Contactor Anti-Telegraphing Feature 1

[hbendillo]

I recently specified a minimum 225-amp, 3-pole, 240 volt rated electrical contactor for a project. It's function is simply to remove power from a panelboard feeding cooking equipment under a kitchen hood when the hood fire protection system is activated. So there will be very little switching for this contactor. It is an electrically held contactor, constant voltage to the coil. Remove voltage contactor drops out, apply voltage contact closes.

Our client has a problem. Come in kitchen to cook and no power to cooking equipment. This contactor dropped out but there is still voltage to the coil. Turn off control voltage at the circuit breaker then re-apply, contactor re-engages. Ok, long explanation.

Bottom line this contactor has an anti-telegraphing feature that basically keeps the contactor from re-engaging for very short term switching. If there is a short power blip, contactor won't re-engage. I can switch the circuit breaker very quickly and it won't re-engage.

Manufacturer says this is a standard safety feature for their contactors of this size and larger. I've never run into this before. I didn't specify it that way. Is it standard for all manufacturers or is this guy blowing smoke?

 

[crthompson]

It is not a standard for ALL manufacturers. I am like you, have never ran into this before. The following is from a patent I found online. "An anti-telegraph circuit for locking out the circuit controlling the flow of direct current in a coil winding of an electromagnetically operated switching device, such as a contactor, from an alternating current source whenever improper control voltages are present which would cause repeated operations of the armature picking up and falling out of its sealed position." All your client should have to do is reset the control voltage... You may want to install a contactor without this feature if it is causing problems, but keep in mind that you won't have the anti-telegraphing feature if you have control voltage problems later... You just have to weigh the options.

 

[jraef]

... standard safety feature for their contactors of this size and larger.

That may be true, so I would hazard a guess that you are using Square D. They, Cutler Hammer, Louis Allis and Allen Bradley, all based out of Milwaukee*, used this circuit on larger NEMA contactors where they use DC coils on AC control supplies in order to keep the coil holding consumption low. But the problem with that is that the coil needs to have the rectifier built-in, and what was called an "economizer" circuit to reduce the coil consumption once pulled in. The next problem then becomes if the contactor control circuit "chatters" (or telegraphs as it is called here), it can be destroyed in short order and may fail closed. So this "anti-telegraph" circuit was employed to overcome that failure potential, with the side effect being exactly what you experienced. Eaton (Cutler Hammer) and Allen Bradley long ago abandoned this design because of advances on other coil designs that came out of their purchasing IEC mfrs. Sq. D on the other hand was PURCHASED by an IEC mfr, who retained the better coil designs in their IEC (Telemecanique) product lines and left Sq. D's NEMA product lines alone. So is it a standard practce? Used to be, but you most likely stumbled onto the only mfr. still using this design for a standard AC contactor.

I should mention though that this scheme is still used in a number of Vacuum Contactors, because for them, the contacts themselves will weld far sooner than the coils will be damaged if there is control circuit chatter.

If you are leaving a contactor on for long periods, a common practice is to use a Mechanically Held contactor. But if this is tied to your ANSUL system I don't think you are allowed to do that, so you only choice is to use another brand of contactor.

*(I mention Milwaukee because there was likely a connection among the engineers or a common professor at Marquette University, the breeding grounds for engineers working for all 4 of those companies at one time.)


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[davidbeach]

I'd forgo the contactor entirely and go with a circuit breaker. I always used a shunt trip breaker, but you could use an undervoltage release instead. The shunt trip is more secure and the undervoltage release is more dependable. Both require manual reset once tripped.