Insufficient breaking capacity 3

[AntonioPerez]

I'm working on a project of an existing facility. There is a main switchboard with a molded case circuit breaker in the incoming whose breaking capacity is insufficient for the calculated short-circuit current. That incoming circuit will be properly protected in its beginning, by a new circuit breaker. I'm affraid that if a future shortcircuit causes the opening of that underrated breaker, an arc flash could happen. I was proposing to change that existing circuit breaker by a switch disconnector but it seems that there is no space in the switchboard and the changing operation takes more time than acceptable. I'm not sure of the effects of a breaking operation of a breaker with insufficient breaking capacity. We've checked if that circuit breaker let the breaking coil to be removed but it is not possible. What do you think about it?

 

[Kiribanda]

Whether you are working in the ANSI world or IEC world, installing a circuit breaker having its capability
less than its calculated duty, then it is a code violation.

 

[eeprom]

I have encountered this situation before, where the existing switchgear was installed for a smaller service. Then, some years later, the service transformer goes from 500 kVA to 2 MVA, and the old switchgear has insufficient interrupt capacity. Install a fused disconnect upstream of the switchgear. This allows you to keep everything in place and still be able to protect against faults.

 

[che12345]

1. In the IEC world: most MCCB manufacturers have rated current up to 3200 A with "Rated ultimate short-circuit breaking capacity, I[sub]cu[/sub]" at 50-60 Hz 380/415 V up to 85, 100...120 kA. Some current-limiting breakers up to 630 A rated 200 kA. These kA ratings would be able to meet most applications/installation, unless the transformer/generators > say 6 MVA.
2. In case that the calculated short-circuit current say > 100 kA, it would be advisable/economical to break the board into separate sections; with each section designed to < 100 kA.
3. Information: It is usually NOT economical/practical to design and test the (LV < 1kV) boards to > say 100kA.
Che Kuan Yau (Singapore)

 

[waross]

Thinking outside the box:
Many generations ago, wireless reactors were used in series with transformer secondaries to match impedances when paralleling mismatched transformers.
There are many ways of reducing the available fault current (Available Short Circuit Current) by increasing the supply impedance.
One project, a similar issue was addressed by using a minimum of 100 feet of supply cable from the source to switch-gear with an rated available fault current rating of less than the source.
I have been able to justify using switch-gear with a lower rating than the source by considering the impedance of the supply conductors.
If you have room, even several coils of each individual supply conductor will act as an air core reactor and reduce the ASCC at the breaker.
Cable and conductors are generally cheaper than a higher ASCC rating.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

@ eeprom (Electrical)22 Dec 23 21:53
"..... Install a fused disconnect upstream of the switchgear. This allows you to keep everything in place and still be able to protect against faults".
I have the following concerns:
1. The Fuse (say 200kA) can NOT protect the under-rated kA Breaker. This series combination behaviour is very complex. It is not possible to based on the Fuse and Breaker characteristics published, unless approved by the OEM and tested by the lab.
2. It is NOT advisable to have Fuse in the incoming. The Breaker does NOT trip when any one Fuse is blown. This would result to very serious single-phasing consequences throughout the whole installation. All running motors will be single-phasing and all motors unable to start up.
Che Kuan Yau (Singapore)

 

[che12345]

1. There are MCCB on the market that certainly would have kA rating higher than the calculated/(Available Short Circuit Current) kA. These higher kA CB may be slightly bigger in physical dimension and cost. The replacement installation time is very short. It would certainly be very much costly and time consuming to install 3 lengths of say 100 feet of 1000A cable, acting as impedance. Note: the R would be << X for large size cables. The X is uncertain depending on the cable laying formation etc.
2. Attention: Replacing the CB with a higher kA does NOT increase the board busbar and the existing CB kA rating. Are the existing CB on the board rated for the new higher kA ?.
Che Kuan Yau (Singapore)

 

[AntonioPerez]

Thanks for your comments.
Upstream breaker is new and has enough breaking capacity. Actually I was plannign to replace the downstream underrated breaker by a switch disconnector. My concern is that the downsream breaker could try to break the shortcircuit current and not be able, causing and arc flash or getting the breaker destroyed.

 

[RRaghunath]

Antonio Perez, I agree with you that it is better to replace downstream MCCB with a Switch-Disconnector.
The consequences will be - for any fault in the downstream of downstream switch, the upstream MCCB will trip and hope that is acceptable to the plant.
With regard to space requirement for the switch-disconnector, I remember you can get from the same manufacturers as MCCB, the switch-disconnector that looks same size as the MCCB but without protection or automatic trip included.

R Raghunath

 

[che12345]

@AntonioPerez (Industrial)(OP)27 Dec 23 16:10
"#1.....Upstream breaker is new and has enough breaking capacity. ... plannign to replace the downstream underrated breaker by a switch disconnector...#2. .. concern is that the downsream breaker could try to break the shortcircuit current and not be able, causing and arc flash or getting the breaker destroyed ".
I have the following opinion for your consideration:
1. Upstream breaker has enough kA is fine, no action to be taken. However, to replace the downstream underrated breaker by a switch disconnector... needs some protection study. A breaker usually offers thermal and magnetic (over-current and short-circuit) for protection; a switch disconnector has none. Check the local Code/regulation !
2. All MCCB manufacturers publish the MCCB to MCCB "back-up" coordination info. As an example: up-stream 36kA, down-stream 16kA; back-up valid up to 36kA. That is the up-stream 36kA breaker protects the down-stream 16kA breaker from "destruction" to any short-circuit current up to 36kA.
3. Attention: Both breakers 36kA and 16kA brakers would trip, i.e. sacrificed on the "selectivity"; which is under a different objective , may have to be taken into consideration.
Che Kuan Yau (Singapore)

 

[Parchie]

@AntonioPerez (Industrial)(OP)
I have seen an old MCC main distribution panel installed with a series reactor at each incoming conductor to solve that problem. It worked.
The calculation of the inductance required is simple:
Inductance = System Line to neutral voltage/(1/MCC SCCR - 1/AFC@POCC)
Better still, there are current-limiting fuses that you can buy.