Contactor versus Breaker
Contactor versus Breaker
(OP)
I am involved in an application where a synchronous generator is being paralleled to the utility across an electrically held contactor. A manual circuit breaker is being used upstream of this contactor to provide overcurrent protection and fault clearing. In all of my experience I have never run into this scenario. In other words the industry standard that I am used to has allways employed syncronizing across a circuit breaker.
I do not believe that this (using a contactor versus a breaker)is a robust design yet I cannot find any reference that discusses this subject.
As I am preparing for an FMEA of this design is anyone aware of a published standard or recommendation regarding the use of a contactor in such an application?
I do not believe that this (using a contactor versus a breaker)is a robust design yet I cannot find any reference that discusses this subject.
As I am preparing for an FMEA of this design is anyone aware of a published standard or recommendation regarding the use of a contactor in such an application?






RE: Contactor versus Breaker
RE: Contactor versus Breaker
RE: Contactor versus Breaker
We installed a small generator and source transfer & synch control system delivered as a package by Cummins at our site last year. They proposed use of a contactor for connecting the generator (350kVA) to our internal 415V network. Their PCCP control system was set up to do this, and they said it couldn't be configured for a pulse-ON pulse-OFF control of a circuit breaker using separate closing and tripping coils, which I struggle to believe. We used a hard-wired solution to convert their relay output to a pair of signals interfacing to our circuit breaker. I didn't like the idea of using a contactor in place of a breaker any more than you guys, and for the same reasons, but it seems that it is not unheard of and may even be standard these days.
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If we learn from our mistakes,
I'm getting a great education!
RE: Contactor versus Breaker
I do welcome any opinions or experiences that you could share on the subject though as I intend to use these resposes in my presentation to the design engineers. Pro's and Con's alike.
Obviously the biggest pro of using a contactor is the cost reduction for the hardware. However the cons are the convoluted logic required to get the contactor to act like a breaker (from the controls perspective not the protective functions), and the extra real estate for the additional hardware.
The application is for a small DG (<500 kw). Also worth noting is that already in the development of this application several events have occurred where out of sync closures have happened and equipment damage has been involved. Unfortunately the fix so far has been to add more devices and protection in the contactor's control circuit.
As you can tell by the flavor of my comments I do not support the use of the contactor. Unfortunately, so far, I'm forced to go along with it. I'm hoping that I can find some real ammo to use in my argument for a circuit breaker.
Thanks...
RE: Contactor versus Breaker
The upshot of that is: what is the fault level on your system at the point where your generator is connected? If the generator can be sync'd to the utility then the fault level could be quite large. You need to consider if the contactor could open under fault conditions, and then ask if it could safely break the fault current. Is there something else backing up the contactor such as HRC fuses?
Most generators can't deliver exceptionally high fault levels because the AVR will act to limit the set's output, but do you trust the AVR enough to rely on the built-in protection alone? I don't, and I deal with AVRs from the 350kVA set I mentioned up to those on the 365MVA turbo-alternators. On all these sets, dedicated short-circuit protection is provided external to the generator's control gear.
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If we learn from our mistakes,
I'm getting a great education!
RE: Contactor versus Breaker
I have also seen contactors "chatter". This is where the control power is somewhat weak or low. If the supply for the coil voltage is marginal, the contactor may attempt to close in but when the coil is energized, it pulls the voltage down a little bit but enough to drop the contactor out. Then the process repeats itself. Granted this is likely a rare event but nonetheless could be very detrimental to your generator (and anyone around it).
RE: Contactor versus Breaker
I'm curious though...the theme in this thread seems to be that breakers are better than contactors for switching high currents. If so, then why do most engineers prefer contactor type ATSs (ASCO, Russelectric, Zenith)instead of breaker type ATSs (Cutler-Hammer)?
RE: Contactor versus Breaker
RE: Contactor versus Breaker
RE: Contactor versus Breaker
Over time there have been a lot of bad designs for circuit breaker transfer switches. There are two general types of circuit breaker transfer switches; electrically operated and mechanically operated.
The electrically operated switches rely on the trip and close coils of insulated case or power circuit breakers, and these breakers also need a spring charging motor. There should not be any particular reliability concerns with this type of transfer switch.
The mechanically operated switches rely on some mechanism to move the breaker handles. In the older designs, this mechanism rotated on a point midway between the two breakers and applied a off-center torque to the breaker handles with the result that sometimes the handle broke. Newer designs apply the force to the breaker handle directly on its axis of motion and should be more reliable.
RE: Contactor versus Breaker
Circuit breakers used for ATS applications should be electrically operated, insulated case or power circuit breakers.
RE: Contactor versus Breaker
The circuit breakers are preferred only when the contactors are not available for the current ratings required, in my experience.
Trust the above is helpful.
RE: Contactor versus Breaker
Thanks
RE: Contactor versus Breaker
RE: Contactor versus Breaker
I think that the application needs to consider the duty cycle involved. I did some low-head hyro generator control systems back in the early '80s where farmers were using irrigation ditch drops to recover electricity (let's not get into that tangent though). We used motorized MC circuit breakers in the arrangement based upon conventional designs, but quickly discovered that MCCBs are not designed for use as a contactor because the mechanical components wear out. When we confronted the manufacturer, Westinghouse, about it they denied responsibility. They stated that breakers are designed as protection devices with limited mechanical life, not control devices that would come on and off repeatedly because the mechanical life attributed to MCCB design is only 10,000 operations (I have never seen that corraborated though). They suggested the exact design you described in the beginning. Breakers for protection, contactors for switching. We did change, and used latching vacuum contactors. The mechanical wear out problems went away.
"Venditori de oleum-vipera non vigere excordis populi"
RE: Contactor versus Breaker