To Instantaneous or Not on LV MCC

[nollij]

This issue keeps coming up in my head and to be honest I can not settle on one side of the fence.

When implementing LSI (Long time, Short time, Instananeous) LVPCB (Low Voltage Power Circuit Breakers) for MCC main breakers, do you utilize the instantaneous function or not?

MCCs are only rated for their Short Circuit Withstand rating for 3 cycles. A short downstream of any of the MCC feeds will be cleared by the MCCBs or MCPs. So, the only issue would be when the fault occurs on the buswork of the MCC.

The argument from a Power Systems Engineer that we regularly hire for our Power Systems studies contends that if there is a fault on the buswork, the gear is toast anyway. So, don't utilize the instantaneous function to maintain coordination. This contradicts the settings he utilized for a Arc Flash reduction project that was undertaken before I started working here where the instantaneous functions are being used.

A short circuit downstream may cause an entire MCC and resulting unit outage if the instantaneous function is used which, is much more likely to occur than a fault on the MCC buswork.

However, I am not so sure that a fault on the MCC buswork would cause significant damage if it is cleared instantaneously. This would allow the equipment to be opened up and the situation potentially fixed. It could be a while for a replacement MCC to be installed, potentially keeping the unit down if there are no steam backups for that unit.


My question to all of you is, what do you do for the MCCs at your facilities that incorporate LVPCBs as MCC main breakers? Or, what do you typically default to if the customer does not specify to which they prefer? Basically, which side of the fence does everyone tend to fall on?

[ron]

For equipment rated for 3 cycle only, such as most MCC's, switchboards, transfer switches, panelboards, etc, I definitely use instantaneous settings. The code requires that we protect equipment at their ratings.

I realize you can buy specialty gear and transfer switches that have 30 cycle ratings, and I do that when I don't want to use instantaneous upstream of them, but when the downstream is 3 cycles, then instantaneous is the way to go.

[nollij]

[QUOTE=ron;1337725]The code requires that we protect equipment at their ratings.[QUOTE]

At the potential risk of a process upset from an uncoordinated fault clearing though?

[dkidd]

You should look at 240.87 for the 2011 Code. There was a good article on NECPlus, but now it is only for subscribers. A web search on 240.87 will probably find more information.

[zog]

Using a maintenence switch gives you the best of both worlds. We do a ton of retrofits of LVPCB's for this specific purpose.

[kingpb]

IEEE 1015 (Blue Book)

General application considerations for the main circuit breaker, i.e. feeding MCC, is that the preferred trip functions for selective trip are long-time and short-time (and ground fault, if required). For coordination purposes, instantaneous should be provided only if necessitated by the circuit-breaker interrupting rating.

The estimated instantaneous clearing time for a LV PCB is 3 cycles.

IEEE Std C37.20.1-2002, “The rated short-circuit withstand current of a LV ac switchgear assembly is the designated limit of available (prospective) current at rated maximum voltage that it shall be required to withstand for a period of no less than 4 cycles on a 60 Hz basis under the prescribed test conditions.

As a result, LVPCBs, ICCBs, and MCCBs must interrupt a fault within three cycles at maximum short-circuit current to protect the bus. It may necessitate using an instantaneous unit on the main circuit breaker to protect the bus at the sacrifice of obtaining selectivity.

If the concern is for arc flash, the maintenance switch, as already mentioned is a good option. But first you must look at protection of the bus.

[nollij]

Most LVPCB circuit breaker Short Time delay units operate around 0.1 sec (6 cycles). So, not applicable for protecting against short circuit current at the level of the rated equipment fault withstand.

I am not concerned about Arc Flash issues (the STD typically handles that quite well, not necessitating the instantaneous unit for that purpose). Merely, I want to know whether people prefer to put the MCC at risk to prevent miscoordination. The miscoordination is a far more likely outcome as bus faults are rare compared to feeder faults. A miscoordinated fault clearing by the main breaker would result in a process unit upset which could be far more dangerous than a broken MCC. But then, what is the point of the LVPCB being there if it is not protecting the bus besides lowering the Arc Flash and providing relatively pointless coordination?

What do other facilities consider to be the bigger risk?

I had not had a chance to go over the 2011 NEC yet. 240.87 is new news to me!

The ultimate solution to this whole problem is to standardize IEEE ratings to be for 6-8 cycles (typical STD times) instead of 4 cycles. I have thought this for a while now.

[jghrist]

As your Power Systems Engineer says, you are not "putting the MCC at risk" to prevent miscoordination. An external fault will be cleared by a feeder breaker instantaneous trip. The main breaker cannot protect the MCC from an internal fault. If an internal fault occurs, the MCC has already failed.

[kingpb]

Why not plot the curves and see what shakes out. It could be that with the Instantaneous set, you might still achieve selective coordination. If not, then you will have to decide if the process is critical enough to risk setting it or not.

As far as Arc Flash, the extended time of the short time setting will directly increase the energy released, so I would not discount that the instantaneous is not required in order to achieve the desired AF incident energy at the boundary.

[templdl]

This is the way I have always looked at this application:
The interesting thing about the instantaneous element of breakers is that should there be a bolted fault on a feeder or branch circuit the fault current will also be seen by the main breaker. Everone on use would like to assume that the breaker closest to fault would trip and clear the fault first before the main trips. But, should that fault current be high enough in magnetude where it would fall within the trip pickup of the main and feeder or branch breaker it would be pot luck on which breaker would trip. Remember that both the short time and instantaneous ajustment of a breaker are calibrated such that they are within the withstand of the breaker. By adjusting the short time delay you may get some coordination with low level faults with down stream breakers, that is the main breaker would delay slightly in order give the down stream breaker a chance to clear the fault.
Usually the instantaneous setting of breakers come set in there lowest setting and most installers that I'm aware of reset them in there highest setting to prevent nuisance tripping.

Coordinating for faults is real ambiguous as there is no sure solution to assuming what they will be. The magnetude of a fault could be anything. Arcing faults are a real problem as they may not be high enough to trip a breaker instantaneously and not enough to trip a breaker on overcurrent. Quite often arcing faults are line to ground where ground fault is a real advantage.