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FC Contactor and Breaker

01/03/2018 1:01 AM

Can any one explain me the difference between FC contactor and breaker. Why we use FC contactor in low power rating transformers and motors? Why FC control circuit use separate relay to energize closing coil?

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#1

Re: FC contactor and breaker

01/03/2018 1:14 AM

Let's say contactors are faster (and more expensive) than breakers. The order of magnitude would be a contactor switching every second and a breaker switching every minute.

For Low Voltage applications you must have a breaker for protection and a contactor for switching. You turn on once a day the machine (breaker) and you start it and stop it 10 times more (contactor).

The control circuits that use a separate relay are for amplifying the control signal, due to high impedance or low power of the source signal.

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#2

Re: FC contactor and breaker

01/03/2018 1:20 AM

Now, what does FC stand for? Do you mean AC (alternating current)?

I totally ignored that, so my answer might change accordingly.

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#4
In reply to #2

Re: FC contactor and breaker

01/03/2018 2:52 AM

Try this.

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#5
In reply to #2

Re: FC contactor and breaker

01/03/2018 4:10 AM

FC means fuse contactor

HRC fuse installed in series to vaccum contactor.

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#6
In reply to #5

Re: FC contactor and breaker

01/03/2018 4:26 AM

On the design documents and the commissioning documents for the installation are some names - names of the individuals involved in the design and commissioning of the installation. These names are definitely the <...we...> in the original post, as these individuals are the best source of information regarding the installation in question. The time to these people and ask the questions directly is now.

<unsubscribes>

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#14
In reply to #5

Re: FC contactor and breaker

01/04/2018 5:04 AM

Then FC contactor means Fuse Contactor Contactor? Double contactor?

If you take the effort to write contactor, then why do not you do the same with fuse?

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#15
In reply to #14

Re: FC contactor and breaker

01/04/2018 5:52 AM

It is a bit confusing - especially with all those other meanings for <...FC...>.

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#3

Re: FC contactor and breaker

01/03/2018 2:46 AM

Ask <...we...>, which is undefined at this juncture.

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#7

Re: FC contactor and breaker

01/03/2018 5:10 AM

I will assume that you are referring to a vacuum fused contactor.

A circuit breaker is intended for short circuit protection only, a contactor would still be required to provide motor starting facilities and overload protection which it would do via attached or separate overload relay.

Some circuit breakers can be also opened remotely by overload relays, but they generally do not have the ability to withstand continuous making and breaking (typically a few tens of thousands of operations and much fewer for S/Cct conditions) whereas a fused vacuum contactor can withstand hundreds of thousands of operations even under S/cct conditions.

Circuit breakers can be operated either manually or, as mentioned above, can be opened electrically. Contactors are only operated electrically.

Fused contactors are generally vacuum type where a bellows is used to close the main contacts. The bellows tries to close the contacts under atmospheric pressure, but is held in the open position by a mechanism consisting of springs and a rocker arm. A separate control relay is used to energise an internal coil to move the arm and thus allow the bellows to close the contactor.

When the relay is off, the coil is de-energised and the rocker mechanism opens the contacts. Power for the controls is often provided by a small transformer in the contactor.

A fused contactor has the advantage of very fast disconnection during high overload or short circuit conditions - a circuit breaker will normally not open in less than 3 cycles under short circuit conditions, a vacuum contactor will do so in 1/4 of a cycle.

The HRC fuses used limit the current to a preset "let through" value melting while the overload current is still rising

A contactor can also provide "No volt" protection which a manual circuit breaker generally can't do

Vacuum contactors are ideally suited for starting large motors or switching transformers, and large ones can be cheaper and smaller than equivalent capacity circuit breakers.

Under short circuit conditions, the fuses on the contactor melt before the contacts open thereby protecting the contacts from severe damage - simply replace the fuses and all is ready to go again. By contrast, a circuit breaker carries considerably more current for a longer period before it opens, and even with arc quench chambers, some burning takes place inside the mechanism.

Hope this helps.

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#8
In reply to #7

Re: FC contactor and breaker

01/03/2018 6:14 AM

<...Under short circuit conditions, the fuses...melt...simply replace the fuses and all is ready to go again...>

Ahem, er, one removes the cause of the short circuit before re-energising.

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#10
In reply to #8

Re: FC contactor and breaker

01/03/2018 12:38 PM

Remember, a fuse typically only protects wires or cables. When a short circuit or arc flash happens with power distribution the cause usually disappears all by itself.

