiting CB X, busbar assembly and including its structure that it had withstood Icw 85kA 1s, then it would be acceptable to claim that the board can retain 85kA 1s rating with the non-current limiting CB X being replaced by a current limiting CB Y . This is based on the condition that the CB Y I2t < CB X. Replacing CB X by CB Y does [not] lower the original tested Icw =85kA 1s. With CB Y, the cut-off is so fast that the board neither had to experience 85kA nor had to withstand 1s. No further test is required per IEC 61439-x.
However, if the board has [not] being tested to Icw=85kA 1s, it would be [wrong] to claim that the board can withstand Icw=85kA 1s , just by replacing the CB X by CB Y [without] going through test.

Che Kuan Yau (Singapore)