1746-OW4 contact rating question

[david90]

For 240VAC, the contact "make" rating is 7.5A, "break" rating is .75A and the "continuous" rating is 2.5A.

Does the rating above mean that my load must not draw more than 7.5A at start up, must not draw more than 2.5A during normal operation, and must not draw more than 0.75A when it is turned off?

 

[sthompson]

For 240VAC, the contact "make" rating is 7.5A, "break" rating is .75A and the "continuous" rating is 2.5A.

Does the rating above mean that my load must not draw more than 7.5A at start up, must not draw more than 2.5A during normal operation, and must not draw more than 0.75A when it is turned off?

Yes, that is what it means. Contacts can handle more current at closing than they can when they open, due to arcing. Also, notice the DC Ratings are much lower, for the same reason. This can be mitigated by MOVs for AC and Snubbing Diodes for DC.


Stu....

 

[Lancie1]

Notice that for relay contacts, the current-handling ability is dependent on the voltage. In other words, it is more a function of power (current times voltage). For your 1746-OW4, if your voltage is really 120 volts AC, then the current level of each contact rises or doubles from what it is for 240 volts (15 Amps making, 1.5 Amps breaking). The continuous current rating is still 2.5 Amps, and that rating more a function of the generated heat from each module- an average current for all 4 output relay contacts so that module power pass-through does not exceed 1440 VA. If you only use 1 or 2 or you use every other output in the module, then each of those contacts could handle more than 2.5 Amps continuously. For example, if you used only 2 of the 4 oututs, each at 3 Amps x 120 volts continuously, then the module is passing 2 x 3 x 120 = 720 VA or about 1/2 of the rated 1440 VA. However I recommend that you always design your equipment so that each output only requires about 85 to 90% of the rated continuous Amps, and also use arcing protection. It will last longer.

 

[david90]

Practically speaking, when do you have to consider the "make" current rating? From my understand, I think you consider it when dealing with devices that have inrush current like a motor contactor.

 

[sthompson]

Practically speaking, when do you have to consider the "make" current rating? From my understand, I think you consider it when dealing with devices that have inrush current like a motor contactor.

Correct, and things like Solenoid Valves that tend to have a high Inrush Current. A bigger concern in my opinion, is the Arcing caused by the Back EMF when the Contact opens, especially in DC Circuits. PLC Output Cards to have some Circuitry to deal with this, but I normally put an MOV on AC Circuits, and a Diode on DC Circuits to help quench the Arc of Inductive Loads.

Stu....

 

[david90]

Correct, and things like Solenoid Valves that tend to have a high Inrush Current. A bigger concern in my opinion, is the Arcing caused by the Back EMF when the Contact opens, especially in DC Circuits. PLC Output Cards to have some Circuitry to deal with this, but I normally put an MOV on AC Circuits, and a Diode on DC Circuits to help quench the Arc of Inductive Loads.

Stu....

Are you saying that arcing only occur when there's a back EMF? What if the circuit is purely resistive?

 

[sthompson]

Are you saying that arcing only occur when there's a back EMF? What if the circuit is purely resistive?

Not at all, there will always be an Arc when you break a Current Circuit. However, it will be considerably less on a purely Resistive Load. This is demonstrated by two Ratings for Contacts in many Manufacturers Data Sheets, one for Resistive Loads, and one for Inductive Loads.

Stu....