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#13
In reply to #7

Re: FC contactor and breaker

01/03/2018 11:35 PM

excellent my friend

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#9

Re: FC Contactor And Breaker

01/03/2018 10:49 AM

explanation of a contactor

This is only the basics. There are all kinds from 110V up to 12 kV or higher.

These are for controlling the application or interruption of normal power to a load that is substantially higher than the control circuit power capacity. Such low voltages as 12 V even can be used to latch in a contactor. These may operate magnetically, or by some electrical means (as in solid state relays).

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#11

Re: FC Contactor and Breaker

01/03/2018 1:05 PM

..." Medium-voltage circuit breakers are favored when: Typical loads include transformers, capacitors, larger motors, generators or distribution feeders Ratings required exceed those of vacuum contactors (400 A or 720 A at up to 7.2 kV) Continuous load current is high (e.g., larger transformers, larger motors) Switching is not very frequent (e.g., weekly or monthly); high endurance (1,000s of operations) is satisfactory Process continuity is critical (e.g., no time for fuse replacement) Reduced-voltage (RV) starting is not needed (RV starting complicates switchgear bus arrangements).

Medium-voltage NEMA Class E2 controllers (fused contactors) are favored when: Typical loads include motors or smaller transformers Continuous load current is low or moderate (e.g., smaller motors or transformers) Switching is very frequent (e.g., daily or several times per day); very high endurance (100,000s of operations) is needed Process continuity is compatible with fuse replacement time Reduced-voltage starting is needed to reduce starting duty (and voltage fluctuation) on system."...

"Historically, circuit breakers have been used for mediumvoltage motors in certain industries, especially in utility generating stations. As these stations have aged, and station operation has changed from base-load to peaking service, many of these motor-starting circuit breakers have experienced total operations well in excess of the endurance required by the ANSI/IEEE standards. As a result, these applications have had higher maintenance costs than if medium-voltage fused contactors had been used originally. In contrast, users in the process industries have long favored the use of fused contactors for such applications, and have enjoyed long service with lower maintenance costs. When applied properly, both medium-voltage circuit breakers and medium-voltage fused contactors should provide decades of reliable service. Applied incorrectly, either can lead to major headaches."

https://w3.usa.siemens.com/us/internet-dms/btlv/PowerDistributionComm/PowerDistribution/docs_MV/TechTopics/ANSI_MV_TechTopics42_EN.pdf

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#12
In reply to #11

Re: FC Contactor and Breaker

01/03/2018 11:34 PM

excellent explaination thanks brother

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#16

Re: FC Contactor and Breaker

01/06/2018 10:11 PM

A circuit breaker (CB), I believe it's purpose is to protect the wire not the device. Contactors are made with heavy duty contacts that can withstand high resistive and inductive loads. If you were to use a CB to switch a heavy load, it would not last very long as it's contacts are not designed for switching loads. When a CB is switched on, there is no load on the circuit when doing so. A light switch can be considered as a "contactor" device used to switch a resistive load or light inductive loads. Light switches should not be used to switch inductive loads, like motors, as the back EMF can destroy the switch. Contactors with their heavy contacts are used for this. Contactors are also fast acting so as to minimize arcing at the contacts. If you were to slowly separate 2 wires on an inductive load, there would be a large arc that would pit the ends of the wires. Pull them apart quickly and the arc would be very small. This is what I think I know about contactors and breakers. I am not an electrical engineer so please correct me if I am wrong.

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#17
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Re: FC Contactor and Breaker

01/06/2018 10:54 PM

You have the correct concept that a contactor is designed to make and break planned current flow while a circuit breaker is designed to break a fault current. But you inverted the term for scaling the load. A high conductance, also known as a low resistance, load will produce higher currents.

Many people are surprised to find out that it is more difficult to safely quench (minimal contact pitting) an arc of DC rather than AC current. AC current goes to zero current 100 or 120 times a second all the time. This is the main reason why our power grid uses AC instead of DC for power distribution. The ease of changing voltages with a passive device is a significant bonus.

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#18
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Re: FC Contactor and Breaker

01/07/2018 2:08 PM

Thank you for setting me straight.

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#19

Re: FC Contactor and Breaker

01/12/2018 2:03 PM

In general, a circuit breaker is built around a design limit. It measures current until the limit has been reached, at which point it breaks the circuit. Protection is based on safety for the type of insulated wire installed.

A contactor, (fused or not) is usually a device designed to be tolerant to multiple occasions of higher inrush current. Contact metal is chosen for reduced arcing at an expected level of current. Contactors are chosen to protect a motor and several may share a circuit breaker. The reverse is not allowed.

Yes, there is some overlap in terms of protection, but contactors are controlled by a single source of energy. Breakers are controlled by humans.

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