Elimination of Incomer Breaker in 600V MCC 1

[cherry2000]

We have a 4.16kV Primary --Step down to 600V via a Xmer and a 600V MCC in the same S/S. A prposal has been received to eliminate the MCC Incomer and rely on upstream fused 4.16kV contactor to clear any fault in the 600V bus. Reflected short circuit current in the primary side is 4.5kA approx and is within the max. short circuit breaking capacity of the contactor. Is this arrangement feasible? Proposal also suggests installing an o/c relay on the 600V side to trip the contactor. Are any codes ( CEC ) violated here?

 

[Eleceng01]

cherry2000,

I am not sure of your location, but if in the U.S. and I understand your situation correctly, you need to be concerned with NFPA-70E - specifically the issue of arc flash protection. Removing the main breaker can cause the arc flash hazard on the MCC bus to rise due to the slow response of the primary fuse. Hope this helps.

 

[Zogzog]

I think there is an arc flash concern no matter where you live

 

[dpc]

If this can be considered an service, you also have to consider the "6 handle rule" (NEC).

The 4160 V contactor itself is useless for a 600 V fault unless 480 V relaying is installed. The 4160 V fuses will be next to useless.

Arc-flash levels for the entire MCC will generally be extremely high if there is no main breaker.

Installing relays to open the contactor might work, but you have to make sure that the fault current will be in the contactor's interrupting range. Also, most new MV contactors have a built-in time delay on opening to give the fuse a chance to clear the fault before the contactor opens. This would significantly increase arc-flash levels.

Also, consider maintenance and troubleshooting issues with no low side main device.

 

[jraef]

If it is already installed, why on earth would someone want to REMOVE it??!

 

[cherry2000]

Well ..all is well that ends well. A decision to go in for the Incomer bkrs has been taken...so this discussion can be terminated. Thanks to you folks for all the valid comments.

 

[rbulsara]

Cherry2000:

A very bad idea IMHO. It is a sepaprately derived system and should be treated as a service entrance. At the best you have to stick to 6-disonncet rule as mentioned by dpc. But the starter disconnects may not qualify for that.

The primary side OCPD does not proectect the seconday side conductors/bus at any rate and so recognized by Code.

Most electrical inspectors will not accept this so you better check with them too.

 

[sslobodan]

As my professor from relay protection was always reminding: "problems on one voltage level should solve protection on that voltage level, not the one on the other side" . Now that circuit breaker is last hope for the low voltage side to switch of in a case of malfunction, and to save transformer from cooking If you take that out it is the question what will do that especially if you have fast short circuit (i.e. Lightning in power cable) and transformer must first change polarity of magnetic field and than let the current flow from LV side to HV, reach disconnecting value and turn of. In that time you have your gear fried on LV side. Cable isolation, transformer isolation,maybe even entire equipment on that end... This is maybe generalized view from distribution systems, but can be applied to answer your question why is there a protection on LV side.

 

[Rodmcm]

For us outside the americas what is the '6 handle rule'?

 

[dpc]

6 Handle Rule: This is a National Electrical Code provision (230.71)that (basically) says that a main disconnect is not required if not more than 6 disconnects (breakers or fuses) are fed from the bus. So as long as the switchboard does not have more than 6 "handles", no main device is required.

Application of this rule is generally a hallmark of design and construction done to the lowest possible cost.

 

[davidbeach]

Application of this rule is generally a hallmark of design and construction done to the lowest possible cost.

That should really read

Application of this rule is generally a hallmark of design and construction done to the lowest possible [red]initial[/red] cost.

Rarely over the life of the facility will it actually be the lowest cost solution. Use of the six main rule has resulted in some very expensive modifications that would not have been necessary if a main had originally been installed.

 

[rcw retired EE]

An overcurrent relay fed by CT's on the transformer 600V bushings can provide phase and ground overcurrent protection equivalent to a 600V Main Breaker by tripping the 4.16kV contactor. A drawback is the contactor's clearing time may be slower than the instantaneous trip of a circuit breaker, but some vacuum contactors have fast clearing times.

A Close/Trip control switch mounted near the MCC would allow local on/off control, similar to a main breaker.

Are there other disadvantages of this cost-effective (cheap) approach?

 

[Eleceng01]

rcwilson,

Off the top of my head, the only one I can think of is knowing what tripped the 4.16kV contactor. Was it a transformer fault or the relay on the 600V system. This could complicate fault determination.

Anyone else ahve any thoughts?

 

[dpc]

The fault current must be in the contactor's interrupting range.

Also, most modern contactors have a built-in time delay on de-energization to make sure the fuse clears any serious high current faults. This would increase the clearing time and arc-flash energy.

Also, the contactor might not be considered a local disconnecting means at the bus.

 

[Eleceng01]

Thanks dpc, I was sure there were more than I was thinking of.