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MEUSA (Mechanical) (OP)
8 Oct 12 18:16
We're looking into upgrading our system. Can we use MV MCCs to feed transformers, or do feeders have to be switchgears? What is prefered and why? Also, can I have transformers feeding LV MCCs directly, and not have to go though a LV switchgear? Is the a specific size HP for LV motors before it's prefered to use a switchgear breaker? I appreciate any assistance on this!
khanengr (Electrical)
9 Oct 12 4:39
According to my inforamtion and knowledge, there is no issue whether you use MCC or Switchgear, but normally the MCCs contains Isolators (Switches) not Circuit breakers and over load relays with magnetic contactors. Which is usually preferred in motors case not in transformers casse, In trasnformers we need a proper Circuit breaker with thermal over load and magnetic short circuit trip units or alternatively micro processors based trip unit if Transformer is of Low voltage and less power.

So, if you have MCC which uses Circuit breakers and not isolator/switch and breaker fullfills requirements of transformer protection then you can use MCC for trasnformer other wise, make feeder in switchgear.

And you can use switchgear for motors as well, with proper rated protection for motors.But MCCs are since especially built and convinent from operation and mainteance point of view so they are preffered for motors. Rest is your choice MEUSA.

This is what i think, rest memebers comment what they feel
Zogzog (Electrical)
9 Oct 12 9:20
If you feed a MCC directly from the transformer you will run into some arc flash issues, something to consider.
MEUSA (Mechanical) (OP)
9 Oct 12 10:16
Thanks everyone. Zogzog, I still don't understand why we will have arcflash issues with feeding MCCs Vs Switchgears. We can have similar arcflash relaying on both systems, right? Also, for larger LV loads (like over 100 HP), is there advantage of using switchgear breakers instead of regular MCC?
ScottyUK (Electrical)
9 Oct 12 11:55
You probably wouldn't consider using a breaker for anything under about 350 - 400 HP, which is around the point where you would normally be using an MV motor for a DOL application. Cost-wise there won't be much difference. Below that size economic considerations would tend to rule out using a circuit breaker for LV motor control.

Zogzog is saying that a directly connected MCC without an incoming brekaer or fuse will have its protection provided from the HV side of the transformer. With a Dyn* transformer the HV side can't distinguish an LV earth fault from an overload, and the likelihood is that a very high arc flash energy will result from such an arrangement.
Zogzog (Electrical)
10 Oct 12 9:02
Yep, that is where I was going, thanks Scotty.
MEUSA (Mechanical) (OP)
11 Oct 12 10:08
Thanks everyone. I have been told that a LV MCC can have an input breaker. If that's correct, I believe we can have the same level of transformer protection, right?
jraef (Electrical)
11 Oct 12 13:50

Quote (MEUSA)

We're looking into upgrading our system. Can we use MV MCCs to feed transformers, or do feeders have to be switchgears? What is prefered and why? Also, can I have transformers feeding LV MCCs directly, and not have to go though a LV switchgear? Is the a specific size HP for LV motors before it's prefered to use a switchgear breaker? I appreciate any assistance on this!

As I interpret your question, you are referring to FEEDING transformers from this equipment, and you are asking if it is better to feed transformers with circuit breakers in MV Switchgear, as opposed to feeding them with "Transformer Feeder Units" in MV MCCs, is that correct?

The difference, for the most part, is nominal but it also depends on what you prefer in terms of "Switchgear". If you want draw out Metal-Clad switchgear, then you will not get those advantages using an MCC. Metal-Clad vs Metal Enclosed is a separate decision making process too tangential for this discussion, other than if you DO want Metal-Clad, then the rest is a moot point. This is, by the way, in reference to ANSI (North American) standards for Switchgear design. If you are not in North America, then IEC Switchgear offers less clear differentiations.

But if you are referring to IEC equipment or were thinking of using Metal-Enclosed ANSI switchgear, then the basic construction is very similar to an MV MCC anyway, so there is only the difference between circuit breakers and contactors to consider. I'm going to assume you would only be considering Vacuum Circuit Breakers (VCB) vs Vacuum Contactors (VC) in this discussion.

The general issue of consideration in that issue is one of application. VCBs are not designed to operate as often as VCs, but a VC needs to have energy on the control coil continuously (excluding latched contactors for simplicity here). So if you are planning on energizing and de-energizing these transformers once per day, I would not use a VCB. That however is typically not how transformers are fed, it is more common that it is energized and left that way indefinitely. That makes a better case for feeding them from a VCB. But in a recent discussion I had with one of our PEs who does Arc Flash mitigation, he brought up an interesting point. If you are looking to use a Protection Relay system to lower your Incident Energy in the down stream equipment, a VC can actually operate faster than a VCB, because there is no mechanical unlatch time involved. I'm not deeply cognizant of all the issues with this because I just heard it mentioned for the first time very recently. But it's apparently something worth considering if this has any connection to Arc Flash mitigation.

From a protection standpoint, if you are going to specify a good Feeder Protection Relay anyway (or a similar level of high-end trip unit in a VCB) there is no reason to consider those issues between the two choices. If you were to want to use the less functional basic trip elements in a VCB, but were willing to allow only fuses in the MCC transformer feeder, then the MCC version is likely to cost you less. I can't recommend that though.

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MEUSA (Mechanical) (OP)
13 Oct 12 11:21
Jraef, that was very helpful thank you. I wanted to collect more information before i make a post. It makes sense that contractors are faster than breakers, but if I understand you correctly, that only applies to electrically held contractors right? The plan is to use MCC to feed a transformer, and an other LV MCC to be fed from the XFMR. The plant likes to use GE Multilin relays on both sides of the XFMR (feeder on MV MCC, and main breaker on LV MCC if possible). Will we have any protection problems in that configuration? Arc Flash is a very important issue.

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