IEC Motor Circuit Protection

[mj1980]

I recently graduated out of school with an E.E. degree, and landed a job with a consulting firm, that specializes in onshore/offshore oil & gas platforms. I'm currently working on a project in which I have to develop one line diagrams for the MCC(Control Gear) and swithgear. I looked at various symbols of the Motor circuit protection, but can find an exact representation of MCP with magnetic and thermal protection. Second, I know heaters are installed on motors and are shown on one lines. But when do you run an individual cable to a heater? Is it determined by the size of the Motor? For example, i know when it's a small HP motor we run cable to both the motor and heater, but when it's a larger motor we run an independent cable to the heater. Please advise, thank you.

 

[EEIre]

Best thing to do is talk to your lead engineer and get his advice.

 

[jraef]

Here in North America, "MCP" means something different, although the acronym is based on the same words. We use the term "Motor Circuit Protector" to mean a magnetic only MCCB, partly because Westinghouse coined the term years ago and it has become universally accepted.

I think what you mean is the IEC device that is a combination of disconnect, magnetic short circuit protection and thermal overload. There is no universal term for that device here unfortunately, because every manufacturer uses their own TLA; Siemens calls them MSP (Motor Starter Protector), others call them MPS (Motor Protector Switch) and some even call them MCPs, which makes it confusing.

But symbology need not be confusing. Just show all three elements in one device and label it; it will look essentially the same as a Thermal Magnetic Circuit Breaker, so the label (and/or a box around it and the contactor) will avoid confusion.

Word to the wise; there are very specific rules to using them in North America that differ from how they are used elsewhere in the world, make sure you read, understand and follow those rules. That's why the suggestion of following your lead Engineer is a good one.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[rbulsara]

Hustontx;

Seanmax's advice is right on. Also a little Internet search helps.

By the way in your second question, what motor heaters are you talking about? Overload heaters are installed in the starters not on the motors. Small (fractional HP) motors may have built-in thermal O/L element,which requires no external wiring. Motor winding heaters (to keep the moisture out) are a totally different thing and requires a separate circuit.

See

http://www.kilowattclassroom.com/Archive/MotorOL.pdf

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Also "heater" OL are used on in NEMA style starters. IEC starters would use Bi-metal O/L relays.

Rafiq Bulsara

http://www.srengineersct.com

 

[mj1980]

Thank you Guy's for your help and contributions. Rbulsara, I was referring to the Motor Winding heater.