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Anonymous Poster #1

Control Circuit Transformers in abTPN MCC

05/17/2018 8:38 AM

We have 3 phase, 4 wire incoming power supply to our MCC.

1. Do we need to install a separate control transformer for this MCC or just tap one of the phase and neutral for control circuits? What are the pitfalls if I dont use a control transformer?

2. We also have some single phase loads on this MCC and can these loads be supplied via control transformer?

3. Or install a slightly a big transformer (around 10-15kVA) and use it as a control transformer and also use it for those single phase loads.
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redfred
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#1

Re: Control Circuit Transformers in abTPN MCC

05/17/2018 9:30 AM

Sounds like you need to hire an electrician or an electrical engineer. There are so many questions that need answers to even attempt to properly answer. Here are just a few:

  • Where is this? We don't even know what hemisphere.
  • What voltages?
  • Do you need a license to perform this work?
  • What are you controlling?
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JRaef
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#2

Re: Control Circuit Transformers in abTPN MCC

05/17/2018 3:14 PM

I agree, more information is better than less. Rules differ around the world.

That said, your use of the term "TPN" (Triple Pole + Neutral) in the title indicates that you are likely NOT in North America because we don't use that term and we almost never switch the neutral in a power system. So making that assumption, and assuming a nominal 400Y230V system, it is (or rather WAS) somewhat common to use L-N within a controller unit to derive 230V single phase control power. There are issues with this however, one being that it requires that your ENTIRE MCC have the N bus run throughout, which is not the norm. Another issue is that ALL of your control devices must operate at 230V, including any automation systems, i.e. PLCs, and 230V inputs and outputs are typically less dense, meaning more cards, larger racks and higher expense. Also for safety reasons 230V control has become less and less popular and may even have been rendered unacceptable (I'm not as familiar with IEC rules as I once was). Most systems have been changing over to 24VDC control wherever possible for these reasons.

As to using one large control power source and feeding it to all of the MCC units, that is also done, usually to try to save cost, but has issues as well. If the single control power source goes down, you lose the entire system all at once, no orderly shutdown is possible.

Using an individual control power transformer (or 24VDC power supply) within each controller unit is considered to be the most reliable option. Remember, what you think you might save in initial hardware cost is often lost within the first few minutes of unscheduled down time when you consider lost production and waste generated by the failure.

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Phys
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#4
In reply to #2

Re: Control Circuit Transformers in abTPN MCC

05/18/2018 8:22 AM

Very good assumption, if the company he works for is not associated with a company from outside of North America, as we are where I work. As time goes by, I find myself using more and more of their terminology, and drawing schematics that have been called "European" style by more than one customer.

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Anonymous Poster #1
#6
In reply to #2

Re: Control Circuit Transformers in abTPN MCC

05/19/2018 11:40 AM

Thanks JRaef!,

What happens if the control transformer is mounted at the line side of the Incoming Main breaker. This Control transformer suppies the control voltage to the entire Panel. Will there be any technical problem later if I put the control transformer at the line side of the incomer if the main breaker trips?; as normally under voltage protection for motor feeders (less than 10kW) is not provided. (or) do I have to think of mounting this control transformer at the load side?
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JRaef
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#8
In reply to #6

Re: Control Circuit Transformers in abTPN MCC

05/22/2018 6:55 PM

it's not good practice to connect anything ahead of a main disconnecting device, especially one that is used with safety door interlocks, unless that device also has it's own separate disconnecting mechanism that is also interlocked with the door. It's far simpler to connect the CPT after the breaker so that when someone opens the door to work on the equipment, everything is powered down, including the control circuits.

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PWSlack
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#3

Re: Control Circuit Transformers in abTPN MCC

05/18/2018 3:19 AM

The <...we...> needs to look at the drawings and ask the Equipment Designer, whose name is thereon, these questions.

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Jpfalt
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#5

Re: Control Circuit Transformers in abTPN MCC

05/18/2018 11:31 AM

The only reason to use a control transformer is to get the right voltage for the control components. If you have the right voltage coming in, you can use it as long as you protect the individual circuits properly and meet local code requirements.
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manindra
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#7

Re: Control Circuit Transformers in abTPN MCC

05/22/2018 7:34 AM

1)Please use control transformer and preferred voltage may be 110v A.C. The use of control transformer has two benefit, first, it will electrically isolated from mains supply and secondly it will reduce the fault level. Please make sure earthing for safety.

2) It is not possible to make comments unless you specify the load details.

3)Control transformer should be installed after main circuit breaker.

Manindra

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Anonymous Poster (1); Jpfalt (1); JRaef (2); manindra (1); Phys (1); PWSlack (1); redfred (1)

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