Protection requirement for 480V MCC main braeker

[elecsun]

Does Main incoming breaker 1200 amps rated, should be provided with undevoltage(27) and negative phase sequence protection circuits?MCC is feeding 460Volts motors maximum rated 100HP,other small motors and lighting transformers and DB's.

 

[DanDel]

It is not required by any code I am familiar with, but is probably a good idea for your MCC motor protection, especially if each individual motor starter does not have UV or loss-of-phase protection.

 

[busbar]

 
Except to inhibit starting, negative-sequence voltage protection for a motor group is virtually useless.

 

[alehman]

Consider negative sequence current sensing (ANSI 46) for more reliable single-phase detection.

 

[RRaghunath]

I think undervoltage protection and negative phase sequence protection for MCC incomers is unnecessary.

Presuming the motors are contactor controlled, the motor protection against system undervoltage is built-in.

Negative phase sequence protection is best provided individually in the motor feeders, though I don't think 100HP rated motors require separate negative phase sequence protection (the bimetal thermal overload relays have inherent fast acting protection for negative phase sequence currents).

 

[jraef]

I think that main breakers 1000A and above are required to have ground fault protection, and I have occassionally seen residual GF protection described as negative sequence protection, but that is a stretch. Where are you seeing this requirement come from?

"Venditori de oleum-vipera non vigere excordis populi"

 

[jraef]

From NEC...
"240.13 Ground-Fault Protection of Equipment.
Ground-fault protection of equipment shall be provided inaccordance with the provisions of 230.95 for solidly grounded wye electrical systems of more than 150 volts to ground but not exceeding 600 volts phase-to-phase for each individual device used as a building or structure main dis-connecting means rated 1000 amperes or more."

I have had several inspectors insist that "structure" includes MCCs, and you can't argue with inspectors. usually they will only apply it if the MCC is feeding power to the building service distribution transformer and load centers, so that main breaker is essentially the "Service Entrance".

"Venditori de oleum-vipera non vigere excordis populi"

 

[alehman]

"the bimetal thermal overload relays have inherent fast acting protection for negative phase sequence currents"

This confuses me. It's a heater. Why is it more sensitive to negative sequence current? Motors will draw more current when the voltages are unbalanced, but the tripping response of a typical starter may be too slow to protect the motor. I agree that it's better to provid protection on each motor, but one 46 relay at the MCC is better than nothing.

 

[RRaghunath]

alehman,

the bimetal overload relay has inherent fast action for negative phase sequence currents due to the manner in which the three (phase R, Y, B) bimetal elements are coupled, called differential bar mechanism. This makes the bimetal relay operate even when the current is lower than the rated motor current in case the current in all the three phases (I should say the heating in the bi-metal elements) is not identical, that is under unbalance load (that gives rise to negative phase sequence currents) conditions.

 

[DanDel]

Just to clarify, isn't ground fault protection correctly described as zero sequence, not negative sequence, current protection?

 

[alehman]

rraghunath - That's interesting. Can you point me to some where that affect is described in detail?

It seems to me that ground fault protection is a separate issue. The NEC requires ground fault protection for circuits >150V to ground and rated 1200A or more. The code allows it to be at the protected equipment or upstream.

A ground fault produces zero sequence and negative sequence current. Traditionally it is sensed with some means of current vector summation. I don't think a negative sequence relay will meet the requirements of the NEC for GF.

 

[rbulsara]

My two cents...

I have not seen UV and 46 protection on MCCs of this size . On utiliy services I have seen 46 relay, but set for alarm only. We in fact had nuisnace trip issue with one of those too.

I prefer to see individual protection on large motors. 100HP motor is not a large motor, imho.

In the USA you can easily specify starters (even in MCC) with relatively inexpesive phase monitoring and UV relays for small motors like the one you are describing

You have to evaluate risk of shutting down all motors upon a false trip vs. providing protection to individual motors.

 

[jraef]

"Just to clarify, isn't ground fault protection correctly described as zero sequence, not negative sequence, current protection?"

Yes, but I have seen quite a number of specifications that incorrectly call for "negative sequence ground fault protection", and when I ask for clarification, they say that they want what turns out to be residual GF as opposed to zero sequence GF.

I think there is a corresponance school out there somewhere that is churning out these engineers who have no idea what they are specifying, but that it a different topic...

"Venditori de oleum-vipera non vigere excordis populi"

 

[dpc]

In my experience, it is quite common to apply loss of phase (negative sequence) protection to an MCC. In less-developed regions, such as the western US, it is quite common to loss one phase of an incoming service for an extended period of time. I am aware of facilities that lost nearly all their induction motors due to single-phasing.

It is not required by NEC, or any other code that I am aware of.

Regarding the ability of simple bimetallic overload relays to protect against loss-of-phase - I wouldn't count on it. It may depend on the Class of overload relay, but I know from experience that the typical OL relay used in the US will not provide complete protection against motor damage due to loss-of-phase.

 

[rbulsara]

negative seq current protection is not inteneded for ground fault protection but for phase imbalances including single phasing with cause negative sequence currents to flow which can cause overheating.

See

http://www.basler.com/downloads/udj7bull.pdf

It is brochure for a 46 relay. Gives some info.

A ground fault may cause it to operate I would think, but it is not intened as substitute for GF.

 

[RRaghunath]

alehman,

You can refer to any LT switchgear vendor for the bimetal relay literature, to my knowledge, all makes of the present day relays have the feature built-in.

dpc is right when he says that the bimetal relay will not give complete protection for unbalance conditions. It is a sort of free add-on only.

In a case such as explained by dpc, I would also prefer to go in for separate negative phase sequence protection for motors. In power plants, the sort of situation is unlikely.